|Jan 19, 2013, 05:28 PM|
FMS T28 part 8: weight and ballance and final decoration
Part 8: weight and balance and final decoration
The picture at the end of part 7 illustrated how a stock FMS T28 could be rigged. Mine being anything but stock I was prepared to see devious numbers concerning weight and balance, especially with the heavier cockpit assembly (215 grams), the MrRCsound system (190 grams behind the wing not even counting the small aux speakers), added elevator servo, added hinges on all control surfaces and flaps, and total vinyl covering. I retracted the gear so I could put the complete assembled model on my precision (digital kitchen) scale and panicked finding out it was above the scale limit. Taking the battery out was sufficient to bring it under 3 kilo so that got my heart beating slower again.
After installing the battery again I attempted to lift the model from the scale with two fingers at the recommended 80-85mm CG position. It was no surprise to find out it was tail heavy, but by how much? I had anticipated that problem so had kept everything free in front of the factory battery tray. I moved the battery against the firewall but even that left the CG around 3cm too far aft. Time to take old lead blocks and squeeze them between the prop and cowling. The model eventually started balancing and I knew I would have to find adequate space for all the lead, as far in the nose as possible. After removing the plastic nose cowl I saw the possibility to glue preshaped lead strips all around the lip.
That amounted to 120 grams, bringing the plastic nose up to 200 grams, all to be retained by 3 tiny screws at the back of the cowling. Luckily the cowling fits tight around the massive foam nose to absorb vertical loads, but it was so tight I had to cut away small foam portions to allow necessary space for the lead strips. After assembly it was time for a more precise balance, but how to support a 7 lbs model on two thin pivot points under a soft foam wing? The solution again was found in the kitchen. Using two wine bottles with slightly rounded caps, aligned with the tiles on the floor, and carefully depositing the model upside down on the bottles, with the wing leading edge also aligned with the tiles, I could find a reasonably accurate balance point without damaging the wing surface.
It still was tail heavy but another piece of used 80 gram lead seemed to do the trick. Off came the prop and plastic nose again, and the search for a suitable space for lead resumed. The foam in the nose part is massive and reaches till the fake engine mold, but did not seem to have any structural function. I thus took out the knife and cut away an hole just large enough to allow me to mount that lead piece without play into that orifice. Contrary to the shaped lead strongly glued within the intake lip forever, the block was just held in place with minimal amount of FMS glue. The idea is hopefully being able to remove that weight later after finding out in the air what the aft CG limit is, or deepening that same cavity out to add even more weight if necessary.
On the above picture you can see the lead glued almost all the way around within the plastic nose lip, the white foam showing where some material had to be shaved off, plus the extra lead block in the foam below the engine. Prop and spinner were already painted to conform the latest changes on the full size aircraft. With the plastic nose tightly against the fuselage and secured by 3 screws to prevent slippage from the foam, there is no way my lead weights can vibrate loose and cause any damage. Waiting for everything to dry and having the canopy removed I tackled the last modification. You might remember some FMS T28 flyers complained from canopies lifting in the back in flight, some losing them while others were able to land with their dangling greenhouse. I still am not sure about the cause, is it from the aerodynamic lift created over the wing like shape of the transparency, or is it the extra pressure from ram cooling air in the fuselage, unable to exit through the small outlets in the bottom of the fuselage. Luckily the cockpit is held well in position by the front foam lip, and although the 3 plastic snap assemblies seemed solid, I augmented those with strong flush mounted rare earth magnets glued in the angled foam surfaces behind the back seater. Such magnets can be very strong and I already broke some just trying to separate them from each other coming out of the spare parts box. With everything in place I again positioned the aircraft inverted over the wine bottles and the slight nose down tendency with the bottles at approximately the correct CG position reassure me for the first test flights when spring comes back to Belgium.
