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Apr 02, 2018, 03:48 PM
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Build Log

Pilatus PC7 Rödel scale 1:5 Parts1 to 3

pic pc7 real

Part 1: Introduction and choices

Purchased unfinished on a second-hand website in 2015, that 1988 Rödel kit had been lying dormant in its box and only assembled after 2010 by a new owner who didn’t complete it. That very rare standoff scale 1:5 model was larger than I had in mind (2m11 wingspan) and was too heavy for the AXI 4163/20 that I had purchased for a conversion of a 1:6 PC9 into a PC7. I sold the PC9 but this PC7 model still had no engine mount nor flaps, aileron servos were missing, and too long legged brand-less electrical retracts with dubious electro brakes installed. Upon inspection I saw that the assembly quality was ok, the model being of period traditional build with balsa or Abachi planking over a foam core for most surfaces. Although it weighed only 5,5kg as acquired, everything was made of strong quality wood with the fuselage painted and wings in Solarcover. White being a good base color for the yellow I was going to apply over it, we quickly agreed on an acceptable price before I loaded the model in my camper. Back home I had to rearrange the hobby room to stock that relatively large model, in the meantime I kept it in the living room in its partially finished Martini display team scheme. This was Jaques Bothelin's lead airplane HB-HMA in which I sat but never flew.

Pic rod pc7 21 jun-2

In 1986 I performed an evaluation flight in the front seat of the yellow/red company demonstrator HB-HMP but as in 2012 I got a back seat aerobatic ride in the last of the 615 PC7s built (privately owned and stationed at our Zwartberg model airfield), I decided to reproduce that very attractive N60LT aircraft (see picture on top of this page) in scale 1:5 . Because of other priorities it remained on the shelf for more than a year but during that time I purchased a complete ESM Pilatus PC7/PC9 electrical retract system and after using diverse calculators settled for a Himax C6320-0250 1700Watt motor driving a 14x9x3 MAS triblade propeller and powered by two 5S4300 batteries in series for a 10S setup. An aluminum 4” spinner for triblade prop and appropriate long engine mount were very difficult to obtain but by December 2016 I had everything in stock plus the many winter days ahead to tackle the job, so I initiated the project using following to-do/progress list:

Order: HimaxC6320-250, MAS14x9x3 prop, 4”spinner, ESM Gear+controller, vinyl
Strip entire wing of paint and Solarcover
Carve wingtips
Make 3-section split-flaps
Make third flap servo cutout
Remove old landing gear
Cut out foam to accept ESM landing gear deeper in wing
Make new stronger gear support beams
Order new gear control unit
Swap wheels and brakes from old to new gear
Install gear and test
Cover wing, ailerons and flaps with vinyl
Install flaps, test and synchronize/adjust flap servos for single channel ops
Install ailerons with proper hinges
Wire wings so gear, flaps and brakes get power directly from separate 2S2100 battery
Strip Solarcover from tail and sand paint from fuselage
Cut cloth hinges from elevator and rudder
Properly install stronger servo’s for elevator and rudder on custom tray
Remove canopy and strip interior of non N60LT conform items
Remove old nosegear
Strip paint from engine cowl
Enlarge prop center and spinner inner diameters from 8 to 12mm
Open air vents in cowling and firewall
Produce engine mount extension for HimaxC6320 and install with 1° side and downthrust
Solder 4mm plugs to Jeti77opto ESC and install with cooling channel to nosegear compartment
Quotation + order of artwork by Callie (45usd)
Measure incidence angles (wing +0,5°, stab 0°)
Underside fuselage to be sanded to be streamlined with aft of flap
Rearrange main gear for more back rake, especially stbd one, check wheel well clearance
Assemble model and measure nosegear install height. Bottom gearmount to ground has to be 25cm
Make nosegear baseplate and Hitec645MG steering servo bracket + modified pull pull system
Adapt lower fuselage cover for minimal clearance nosegear movement and cooling exhaust
Get Du Bro 3-3/8 1:5 Cub wheels for nosewheel and install
Correct shape of all tail surface fairings plus top of the tail
Prefabricate new tail hinges
First rough weight and balance: 7,2kg +bats=about 8,5kg , CG gear up about 13cm
Make adjustable battery supports
Make battery cables for series 2x5S with single connect and anti-blitz between both
Test drive train for correct operation on 10S and measure power output
Make cockpit interior as per N60LT with front seat pilot buste
Cut shape of clear canopy to conform correct front and back rakes
Test different prop combinations for power (Engel16x10x3=36,4V, 50A=1820w)
Install 6xELCO 63V 4700uf in parallel at ESC (long power cables)
Balance prop plus spinner cone and vinyl the latter red
Produce conform turboprop exhausts
Add third wing dowel at leading edge
Complete all electric and electronic wiring and connectors and install Rx + failsafe
Final trial assembly plus second W&B
Vinyl fuselage and apply second version of Calliegraphics
Glue tailplane hinges in position and tighten servo arms
Final W&B (9100ge, 11cm from L.E.)
Trial full connection and program TX plus telemetry
Design and make transport cradle for camper
Range test
Maiden flight

Part 2: The wing

I started with the wing (a one-piece 2m10 affair) and was glad the Martini stickers came off relatively easily because I wanted to keep the white Solarcover as a base for the yellow vinyl. The four servo access panels had been taped to the wing and upon removal I discovered there were two quality Volz FS power 2BB 39N/cm (4kg at 6Volt) servos glued to the panels in front of the wing flap positions . No servo arms, no flaps nor cutouts were available. Wiring ran through the entire wing but had been cutoff at the entrance of the aileron servo cutouts which for the rest were empty but still had glue from servo's that had been removed. Somebody at some time removed the servos and just cut the wires with scissors leaving nothing usable to work with, thank you! I drilled holes for 4 wood screws per coverplate and fixed everything properly so I could turn the wing around at will for the rest of works to be done. I also saw that they had used overlapping Solarcover between the middle and outer wings, and at the trailing edges. With a sharp knife I carefully separated the overlapping part because that would show through the vinyl I was planning to cover the entire airframe with.

The red wingtips were just squarely cut and detracted much to the scale looks. After removal of the red Oracover I discovered the wings to be covered in thin but stiff Abachi planking, but the 3cm outer tips were plain balsa. Luckily I took tons of pictures of the real PC7 at our airfield so it wasn't difficult to find out that the extrados of the tips could remain untouched but the intrados had to be angled considerably inwards, and that in turn would cause the leading edge to show a more rounded tip. With the aft being a flat fishtail plywood reinforcement in the prolongation of the ailerons, it was obvious the underside of the tip had to be carved in a smooth continuous twist. My first attempts using a saw or knife were unsuccessful because the balsa was so hard I hardly got through, the only solution was to peel-off thin slices of balsa with a sharp knife and finishing the shape with 40grit sandpaper on a flat block. That wasn't the easiest of jobs and I ended up bleeding after slipping with the knife. The result (on the tip, I'm not a vampire) was pleasing but the sanded balsa didn't seem adequate to cover directly with vinyl. I spread PU glue over the entire tips and after sanding got extremely hard wingtips (essential for avoiding transport damage) on which vinyl would have serious grabs.

wing mods 2

The real PC7 has carry-through recessed split flaps but as this wing is Abachi over foam, it became a dilemma between structural integrity and aesthetics regarding flap installation options. Any cutting in the wing's aft section to recess any thickness of flap profile would tremendously reduce overall strength. The only solution was to make split flaps by having the thinnest possible flat sections rest underneath the existing wing structure. As the centerline flap section was 50x5,7cm and would be actuated by a single servo in the middle, warping was a major concern. A test plate out of 2mm multiplex showed it couldn't maintain its flat shape with any serious airstream beating it at the extremities. I settled for a 1,5mm thickness aluminum strip that I finally found in one of the many do-it-self shops... in the household curtain department! It was 6cm wide but I had to cut off the 1cm squared corner anyway. As the strip was 240cm long the excess cornered material was used to produce the control horns that I bolted to the three flap sections. The Proxon table-saw got through the aluminum relatively well but left quite a mess of metal debris all over the hobby room. Dremel grinding plus hand file were necessary to produce edges that wouldn't hurt anyone at the back or sides, but leading edges were sharpened for the transition with the existing wing's intrados. The price for the stiffness of the flaps is weight, a hefty 240 grams for the 115cm total length of the 3 flap sections (versus 90 grams for plywood), and that is without hinges, screws, horns nor servos.

