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Oct 06, 2014, 06:44 PM
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Build Log

Hangar9 B25 Mitchel full electro

For reasons I still am unable to pinpoint or identify, the B25 Mitchel bomber has always been an absolute favorite of mine. After becoming proficient in R/C flying in the mid-seventies, I ordered the then nec-plus-ultra Royal B25 kit and 2 K&B40 engines. Unfortunately a partner and building a house caused me to sell it before I even started assembling it. Just before the turn of the centuries I had the incredible luck of captaining the real B25 N320SQ during a display season. A decade after I quit real flying, I resumed R/C and it didn’t take long for me to acquire a couple of FMS foamie 1m40 B25 Mitchels, which I painted as Lotys II and Sarinah, the two schemes I flew in for real. After finding out the correct mix between throttle and elevator, I happily flew them for 3 years because they were practicable, stable and forgiving, even in moderate winds.

Here is an onboard video of the model flying the identical display as I flew it with the real B25 of the DBAF anno 2000.
B25 routine 010814 (6 min 37 sec)

All that time I was dreaming of a larger size B25 model, but it had to be electrically powered and affordable, and preferably second hand with electric retracts. New ones were unavailable because the Hangar 9 ceased production of it. During the summer of 2014 I saw a Hangar 9 model advertised in almost new condition for a more than reasonable price. After a few weeks of bidding, I got it and collected it at the second owner’s place. Seeing his other models I believed scale flying was not his thing, and the reason he sold the model half a year after acquiring it second hand. A couple of weeks later I found time to make a detailed inspection and inventory of that purchase.

Inventory and inspection:
Engines: 2x Dymond AL 4260 profi, 600Rpm/V, 12-24V, 900W, 5mm shaft, 4-5S 42A
ESC: 2 x Hobbywing Pentium 60A UBEC 3A5v, DC 5,6-26v . Props: 2x13x8E twoblade (recom 4S 12x6)
Servos: NWS: Hitec311. Elevator: 1 Hitec225MG (4,8kg6v). Rudder: 2x Hitec HS81 (3kg/6v).
Ail 2xDymond 5000. Flaps: 2 x dymond 5000 (3kg). L/G: HK. Nosegear door unknown

Wings 1980gr each, spar 136gr, fuselage 2040gr. Weight dry: 6140gr . Scale 1/10th
cg 95mm gear up, AUW 7,3 kg Wing bolts: hex 5mm, tail bolt: hex 3mm, engine nacelles: 9/64th
Control throws in mm: elev 16,rud 21, ail 14, t/o fl 26, land flaps 49
separate bat for gear&flaps. bec + 3S2200 in nose, Nimh 4,8v 3000mah in nose, 2x4S4000 in wing.

To do list:
Design transport/battery change cradle with repaint in mind
After initial flighttest:
Replace 2 blade props with 3 blade props.
Remove gun upper turret, remove half of the cockpit side guns
Install wing oil coolers and tail bump plastics.
Repair crack around port l/g opening
Install twin sound system
Redecorate the model to portray N320SQ anno 2000 Loty’s II scheme

Hangar 9 produced a very representative wood and modern cover 2 meter reproduction of a B25 bomber. The planked wings and glossy cover material might be attractive for blink-blink attracted people, but are a far cry from my desires for a realistic representation of a paneled and riveted WW2 weary warbird. Even looking very carefully, I could find no traces it ever had been used. After dismantling all the parts (held together with just a few but seriously sized hex screws), the interior showed no surprises. Everything had been glued correctly and adequate servo’s installed. Although the Hightec 225MG servo has a good powerful reliability record, I would have preferred dual channels and servos for the elevator controls. The Dymond AL4260 engines with 60Amp ESC’s seem adequate to power that 6kg+ (weight without batteries or accessories) into the air in a scale fashion with twin 4S4000 batteries. I also opened up all other servo receptacles and noted the types installed, checked their installation and looked up their figures and reliabilities on the internet.

With no structural worries, I now could concentrate on the mechanical and electrical aspects. Mechanically the flight controls were ok, but the Hobby King electrical gear had excessive play. My model was deliverd with (spare?) high quality expensive Robart air-retracts with less play, but these had been taken out by the previous owner and were in a box . Both systems have sprung gear legs. I much prefer electrical gears to pumping up the pressure on a bottle, but only the operation from our hard surfaced runway will tell me the way to go. Connecting an additional 4 airtight hoses each time I attach the wings doesn’t sound tempting either. Selling the Robarts and my old FMS airframes might cover some of the price of the second hand Hangar 9 model, so I’ll see how that situation evolves when I get the Hangar 9 model to perform to my likes. The fact the air system legs without wheels weighs more than 600gr also plays a role. The builder installed a large pivoting nosegear door actuated by a separate servo and sequencer. That might have worked on the snappy air retracts, but didn’t with the slower electric retracts. If the previous owner ever flew it, it must have been with the gear down or a sequencer with extra-long time interval.

Hangar 9 recommends to make a shaped cutout in the geardoor, then fix the door with screws to the airframe. On a real B25 the large gear doors only opens during the actual gear travel, but are closed when either gear is fully up or down, leaving just a small slit open for the gear leg and actuator rods. The open nosegear door on the model looks awful on the ground (just as on the FMS model), so I’ modified that to mirror the cutouts for the maingear in the engine nacelles, thereby eliminating possible mishaps with the sequencer. After obtaining a good fit with minimal cutout, I discarded the nosedoor servo, and before mounting the door I investigated if I could correct some problems with the nosewheel. In the real B25 (and on the Robarts) the nosegear has a forward rake. This Hobby King one is mounted at 90° and with the play it even leaned seriously backwards. This meant taking the nosegear assembly out, and also attempt to minimize all plays, and reinstalling it at an angle after mounting it on a wedge. I didn’t glue the wedge in because if the Chinese retracts prove to be too light I want to keep the capability to reinstall the Robarts, which apparently can be modified to operate electrically (but at a serious cost). There was hardly space for me to unscrew the locking nuts from the bolts, neither side being sufficiently wide to use the necessary tools to tackle the job with standard tools. It took me quite some time and experimenting with various tools to dislodge the retract from the plywood. I discovered the latter to have been weakened a lot by making large holes to accommodate the Robarts and pressure hoses. The nosewheel steering also had much inherent play and was disconnected. To completely take the nosegear out I also had to use knives and screwdrivers because the previous owner managed to glue the servo electrical plug connection with its extension piece deep into the wheel well, using a liberal amount of epoxy! I think it was a blessing for aviation when he decided to become an airline pilot instead of a mechanic.

After opening the retract mechanism, I discovered everything to be in metal, but although brand-new, had excessive tolerances which allowed much play in the down and locked position. I couldn’t reforge those pieces, but using thin scrap metal plates I was able to eliminate most of the system play. Removing the strut and the steering mechanism, I noticed no flattening where the Allen screws grabbed the axles, nor thread lock to keep things in place in the long term. To eliminate wheel wobble around the axle I used a copper tube as a bushing. What could be done without extracting the main gears from their pods was also tackled. The net result was a dramatic reduction in overall landing gear play in the down and locked position. I had to glue some guide plates around the nose gear so it retracts and extends jam free from its narrow confines within the door cutout. I purchased longer bolts with Allen heads to fix the gear to a wedge shaped plate at the back of the front frame, and also tools designed for R/C cars to be able to tighten all nuts in the tight recesses. Wires got rerouted and labeled, and the remains of the nose door inserted on one side with the hinges, the other side being bolted down for possible later access.

Before connecting the wings I started labeling the existing servo cables by using a servo tester. Flaps and aileron throws were measured and proved correct. Checking the tail I noted the rudders not to be in a mechanical centered position, and their throws to be less than recommended. After opening up the rudder servo trays on the bottom of the horizontal stabilizer I noted the starboard one to be jammed and distorted by glue, and attempts already made by the previous owner(s) had damaged the wood plates and covering, and this was first rectified.

Both servo arms had been mounted off center and that was first rectified. When opening the quick links to engage them closer in the bellcranks to augment the deflection, one broke right away and I noted how brittle it was. A replacement link was installed correctly, and although the rudder throws are now larger than recommended, I prefer that in order to better handle single-engine situations, and crosswinds during taxi. After removing the complete tailplane I got access to the elevator servo which had been mounted on the horizontal stabilizer, and I also reset the arm and kwik-link, thereby reducing elevator throw to half of what it was, but still a bit more than the recommended high-gain figure. The tail was bolted to the fuselage again using a single solid bolt at the back, and a single dowel at the front. With the elevator servo underneath, epoxying the tail to the fuselage for added solidity is no option anymore.

Each time I turned the fuselage over, I got annoyed by the bombardier/nose gunner’s compartment falling open. Somebody somewhere modified the system so it pivoted upwards around a single hinge point that had been solidly glued in position, and seemed impossible to detach anymore. Nothing but gravity held it down and closed, well sort of, because there was a serious gap between the transparent plastic and the fuselage sides, which reinforced my conclusion that the model never had been flown in its latest configuration (or was it flown just taped-up?) but why?

Because apparently the nose needs additional weight, I decided to cover the glue a piece of custom shaped triplex between the lower window frames for the following purposes; cover and secure the balance weights, battery for the receiver and the one for the landing gear retracts, be attached to the sides to reinforce the forward canopy shape to hug the fuselage shape, and at the front form a solid base for a system to keep the nose compartment closed in flight. The whole assembly had to be painted in advance because the visible parts wouldn’t be accessible once glued. Using canopy glue that had to be pressed and left unattended for 24 hours. The glue wasn’t solid enough, and I still ended up with ungainly gaps between Perspex and fuselage, which magnets wouldn’t be able eliminate. I thus reverted to a recently purchased glue gun and after a few tries got the new base plate right. I still had to cut away parts of the nose plastic to make it conform around the nose shape, and produced a custom-shaped hard wooden piece that was glued at the bottom and allowed the plastic to be tightened and screwed around it to keep everything in place. Figuring the nose batteries would have sufficient power for a couple of flights, that nose could be kept shut tight for the weekend flights if an on-off switch got placed close to the engine batteries that had to be taken out and charged after each flight. It wasn’t the best solution, but would at least allow me to test-fly the B25 before the winter makeover. With the extreme front and back ends being rectified, it became time to cater for everything in between.