Back on the scales the model now weighs empty 3,25kg or 7,2 lbs. To that I will have to add another 450gr or 16 oz of batteries and the model remains well short of 4kg. The manual mentions 33,4 dm2 wing area, but with my aileron and flap modification that got reduced to about 32dm2. Dividing 7200 gram gross weight by 32 wing area results in a staggering 115 gr/dm2 wing load (compared to the 74gr/dm2 for the stock model, but it is unclear to me if those numbers include battery weight). This traditional prop model approaches or surpasses the wing load figures of my B25 bombers or EDF fighters and promises to be a handful to handle, with very limited speed range. The thick wing and efficient flap system probably will develop sufficient lift to get it from (and back on) the ground. I also wonder if the stock engine will be strong enough to keep it in the air at anything but wide open power, and am glad I exchanged the ESC for a heavier 65Amp one to cater for the expected higher power use. The interior has been rearranged to cater for batteries positioned as far forward as possible, the main one at an angle for not obstructing cooling airflow from the engine compartment. Cutting a bit of foam away according to the battery shape at the front and rear, pretty much ensures it will remain in position during normal flight. due to its gentle squeeze. It means I will be restricted to the use of this brand or exact battery seize but I can live with that. To the right of it and standing up in the corner I got a perfect space and fit for the smaller 2S950mah flight control battery that is connected to the 10Amp BEC positioned above the ESC (not shown on the picture below).
Thin black striping was applied to accentuate the individual canopy parts. I finally got my decals (delay not Calieís fault) and after applying them obtained a splendid semi scale (standoff scale?) foam model of which following pictures give a better idea. Spending 3 months of daily work on an ARF model maybe isnít everybodyís cup of tea, but it provided me sufficient challenges, dexterity with foam, and experience in vinyl covering, that I feel confident and ready to tackle even more unique foamies. Please be patient for pictures besides the real N1328B, and videoís about the sound system and maiden flights. Iím waiting for warmer weather and calm winds down the runway, a capable cameraman who can edit and post such movies, and all that together might take a few more months to happen.
Specs and settings on Spectrum Dx10 transmitter before first flight
Empty: 3kg250 = 7.2lbs both batteries: 450 gr or 16 oz. Total 3,7kg
Wing type: 2 Ail, 2 Flap; Tail: Normal
1 : throttle = throttle. travel 100 over 105
2: right aileron: Reverse; trav 100 over50
3: elevator: Reverse; trav 100 over 100
4: rudder : Reverse; airplane rudder on aux 3 no expo trav 100 over 100
: nose wheel steering on rudder 90% expo trav 115 over 115
5: gear : left flap, Reverse; trav 80 over 110 Subtrim up 168
6 : left aileron ; Reverse; aux 1 trav 50 over 100
7 : Right flap: Reverse; aux 2 trav 100 over 110 Subtrim Down 10
7 : aux 3: Input Rudder; airplane rudder (slaved to nose wheel steer) trav 100 over 100
8 : aux 4 : Input throttle; throttle sound trav 95 over 100 Subtrim +10 ; Speed 2,0 over 2,0
10 : aux 5 : left flap switch A. Reverse; system assigned no aux5 in rx trav 81 over 86
11: x+1: Switch N ; port cowl flap trav 115 over 120
12: x+2: Switch E; landing gear: switch E trav 100 over 100
13: x+3: Switch N; starboard cowl flap trav 130 over 120
Mix 2: aileron to Aux 3 (rudder): 15% 15% Switch ON
Flap sys: pos 0: -100% elev 0%; pos 1: -25% elev 15% ;pos 2 = 100% elev 20% switch A ; speed 3,0 sec
Ailerons: 20% expo switch ON
Elevator 20% expo Switch ON
Rudder (in fact only nose wheel steer) 70% expo Switch ON
Timer 6 minutes
|Jan 19, 2013, 07:59 PM|
Joined Jan 2007
A lot of time and effort has definately gone into making this a work of art.
I myself have spent a lot of time with mine especially in the engine bay.
Do you intend to possibly sell kits at all. If so you can count me in on at least 2.
This year I intend to strip my T-28 back to bare bones and possibly do some similar mods and a total paint change. Nothing like something different on the flight line at the club and especially when your out somewhere else.
I'll be looking forward to seeing the video's etc when you can get them done.
|Jan 20, 2013, 04:52 AM|
Thank you Peter for your nice comments.