wing mod 4

For hinges I used standard plastic Kavan hinges with their metal pin, These are solid units with 4 predrilled holes to fix each half to whatever material. Two hinges per wing flap would have been sufficient but I like the safety of a third one to keep the flap in place should a single one fail. Along the wide center section I used four hinges, two on each side of the control horn. So that makes 10 hinges and a total of 80 screws! How do you fix nylon hinges to a thin aluminum strip? Nuts and bolts seem logical but produce an awful sight. My solution was to use self tapping screws with a relatively wide and flat head on the nylon side (this screw type is much used to mount servos through grommets on wood). After predrilling the the aluminum, the screws were engaged fully tight but without overtightening to avoid the screw thread slipping in the aluminum. I then turned the flaps upside down and carefully grind away all the screw length sticking out, leaving a perfectly smooth aluminum flap on the outer side, and just the minimal thickness of the hinge material and the flat screw-heads on the inner side. I figured that with the force on the flap hinges being spread out over 40 screws, even as less as 1,5mm grab in the aluminum would be sufficient and the grinding hopefully melted the screw bits sufficiently for them not to come loose due to vibrations. Yes the grind screws are still visible with the naked eye on the picture, but everything will disappear when vinyl will cover that flat (to the touch) surface.

Of course the protrusions on the other side prevented the flap sections to lay totally flat under the wing trailing edges, but minimal tailored cutouts in the Abachi hardly weakened the wing structure but allowed these protuberances to penetrate the wing surface just enough for the split flaps to lay perfectly flat and causing only 1,5mm extra thickness, hardly noticeable with the flaps in up position. Expecting serious forces on the flap horns in anything but up position, I elected to use nuts and bolts to keep those horns rigidly against the flaps. With the relatively flat heads on the outside and the rest in a small but deeper cutout in the foam of the wing, this setup doesn't attract too much attention. The horn positions have been chosen so all three servos lay in exactly the same way in their wing cutouts. Using servos of the same brand with their arms in the same angles allowed me to connect all three servos on a single channel (individual flap adjustments can still be made at the quick-links at the servo arms). With all 3 flaps on a single channel, it eliminates an electronically induced asymmetrical flap cause, they will either all move in unison or none, but individual servo failure is of course remains possible.

Then came the problem of fixing the other end of the hinges to the wing itself. As I already mentioned, the foam wing cores were covered with 1mm Abachi glued to them. I trust crews in thin aluminum but not thin wood (although for positioning and testing of the throws the screws held well). Using long screws in the brittle foam didn't help and an attempt to reinforce the foam locally by injecting thin CA in the screw-hole only resulted in the foam melting away. As that side of the hinges had to be mounted on top of the existing wing and covering (maintenance restriction), reinforcements could only be made in the foam core inside the Abachi. Screw heads and hinges had to be exposed because flaps could only be removed at that side (other side of the hinges had been grind away). My solution was to cut ten 2mm plywood plates the exact size of the complete hinges. I then used an extended sharp knife to separate and take away a little foam from under the Abachi through the cutouts I had already made. After applying expanding PU glue on top and bottom of the plywood plates I then could slide them through the cutouts to fill the gap I created under the Abachi. When I then pushed the plates flat into the recesses for the hinge part of the flap, the plywood came against the Abachi and after drying created a sufficiently solid grab area for the wing part of the hinges. The remaining half of the plates fell flat in the recesses and provided a spread of the forces along a larger foam surface. All that was left was to finger drill the holes in depth for the long screws.

wing mod 11

With the flaps temporarily screwed in place I installed the servos and made the pushrods. I deemed the two installed 4kgcm torque Volz servos too light for the job of pulling those large flaps down into the airstream. Split flaps are very effective in creating drag but need serious angles, watch the split flaps on Spitfires extending almost to 90°. On a DC3 or T6 the flaps can be lowered to 45° and then produce a 35% increase in lift and a staggering 250% increase in parasite drag. The split-flap layout and angles on the PC7 are nearly identical so I am convinced their weight and build complexity are worth the penalties. The online e-calculator provided the figures of 52km/h for stall and 102km/h max (with my power setup and estimated weight of 8kg) so I knew that extending those flaps against the airstream would require a hefty pull. With the inherent design providing a mechanical uplock, the challenge would be to pull and maintain the full down position. I therefore opted for very powerful metal gear analog servos and didn't care much about precise centering nor speed. I my stock I found cheap German analog metal gear Yuki CYS-S0130 servos that produced 13kgcm torque at 7,4V and installed all 3 of them in the same orientation.

The central servo placement was a bit of a problem. It had to be mounted forward of the reinforcement plate that housed the 2 long M6 bolts that kept the wing against the fuselage. After I marked the cutout for the standard servo carrier/cover and started cutting through the wing, I discovered that aft of the wheel wells there was a solid wooden beam that catered for the loss of wing strength due to the cutouts for the wheels in their up position. As I unfortunately already cut through the hardwood, I had to remove additional foam just forward of the beam to glue an additional hardwood beam against the original one to ensure carry-through strength. Being unable to move the servo further aft, I had to seriously reduce the size of the coverplate and adapt the inside of that well to be able to solidly bolt the complete assembly in the very limited space available and still allow the servo cable to be routed through the wing. Z-bends were made in the pushrods on the flap side, but adjustable length on the servo arm side. The arm orientation and pushrod lengths were chosen so all 3 arms would be at nearly overcenter position at full flaps, minimizing the tendency of the airflow to move the servo. After much trial and error I obtained an acceptable result and weighing the total wing with all flap components installed I read 2620gr which meant 470gr extra for the aluminum, screws, 3 strong servo's and the actuators, a figured I could live with.

wing mods 14

Part 3: Gear change

The installed retracts didn't satisfy me. The legs were much too long (for scale appearance) and the arm thus became longer and would cause even more torsional forces on the supports when operated from grass runways. Unlike on the real PC7, the wheels on the model were inboard of the legs, a solution probably chosen to keep sufficient foam integrity because the cutouts for the legs in the wings were then less deeper. Electric brakes were mounted and the wheels looked good but one of them ran very eccentric on its axle. The secondhand ESM PC9 gear that I purchased had much shorter main gear legs but came with atrocious wheels and an erratic gear control unit operation. It rotated 100° allowing the wheels to be fitted correctly on the outside of the legs and still be fully retracted in the gear well cavities. Just positioning both next to each-other indicated that a lot of modifications would be necessary to swap these much different electrical retracts, everything had different dimensions. The gear-wells being at a given position, all the rest had to be adapted to fit but I had no drawings whatsoever of how the foam wings were reinforced to support the gear or ensure the dihedral just outside of the gear. The existing gear also had been mounted too much forward and due to that the model will have to be flown with a forward CG and will be bouncy on its nose gear, but again, the position of the deep wheel wells and unknown inside wing structure made it nearly impossible to modify that. Here is a picture of the old retracts still in place and the new maingear below it, at the position it should have been mounted for scale and better ground handling.

wing mods 5

I first wanted to be sure I could get the new gear to work before I attempted any cutting that I couldn't undo. The old gear control unit being a double electrical IC circuit, nothing could be done to repair the erratic one and a replacement IC unit of this old V2 gear was unavailable even after searching the net for a long time. I finally found a Dutch online merchant specializing in large models who had a more recent version 5 controller that seemed compatible, but at a hefty 89 euro cost. When it arrived I tested it and the 3 gears worked like a charm, with the bonus of a simple button on the controller to operate the gear without having to use the transmitter, a great help when switching the model from flying to transport or storage mode.