With the nose drying, the only things I could do was label some more wing wires, test the main landing gear operations, and install the oil cooler intakes on the leading edges of the wings. Flap and aileron operation were through multiplex connectors to the fuselage, but the latter had to be glued again in their receptacle because they came apart upon pulling the plugs out. Looking at the main battery emplacement below the wing, I saw that those two heavy separate 4S4000 (one for each engine) were supposed to be suspended from the roof of that tray by a simple rather narrow Velcro strip, has it really flown that way? I decided to replace those with twin strong straps for each battery, because either G’s or a hard landing could have dislodged those 400 gr batteries allowing them to fall through the light balsa fairing below the wings. The battery cables from the nacelles were shortened as much as practicable and Deans plugs soldered at the ends. I chopped off half the plate length extending forward of that compartment cover. It wasn’t necessary for strength and prevented the batteries from being placed as forward as possible for CG reasons. A few obstacle blocks were placed around the batteries to prevent them from sliding in flight.

The previous owner was afraid that a jammed electrical landing gear could drain the battery, and opted to install a clever self-designed system so that the positive and negative gear servo cables are connected to a separate 4,8V NiMh 3000 mah battery in the nose, and only the signal wire is connected to the receiver which is powered through a 5v Bec from a 3s2200 Lipo, also in the nose for balance purposes. His device having provision for 4 servos, I figured that using the glued-in 2- and 3-channel Y cables, I could also use this setup for the flaps which are noticeable power guzzlers on such a large model.

Connecting everything to a Spectrum AR9000 DSM2 receiver was not too difficult but caused a high concentration of wires and additional labeling was produced to eliminate guesswork for field assembly. During all the turning upside down of the model I managed to accidentally dislodge the front canopy hinge. This was lightly glued back on with cold elastic glue so it can be removed after the testflights, and the complete system revised to hinge from the bottom, with strong magnets to keep the top back in place.

When all electronics were working properly, the model was put right side up on the Sig balancer that had been set for 95mm back. With the 3 gears retracting backwards, I balanced the model with the gear up. By swapping the 2S for a 3S flight battery, it didn’t require too much additional weight in the nose. Lowering the gear brings the CG forward which should give even more stability during the critical takeoff and landing phases. The ready to fly weight is already a hefty 7,3kg which leaves little compression margin for the main gear legs. After the test flights, the lead in the nose will be removed and a Mr RC sound system especially designed for twin engine B25 sound (separate startup) will be installed under the pilot compartment. As is, the model is ready for some test and shakedown flights before its further makeover during the winter break.

I was glad to get some help from Eric at the field to assemble and turn this (for me) heavy model around. After a thorough preflight and range check, I neglected the strong wind hoping the weight of the plane would be sufficient to counter it. Taxiing out towards the runway I noted it didn’t track straight but moved in a crab. A bit of torsion on the main landing gear leg corrected that, but also proved that I should also have dismantled the main gear gear legs to flatten the spots were the Alen screws held the legs tight into the mechanism. I just crossed my fingers these would hold for one takeoff and landing, but now agree that this was a serious risk during the short flight.

With everybody else on the ground (standard procedure in our club during maiden flights), I positioned straight behind the Mitchel so I could monitor any crabbing tendency and abort. Acceleration was more positive than expected, and liftoff achieved slightly past the halfway mark. I kept the gear down, and made a shallow climbing turn to downwind, trimming as I could in the turbulent air. Flight controls seemed to respond in a normal way, and I even was able to reduce the throttle during some level maneuvers. It had little or no adverse yaw during aileron reversals, didn’t bleed off much speed nor loose any altitude during steep turns, and could be flown safely at slow speeds (without actually exploring full stalls). The model didn’t stand out well against the dark threatening clouds, and I made a mental note not to fly it anymore without invasion stripes. In the meantime Erik (a very experienced scale pilot who competes in the Benelux cup) had joined me to comment and help if needed. With this headwind I had no plans to explore the flap operation range, and with uncertain engine consumption and gusty winds, I was satisfied of what I had seen and entered the pattern for some no flap approaches. The idea was that when I was satisfied and steady, I would land it, otherwise I would take it around and make as many short patterns as necessary.

The first pattern worked out perfectly, and once on a straight final I had it steady on the glide slope and well aligned. I didn’t let this occasion pass, and started flaring when about a foot from the runway. The combination of a bit of power, lack of flaps, and headwind, made it balloon up to half a meter, but I was able to catch the ensuing rate of descend and touched down just before halfway, the model rapidly coming to a standstill due to the headwind…and the main landing gear legs pointing a bit outward. I noted the flight time had been 4 minutes (plus ground power and range check at full power bursts), and after taking the belly cover off I measured the batteries, which still indicated over 60%. In the future I don’t have to be afraid to fly for 8 minutes and that surprises me.

Although the turbulence and wind prevented me from executing the normal maneuvers on my maiden flight test card, I was very satisfied of what I had seen (except the main landing gear off course). It flew, and well, and thus could be further finished (mechanically and cosmetically) before resuming detailed test flying. Despite its reasonable 2meter wingspan, it is a hefty model when compared to the 1m40 FMS Mitchel and a 4m span 11kg Blanik glider (more about the latter to be posted soon).

I’ll complete this build log after the winter restoration and spring 2015 test flight program.
Last edited by BAF23; Dec 22, 2015 at 07:06 PM.
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Nov 10, 2015, 06:40 PM
The sky is the limit
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Thread OP

Improving the Mitchel during winter 2014-2015

1) Landing gear

After the autumn 2014 single test-flight I’ve had enough of those cheap Chinese retracts that didn’t look good and were probably not meant for 8kg aircraft. I took the robust Robarts out of the spare parts box and started searching the web for kits to change them from air to electrical actuation. It didn’t look like straightforward swaps, and most suggested to send the air gears in for them to make the modification. As prices were roughly the same in the US and France, I choose the latter so I wouldn’t face additional import fees by customs upon return. I would hate to pay 11% import plus 25% luxury tax on a 450$ Robart gear that already had been in Belgium, plus on the 400$ electrification of it. After a confirmation from the French LADO people that if I sent them my gear, they would electrify it and send it back after a week for a total sum of 390 Euro, that became the best feasible option. Upon return I noticed they had done excellent work machining new sliders etc, but was less enthusiast about their electrical engines. The long worm axles were not straight and caused the engine-case to wobble during motor operation. Because the engine-case is mounted on pivots to allow the custom copper sliders to move through their shaped guide rails, this is not a real problem but looks and sounds awkward and was not expected for that steep price. I also expected these motors to be a lot stronger. The LADO people ascertained me that they are powerful enough, time will tell, but they don’t need much resistance to just stop the movement and cut the electric power. With 100gr wheels that have to be pulled up, I wonder how well that will work. Using a 6V 5-pack NiMh instead of my usual 4,8V testpack seemed to help. Advertising the engines to be good till 8V and having a separate electrical power system, made me consider using a separate 2S Lipo battery for gear motor operation.

Following picture illustrates the LADO modified Robarts with the red electric motors in place, and their individual electronic control circuitry next to them on the white foam. Below them are the Robart wheels that I purchased, and on the red carpet are the pneumatic system components that were discarded, including the 3 Robart air actuators on the right.

The wheels the Mitchell had upon delivery were much too small to look scale. After scaling down the results of an Internet search for tire sizes of the real aircraft, I came up with reasonably close 2-1/2 inch (63mm) for the nosewheel (fitting exactly under the nose-leg arch) and 3-3/4 inch (95mm) for the main wheels. Robart has both sizes in the correct Diamond thread series low-bounce scale wheels with nice hubs. Finding these sizes in stock proved difficult and I had to order from 2 different dealers, Austrian Schweighofer and French Ecotop. These wheels are packed in pairs, with alternate flat dish or spoke type hubs for each. For the highly visible nosewheel I drilled out the center of a spoke hub to 10mm so I could apply the spoke hub on both sides of that wheel. On the main gear that was not possible because I only had 2 spoked hubs for 2 wheels. The carton mentions additional hubs and caps being available, but I haven’t found a place to order them, even not on the Robart website. Anybody out there wants to swap a pair of my full dishes for their spoke variants? In the meantime I drilled out the center of the dishes and mounted them on the inside to cover the ungainly hollow inside hub.

Swapping the nose gears was not a straightforward job. With the Robart one traveling almost 120° to obtain that forward rake, the original single-side pushrod steering could not be used, and some sort of double cable pull-pull system had to be installed. When the gear retracts backwards, the nosewheel effectively then gets disconnected from the steering inputs, but is kept centered by a clever system of a piano wire in a slot. The forward rake of the leg also necessitated a reshaping of the former nosegear door cutout, but also meant the cables would get a slack of 3cm when retracted, with the danger of the loose cables getting entangled and hampering gear extension. It took some experimenting before I found an acceptable solution. 3cm slack was impossible to catch by the use of springs, I therefore opted for 1mm uninterrupted piano wire all the way back from the steering on the nosewheel till the servo arms. The wires are not connected to the arms, but allowed to slide through rings at arms end. Adjustable end stops on the piano wires are kept tight on the servo arms with the gear down, with small rubbers dampening the shocks.

With the gear up, those stops slide 3cm backwards, thereby allowing free movement of the steering servo along with the rudder inputs. The servo also was changed from a simple HiTec 311 plastic gear low power one into a more powerful metal gear Dymond 5000 that I recuperated from the flap mechanism.

After painting the wheels they looked nice but I had to use spacers in order for the tires not to rub against the maingear leg compression triangle construction. Using the provided plastic axle adapters still allowed too much play on the 4,5mm axles on those Robarts. Probably these were not original, and 5mm axles were too large. Measuring with a slide rule revealed everything to be designed for 3/16th axles, but finding such wires on the old continent was difficult. Luckily I found an old screwdriver (that I purchased at a USAF PX) in my garage that had exactly a 3/16th steel metal shaft. After hammering the plastic handgrip away around the steel, and Dremeling the ears away (they prevented the metal from rotating in the plastic), I had a piece of strong metal just long enough to make my two axles. I of course grinded flat spots where the screws of the collars and legs would keep the axle tight, and got rid of the superficial rust that had formed during the 30 years the screwdriver spent in my garages. The fit for the wheels was close to perfection so they wouldn’t wobble and required less spacers to keep away from the legs.