I first purchased a set of Du-Bro 3-3/4inch low-bounce wheels but couldn't believe how heavy those were, especially with only plastic hubs which had too small diameter holes for the 5mm wheel axis both retract sets came with. The Du-Bros quickly got relegated to the scrap box. Still in the running: the rather thin and toy-looking wheels that came with the ESM gear, or the model's nice aluminum wheels with brakes. The choice was not straightforward because the latter's brakes and wheels were wider than the available 5mm diameter length of the axle, and one wheel was grossly eccentric when rotating. After finally finding the special key to separate the wheel flanges I discovered that the previous owner already made attempts to correct the problem because rubber had been grossly scraped away from the hub but apparently without any other results than having the tire slip over the flange thus completely eliminating the effect of the brake. By using a buildup of scrap red plastic inserts I was able to take away the slack and ensure the outer rubber ran perfectly circular again. Once the ideal position was found I injected PU glue to block everything in position and fill the cavities of the previously removed rubber.

pup coc 001

The flange was screwed together again before the glue could expand and I now have a perfect lightweight wheel. Luckily the other wheel had not been tampered with and was perfectly centered. The wheel axles were a factory inserted affair that couldn't be loosened from the leg anymore so I was stuck with the available length. Cutting away part of the brake coil was no option so I had to grind away a few millimeter of thicker axle root so it became a constant 5mm up to the leg. The brake could now be pushed all the way to the leg and with the wheel in place I had just sufficient space to fix everything with a self-locking screw to cap-off the wheel assembly on the ESM retracts. I then knew that the merging of the old and new gears worked and would be a major improvement if I could fix it correctly in the wing. Prodding with a knife through the surrounding Abachi I could feel no wood except for the small blocks to which the previous gear legs had been attached with 4 minimal screws. Careful removal of the wood revealed nothing but foam underneath. A hollow wooden block formed the depth of the gearleg space.

gear change 008

My initial thought of keeping as much existing Abachi as possible in place while attempting to slide new wooden gear beams underneath proved impossible. Furthermore, the electric motors at the end of the retracts were much longer and had to go deeper into the wing, even past the dihedral joint which I had no idea of how it had been achieved. As can be seen in the pictures, the previous owners use of stiff household electrical wire-gutter also complicated the deepening of that hole. All I achieved by trying to have it fit is that I broke-off one of the two electrical pins at the back of the sealed motors. Carefully using a long 10mm drill slowly destroyed about 2cm of the plastic gutter but the noises that it made in the wing were anything but reassuring. I also found out that both foam wing planes had been simply glued together at the dihedral line, not making me any wiser about how and where the dihedral got its real strength.

gear change 001

It also became obvious that in order to have the wheels completely in the wells would require the gear supports to be mounted at an angle and the leg to be very deep, almost against the Abachi op the top of the wing, necessitating almost all the foam to be removed from the new channel. On the other end, the motor would touch the Abachi on the intrados where all the foam had to removed from the interior. I quickly abandoned the idea of a conspicuous approach and decided to go for a wide and deep trench in which I could drop and glue strong gear support beams mounted at an angle. In fact I worked a bit in reverse, first attaching the gear to the beams with long screws, then cutting the necessary wood and foam in the wing. After positioning them at the correct angle I then glued a plywood baseplate and side walls to the foam, all at the precise angle and in-between I could slide the beams in and PU everything solid to the surrounding foam. That method also ensured correct wing integrity and carry-through strength despite having removed a lot of foam almost from the complete depth of that wing section. Once everything was in position and had dried, I could just unbolt the 25mm long attachment screws from the beams and the gear could come out to facilitate further work.

gear change 006

Previous pictures give you an idea of how invasive those cuts were in one half of the wing compared to the original other half. The hardwood beams I used were glued to eachother in a T-shape for strength and better grip when they got glued into the angled cradles that had been solidly pu-glued against the foam. Additional glue was injected to be sure every cavity got filled before joining the sub assemblies. After the difficulties I encountered on the first side and knowing nothing but foam kept the wing panels together, I decided to cut the other side to include the electric motor space. That much facilitated the removal of the conflicting plastic gutter length. In following picture you can clearly see how angled the beams were glued into the wings in order to have the outward pointing wheels completely in the wells when retracted, but the legs totally vertical when extended to their 100° point. I intentionally placed the old gear next to the new as it had been mounted when I got the model. That little wooden block with still some foam sticking to just one side was the only thing that was supposed to make up the structural strength of a 7kg model on 15cm long gearleg stilts! That gear had been bolted in the not too hard blocks with screws that only penetrated 8mm. These minimal wooden blocks that actually had to absorb all the forces came off all too easy together with some brittle adjacent foam structure. That old landing gear would never have survived anything but tiptoe landings on a pool table surface.

gear change 009

Next step was to close all the buildup cavities to end up with the minimal openings as per real aircraft. I first constructed the four fixed boxes between the gear attachment plates and the wheel wells, using 3mm balsa for the sides and 1,5mm plywood for the covers. I then glued a new plate to cover the cutout I had made for the motor. After all dried I used lightweight filler coats to obtain smooth surfaces and joints. After sanding, I applied pore filler to the entire gear assembly interior. I then cut plywood to fill the space around the gear-legs and leading to the motors. Those plates had to be made removable in order to allow later work on the gears. I therefore glued some piano wires to the underside of the front panel and drilled holes for those just under the fixed plywood plates. After sliding them in, those cover plates could then be affixed directly to the wooden beams by means of 2 flathead screws.

After a last coat of lightweight filler to smooth out all transitions between the remaining white Solarcover and itself or bare wood, I applied yellow paint on all inside surfaces that would not be covered in vinyl. A new wiring loom was produced because I wanted those heavy current guzzlers (gear retract/flaps/brakes) to get their power from a separate 2S2100 battery, their operation signal was the only thing to come from the receiver so in the event of a burnout or so, only these accessories would run dry but wouldn't affect my receiver voltage stability nor flight control servos. Of course I wanted to try everything out before pulling all those wires through the wings. Because of the little space available I used JST type connectors in the wings but swapped male and female sides for the brakes so it wouldn't be possible to to make mistakes between gear and brakes, both being operated through their specific controllers. Here you see a picture of the gear and brake testing, flap servo installations had to wait till the wings were covered in vinyl. Retract and flap wires are 0,35mm across to cater for the heavy electrical loads.

gear change 012

After testing everything I cleaned the wing and attempted to apply the vinyl over the existing white Oracover type material. Having only two hands this proved impossible because I was unable to apply the material over the curved wings without trapping bubbles in between the layers. I therefore took the decision to remove ALL the white cover material in order to end up with nothing but wood (that allows some air to pass through). Luckily the material came off (without heating) relatively easily with minimal glue traces but taking away some wood that had to be filled again. I decided to then first cover the flat metal flaps and the multiple access covers, none proved difficult with the use of old credit cards to push the air bubbles out during the application. I never had been happy with the fit of those ailerons and took the opportunity to cut through the paper hinges en replace them with proper Kavan hinges through correctly positioned new slots in the wood. After dry fitting I removed everything again in order to apply the vinyl to as flat possible surfaces and providing sufficient grab around the edges.

I then took lots of measurements on pictures and wings and drew relatively thick pencil lines to indicate where the color separation should be. The lines at the wing tips and outer ailerons were not easy to mark because none ran parallel to anything else. A week later the wing was ready (except for the large brand markings) and I trial fitted it to the original fuselage to get a better feel for the model. Unfortunately during my Italian ski holiday in March 2017 I broke my right collarbone in 4 pieces which forced me to stop physical labor for months an thus the PC7 works were at a standstill for a year.


Continued in Parts 4 to 7 here under
Last edited by BAF23; Apr 02, 2018 at 04:47 PM.
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Apr 04, 2018, 04:00 AM
The sky is the limit
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Pilatus PC7 Rödel Parts 4 to 7

Part 4: the fuselage

Shortly after NewYear 2017-2018 I resumed the work on the PC7 by first stripping the fuselage. The stickers came off clean cold, but for the rest of the white and coloured Solarcover, a paint stripper set at 150°C was necessary to separate the paint from the wood on the tail. The rest of the fuselage had been painted white and that was lightly sanded in order to allow the complex vinyl shapes to be applied without too many air bubbles over the curved surfaces (porous underground helps). I cut the fabric type hinges from the elevator and rudder as these will be replaced by proper hinges after the surfaces are covered again. In the tail I saw two thin small servo’s glued next to each other on a plywood plate. As I didn’t trust that, I pried the rudder servo free with a screwdriver. Not only was I surprised that the black glue was super-strong (the ply came apart before the glue), but it was a Graupner DES448 quality digital servo with ball bearings and metal gear that despite its very small size (9,5mm thick) produced 4,7kg torque at 6V and received high marks in the online reviews. The elevator servo was even smaller but was an even more expensive quality Futaba S3150 digital servo that should not be operated above 4,8V (at which it produces only 3,7kgcm torque).

Pic 4492

I consider the DES448 sufficiently strong for the rudder but for the elevator I prefer a Hitec645MG that produces 7kgcm or torque at 6V and uses less power because it it analog. The rudder servo was again glued into place but with a retainer strap fitted around it for safety. The stronger elevator servo was mounted with screws on a custom plywood bracket. The latter being at a different position, required a new pushrod to be made.

I then removed the too long and flimsy nosegear assembly plus removed the canopy and stripped most of the cockpit because it was not representative of the PC7 I was reproducing. It again became clear that everything had been very strongly glued and removing the rear-view mirrors, incorrect seat headrests, flat pilot figures, top control panels and pictures of side panels and glass cockpit instrument panels was more tedious than anticipated. A more representative cockpit layout was produced later. Here is a picture of the old cockpit interior just before it was stripped.

Pic 4493

I made a picture of the basic fuselage with all the individual components that had to come together. The “drivetrain” was put one after the other but it rapidly became clear that the extension engine mount was too weak and too short for the long nose. The spinner back-plate had to come 19,9cm in front of the existing plywood firewall. Motor, spinner and prop weighed about 800gr, mounting those ahead of a 12,5cm lightweight universal engine mount did lot look like a good prospect regarding g-forces, gyroscopic forces or inadvertent contacts. I took a couple of days to figure out how I would tackle the complex problem of engine mount and cooling, including the ESC and integrate it in the half completed model that I bought.


A brainstorm in a fellow modeler’s atelier led to producing an engine mount consisting of four 5mm threaded wires that pull straight against the back of the existing solid firewall. 4mm plywood would then be glued in between, forming a strong compression box. All this had to be mounted a few mm up and port of the center line so the motor could be adjusted in front for the side- and downthrust, keeping the spinner exactly in the center of the cowling. All this was much easier said than done because access to the aft side of the firewall was limited through a gap above the forward wing former that was a full foot behind it, and the top of the forward fuselage consisted of a thick foam arch that was planked on the outside. The dimensions we calculated and marks we made on the plan were unusable because the shape and dimensions of the actual cowling and firewall didn’t exactly match the plan.

I thus decided to work directly on the model and built it up temporarily so I could make new measurements for everything. After putting the main landing gear down and using blocks to keep the fuselage in the correct ground stance, I measured the exact required distance between the forward fuselage baseplate and the ground, that had to become 25cm. The expensive heavy ESM nosegear had been conceived to fit into their ESM GFK fuselage so it was a real challenge to find a way to adapt it to the existing wooden nose assembly. After replacing the nosewheel by one of 80mm diameter, the distance between its mounting plate and the ground was 23,5cm. The width of the mounting plate also meant the old high but narrow blocks had to be removed (glued solid with dowels and polyester matting), new blocks of 1,5cm glued and the baseplate had to be cut so the retract mechanism and motor would be halfway embedded. The long careful cut in the 3mm plywood provided an extra visual- and finger-access for mounting the T-nuts for the engine bracket metal wires. To benefit from this temporary access I delayed the mounting of the nosewheel hardwood support till the motor mount was completed.

I then tackled the cowling. I cut open the intake below the spinner before eliminating all the paint from the cowling (making it lighter and more flexible). I then cut the exhaust flap from the aft lower part and had to force things into shape by gluing wood on the inside before it could be opened at the correct angle. For the lower cowling to hug the bottom fuselage I had to glue additional mounting blocks to screw it tight. Only then could I make wooden custom shaped blocks to fill the sides of the partially open flap. I didn’t do that for scale (which it actually is) but mainly to provide an additional exhaust for the cooling air for the ESC. That flap exhaust area being much smaller than the intake lip, I then also removed a good portion of the firewall under the gear baseplate. This was no easy task because at that place the firewall was a buildup of 10mm plywood. Hot air could then also pass through that opening and exit the fuselage through the opening of the nosegear well (if no geardoors are installed). Those preliminary works allowed me to position the cowling against the firewall and make marks where the holes had to be drilled through the firewall for the engine mount extension.


I then glued the T-nuts tight to the back of the firewall and cut four pieces of 3mm plywood that were then hollow shaped at their sides so they could be glued in between the long threaded wires for overall strength and stiffness. Holes were then drilled in the sides to allow motor cooling air to exit. A front engine attach plate was made with a large hole for the motor axle, four 5mm holes for the threaded wires to pass into the metal crucifix motor mount, and side cutouts for motor heat evacuation. It took me a couple of days of dry assembly and lots of small adaptations to the wooden box plates before I finally was able to glue everything together in one go, with the spinner baseplate being centered in the middle of the fairing and with the recommended side- and down-thrust. I then used 2,5mm plywood to fabricate the angled plate on which the finned ESC rested in the full cooling airstream. The sides of that latter assembly was tilted out to augment the down and side force absorption of the long engine mount. A slit was left open in front and aft to help evacuate hot air. The Himax motor had different 4mm plugs than the ones on the ESC so I unsoldered the ones on the ESC and replaced them with the ones that were provided in the Himax accessory bag. The ESC then was bolted to the plate completing the powertrain in the nose. I used the space on the lower back to block foam surrounded two 63V 4700µF condensers to absorb power surges that almost certainly will occur with such long power cables. These were soldered in parallel to the power supply cables through an independent Deans plug in case I have to replace the ESC.

Pic 4540

Back to the nosewheel assembly. The 15mm square gear blocks were custom fashioned so that the steering arm could rest in the remaining extension when retracted. The aft prolongation of the wood provided additional strength if the nose wheel got stuck on an obstacle or hole, while the reduction in height allowed the pull-pull cables for the steering to run free. Everything got PU glued and bolted with 3mm bolts and T-nuts on the plywood. For the retracted position a hole had to be made for wheel and sufficiently wide to accommodate a wider (scale like) balloon wheel with smaller diameter hub. I used the remaining space between the main bulkhead and the tire to mount a strong Hitec 645MG servo for the steering. This required an unorthodox approach of mounting the servo inverted on an assembly that kept the servo sufficiently far from the bulkhead for the long arm to fully deflect each side. This arm had to be wide so the cables would pass left and right of the (retracted) wheel, and at about the same level of the nose wheel support wooden blocks. Once the servo is mounted on the bracket, the screws are not accessible anymore so an integral plywood backplate was used to mount this complete assembly with four screws to the bulkhead.

Pic 4518

Because the steering arm of the over-center nosegear moves back about four centimeters during retraction, the steering system itself was finally made of adjustable length long 2mm iron rods on the servo side, and two springs mounted one after the other on the gear side. Elastics mounted all along and redirected from the side of the fuselage ensure everything is pulled outside sufficiently to take up the slack and not jamming during the gear movements. The large fairing over the bottom nose did not fit well with the rest of the contours and had to be rectified. A few cuts in the subframe permitted the radius to be altered and when conform again, the gaps were filled with bits of ply and PU glued shut. Previous owner made a much too wide cut to install large nosegear doors over the complete length. On the real PC7 the narrowly cutout opening leaves the wheel uncovered and only two small narrow doors cover the gear leg. Because these are hardly visible to people around the model on the ground and to avoid complication, I elected not to install operating geardoors. The added opening also creates a larger exhaust capability for the cooling air. 3mm balsa was shaped to exactly fill the void between the nosegear and the plastic fairing, it was precision work but the result makes a great inprovement versus the old huge opening.

Pic 4519

Whilst taking the measurements for the Caliegrafics artwork on the tail I compared it with pictures of the real aircraft and found out that the top of the fin and rudder didn’t have the correct shape. Luckily that was massive relatively hard balsa so grinding and sanding produced a faithful reproduction. That made me decide to also correct the 2cm wide almost flat leading fin extension. A lot of material was taken away to produce a sharper fin with better overflow onto the vertical fin. The rudder then also got a thinner top which looked great compared to the very thick leading edge extensions between the horizontal stab and the fuselage. I carved an enormous amount of material away before sanding for hours to get that tail assembly a lot more elegant compared to the former state of 2cm thick rectangular balsa planks that just had been glued and only minimally rounded off at their leading edges. I couldn’t believe the 3 fairings had been wider than their respective fixed tail surfaces. Besides looking much better, I am also convinced I seriously reduced overall tail drag.