To facilitate working on the model, I split both wings and unbolted the engine pods. I then unscrewed the Chinese gears which probably will find their way in a 5kg aircraft. A single main-gear with wheel weighed 150 gram, a single Robart assembly a hefty 280 gram. At first sight the gear change will add another half kilo (1 lb) to the model. Dry fitting the gear assembly revealed that just as on the real B25, the model’s wheels stick out just a bit below the gondola. On a real B25 they made small bulges in the geardoors to accommodate the large tires, but because I did not want to use door sequencers (to close the doors again after the gear had traveled to the down position), I’m happy with just a few mm of the tire sticking out of the gondola when retracted. Those big tires also forced me to cut a few additional millimeters of the gondola cutout, but also meant that I had to cut away excess length of the bolts holding the gondola to the wing. The landing gears look much better now, but I hardly dare to say that the cost is twice that of the rest of the model. Would I do it again? Probably not unless the electrification of Robarts becomes more reasonable in price and stronger in operation.

2) Flap system

In the fully retracted position, the main gears also pushed against the system that actuated the flaps so it had to be modified. A closer look at the operation revealed that there was much more flap angle possible with the mechanical hinge setup, but it was hampered by the metal of the arms fitting too tightly within the hardwood cutouts. After cutting away only a tiny bit of those wooden sides, the flaps operated freely over their complete range. To augment the flap angle range, I rotated their plastic loop actuators as far in as possible on their threaded rods, and that allowed me to grind those flap arms sufficiently so that the tires couldn’t touch them anymore. Increasing the flap angle also meant that the servo would have to work harder against the airstream to get and keep the flaps fully down. The flaps up position was mechanically locked by gluing bits of wood at the back of the cutouts, so the up position under G-load doesn’t cause any strain on the servos anymore. I then changed the servo arm position so that flaps fully down coincided with the arm fully aligned with the actuator rod (see previous picture). Whilst not allowing for the longest possible throw, this method moves the flaps down between up and halfway (less airstream drag) using the full servo angular force for the first 45°, but allows more servo angular movement to move the flaps further into the full down position. When attained, any airstream force on the flap will not cause the servo to rotate anymore, thus in fact creating an artificial mechanical down-lock.

Whilst the Dymond 5000 servos are sufficient for aileron operations, I had doubts about them being strong enough for the flaps (probably the reason why the builder restricted flap angle deflection). Searching the internet for that basic servo produced few figures, and never more than 3kgcm. The servo arm length on the model was used to its fullest extent of 17mm to obtain the flap angles, so 280 square centimeters of flaps were supposed to be brought down against the airstream with about 2kg pull. I therefore decided to replace the Dymonds by Futaba S3010 servos of 6,5kgcm pull that I had removed earlier from my Blanik glider. This will give me more chances that my flaps at least get to the full down position in flight instead of buzzing to get (asymmetrically) to some kind of random deflection angle fighting the airstream. Luckily the servo arms between both brands were interchangeable so the setup was just transplanted. With every change affecting something else, it quickly became clear to me that this would become a major update of the basic acquired secondhand model.

Previous owners opted for both flap servos to be actuated by an electrical y-cable, and the mechanical movement setup had been mounted at an angle on one side in order not to use a servo reverser. The fork system used to actuate both flaps on one side by one servo, had been twisted so much for that, that I found no way to attain symmetrical flap angles (even between inboard and outboard flaps) and couldn’t even adjust the kwik-links because they had glued metal metric ones to the factory different thread fork extremities. I opted to use fully separated flap channels and thus aligned the actuators in a proper way. To test the movements and symmetry I used a y-cable and a servo-reverser, but after everything was fine-tuned, I labeled the servo cables and they will be connected to individual servo channels which I can reverse in the transmitter.

On the wing intrados, the gap between wing and flaps was enormous in the up position. I used soft clear plastic food container caps to cutout strips that were secured with double sided tape to the fixed part of the inboard wing. The free part of the plastic runs over the gap and is allowed to slide freely over the flap during its operation. Because the outboard flaps are completely in the zone of the to be applied invasion stripes, the latter have to be applied first in order to have the black and white lines sliding over each other without gaps. Whilst removing the stock Ultracote from the flaps, I noted that the so called hinges just slipped out of their slits in the balsa. They had not been punctured and I only saw CA remains on the last millimeter, the rest of the hinge never saw glue. Taking them out completely was impossible because of the solid flap actuator torque rods. After removing the rest of the Ultracote on the wing portion receiving the invasion stripes, I drilled two vertical 2mm holes from the top of the balsa through the hinge flaps, then further down till they exit in the flap cavity, and repeated that for every hinge. I then applied some expanding PU wood glue on toothpicks and pushed them through the drilled holes. Just before max glue expansion, I used a knife to cut the top of the toothpicks flat with the balsa, and wiped off excess glue. Even if the glue loses grip, the toothpicks will keep the hinges in position. Next morning I sanded the top surface smooth and cut the bottom part of the toothpicks off in the cavity. I then applied white vinyl on the underside of the flaps, from back to front and pushing it deep into the large opening and back up on the wing part. This now forms a second protection against a flap moving backwards.

Following picture shows the enormous flap gaps on one wing, on the other one the flap already covered with invasion stripes and the red soft plastic over the gap. On the inboard flap, that plastic already was covered by recuperated gray material making the gap completely invisible and aerodynamically smooth and closed with flaps either up or down.

Decoration concerns:
After the completion of my Windrider Boeing 737 early February 2015, I resumed work on the Hangar9 B25 and carefully removed all the USAF decorations. The ones on the vertical stabilizers came off easily, but were applied on white and blue Ultra-cover that also covered the rib structured rudders. With nowhere to find the exact Hangar 9 shiny green Ultra-cover material, this caused a problem and I needed every bit of the recuperated wing and fuselage green to cover these large outboard vertical tail surfaces. I also carefully cut away the top turret (N320SQ being a post war TB25N airframe never had a turret installed), which made it easier to have the model rest inverted on its twin tails. I also temporarily removed the clear plastic of the nose gunner’s compartment because I had other plans for the nose end. I also discovered that the builder had omitted to follow the instruction to have the waist gunners blisters staggered (In the real aircraft the fuselage wasn’t wide enough for two people to stand behind their guns without rubbing their backs together). The starboard blister had to be completely ahead of the invasion stripes, the port blister half-covered by the first stripe. This feature was very visible and had to be catered for. Removal of the blister caused a lot of green material to be torn away, and my first idea of actually cutting the balsa out behind the blister proved impossible because balsa ribs and stringers run all along the back fuselage and they seem important just behind the wing gap area.

Removal of the Ultracote from the wings was done very carefully. After marking the lateral limits of the invasion stripes, I used a sharp knife to make the cut through the ultracote, but not deeper because the balsa wing planking is only about a millimeter. I then lifted a corner of the material at the back of the intrados, and pointing a hot hair dryer on the material, slowly peeled it off. Tearing it off cold also works, but leaves more balsa remains on the Ultracote, That is ok when you don’t recycle, but the purpose of my removal was to recuperate that green colored Ultracote for covering the vertical tails, and the gray for a variety of parts on the bottom that needed to be filled (such as the flap gap clear- or red plastic strips). Those were covered with an overlap on the gray of the wing, not only to help hide the step, but also to better secure that plastic on the intrados instead of relying only on the double sided Scotch tape. Trying out an old flap area on this later less visible spot to find the correct temperature to reapply used Ultracote, and what temperature had to be applied to make it shrink again, proved that it was feasible.

Back to the removal of the large wing panels. I found out that by lifting the wing barely off the ground holding the already loose Ultracote, I could use my other hand to point the heat gun, and allowed gravity to separate the rest of the material. Because at the factory they overlapped gray and green at the leading edge, both materials kept together when rounding the front and working further back towards the trailing edge. The big advantage doing it that way was that I not only ended up with the largest possible single piece clean materials, but those still-joined parts provided a perfect pattern to cut the single white vinyl panel that needs an angle between top and bottom because of the wing geometry. Only after that was cut and verified, I just used scissors to cut the green from the gray.

With the flap problem in mind, I also checked the ailerons and sure enough, one of them slipped out of its hinges, the other one being solid. With this portion of the wing keeping the stock Ultracote, drilling holes through the balsa was a no-go. After disconnecting the pushrod, I took the complete aileron out, verified the hinges were bonded solid in the aileron, then I drilled a couple of 2mm holes in each of the hinges, filled the holes with PU glue, and carefully slid the aileron hinges back through the slits in the balsa wing. That way no PU glue got pushed out of the slits, but the glue in the holes will expand vertically and bind within the slits.

Covering the highly visible twin vertical tails was no sinecure using the old Ultracote, because the panels I recuperated were too small to cover either of the four parts in one go. Luckily the overlaps are hard to see, and I have no wrinkles. I then applied the very large base vinyl white custom shapes of the invasion stripes to fuselage and wings. The one on the aft fuselage was especially tricky to apply because all the lines had to be perpendicular to the flight path, yet the fuselage tapered in height and width, and the white also covered almost half of the port waist gunner blister. It took me two hours doing it. I then used the shaped backing paper to draw the black line pattern on, then cut them away so I could draw the contour on the back of the black vinyl.

The same plastic material as for the flap gap cover was then cut and glued to the inside of the engine nacelles to reduce the width of the gear opening. Putty was then applied to soften the step, and when everything was smoother I used old gray material to cover that regained area. Foam was used to make a removable plug that covers most of the void over the gear attachment plate (see previous picture). This only needs access for gear removals, for the rest of time just a small slit to allow the gear leg into fully down position is sufficient. Remember I don’t use gear-doors and thus wants to keep the opening as tight as possible. With all the clear plastic removed I also took the opportunity to glue some transparent green sheeting on the windscreen to form a sunshade screen. This was very prominent (and useful) on our real aircraft, but even at this scale it is rather visible and produces a nice scale touch. At that stage I still toyed with the idea of making a pivoting canopy held by strong magnets to replace batteries somewhere in the cockpit, Works on the Mitchel were then stopped because of glider restoration works that had to be ready before spring. At that stage I already applied the Calliegraphic produced stickers and showed the model at the end of winter club gathering (where members show their winter realizations).