As I had been toying with the idea of making a sliding canopy I looked closer and discovered that the model’s fuselage was not conform. A real PC7 tapers open from the nose to the front of the cockpit, then remains parallel to the aft of the cockpit section, then tapers close to tail. The Rödel model it that way on the underside but the top of the fuselage tapers open from the nose till the aft of the cockpit, then close again to the tail. The fuselage sides not being parallel along the canopy prevents me from installing a sliding rail system. The only possibility is to screw it against the tapering fuselage. I removed the white incorrect frame lines (using my nails because nothing else worked!) and further cut all the contours to correct the rake and depth in the front, back and sides. I drilled new holes for the screws at their new positions and made new blue fake framing. Just another complete workday for a simple fixed canopy.

On the interior side I was limited by the old plywood base that couldn’t be lowered without major work and was essential to ensure stiffness of that fuselage portion. With the batteries below it I had no possibility to make a proper cockpit tub but at least tried to produce the correct length proportions as seen through the canopy from the sides. I cut away the incorrect box behind the backseat and reinstaled the latter higher up at the correct rake angle after having been remodeled to a more correct shape. The aft instrument panel housing was kept and just painted, the front seat headrest being remodeled but kept at the incorrect rake. The complete front instrument panel housing was cut out, reversed and the panel put deeper with raked more pronounced glare shielding. Pictures of the instrument panels of the real aircraft were reproduced on photo paper in the correct scale and glued in the respective cockpit areas. A pilot buste was raised to the correct height by gluing balsa blocks that also formed his arms in a normal flying position. I couldn’t change his head because it was molded in plastic so painting it was the only sensible thing I could do to correct the flaws.

Pic 4537

On a first attempt to mate the wing to the fuselage I discovered that the wing trailing edge went in so deep that the middle flap jammed and an undesirable 5mm step was highly visible and aerodynamically undesirable. Lowering the aft of the wing would hurt the incidence equilibrium between wing and horizontal stab, and lower the fuselage angle for touchdown, thereby augmenting the risk of landing on the nosewheel first, especially with flaps down. I therefore sanded away the 5mm of the underside of the fuselage till a smooth transition between flaps up trailing edge and fuselage was obtained. It was also only at that late stage that I noticed that the rake of the main gear legs was different, the starboard one being so much forward that the wheel coincided with the prescribed CG position of 12cm. To correct that, required cutting a portion of the aft wooden support dowel and eliminating the plywood crossmember at the bottom of the retract seating so the complete gear could be tilted in its cradle. Some more cutting then had to be done in the wells and cover plates to allow free operation of the legs. The very limited accessibility and already vinyl covered wing surfaces caused this wheel rake affair to cost me a full day of work. Both legs are now raked slightly backwards what will hopefully provide sufficient weight on the nosewheel for steering to be effective. The nosewheel was swapped for a more faithful looking DuBro one that amazingly was intended to fit 1:5 scale Piper Cubs.

As I had cut the soft hinges of the tail feathers to work on the plane and properly cover those surfaces, I now had to prepare the installation of hard hinges from Kavan. Removing the old hinge material proved impossible. I bought the DuBro hinge slot cutter kit but attempting to make a new slot along one of the sides of the old hinge only resulted in bending the tools. These seemed only suitable to handle virgin balsa, not balsa that had been hardened by glue used to affix the former hinges. The plastic handles look cheap and are barely able to keep the metal cutters in place by way of a single clothing type rivet. The metal is thin and sharp and doesn’t endure anything but soft wood, but is capable of being bent back into original shape with pliers. The whole kit is not worth the 14 euro it costs. I thus decided to make new hinge slots in direct prolongation of the old ones. This worked out fine, the old ones were plastered in but the new ones were left out at this time and kept aside till all surfaces were vinyled.

Part 5: Drive train testing

Before the fuselage was completely vinyled I took it to friends to test the power setup. I was seriously disappointed when we proof ran the drive chain with the MAS 14x9 triblade prop in that it only produced 31,9Amps at 36,2V on the 10S batts. That was only 1155 Watts and barely would be sufficient to get the model airborne. The e-calculator proved to be way-off and we then tested an old 16x8 wooden triblade that was laying around. That produced 36V and 44Amps at 7470rpm which meant 1584 Watt, much closer to the 1700 Watt I need to power my 8,5kg model. As the props were balanced on the spot and vibrations were still felt we ran the setup without spinner-cone, that mostly cured the problem and the 130gr spinner assembly was further balanced (quite a bit with about 7gr and 5gr of lead that had to be glued on the inside with UHU Endfest). That prop could already be suitable for the development flights but I then ordered a non-scale looking 16x10 carbon triblade from Engel for test purposes. With that new larger prop the motor produced an amazing 1820 Watt (36,4V and 50Amp) with 2 Elco 63v 4700 µf condensers installed in parallel along the drivetrain (another 2 were later installed further back). All this testing proved that whatever is published by the brand sellers or available on online calculators can be far off from what power is actually available on the actual ready to fly model. Measuring the values are really essential before attempting to fly.

Part 6: Vinyling the fuselage and artwork application

Because the fuselage had been previously painted and the plastic forward fairings also were impenetrable, applying the vinyl without air bubbles was a very difficult task. With very few straight lines and lots of compound bulbous shapes it was a formidable task to cover the model in vinyl and I admit I should have chosen easier materials or for just painting the fuselage. I started by applying non-stretching satin black (kitchen use) vinyl as anti glare panel on top of the nose and that was done in two pieces, one for the fuselage and one for the cowling, plus another piece on the bottom of the front windscreen to help conceal the kit’s incorrect canopy shape.

The rest was all done with high quality stretching Qualicast (made in Belgium) vinyl. I had no other practical choice than to work from top to bottom and without overlaps, meaning shoulder to shoulder precise positioning of the various colors. To achieve acceptable results required me to first cut pieces of paper and hold them against the curved fuselage and previous separation lines to draw the desired lines, before transferring those lines on the back of the flat vinyl for cutting. Once the correct shape was cut, it was kept as a reference for the curved line on the next color pattern paper. It was a tedious work as many individual panels had to be cut per color because a 2 meter compound curved fuselage cannot be covered in one length. I thus decided to use minimal (few millimeters) overlapping same color panels that were applied aft to front so airstream or ground encounters wouldn’t peel those joints loose. The panels are between 20 and 40cm long and overlaps lines were chosen to concur with what would be the rivet lines along the fuselage frames of a real fuselage. Using the same lines for all the colors produced in a very realistic result on the model.

As I applied the blue I also cheated quite a bit on the clear canopy outlines to achieve a better scale look. Next came the white because the old paint had yellowed too much over the years. Applying the red on the nose fairing and intricate functional lower intake/cooling lip was a real challenge but was finally managed in trial and error method in just two left and two right vinyl stretches that overlap horizontally where the access hatches around the exhaust open on the real aircraft. The yellow was easier to apply except that the symmetry between left and right was challenge on the curves, and that the top and bottom of the horizontal tail were different from each-other. After two weeks of work and square meters of pattern paper shapes and vinyl support and peelback paper (plus hundreds of very small vinyl strips and cutouts for corrections) all over the house, the model looked stunning (but not perfect). Using chrome paper strips I created look-alike canopy slide guides on the fuselage aft of the cockpit.

In the meantime Calliegraphics had produced the artwork and I needed the helping hands of my dad to apply the vertical tail thick vinyl stickers. I had them produced slightly larger because I had to wrap them around the tail and rudder (up to 2cm thickness) but that made it extremely difficult to align the eagle with the corners and get the text bow aligned and readable. I couldn’t have done it without somebody else keeping the remaining sticky material away from the tail whilst I was eternally re-positioning it to concur as much as possible to the pictures I had collected of the real aircraft. An afternoon later all was applied and I used similar looking cutoff side excess material to complete missing areas within the tail-rudder gaps. The tail to fuselage fairing was an anything but smooth continuous curve that was extremely difficult to cut away with a sharp knife. With only one left and right tail art, missing was not an option, but for me the result is acceptable (I see many more flaws but as long as I don’t mention them to others, they don’t seem to notice).

As I trial-positioned the registration marks on the wing something didn’t look right. Sure enough they had been produced much too small. Measuring the rest of the artwork revealed other flaws and when peeling back the protective paper I noticed the blue didn’t match the one on the fuselage and wings. As I presented the model on the annual post winter club gathering I had to do it with incomplete artwork. While I was on a ski holiday to Italy, Callie produced new artwork in correct dimensions and colors and sent it over for free. It was in the mailbox upon my return and was applied in the following days, seriously enhancing the overall look (and attracting the attention away from other minor visual flaws).

Pic 4535

Part 7: Electrics and electronics

With the batteries almost above the CG position within a fuselage portion that is completely open at the bottom and only relatively thin plywood at the sides and top, the most logic position for them is on the top of the wing. For structural strength there is no way openings in the sides or top of the fuselage (cockpit area) above the wing could be made, resulting in the one-piece wing having to be separated from the fuselage each time batteries are changed or connected. Electrical connections between wing and fuselage therefore had to reduced to a minimum which was not obvious with a two times 5S in series connection between batteries and ESC, and higher voltage for the nose gear retract than for the rest of the fuselage servos.

After much thinking I came up with the idea of connecting the batteries with their EC5 plugs to the ESC, but using a single XT90 anti-blitz connector to complete the single series wire between the batteries as a last connection just before flight. The second wire in that XT90 has no anti-blitz feature but I use it to close the circuit between my 2S2100 accessory drive battery just before flight. All the fuselage servo wires were rearranged with common 6V + and - plus individual signal wires so a single black Emcotec 8-pin connector, sufficient to connect all the fuselage electronics in one go to the wing “power station”.

As with my B25 bomber I found it easier to have the main electrics and electronics all grouped together on top of the wing, encapsulated by the fuselage, without any access doors nor external switches. Power for the motor comes from 2x5S4300 in series. Power for the retracts, flaps and wheel brakes come from a 2S2100 battery. Power for the receiver and flight control servo’s plus signal power for the retracts, flaps and wheel brakes come primarily from a separate 2S900 battery through a CC10A 6V BEC, but alternatively through a diode from the 2S2100 battery through a Graupner CPRX4 5,5V for backup. All 5,5V, 6V and 7,2V negative grounds are interconnected. All 3 flap servos are connected together and operate in perfect unison on a single channel. As aileron hinges had been installed to produce some mechanical differential, a Y-cable could be used to connect both servos on a single channel. All this allows the model to be flown on a single X8R 8-channel receiver, including a separate nose wheel steering and separate wheel brake channel. For my telemetry I also use a FAS100 Amp meter in the motor power line, a voltmeter for one flight battery, a non-precision variometer for telemetry protocol translation and altimeter reading, a large gear controller unit, and an electrical magnetic brake controller. All this had to find a space and be connected efficiently within the confines of the space available on top of the wing forward of the wing attachment block. I soldered and isolated all those wires as I was developing the layout but waited till everything worked before I put it schematically on paper for reference if I later have to work on it. I know engineers prefer to work the other way around but that system suits me fine as I often have to change things during the build.

Pic PC7 wiring diagram

Of course the schematic depiction looks a lot different to the compact layout in the model. As I also wanted a way to keep the flight batteries in place without having to fasten them, I glued guide panels with a “roof” on them, keeping in mind the port battery was going to be at the forward edge while the starboard battery could made to move further back in its kind of tunnel to make adjustments on the CG if necessary after the testflights. I ended up with a strange contraption to which all the electronics and batteries are either (semi)-permanently fastened or are blocking it during flight (because the fuselage sides slides over them when all connections for flight have been made. For the electronic connections, the wires are sufficiently long as to get everything done with the wing on the mounting cradle and the fuselage tilted on its left side besides the boxy assembly. Because of the strain of the weight of the wing during maneuvering I added a third central dowel in its connection with the fuselage. Two long M5 bolts are all that is used to affix everything together for flight.

Pic 4538

Although the plans call for a 12cm CG, for a 38cm deep root it seems too far back and for the test flights I balanced it with gear up at 11cm which is still 29% MAC and as far back as I dare for a maiden. I also weighed the model in the finished state with everything installed and came up at a whopping 9,1kg. After calculating a wing area of about 60cm², this translates in a wing loading of 151gr/cm², exactly the same as on my Hangar 9 much modified B25. This definitely indicates she will be a stable aircraft that I can handle when keeping in mind to maintain sufficient airspeed. With 200watt/kg it predicts to be flyable and capable of scale like PC7 flowing dynamic aerobatics.

The kit exhausts were a simple too small 90° elbow circular polyester affair. There was no way I could heat it to get it into a shape comparable to the real compound curved aerodynamic exhaust of modern Pilatus turboprops. After much thinking, I opted for a 32mm round hard wooden base mated to hand carved balsa 10mm planking glued together to get sufficient thickness. I know it will not be very solid (for transport) and will only roughly approximate the intricate shape of the real thing and even after trying to interpret close up pictures from different angles I’m still not sure about the conformity of my scale rendering. Two wood screws from the inside of the plastic cowl keep each in place and eventual replacement with better stuff in the future will thus be straightforward.

I then applied vinyl to the spinner and its baseplate outer edge. This was a very tedious job that had to be done in 3 overlapping parts, I wonder if it will stand the centrifugal forced at 8000 rpm during its lifetime but a first one minute full power test showed no signs of peeling so I will try to fly it that way. Imagine the imbalance if one of the panels slowly separates while the others stay in place. With that job finally done, the nose of the airplane looks much better and rather pleasing.

Pic 4532

I then spent a couple of days finetuning the elec power setup and programming the transmitter. One of the things I added was an audio warning every 2 seconds if the magnetic brakes are activated (by retarding the left slider aft of neutral). Unknowingly keeping that activated could rapidly drain the accessory battery. I programmed the nose wheel steering servo to be only active when the gear was down, thereby reducing the electrical load and friction when that gear is up and centered into its confined space. When everything (including the telemetry) was set and tested to function correctly, I stored a first failsafe setting, with the gear and flaps up in order to augment the chances of those components to survive a crash situation. A second failsafe setting will be applied after the trimming of the test flights. After final assembly of the model I used touch up paint for all the hinges and screws so they blend in with the rest of the colors at their respective backgrounds. That was the last job before the actual range test and trial taxi tests before the maiden. Not having flown all winter I will first regain my currency on simpler (expendable?) models before attempting to fly that complicated heavy Pilatus PC7. Stay tuned for further reports later in the 2018 season.

Pic 4534
Last edited by BAF23; Apr 04, 2018 at 04:35 AM.
May 16, 2018, 03:23 AM
Registered User
Wonderful rebuild/restoration/modification, congratulations!
Looking forward to your report about the model's flying qualities with the added flaps.
And hopefully some photos of it flying, too.

Good luck with the maiden flight and best regards,

Aug 05, 2018, 04:46 PM
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"Family" pictures

Although I regained my currency and confidence after flying my Machi MB339 and B25 Mitchel, I had not dared to maiden the PC7 before pictures of the model had been made with it’s full size example. The occasion came the evening before the annual fly-in July 2018 at our home base EBZW. My PC7 was displayed together with some other models in a corner of a hangar. The owner of the full size model got word of it, came over and admired the model, promptly offering to buy it on the spot, and proceeded to open his hangar doors to tow his PC7 out on the apron for some family pictures. He insisted we both also were pictured in between the large and small, and on some shots you see him take many pictures of this unique occasion. It didn’t take long for other enthusiast pilots to come along to immortalize this frame, even if the light conditions were already less than optimal. I now share these magic moments with you through a selection of the many shots I took with my cell phone, hope you will enjoy it as much as all the people on the spot did. I still intend to fly it during autumn, after intense training with my almost completed third scale Tipsy Nipper.
Oct 09, 2018, 05:12 PM
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Successful maiden flight

By the end of September 2018, with 7 successful development flights of the Tipsy Nipper behind me, I felt confident I could attempt flying the PC7 and awaited a suitable calm day with a little wind blowing down the runway at the model field of Zwartberg. After the assembly of the model and checking all the systems, I put the model on the open field and performed the range check around it. As no weak sectors were found I then pushed it to the holding point (as I wanted to spare the power batteries for flying). I thus skipped the taxi trials and as there was a lull in the model flying, I decided without further delay to fly it. As I didn’t know the lift/drag balance of split flaps and with a 90m runway, I elected to takeoff without flaps.