Last edited by BAF23; Nov 11, 2015 at 09:33 AM.
Nov 11, 2015, 05:52 AM
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Resuming the works during fall 2015

Installing a MrRcSound V4 sound system

Unfortunately, repair work after crashes and getting the rest of my fleet airborne after receiver swaps, delayed further work on the B25 till the end of September 2015. In the meantime I had purchased a MrRcSound V4 sound system with an extra sound card especially programmed for twin engined B25 separate engine startup sequence. I also ordered two TT25 transducers in lieu of speakers so I wouldn't have to make holes in the fuselage. Such transducers are smaller than normal coned speakers and are glued directly to the balsa fuselage causing the complete structure to vibrate as speaker. The 3M silicone attachment was discarded because reputedly too weak, and the cone of the transducers was glued directly to the balsa with slow drying PU wood-glue. The constraints of cable length, one speaker for each engine, the need to be affixed on a flat surface, heat dissipation and CG, left only one (not optimal) spot on which to glue the transducers, and that is behind the guns just aft and below the cockpit. With the system also programmed to produce the sound of guns (and whistling bombs), this position seemed feasible but necessitated access holes to be cut to place them.
During that single test flight in the original configuration I found it very impractical having to turn that 8kg model upside down to install the 2x400gr engine batteries under the wings, then use heavy NiMh separate batteries in the nose for the gear and receiver, but also used to balance the plane. After some brainstorming and experimenting sliding the engine batteries on top of the fuselage with the model on the Sig balancer, I figured that by mounting those heavy batteries just behind the pilots, the plane balanced correct without the need for NiMh batteries in the nose. Being able to discard the latter meant a weight save of around 400gr, largely compensating the heavier landing gear weight. The space was available, but there was no access to it due to solid triplex fuselage formers and bulkheads. I then figured that if I could cut the cockpit canopy free from the fuselage and reinstalling it with a hinge and some magnets, then cutting away a portion of the behind-pilot bulkhead, I could slide the batteries in on a custom built solid tray, but also had a way to insert the transducers with the use of long curved pliers for gluing them in position.

Next came the selection of the spot for mounting the 3x2inch sound electronic plate that is said to become very hot. I first contemplated mounting it on the side of the nosegear compartment because the open space got sufficient cooling when flying, but it would be prone to FOD and I would have to lengthen the transducer cables. The solution I choose for was to drill 7 large cooling holes in the backplate of the nosegear compartment and mount the sound unit transversally between that bulkhead and the nose wheel steering servo, behind an access plate that allowed easy programming changes, and made connection swaps to the print-plate possible. When all was installed I made another check of the sound system using a 3s battery to power the plate, and a servo tester to produce the signal. With the transducers rattling the balsa I really was impressed by the overall noise level, but the sound itself couldn't be assessed until the model was completely assembled. Because the engine batteries had not yet been wired, I wasn't able to assess if the sound signal had to be ran of a dedicated servo channel (with a bit of offset) or could be y-cable wired to the throttle signal, because I like to have the start sound coincide with the movement op the propellers. With the batteries out of the belly, a possibility opened up for installing an operating bomb bay.

Sorting out the electrical power system

I wanted the power system setup with following desiderata and system limitations:
-The X8R and X6R receivers powered by 6v (with 5,2v backup) with dual battery redundancy, and providing the power for the elevator/ailerons/rudders and nosewheel steering.
-power for the 3 landing gear motors and both strong flap servos had to be non-essential 6 volts not to be drained from the receivers, each flap needed to be on a separate channel to obtain symmetrical movement and mechanical over-center stop in the full down position.
-power for the sound generator has to be minimum 12 volts when using 2 transducers, sound signal from receiver has to be received before main power is applied to the generator.
-engine battery power was calculated versus the 3-blade props and existing ESC's and came out to a 4S4000 battery for each engine (5S was too much for the ESC's).
-Installed port ESC has inoperative BEC (measured at esc), starboard ESC has a BEC delivering only 3 Amps at 5,2 Volts.
-Preferably no extra batteries to reduce overall weight and compensate for the heavier landing gear.
-Although the wings would mostly be kept together, separating them occasionally (both physically and eclectically) for maintenance or repairs had to be possible.
-Minimal connections at the field, and no more turning the heavy model around to assemble/disassemble/swap batteries
It quickly became clear that those requirements were nearly impossible to attain using the available wiring. I already disliked the previous owner's spaghetti setup with many field connections between the wings and fuselage, and it certainly would become even worse with my added requirements. After spending hours drawing schematics and looking at the existing hardware and wires, I came to the conclusion that rewiring everything with a different philosophy was the only solution. Instead of connecting the separate wing systems to the fuselage mounted receiver and batteries, the new layout would consist of the receivers mounted on one wing with all their fixed connections, then a transverse connector between both wings (remaining connected most of the time), and a connector between the wing and aft fuselage (tail servo's) plus one for the front fuselage (nose gear and sound system). A separate connector was foreseen for the future (completely removable) bomb bay assembly. For diagram simplicity none of the ground or signal wires are depicted on the scheme.

My initial idea of mounting the batteries in the fuselage might have suited the CG and proved easier for field change, but that meant the power leads between the batteries and ESC's would be close to one meter long and require multiple connectors. For such lengths, additional capacitors have to be inserted between the leads at regular intervals. A couple of club members regularly fly their Hangar 9 Twin Otters and I saw them swapping their batteries in the engine gondolas via the top of the wing, dramatically shortening the length of their power leads. A closer look in my B25 revealed that even with retracts installed, ample space remained in the engine pods, but the obvious space was unsuitable because the batteries would have to be loaded from underneath the wings (again requiring the model to be turned upside down each time), and the batteries would end up at the same CG spot as before, thereby requiring a lot of waste lead in the nose.

I then removed a nacelle from a wing, and the plastic cowling from that nacelle, exposing the motor and ESC in front of the bulkhead. Using a 4S4000 battery I checked for possibilities of mounting that as much forward as possible, taking into account the two consecutive existing forward plywood bulkheads. The most forward bulkhead was the GRP/GFK front of the nacelle, to which a double 3mm triplex had been factory glued, and to which the extenders for the electric motors were bolted. Making a battery-sized opening through that structural bulkhead was possible, and allowed a battery to be sled against the second bulkhead/frame that provided the rigidity of the nacelle-to-wing attachment. With the fake air-intake on top, and the engine-mount in diagonal, it limited the choice of battery location to the 45° angles either side of the vertical. To change batteries would require either the complete cowling to be removed (after the prop had been removed), or an access hatch to be created in the cowling. I choose for the latter and because the hatch would be visible to anybody looking at the model from the top or the side, I opted on a 45° installation on the inboard upper of each cowling, just behind the forward ring that was kept mostly intact for shape integrity and solidity. The battery length is so critical that I either had to discard the fake engine cylinders or attach the complete cowling 2cm more forward. Opting for the latter I glued plywood extenders over the old attachments holes for the cowling, these paddles not only allowed the cowlings to be mounted more forward without obvious visual traces, but also created a venting ring all around between the nacelle and the cowling. This will enhance the motor and ESC ventilation, and hopefully provide sufficient cooling for the encased batteries.

The chosen solution meant that the battery had to be inserted above the ring (and thus at an angle) before it could rest flat parallel to the motor extenders. The angled insertion meant that the hole in the bulkhead had to be slightly larger than the battery cross-section, leaving almost no material at the outside, and very little plywood along 2 of the four motor mounts. Plus-points of that setup are very short leads to the ESC, batteries at the same forward CG distance than they would have been just behind the cockpit, batteries being easily changed with the model on its wheels on the ground, and no more power lead connections between the wings and fuselage except for the sound system and a few servos. Minus-points for that setup are a serious weakening of the engine support bulkhead, but I figured that had been made 7mm thick to cope for heavy 2/4-stroke engines. The 280gr lightweight electro's at the end of their extended motor mounts certainly would cause less forces on the wood. To compensate for the weakness and ensure the batteries wouldn't move and touch the outrunners, I fabricated custom 3mm plywood cradles that somewhere reinforced the weaker spots around the battery hole, and provide a solid positive- and lateral g-force movement physical limiter for those 400 gram batteries. With all that added weight in the front of the nacelles, I decided to use a pair of additional wooden dowels to augment the original pair in order to solidify the nacelle to wing mount.

An additional bonus was that the setup allowed most of the receiver wiring (no thick heavy battery leads anymore) to be concentrated around the receivers mounted on top of the port wing center-section. The former screwed-on plywood plate to mount the receiver and accessories to the fuselage thus became obsolete. With sufficient holes in it, eventual additional wires could be routed over it in a simple way, but gluing that large 26x13cm plywood plate to the fuselage and cross-members created a box structure for the fuselage-saddle above the wing, instead of just a single carry-through structure limited to the very top of the fuselage. I'm convinced that modification made the fuselage structure at least 3 times as strong as before on that critical spot. In the meantime I came to the conclusion that this model better be equipped with a FrSky X8R and X6R receiver so every servo has its own channel , differential aileron possible, no more servo reversers, individual NWS channel for expo versus no expo on rudder for single engine situations, engine startup sound to be programmed with offset to coincide with prop rotation.

This new setup left the complete belly below the wing fully empty. That vast part being hollow and easily removable thanks to a spring operated lip, got me thinking of modifying that with operative bomb-doors and eventual bomb release mechanism at a later stage. I still have an e-flite bomb release servo that I took out of a Ka8 glider (factory-suggested but unsatisfactory for tow release), and the sound system also includes playing the whistle of the bombs falling down. In addition the system also produces the sound of the side fuselage mounted cannons with a connection for synchronized LED lights. All those gimmicks won't be installed until the shakedown flights have been completed, but (receiver and wiring) provisions are already taken care of at this stage.