As soon as I applied power, the model started to swing to port. I wasn’t prepared for that on a tricycle gear aircraft, but a few positive rudder inputs got her back on center-line and as the speed increased, she tracked straight again, so I completely opened the throttle and went for it. To my surprise, motor/prop noise varied little with throttle movements making power settings guesswork. As I approached the end of the tarmac I cautiously applied some up elevator and she rotated very gently in the air, climbing steadily with a realistic scale angle. As for any maiden flight, I elected to keep the gear in the down position for the complete flight. The turn to downwind also was smooth, my control throws and expo’s seemed to be on the mark.

As the model didn’t really accelerate in downwind, I kept full power and climbed higher for some basic handling. At about 100m I bought the throttle back slightly but that didn’t affect speed much, is gear down creating so much drag? Lateral stability and control-ability seemed perfect, longitudinal stability seemed a bit too neutral and elevator control was a bit touchy, but to my surprise I did not have to trim the model at all. With the gear down and not so much longitudinal stability, not much else could be done at altitude but extend and retracts the flaps to half. This again did not require elevator nor trim changes so I brought the model down to about 60m to redo that test right in front of me, same result. I then flew a few patterns during which I came lower but still at speed to get a feel of the machine. It was very docile but I was surprised not to hear the power consumption figures through my earphone telemetry and as I saw no use in further stretching that maiden (all primary objectives had been met), I decided to land it if the approach looked good.

In downwind I lowered the flaps to half again but that didn’t produce much drag so in the final turn I had to considerably reduce the power to keep the speed from increasing. The final seemed very steady but I was close to idle and even noticed my prop stopping in short finals. A tad of power was added (I hadn’t performed stalls yet) and as the attitude looked right, I let her come down again and cut the power because I knew with the light headwind and the inertia of the 9kg machine it would be difficult to stop. The airplane responded well to the elevator during the flare and settled gently on her main gear but after eating up half of the runway. During the roll-out I forgot to apply brakes (my first a/c with brakes) but remembered the aircraft had a tendency to rest on its tail so aerodynamic braking was not performed, so the aircraft went past the runway in the grass where I gently turned and brought her back. With the power applications I got my logic together again and realized what had created the swing during the acceleration: the lack of weight on the nosewheel (friction) had caused the prop-wash on the tail to point the nose to the left. For next flights, either I will have to use rudder as on a taildragger, or apply down elevator during the first half of the takeoff roll.

Back in the parking I engaged the throttle kill switch and lifted the model on the assembly stand for a first look-around inspection. All controls were streamlined, all my trims were still neutral, no play was found on the gear, and the model was totally intact. My telemetry showed no power consumed nor Amps so there was something wrong there and I was happy to have brought her down without further delay. After separating the fuselage from the wing I saw the culprit Due to the stretch on the wire between the Amp sensor and the altimeter/converter, it had snapped at the latter’s connector (lengthened after that mishap). I then measured the capacities of my four batteries. Both 5S4300 (in series) flight batteries still had 60% in them, the 2S2100 accessory battery was at 92% (with just 2 flap cycles, no gear nor brake discharge), but my 2S900 primary receiver and flight control battery was down to 50%, something I didn’t expect and will have to investigate further. There is sufficient space available to replace the 2S900 battery by a 2S2100 one, this will also move the CG a bit more forward.

I can only say the apprehension I had for months to fly the model had no grounds, this had been an exemplary maiden flight that revealed no basic flaws and encouraged me to quickly proceed further into the test program, but with autumn and winter ahead this might be delayed. The 7% throttle to rudder mix was increased to 10%. The 30% elevator expo was either insufficient or the elevator throw was too large, but the longitudinal stability even with the gear down (nosewheel retracts backwards) was so neutral that I added 60gr of lead in the nose around the ESC plate. That might not sound much but at 55cm ahead of CG it will make a difference and also help to increase nosewheel pressure on the ground. The elevator expo was increased to 40%. The ESC has been reprogrammed to freewheel and motor power plus its telemetry tested so she is ready for flight two.

The maiden was filmed by somebody with a cellphone, but put online unedited. I am glad I can share those few moments for you. You first see me wait until a police helicopter departs after maintenance. Because those pilots think they are above the law, they sometimes just depart or arrive directly over our model field, hence me waiting and observing it’s departure before I takeoff at 1’20”. After 2’05” I climb out of view for some testing so you can skip to 2’25” for some passes till 2”45. After more high level testing that you can skip I perform a low approach gear check as from 4’00” then land and taxi back till the end of the movie at 6’07”.

Laurence Adriaensens 27 9 2018 (6 min 8 sec)

A recurring medical problem called “frozen shoulder” prevented me from performing further testing because the doctor recommended me not to strain my shoulder. Manhandling the 9,1kg PC7 will have to wait till spring 2019. Stay tuned for further updates on the development flights and rest assured, I won’t let the owner of the real aircraft buy my PC7 soon just to have it hang in his office, it just flies too nicely for that and offers me the joys and satisfaction to fly a model that is rarely seen on the airfields.
Oct 18, 2018, 04:12 PM
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Second and third flights

With the summer showing no sign to end, halfway October I took the PC7 to Zwartberg again on a 25°C day with very little wind. Assembly was straightforward again and soon I was getting ready for takeoff. The augmented pressure on the nosegear made little difference and I aborted the takeoff when directional control was caught too late. I did the second attempt standing behind the model and that was easier. It caught my eye how narrow the main gear was, a possible problem on uneven soft fields. Even without headwind, the model got airborne in about 50 meters, encouraging regarding power availability for takeoff on grass. Even after a straight ground roll, the plane banked to the left after liftoff. I let it continue because we flew a left hand pattern anyway. I immediately raised the gear and allowed the plane to climb at full power to altitude for some testing.

With the gear up, power could be reduced much more and still allow a comfortable level cruise speed. The new forward CG necessitated some serious up trimming. When everything seemed in equilibrium I initiated a CG dive check but that showed only a rather neutral longitudinal stability. That was sufficient to perform stalls with the gear up (nose gear backwards). The clean stall was benign with a positive dip and a light left wingdrop. Flying downwind I lowered the flaps to half, then to full, neither producing noticeable pitch change. After raising the flaps again I came down and opened up the throttle and performed an aileron roll. The nose hardly dropped and the roll-rate was realistically scale. I re-positioned, took some speed and gently pulled for a looping. I wasn’t prepared for the rapid pitch response and the pull to the left, the loop was much too tight and corked. But proved the power was adequate for vertical maneuvers.

I came down to traffic pattern altitude and after a normal circuit I lowered the gear and flaps to half for a visual check during a low-approach. Those retracts are so noisy during operation that I heard them over the engine noise. With the gear checked down, I climbed to downwind and performed a final turn, observing the propeller kept turning even after a temporary intentional throttle reduction to idle. I kept the gear and flaps down during the go-around and had no problem climbing to downwind again. That is where I lowered the flaps to full with the intention of landing. The increase in drag allowed me to keep about a quarter of throttle on all the way down. With no headwind and a variable crosswind component I played the throttle and kept some speed. This caused me to balloon during the flare but I caught it and settled about a third down the runway and had no problem stopping. During the taxi-back I tried the electro-magnetic brakes but those were really disappointing, definitely too light for such a heavy model. Fellow modelers applauded when I came taxiing in, I was so glad I still took it up before the winter break.

Both batteries indicated 40% remaining but I was even more satisfied to note the less than 10% consumption from the 2S2100 receiver and accessory drive batteries. Benny had the impression elevator and pitch were over-sensitive, so during the charging of the engine batteries I augmented both expo’s to 40%. A cookie-break was urgently needed and it was relaxing talking with club pilots I hadn’t seen for a while.