That radical design change change set me back 3 weeks but eliminated a lot of problems regarding CG, weight, electrical conductivity, practical battery change, and electrical connections during field assembly/disassembly between wing(s) and fuselage. Resolving confusing and impractical options is what makes this hobby so rewarding. Off the shelf (even secondhand) proven hardware can be modified and even improved thanks to personal creativity and sound analysis. Nobody but you sees any difference on the field, but you know which personal touches went into that model and how much more practical/solid/safe you made it during all those long hours back home.

Drawing a simplified electrical schematic (mainly consisting of the different power supplies) is one thing, producing the actual complete wiring looms and solder all the connections and connectors was a different ballgame. Initially things did not go fast, because I continuously had to stop and rethink, or think much in advance where things would end up and where to split or join the myriad of wires. After a couple of days I had the basic setup completed with all the connectors for actually flying the model, but none of the bells and whistles yet. It still looked very messy because I wanted to have access to any connection so I could change or repair things after a first power-up for initial testing.

To keep things simple I started by just assembling both wings and plug the wing connector in, no fuselage yet, After making sure none of the many exposed wires touched anything they weren't supposed to, I switched the Taranis transmitter on (on which I had programmed the 14 necessary channels) and holding my breath, connected the starboard battery. As there were no sparks, smoke or funny noises, this was a first sign of big relief. The transmitter voice repeated the low battery voltage alarm as I had that set at 5,5v and the BEC emergency power only produces 5,2v. As I was able to move the ailerons (not yet as they should), I seriously exercised them whilst watching the receiver power displayed on the transmitter screen. I was glad I saw no further drop and both receiver lights remained steady green as grass. Actuation of gear and flap switches did nothing (although the signal went to the servos) as no 6v non-essential power flowed to power those servos. As the ESC had armed, bringing the kill switch alive allowed me to obtain normal starboard engine rotation.

As an aside I would like to mention that after the initial binding process, I made separate throttle calibrations for each engine, each with a slightly different idle throttle trim setting, The result is that when both engines are connected, the port one start one trim click earlier than the starboard one. This feature can then be combined with the mixed engine sound to produce a fully individual startup sequence because the sound card produces a stereo 20-seconds duration startup sequence for 2 engines. All these little gimmicks and tricks will enhance the ground show of this impressive model.

Back to the initial wire-testing. I plugged the port battery in, and despite the 3 separate voltages in the wiring, didn't produce any undesirable effects. As I had set the Castle Creations 10Amp BEC on 6v, the voltage warning went away and the telemetry showed receiver voltage to be 6v. Gear and flaps now moved but the transmitter had to be reprogrammed to obtain the correct directions and endpoints. Port engine was also brought alive and the subtle differential rotation start between both engines verified (differential thrust at higher throttle settings will be negligible if any). I then brought the fuselage and connected the tail servos to check their operation. Because I programmed by Tx with crosstrims, the neutral point of the elevator had to be mechanically adjusted (the throttle trim has no center point and as such, with the trim centered, the servo neutrals at 1550µs instead of the normal 1500µs).

I hardly could believe that I had not made a single mistake in this not so simple wiring, but after verifications and adjustments (also redefining the failsafe receiver settings in the process), I then disconnected the starboard battery. Except for killing the starboard engine, everything else kept operating and no warnings were provided. Even operating flaps, gear and all flight controls solely on the port power system, no voltage drop at the receivers were noted indicating a single engine landing could be made with only the port battery working. Playing with the batteries on and off, it was obvious that with a port battery failure, the Mitchell could be flown single-engine, but a flapless gear-up landing would have to be performed.

With this important wiring verification completed, next step was to use liquid rubber and shrink-tube to insulate all the open connections and soldered connector pins. Having 2 receivers, I then made holes to insert the vertical antennas through the wing and used silicone double sided adhesive strips to keep the other two antennas, diode and common negative wire connector on the narrow available port wing upper surface area. It now was the turn of the wires towards the nose section and bomb bay connectors to be soldered, and after completion of that task it all looked pretty neat and handy compared to the spaghetti setup when I bought it (see illustration in chapter 1). Furthermore, I can completely assemble the model on its wheels on a table, and bolt in the two screws from below with the ample clearance between the wing and the table. No more turning the heavy model upside down anymore, even for bombing-up but that will be discussed later.

The unusual small number of pins from the secondary receiver are because 3 of the channels only produce a tiny signal to the sound unit, and a single plus and minus normal power voltage is sufficient to power the sound module and nosegear. The same is true for the bomb assembly, just a non-essential 6v power and one signal wire for the door actuators and one for the bomb release mechanism (probably twin bombs with a slight time-delay). As I found a way to store and transport the fuselage and the joined wings with their gear down, field assembly was performed in a matter of minutes. The model not having been balanced, the idea was not to fly it, but to perform all the essential ground checks that had to be done prior to the maiden. That for instance included range tests, taxi trials, acceleration checks and full power checks. Here is a video of the ground tests taken at that time, please open up your speakers to hear the sound system work on the ground, including a short burst of the guns and the bomb whistle.

B25 engine test (2 min 32 sec)

The power check revealed that the props were not providing sufficient pull for the model. Whereas the prop calculator from suggested a 13x8 triblade that would produce 750Watts with 54 Amps, my telemetry only showed only 35 Amps with the MAS triblade, but 39 Amps with the old twoblade 13x8 it already had flown with. Back home I took a MAS triblade 14x9 and that already indicated 45 Amps or about 660Watts per engine . I only could find one model store having those props in stock, it was in Germany but close to the Dutch border so I drove there to get one to be able to flight test my model during the presumably last weekend with mild temperatures and relatively calm wind.

Achieving the ballance

I balanced and painted both props, then put the model on the scale and balancer. With the 3 heavy gears retracting backwards, it was imperative to measure the CG with the gear in the up position. I was not happy to see that I needed about 120 grams extra in the nose end. During the ground tests I noted there predictably was no difference in frequency between both TT25 sound transducers, which was not conductive to make the startup sound clear between both engines. Luckily I had a heavier bass transducer in stock and it weighed 130gr. After opening the nose I saw that it fitted well on the plywood panel with the nosegear mounting screws. I ran the cables along the sides and replaced one of the existing speaker pins with the bass transducer pin. A first test confirmed that having the base transducer kicking in as second engine startup noise now made it very audible whilst producing a deeper overall twin-engine sound and balancing the plane on the recommended 92mm mark.

The maiden after the extensive modifications

Following weekend I had the very knowledgeable Benny check over my model whilst I ran over the new-model verification checklist. He found 3 minor discrepancies that would not compromise the maiden flight and were corrected later. During the fail-safe check (switching off the transmitter with running engines) it was funny to see the gear slowly retract (as intended) but the props freewheeling to a stop were not obvious with the artificial engine sound still rattling at steady frequency and loud volume. As he declared the model flight-worthy, I was awaiting a lull in the fight operations on that busy day (everybody seemed to jockey to get airborne on probably the last nice flying Sunday of the season), and had to personally ask some for a break to allow a free sky for my maiden. Here is a picture of the model with the new larger props and the battery hatches unlocked awaiting the final connection when it was my turn to fly.

As many pilots still were in the pilot box, I asked Benny to accompany me a few meters further so we wouldn't be distracted by their comments. With tail-draggers I usually position myself behind the aircraft during the takeoff roll, but on the B25 I thought directional control wouldn't be a problem so I preferred to watch from the side as to have a better visualization of the acceleration and takeoff speed. I already had briefed Benny about my sequential intentions for this test flight and I told him all along what I would do. As with all my maidens I took off without flaps, but was surprised the model drifted to port requiring me to rotate earlier than planned before the model would roll into the grass just past the halfway mark. It got airborne in a normal no-flap attitude and as I slowly banked towards the runway axis Benny reminded me not to climb too steeply. After an easy turn to downwind I leveled off and was amazed I hardly had to trim. The model was very steady in pitch and yaw, but a bit sensitive in roll. As I commented that it didn't increase the speed much, Benny told me the props maybe produced more torque than speed and I followed his suggestion to raise the nose into a climb. He was right and I was able to maintain about 20° of climb without losing airspeed.

I then continued flying traffic patterns at altitude and contrary to my habits, decided to retract the gear because I wanted to check the slow speed handling with the CG in aft position. I didn't let it go in a full stall but recovered when I deemed the speed low enough for landing. On the next downwind I decided to select half-flaps to note the pitch change. Although the flaps-one deflection is considerable, I hardly had to correct in pitch and let the speed drop off slowly again till close to the stall. She still flew steady as a rock so I raised the flaps again and brought her down for a pass over the runway. Next downwind I dropped the gear and made a visual check during the low approach. Everything seemed normal and following downwind I selected flaps one and brought her down the final turn for a first landing attempt. I was a bit steeper than ideal but I thus kept sufficient speed to counter for wind surprises and the usual downdraft on short finals on that specific runway. By coming in steeper none of that materialized, and a slight flare brought the nose up so it touched gently about 15 meters down the runway on the main gear.

During the roll-out one of the fake wheel covers separated from the rim, and I wasn't able to stop before the end of the tarmac but rolled about 5 meters in the grass before taxiing back. With the wind in the back it didn't slow down and I had to steer voluntarely in the grass to get to a stop before hitting the bystanders watching me taxi back. I kept it at a very sedate pace and taxied to my spot as a bomber would do. The shutdown sequence of the engines one by one sounds very realistic and I really felt proud and happy about that majestic model. As I noted trims and power consumed (only half the battery in 5 minutes), I removed the batteries to measure them separately and was surprised how much power the sound system appeared to absorb, left battery 44%, right battery 59% remaining. Temperatures of batteries, motors and ESC were normal to cool, so the prop setup seems to be good. The model now goes back to the hangar for further finishing during winter.

Here is a picture of the DBAF real TB25N (turretless) aircraft anno 1997 somewhere during an airshow.