Takeoff for the third flight was uneventful and the climb more steady with the new expo. At altitude I performed a couple of stalls again, with stick half back and then full back. For the latter I brought the flaps up again during recovery because the nose had fallen quite a bit, but nothing to be afraid of (at altitude). The added expo made the model more relax to fly, but it still reacted too much in pitch during the looping. Unless that was fixed I felt no need in further exploring the aerobatic envelope. I came down with power because I wanted to know if the motor/prop combination was able to produce realistic high speed passes. A real PC7 produces 250W/kg but my model only 200W/kg, it is logic it wouldn’t be a screamer but the result looked quite scale during the two level passes. At that speed it was stable and could be flown very precisely during a low level top-view pass. A former club president later commented you couldn’t see it was a model passing by.

I then performed a pattern during which gear and flaps-one were lowered for the visual gear-check, the next one intended to be the full-stop landing. A full-flap powered approach was flown and everything looked steady up to the flare. The model then again started to bank left and due to the increased aileron expo my correction was insufficient. Not having explored the go-around capabilities with full flaps, I allowed the model settle at an angle on the grass along the runway. Having pulled much up elevator to spare the nose wheel it stopped resting on its tail. I raised the flaps and applied power while applying down elevator and it started taxiing back. The narrow track didn’t seem to affect the lateral ground stability much and soon I was on the tarmac again. A quick look at the telemetry showed I even hadn’t consumed half the battery capacity so I taxied to the begin of the runway and took off again. That last landing was no way to finish the Pilatus flying day so I took it around and brought it back in for another landing. This time I was quicker catching the left wing lowering and landed one third down the runway, stopping at two-third with brakes.

I was much satisfied about those flights and noticed the engine batteries to be down to 35% with 45 seconds to go on the timer, that was perfect. Telemetry figures displayed correctly on my transmitter but the audio calls have to be reprogrammed to correspond. Even without recharging the two other batteries, both were still above 80% meaning I didn’t have to worry about servo or accessory consumption, making lighter batteries a possibility again if CG can be allowed to move back.

Back home I mechanically reduced the elevator throw hoping to obtain a less nervous elevator response during next test-flight. A to-do list was made and will be taken care off during winter. Bystanders lift their eyebrows seeing me connect four batteries to a rather complicate electric assembly, and having to separate the fuselage from the wing to do that seems illogical to them, but to me it is simple and the complete operation only requires two M6 bolts to be inserted to hold everything together. Transportation and home-stocking is a piece of cake so to me this model feels perfect for casual (versus special occasion) flying, something I never anticipated.

To do before flight 4:

Mechanically reduce elevator travel
Mechanically install some up elevator and zero ele trim (if wing incidence unchanged)
Reduce ele expo again (30%)
Check if throttle-rudd mix affects nosewheel tracking straight
Reset rudder trim without affecting NWS tracking
Reset rudder subtrim center more to the right
Eliminate tendency to bank left with up elevator
Find out why still no CNSP calls during the flight. (call landing during initial climb)
Why battery critical single call during normal flying?
Total power consumption receiver bat: 290mah, accessory bat 83mah after 2 flts !!!!
Review receiver battery attachment?
Think about wing incidence, now 0, normally 0,5°.= 0,35mm spacers aft, incline main gear aft
Rename gyro as brakes on tx
Reset failsafe with gear and flaps up after completion of flight testing
Oct 21, 2018, 05:01 PM
The sky is the limit
BAF23's Avatar
Thread OP

completion of test flight program

Flights 4 to 6

I kept the momentum going and a few days later took the Pilatus out again to evaluate the recent modifications. Knowing I had more than sufficient endurance I taxied out to the runway, stepping behind the model to observe the tracking of the nosewheel. It was obvious my steering system with the rubbers and springs was not ideal to ensure consistent neutral centering, but once it tracked straight, it kept the direction well so the trick was that after lining up I had to roll a few meters hitting the rudder till it tracked straight. With the countdown timer reset to 5 minutes I stepped to the side and slowly opened up and only needed a short steering correction during the straight takeoff roll. After rotation it climbed a bit too steep so I reduced the angle and raised the gear, turning away to better monitor things. A few horizontal passes were necessary at pattern altitude to re-trim the model but the modified wing angle seemed adequate.

With the model trimmed, I climbed for the stalls and slow flight. The model now was much more docile in pitch stability. I observed a slight ballooning when lowering takeoff flaps, but no further changes extending the flaps to full or retracting them in one go (that still takes 8seconds). I heard the audio altitude above 120 meters (legal limit in Belgium), the power consumption every 1000mah, and the minute countdown calls according the throttle setting. I also flew a couple of overhead passes during which I retarded to idle and accelerated to full power. The model tracked straight as an arrow and finally remained wings level, proving my guesswork on resetting rudder and its mix to the throttle was perfect. The CG dive check brought me a little lower where a couple of loops and barrel rolls were flown much more fluently thanks to the the augmented stability and perfect expo’s at 40%.

I then came back down for a few low passes over the runway at various power settings and was surprised I didn’t have to trim the model according the speeds. Overall it flew much more stable than before and whatever I did, it didn’t display undesirable secondary effects anymore. By that time my timer had already passed zero but I had the power consumption calls to go by. Gear down for a low approach to confirm, then another pattern this time with takeoff flaps, and after the 3000mah call I lowered full flaps for an excellent landing. With the remaining battery I position behind the model and performed a few accelerations from standstill to full throttle with elevator slightly down, neutral and up to observe any tendency to swing. Slight positive push on the nosewheel after brake release helps the model to track straight during those critical first meters. Back in the parking my timer already had been counting up for 1,5min and as the batteries had only depleted to 25% I changed the countdown timer from 5 to 6 minutes (throttle sensitive). It had been a real long flight close to 10 minutes during which all objectives were met.

While the batteries were loading I mechanically readjusted the elevator kwik-link to obtain a neutral elevator trim again but had little else to correct. 2800mah were needed to top up the 4300mah batteries so they had definitely not been excessively drained. A fifth flight was flown just to check the validity of the new elevator neutral setting and it proved better, the model not having tendency to climb too steep after liftoff. By that time, the sun in final made it more difficult to fly patterns but I still managed to considerably reduce the speed in final and even without headwind or brakes, I managed to touch closer to the threshold and stopped in about 40 meters. I called it the day, reset the fail-safe settings on my FrSky receiver, disassembled the model and declared it ready for the next step: operating from the well groomed grass field of Tongeren.

A few days later the weather for that was perfect except there was no wind, a little headwind would have helped. Initial takeoff acceleration was good but even on this smooth recently welled grass I saw the model bump more than I wanted. I rotated and got airborne in about 90 meters. I performed a left turn out and flipped the gear button to up but as I passed downwind I still saw something sticking out underneath. I came down a bit, flew overhead and sure enough, all three gears were still down. I recycled the switch but as nothing moved, I flipped the gear switch to down and proceeded with the flight. I was able to fly a loop and a couple of barrel rolls but didn’t like doing aerobatics with the gear down so I came down and performed a couple of level low passes and a top-view pass before flying a couple of circuits.

As there were a lot of pilots watching that new bird on the field, I decided not to experiment full-flap go-arounds and flew a normal full-flap landing. Touchdown was one third down the runway and even without brakes, the model quickly stopped but I was surprised how much it wobbled on that reportedly flat surface. During the taxi back it was obvious that the narrow track of the gear was ill-suited for regular operations from grass field. I got lots of compliments from other members about how well that PC7 flew and looked but in my mind, I already had taken the decision to operate it only from tarmac except maybe for a fly-inn or so. After opening up the model I discovered that a loose power wire had been the culprit for the gear problem and I was glad it happened like that instead of causing a gear-up landing if it had happened in flight.

That sixth flight completed all I wanted to explore in the test-phase. I never expected it to be completed that late in the season and with so little problem areas. The model will get undergo some refinements during winter but together with my recent Tipsy Nipper will provide the backbone of my powered models for 2019. Do not expect any further entries concerning this PC7 except if I get good flight pictures of movies from it.

To do during winter:

If AOA is better (watch ELE streamline), then modify pinholes at front of wing for new AOA
Make fairing between lower leading edge of wing and fuselage
Reset failsafe with gear and flaps up after completion of flight testing
incline main gear further aft?
Rewire and adapt to LiFe bats for Rec and accessories
Redo wiring for gear ops at deans connectors.

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