Following update spring 2016 after completion of the bomb mechanism, guns, cockpit interior and more realistic drab finish.
Last edited by BAF23; Dec 23, 2015 at 06:20 PM.
Jan 10, 2016, 05:01 AM
The sky is the limit
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Winter 2015-2016 updates

Dropping bombs

As told in previous entries, the move of the batteries from the bomb-bay location to the nacelles freed the former for other use, a functioning bomb bay being the most logical. After comparing pictures of the bomb-bay doors of the real aircraft with Hangar 9's factory-made under-wing cover it became obvious that the existing inside plywood formers and balsa stringers could be used to their advantage. A set of HobbyKing geardoor hinges looked suitable and these were positioned on the inside of the intact assembly to draw the long cut for the doors with a ballpen. The length of the bay was driven by the space between the forward assembly bulkhead and the third plywood former, the distance being rather close to length of a scale bomb-bay. Being afraid of the balsa skin losing its very curved shape upon cutting, I first made 4 extra formers that were glued to the interior of the doors just inside the cut, these ensured structural rigidity and shape, and would become part of the box-like door structure after scale-like interior panels were glued to them.

When the glue settled I cut out parts of the intermediate former (kept in place for rigidity) before actually putting the knife along the blue pen marks to cut both doors (still together) out of the original part. The long stringer still joining both doors was then cut in half vertically to obtain the two separate doors. The sparse factory glue had to be augmented by copious amounts of PU wood glue all around the various parts to ensure door rigidity, not only in the airstream, but also as a floor for candies that may be released upon door opening during special events.

With the bomb doors becoming a prominent feature they obviously will attract a lot of attention, both during low level passes or actual drops, and during re-arming on the ground. Furthermore the forces of the candy resting on them until drop, and the side forces due to uncoordinated flying in the air, made me decide to spend quiet a bit of time to fashion the inside of the doors as per original design. Pictures of the real B25 clearly show the interior of the doors being made of perforated concave sandwich panels. I made those of 1,5mm balsa that I perforated with 10mm holes on a raster-marked template by means of a column drill. Just punching the holes in one go made a mess on the underside and had the balsa split all over the place. I resolved the problem by first drilling over a slightly larger hole in a piece of hardwood that was judiciously clamped so I could slide the balsa over it. That only resolved the splitting problem, to obtain a clean hole necessitated drilling only halfway down the mere 1,5mm balsa with a center pinned drill, then turning around the balsa to drill the rest out from the opposite side. Now the cuts were clean but that necessitated a total of 224 delicate drilling's to obtain those fake inside panels.

After first painting the original inside of the doors I then glued the perforated panels between and along the extracted doors to obtain a more solid lightweight concave box structure as per original. Those funny small HK door hinges were then glued in between the structure, but had to be carefully cut out again the next morning because they were designed for straight surfaces while here everything was curved and a dry fitting revealed the angles impossible to glue to the fixed but curved side walls. Luckily after 6 hours the PU wood glue had not completely settled and using a sharp knife I was able to pry all 6 hinges out without any damage to the outer door surface. That made me think I better ensured more grip than just surface glue on the next try, so I used a sharp knife to roughen those flat nylon surfaces, and finger-drilled four 1,2mm holes into each baseplate so the PU glue could grab into it. At that point an additional 48 holes were just peanuts. Both doors were then positioned in the closed position within the assembly before the hinges were glued into them with their other side resting on the curved wall.

Next step was finding a way to actuate those doors. There was no way I could find an acceptable solution to actuate the doors on the front (logical) side because of the simple triplex plate offering no depth because it had to be mounted flat against the fuselage former with the 4 holes for the forward wing attachment. The moon-crescent plywood former at the back offered few possibilities either so I decided to cut a new custom shaped half-moon former where the servos could be mounted on the upper part, and then the complete assembly could be glued against the existing minimal cross member. After several dry-run trials with the servo-tester moving various servo combos with different length arms through pieces of wires against various pivot point locations on the doors, I settled for for following solution: two Hitec HS82 servos (one of which I opened up to swap the inside wires to operate in reverse direction) with adjustable length HS225 servo-arms actuating scrap adjustable aileron connecting rods attached onto modified cross-shaped servo-arms that were solidly glued to the plywood aft door former. The geometry of the servo-arm was chosen as to have the arm aligned with the rod in the doors-open position so as to form a mechanical lock against side forces on the doors. Most of the features discussed since the last picture can be found in following picture, except for the sliding servo arms, the empty gel-dispenser will serve as base for the bombs.

The sub-assemblies were then painted before the hinges were first spot-glued one side at a time to the walls, using rubbers bands to keep them in correct closed position and carton strips between the doors and walls plus doors themselves to obtain sufficient play for the mechanism to work freely. After the spot-glue had dried and the mechanism was tested for freedom of operation, additional thin balsa custom-made caps were glued over the hinges to increase system rigidity, both under static load (candy) as with dynamic side-slip loads in the air. All had to be glued in small steps at a time and knowing that it takes 2 hours for that PU-wood-glue to firmly grasp, 4 hours to become solid, but 24 hours to be completely cured, it gives you an idea how long it took to just get the gear doors to be completed to my likes.

I then had to make a bomb drop system that preferably released both bombs with a small interval for spread and to coincide with the multiple whistling noise of the sound system. I also had to find a place for the MPX plug and take care that the intentional long wire coming from under the front of the wing wouldn't get entangled with the bombs. As pictures (of the bomb-bay) on plastic models are usually a good source for info, I found one that showed a box structure at the top and front of the bay of a 320sqn period B25. Note the following features on that plastic model: perforated interior panels on geardoors, box structure at the front in bomb bay around the main wing spar, minimal geardoors visible when gear fully down, bulbous protraction on aft geardoors where the large tires had to fit in, serious flap angles and wide invasion stripes all along them. Bombs stacked 3-up in deep bay. The model displays wing and tail leading edge black covering and a lower gun turret, all features not found on the DBAF TB25N that I flew in the similar colors.

The boxy structure in the forward part of the bomb bay seemed a good starting point to incorporate the plug, excess wire and the drop servos. A plywood plate was thus fashioned and glued to the strong beams along the side panel. Additional transverse plates were made for the bomb attachment/release mechanism in the middle of the bomb bay. The forward plate is a sandwich of balsa between two plywood beams, cut in a slight V to accommodate the wing dihedral. The balsa is only 1,5mm thick and was cut to allow nylon hinges to freely slide vertically in-between the plywood.

Bomb drop mechanism and bombs

Although a real B25 stacks small bombs vertically, the wing of the model doesn't allow that and such small scale bombs would be hard to find on the terrain. I therefore choose as large bombs as could be accommodated side by side in the bomb-bay and painted them blue (like any inert exercise bombs) as a compromise between visibility (also for recovery on the field) and reality. To simplify the drop mechanism I choose for the bombs being suspended by a fixed pin at the back and a hook at the front. The strong plastic gel containers I use as body for the bombs have a kind of recess hole in the back , on the side of the baseplate. A piece of close to horizontal fixed wire inserted at that point can easily hold the bomb up as long as it is pushed against it. With the recess hole on the top, if the bomb is released at the front, it will pivot downwards and automatically disengage from the wire so it can free-fall vertically. To eliminate any chances of airstream holding the bomb back against the wire during the release phase, I allowed the other side of the wire (after looping the cross member) to push against the middle of the baseplate. That way as soon as the bomb pivots down, it will do that around the middle wire and thus for sure disengage from the upper holding wire in the same process. Following picture of the assembly with the bomb doors in open position and port bomb in the lugs shows the described sub-assemblies with double wire holding the bomb in the back, cross-member with slit for the suspension hinge, sliding wires with adjustable length for time delay, both release servos rotating the same way, angled MPX plug for easier insertion in the compartment for the long connecting wire to be rolled up.

The gel container is sufficiently solid to survive multiple drops, even on tarmac if a suitable soft nose is mounted. The diameter of the container is just over 40mm which incidentally is the diameter of squash type balls. The rubber provides a nose-heavy bomb and can be easily inserted/replaced without the need of glue. The black ball with a red dot) is 6% larger than normal and squeezes firmly into place without risk of going deeper thanks to some small rings on the inside of the container. The blue balls unfortunately are 12% larger as normal and do not fit in. After the rebound the bomb will land at any angle so it was important that the forward hook and rear fins be sufficiently flexible to absorb the secondary impact forces. I used the grinder to make slits in the container for anchoring these protuberances. The fins were cut from a soft acetate sheet and are only 5x1cm because they had to fit in the bomb-bay and also wouldn't create too much side forces on impact. A nylon hinge was pushed halfway down into the forward perpendicular slit so the exterior half can pivot. The interior part was fixed with toothpicks through the nylon holes whilst a larger central hole was drilled into the outside part to accept the sliding pin of the bomb release mechanism. All these parts got PU-glued to the container. Two small servos were mounted in the forward boxy structure and exact lengths of wire run into guides glued on the forward traverse before gliding in and out of the sandwich structure through which the nylon hinge can be inserted. Although both servos move in unison, individual wire lengths can be adjusted through the clevises, and that together with the servo slow function allows for a slight delay between the drop of the 2 bombs, a desirable feature because of the multiple whistles of the sound system after release. This is the assembly with the 2 bombs locked and the doors closed, ready for slising into the fuselage with the front plywood pin and locked in position by a single latch pin through the hole in the back, the tight fit along the bottom of the wing ensures no lateral movements can be made. Spare bomb assembly ready to be painted is shown along.

The 4 servos in the bomb-bay were then connected to a single green multiplex plug that was affixed into the forward tray. Servos were physically isolated laterally from the long wire that has to be stowed in the tray after the bombs (or candies) have been reloaded in the removable bomb-bay. The relatively long wire from the bottom of the wing to the MPX plug in the tray allows me to simply lower the complete bomb-bay assembly with the airplane resting on its gear, then slide it sideways with the power to the accessories from the left battery to operate the doors and mechanism during the reload, all without moving the airplane at all. It would have been simpler to mount the release mechanism directly in the bottom of the wing, but in that case I each time would have had to turn the 8kg model upside down to re-arm or load candies, tennis elbow guaranteed after a while. I intend to make an identical fake foam shape to use when the heavier (250gr empty plus two 49gr bombs) bomb assembly is not used during pleasure flights.

Enhancing the cockpit

I always hated airplanes flying around without a pilot visible at the controls. Although in this case there was sufficient space for making a real scale cockpit, practical concerns such as weight and effort made me chose for a simpler option: just pilot bust figures on a black balsa plate at canopy level. I had two identical white foam pilot busts in the correct scale and wearing caps, and having been captaining the real Loty's II at the turn of the century, it was an inevitable choice that the left pilot should have my period looks, including the red cap with pins and the David Clark headset wearing standard Nomex flying suit. The copilot got a standard black Duke of Brabant cap and different look on his face. The headsets were made of lightweight filler that was shaped whilst drying, the rest was purely a paint job.

In my old pictures I had quite a few cockpit shots of our aircraft and the one you see served as a base for printing-out in the correct size for the model. For real, the instrument panel is completely vertical and pedestal horizontal, whilst on the model I was stuck with a slanted upper half of the instrument panel. After some experiments I found a way to cut the lower pedestal parts away, leaving a realistic upper part transiting in the horizontal pedestal with most of the engine controls and the control columns visible. In regards of the simplicity it produces a very pleasing visual enhancement of the complete model. Because of the relatively delicate built of the nosegear, nobody was allowed to sit in the nose or tail gunners compartment during takeoff or landing, so those could remain empty on the model. Our crew/passengers were either seated on the bomb bay box behind the pilots and had to stand on their feet to see through the windows behind the pilots, or in the waist-gunners compartment but as on the model the windows were applied over the structurally plain balsa sides, no other crewmembers were made. The model thus is in the configurations we used to display during airshows: pilot, copilot and crew chief (monitoring the instruments and pedestal from below).

Fuselage mounted fixed guns

On the previous picture you see how I had used the Dremel grinder to get rid of the twin .50 barrels in each side fuselage mounted gun pod. I did that so I could reproduce the single .50 barrels sticking out of the four side mounted forward-firing gun pods that were on our aircraft until the year 2000. On the picture underneath you can see our side guns next to the cockpits, but also a fake nosegun kept in forward direction by elastic straps, the lack of dorsal turret, the bomb doors in open position and the huge propellers. This picture was taken with our crew at the Czech airfield of Rodnice during an airshow where we arrived with 3 pilots (I still was under training) and 5 mechanics.

You might remember that my MrRcSound module V4 also included the sound of guns firing. Besides activating the speakers, it also gave electrical power to 2 pins that could be connected to LED lights. With only vague data to be found of power output or required LED characteristics, I ordered a variety of LED lights in yellow, amber and red, varying beam angles and intensity, some 3mm to completely insert into the muzzles, some 5mm to mount sticking out of the muzzles. These muzzles were simple 5mm hollow carbon tubes through which I could route the electrical wires. Because the Hangar9 gun pods had been firmly glued to the fuselage cover material and contained the plywood plate with the inner remains of the twin guns , it wasn't easy to finger-drill the new 5mm channels to house those barrels parallel to the finished fuselage.

After passing through the plastic and plywood, I then somewhere had to get through the side fuselages to pass the wires inside, but you might remember that was the only flat surface I had found to glue the sound speakers on. I thus had to be very careful as to where I would penetrate into the fuselage, all from within the 5mm hole in the front of the gun pods. Inside the fuselage I ended up behind the permanently glued sound card so that I needed custom-made wire-hooks to retrieve the wire ends and pull them towards a finger-accessible spot near the sound card. After much thoughts I decided to first solder the LEDS to their wires, then through the still lose barrels into the pods, then pull the wires inside the fuselage before attaching the JR plugs to connect them on the sound card.

With the sound-card fixed into the fuselage and the signals and power to it being soldered into the fuselage-wing connector plugs, I only could test the LEDS on the completely assembled model powered by both batteries (remember my wiring diagram?). Connecting the batteries required the model to be right side up, but the access panel for the soundcard was at the bottom of the fuselage. Testing of the many LED types thus required the model to be continuously turned around within the confined space of my hobby room (because my soldering equipment is semi-permanent on that table. I resolved that problem by buying an aluminum folding frame that musicians use to put their electronic organs on. Those are sufficiently strong to support my 10kg model on the field, and their geometry also allows a model to be rolled from one side of the fuselage to the other without having to lift or displace it. Here is a picture after the LED equiped gun muzzles had been installed.

That setup allowed to connect the batteries so I could send the electrical power to the various LEDS to compare and figure out if I needed serial resistors and of which value. I finally opted to mount 5mm LEDS in front of the muzzles and got the best visual effect with amber 3,1v 30° spread 7800mcd intensity 20mA LEDS without using resistors because the pulses were short and the guns only would see use in relatively short burst with good cooling in the air. As I wanted a capability to replace defective LED's I opted for shrink sleeves over the soldering of the cathode and anode pins to the wires. That produced just sufficient thickness so it was kept rather firmly into place in the barrel, yet could be pulled back out sufficiently to solder a new LED on without having to pull all the wiring out. I could find no info as to why there were two set of LED pins on the sound-card. I hoped they would alternate in signal so I could cross connect each pair of guns on each fuselage side, but testing showed that this was not the case and both pins gave the same signal at the same time. I then (parallel) connected both fuselage ends of the LEDs of the same side together in one JR plug, connected the JR plugs from each side in their pins on the card, and made a final functional test of everything before I pulled the LEDs tight against their muzzles and glued the barrels into their pod orifices. Following picture shows the upper LED already pulled tight against the gun barrel but before insertion into the pod, the lower one still having to be pulled back into the carbon tube. Soundcard access is through a hatch at the bottom of the fuselage.

Such complications for a simple visual enhancement is what makes semi-scale modeling such an interesting sport. It takes wild imagination and sound engineering (both mechanical and electrical) to figure out how to produce and make things work the way you want, mostly without the benefit of guidance or precise known values. It is up to the modeler to translate wishes into practical reality. There are no limits as to what can be achieved with ARF (Almost Ready to Fly) kits that you buy. The Asians mass-produce relatively cheap basic kits that many RC fliers fly after a few hours/days of work, but the real modelers can spend challenging weeks improving such kits to become little marvels that differentiate them from the fleet of nearly-identical models found on the flight-line of most clubs. What a great hobby.
Last edited by BAF23; Jun 05, 2016 at 02:43 AM.
Jan 23, 2016, 05:36 PM
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11dryver's Avatar

Incredible Blog!

Sir, that is an incredible amount of info that you've included! I wish I had a 1/100 of the time that you have to be able to do all that work AND to take the time to share with us all the details. Thank you for sharing. My father was a B-25 pilot in WWII, so I, too am a big enthusiast. I'm hoping that before I'm too old that I pilot the real thing too.
Jun 04, 2016, 06:52 PM
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BAF23's Avatar
Thread OP

Final tests and cosmetics

May 2016 the weather got just a bit better and my new club asked if I could drop candies at an event where kids bring their puppets or plush animals for an air-ride in large models at the end of June. That was the trigger for me to get my B25 Mitchel ready for action. On a windless day I took it to the club with the tarmac runway to check the takeoff roll and bombdrop mechanism. I somewhere goofed during the programming of bombdoors and drop, so I spent some time getting that right. It wasn't as straightforward as you think because these servos run on non-essential power, requiring me to hook up both batteries, but that also triggers the very loud sound alarm after 3 minutes of no throttle movements. Each bomb-drop signal also started the loud whistling sounds. I suppose I also triggered the nerves of many other modelers that day, but not disturbing the peace in a quiet apartment was also important, so it had to happen on the airfield. I finally got it working, but not with the switch positions I had in mind, and the audio calls in my Tx didn't match what was happening on the plane, but at least I could try an actual bomb drop.

Because of the heavy 8,3kg weight, temperature and no wind condition, I opted for a half-flap takeoff. To my surprise the bomber lifted off by itself slightly past the halfway point and I had to check the nose so it wouldn't climb too steeply. Even full down trim was not sufficient to keep it level. I raised the gear and in downwind the flaps. Without flaps it lost its tendency to climb, but full down trim was still necessary. I never expected the addition of the bomb system and guns to change the CG that much. Turning upwind again for a low observation pass, I noted that the nosegear had jammed halfway into its cutout. Recycling the gear produced the same jam on the way up. Aerodynamically this wouldn't hamper testing the bomb drop, but I didn't know if there was a cutout for the electricity into that Lado worm-wheel motor. Maybe the motor could burn through or drain the battery to deplation. I already regretted having wired the nose retract to the essential power bus for ease of connection between wing and fuselage (the mains are powered by the non-essential bus).

I decided to just release the bombs and land immediately for precaution. I choose to drop the bombs in the grass and after lining up at about 30ft AGL, observed the bomb doors to open correctly. When in front of me, I gave the drop signal and was surprised there was hardly any delay between both bombs. I closed the doors and proceeded downwind for landing. Remembering the aggravation of the out of trim situation caused by the flaps, I had no envy to try full flaps and after lowering the gear and checking down and locked during a low pass, I made a short pattern and landed with takeoff flaps. Without stalls on this 151gr/dm˛ wing-load model, I didn't risk slowing down or flaring too much and touched on the mains about 1/3rd down the runway. With that weight and no headwind, there was no way I could stop on the 90meter tarmac and rolled in the grass for a while before I could turn around and taxi back onto the hard.

The short flight had left 70% in one battery, and 60% in the other. I then collected my bombs which had fallen only 1m apart and seemed to have impacted at a 30° angle. After taking the little earth and grass away between the squash-ball nose and the bomb body, they were intact and immaculate. With the model on its stand I actuated the gear and noticed that it jammed at two points where the clearance was minimal. After helping it manually laterally, I got it in completely so I could remove the former door assembly and allow the gear to operate during a second flight. Because the servo actuation of the elevator is completely inside the tail, I decided to dismantle the tail assembly at home to adjust it mechanically. I just changed the electronic neutral value on my TX so I could fly without pushing down on the stick. Bombs were not carried and door mechanism not connected for the second flight.

There was slight wind blowing so I took off with takeoff flaps from the opposite runway, prepared to catch a climb. This went smoother and I raised the gear in downwind, confirming full retraction passing overhead. That was the signal to climb to altitude for a CG dive check which revealed a neutral stability by continuing in a straight line. I climbed higher for some stalls in the various configurations. It was very docile and clean with takeoff flaps, but just as on the full size B25 it vigorously dropped both the nose and a wing when gear down and full flaps. The stalls happened after quite a while, proving the aircraft could be flown slower than I thought. I found it strange that at slow speed and high power I needed much left rudder to prevent it from yawing because it hadn't shown that behavior before. I came back down and performed some nice low passes and a top view pass. What a beautiful and docile stable model this was. I dropped the gear and takeoff flaps for visual down and locked pass, then made a short pattern and when turning base I selected full flaps. That didn't upset the pitch much and I flew a nice powered approach with the nose slightly high. I still had sufficient authority to flare and touch down a few meters beyond the threshold, this time bringing the aircraft to a stop before the runway end.

Because of the sound system I programmed my props to still rotate even in idle. When in the parking I reduced the throttle trim to initiate the audio shutdown sequence and stop the props one at a time. To my big surprise, the starboard prop came loose from the shaft, together with the spinner cone when the motor stopped turning. That explains the rudder inputs in the air, the starboard prop already was loose and probably slipped along the axle. The spinner stayed on just because of the engine rotation direction. When that stopped, prop and spinner just continued turning till they separated from the axle. That had been a close escape, but also explained the difference in battery drainage. Thinking about it, I was lucky that the climbing turn after my gear check had been into the good prop, and I am still surprised I was able to perform a climbing 180° turn in nearly single engine condition. That was a tremendous confidence booster. I tightened both props securely on their motor shafts, inspected the model and was satisfied everything was still ok. I dismantled it in a few moves and drove to my other club to see if it was possible to operate it from their grass surface.

Due to the humidity they hadn't mowed the lawn and my props only clear the ground by one inch, but I gave it a try (without bombs). Using takeoff flaps the roll distance was only about 50 meters and I flew some patterns, playing with the bomb-doors and guns. Contrary to the first club whose members tend to neglect me and don't comment or come to look closer, the pilots of the second club watched in admiration, both the model and all its features plus the way I fly it very realistic. I must say that my confidence in the model grew stronger but I still didn't have the courage to undertake the display sequence I flew with the real B25 or the smaller FMS B25 model. I performed a perfect landing with full flaps and touchdown just in front of me. It definitely stops much better on the grass than on tarmac. The props didn't come loose anymore and taxiing in high grass posed no problems. I was surprised that the yellow propeller tips hadn't turned green from the grass. I flew a few other flights with the PA18 Super Cub, simulating glider towing, and then had a few beers with the other guys before driving home, super satisfied of my day and my scale models.

Next day I cut off minimal bits of the nose gear opening and bent the gear so that it now moves freely up and down. Because the retract problem happened immediately after takeoff from the tarmac, I suppose the gear pin was bent when I hit something during transport. To facilitate assembly on the field, the model is stocked and transported with the gear in the down position, I'll better pay more attention passing through doors and perform a retraction test before starting a flying session with the Mitchel. I also glued the tail bumper in its position.

With the results of the CG dive check and general handling in mind, plus noticing the elevator streamlined with the horizontal stab with the trim full down, I decided to leave the CG as such and dismantled the tailplanes to mechanically adjust the elevator neutral point with all the TX elevator settings and trims in neutral. I also discovered that the clearance between my elevator and fuselage was so tight that it had friction and might have hindered return to neutral. A sharp knife quickly corrected that and the tailplane was assembled again (it stays on the fuselage all the time). To further eliminate the nose-up tendency with half flaps, I mixed in some down elevator for just that setting because full flaps brought the nose back down. I off course also used the 4 second slowdown of the movement for it to coincide with the flap movements.

I then spent some time getting the bombdoors and release mechanism to work conveniently on a single 3-position switch with closed and locked in the forward position, doors open in the middle, and bombs released in the rear position, each with audio feedback in the Tx and bomb whistling upon drop by the airplane sound system. I further slowed down the operation of the drop servo to obtain more time separation between the releases of both bombs. Changing all that reminded me to reset the failsafe settings on the receivers, but that wasn't easy because the Rx's were only accessible when the fuselage was away from the wing, and both and their systems had to be powered (from the nacelles) to make the setup. With the model on the stand and the fuselage just to the side of its normal position, I powered it on but during the setup I incidentally hit the throttle a bit which started the engine sound start sequence. From previous experiences I knew that putting the throttle back to full idle wouldn't stop the noise, but by advancing the throttle to low power was sufficient to allow the sound system to to reset in order to initiate the shutdown sequence. With the model still high on the stand, the little throttle opening made it tip over forward. I was laying down below the model to observe the bombdrop/bomb release actions when the Tx power was switched off, so I quickly got up and received the idle turning starboard prop into my my left cheek. Luckily this happened in a parallel direction and except from some minor bleeding from the scraping marks over the full length of the cheek, I got no further harm before cutting all the power. Initiating the sequence I had the engine safety cutoff actuated, but that has nothing to do with the sound activation. Sound deactivation can only be controlled when engine power has been effectively applied, hence the rotating props. Luckily there was only damage to my cheek and none to the model (some won't understand my reasoning but so be it).

I reprogrammed the failsafe standard but with the gear down and bombs locked plus doors closed. The reasoning for that is because often when I have the model ready to fly and either take it off its stand or bend down to push it to the hot area to taxi it, the Tx with tray on my belly gets too close to the Rx and the system goes into failsafe. If the failsafe position for the gear is up to minimize the gear damage in case of crash, it will start to retract and if the bombdoors are failsafe open with drop, that is where the model would rest on even if the signal is lost for only a second. That is why I opted for a failsafe gear down and bombs closed.

With all the essential stuff done it was time to experiment more with means to get the factory gloss Ultracover more dull. After many tries with products that didn't adhere well enough to the surfaces, I found a local product to fix charcoal drawings on paper with a dull reflection. I was amazed that grabbed very well on the very brilliant Hangar 9 Ultracote and after only one pass it already produced a satin shine. A second pass (experimenting first on the bottom gray) showed it really became Matt so I did the whole plane. I had to avoid applying it to the clear cockpit perspex but used it delicately on the other clear surfaces to camouflage the voids behind it. My dad caught that and asked me why I hadn't detailed the interiors more besides the cockpit or had put crew figures in place, but I explained him that in real B25's crew were not allowed in the glazed nose or tail Barbette during takeoff and landing because the nosewheels were prone to collapse. I wanted “my” B25 to portray the aircraft I flew displays in during the turn of the century. Although during the travels our refurbished TB25N (factory delivered without dorsal turret nor waist or nose guns) carried support people, our displays invariably were flown with minimal crew comprising pilot and co-pilot in their visible positions, and the crew chief/flight engineer standing behind them on the lower entrance deck, invisible to the people around.

The drawers' product worked well on the Ultracote covering, but not on the many plastic or acetate accessories that already had a different green color to start with. For those parts I used two layers and two pots of flat varnish from Revell to have the plastic blend in with the rest of the model. I really like the end result, it finally became a rendering of our workhorse instead of a model of overly restored museum airplane. After five adjustment flights I hope to have it ready for years of pleasure. The sixth flight should confirm the previously changed settings to be near the perfect mark, but that is a minimum routine I have often encountered with any new or secondhand model. The Hangar 9 B25 is a good solid base to start improving into a very nice looking and impressive flying model. The poor fitting plastic nose and canopy won't allow it to compete in scale competitions, but on the field it will be a head turner that is very pleasant and rock solid to fly. Hopefully I'll soon find somebody with a video camera so I'll be able to share those pleasures with you. In the meantime you'll have to do with the latest still images spread along the text of this entry.

Last edited by BAF23; Jun 05, 2016 at 02:46 AM.
Jul 13, 2016, 02:00 PM
The sky is the limit
BAF23's Avatar
Thread OP

Candy bombing

It had been raining a lot for weeks and the grass strip at Tongeren was not as flat as it should have been. The club was running a successful first event where kids could have their dolls or plush pets taken in the air by large models. My B25 was the smallest plane on the field and was only used for dropping candies twice that day. I therefore took the bombs out and poured a pound of candy in the bomb bay. This is where my ingenious system of a modular bomb bay proved very handy.

pic 1799

After an extensive safety briefing for the pilots and kid's escorts (no risks with 65 kids on the field ) the flying operations went into full swing. It was a challenge because all day we experienced a stiff full crosswind and often threatenings skies (out of which rainfall occurred during the afternoon).

pic 2086

For the morning drop I paired with Danny's Wilga which can be fitted with two large candy boxes

pic 2106

During our slot time all other flying was stopped and the kids gathered at the gate to observe the drop spots. Danny and I lined up line abreast and we performed a formation takeoff.

pic 2113

We both had a coach which looked as much in the air (for separation) as on the ground to make sure no kids trespassed the field during takeoff and landing, and that we dropped on the agreed line and spot.

pic 1804

In base leg we took some spacing for a low pass to verify drift and lineup. Even at such little altitude, the crosswind and turbulence played havoc with out flight paths, but it resulted in a magnificent picture of my Mitchell arcing the field in a top view pass.

pic 2124

The kids becoming impatient, we cut out the crab and decided that we would drop everything during the next pass. I dropped first but the photographer only took pictures of the larger Wilga during his drop. His 2kg drop looks rater impressive in this shot.

pic 2117

He landed immediately out of a closed pattern while I flew a large downwind for separation and setting up for a long (bomber type) final. Due to the strong crosswind I came in with ample speed and wing low into the wind.

pic 1833

Taxiing back I noticed my nosegear was anything but straight, I still don't know if that occurred during the slight bounce or because of an almost 3-point touchdown in crab.

pic 1849

After I taxied back and cut-off my engines they released the kids who ran over the field and spent the next five minutes collecting the candies from all over the field.

pic 1851

My “little” B25 certainly wasn't the favorite on the flight-line, but I was glad I had a workable scale model that could be of use for clubs for candy drops, starting with one of my own clubs to prove the concept could work. The nosewheel was just bent back by hand and as long as I choose only perfectly flat and groomed grass strips, further demo operations (either bombs or candy) are a possibility. My props being only an inch from the ground resulted in green prop leading edges and a lot of (wet) grass sticking all around my B25 and even in the gearwells.

As soon as I get somebody to shoot a flight video I'll post it here.
Last edited by BAF23; Jul 13, 2016 at 02:28 PM.

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