No, I have no plans in marketing, I just do such elaborate modifications for my own pleasure, and to prove the guys at the field scale models are not a prerogative of balsa builders. Foam builders can also have challenges and come up with little pearls (as you can see on my blog pages of the DC3 and T6 with many more following).
Good luck on your T28 remake, Laurence
PS: do you already have a colorscheme in mind?
|Jan 20, 2013, 03:52 PM|
Joined Mar 2010
I have followed your build logs on the official T-28 threads with much interest. Why some folk object to these on the main thread is weird, because that is exactly what it is about. I think that a lot of folk will miss out on some interesting info. I also love tinkering and spent last winter also working daily on a Silver T-28 , whose scheme I kept but did lots of mechanical work. I fully glassed the whole plane, also rehinged all the surfaces and shaped the control surface hinge lines to be more scale. I considered doing your full mods, but as you have noted, it would have been at the expense of surface area. Once the plane is in the air, these details are never noted, and my experience was that it would make no difference to flying characteristics, but it is always nice to have it just right like you did. I am the type who does a perfect paint job behind permanently built in kitchen cupboards for example! I put on Oleos (not scale of course) and the bigger 514 retracts with metal trunnions -- as Sandancer did, and noted that a decent job of this takes a lot of time. The lights were upgraded to something that was actually visible in full daylight. Mine came out at about 3200gm with 3600 4s and flew very nicely at about 3/4 throttle. Landings without flaps are hot but the tricycle under carriage if doing a 3 point landing works really well. Flaps (mine were rehinged in the stock manner) reduce speed significantly at about 35 deg deflection. I sold this model to a flyer who was also planning on using the Mr RCsound system. I have not heard how that has gone, but he flies 1/4 scale and at about 3250gms felt that this plane flew rock solid and landed exactly scale.
I have since built another with similar mods but only partial glassing of the wings alone, but a final weight of about 3050 with a 3300 5s and a huge 4020 Scorpion motor. This generates that same watts on 4s as the FMS stock motor, but pushes out a wopping 1500W with a 5s. My 80A pentium esc gets pretty hot even with all the cooling mods. Moving to 5s with even the stock motor makes a significant difference. This motor added about 60gm to the stock motor weight.
On this model I managed to balance with bat in the stock location without lead. The fully glassed version balanced with the bat just aft of the firewall. Clearly glassing or adding vinyl aft of the CG makes a significant difference to balance. I was planning on doing vinyl just like yours on this second build, but decided to try first with a stock fuse. The only real difference is that landings are a fair bit slower.
I will note that the heavier versions (mine flown by some very experienced pilots) all need a longer take off and landing run than I and they expected, but all very scale. I really do not think that your higher wind loading will be an issue, as the lift generated by that wing section is huge. I would definitely go 5s and just use a smaller battery. I easily get 7 minutes from my 3300 5s with at least 10 WOT runs and 1/2 throttle flying for the rest. I usually use smaller bats x2 flights rather than one long flight at higher throttle. Due to the natural airframe drag, there is not a huge difference in top speed between 4s and 5s, just a lot more grunt.
She likes to land under power at about 1/4 to 1/3 throttle, and with flaps just descends beautifully and predictably.
Sorry for the long post about my experiences, but you may find that this settles pre flight concerns. I was very worried about wind loading, but found it a non issue other than that she handles the wind without even a wobble. She is a vice-less flyer and one of those rare triumphs of the modelling industry with FMS getting her just right.
I look forward to your maiden observations.
|Jan 21, 2013, 03:46 AM|
A pity the readers of the "normal T28 forum" will miss all that valuable info you wrote down. Your message is full of extremely useful info and it also reassures me a lot before undertaking the maiden later this year. What remains is still being puzzled about the power. Will the stock engine with stronger 65A ESC be capable of handling 5s power? How much modifications are needed to attach all the different suggested engines? Are they still capable of turning the stock scale hub and propellers without adapters, and is that prop capable of handling the extra power. Many of those questions never were answered on the forum and I got disappointed by its contents lately.
What concerns aircraft balance, I don't think the vinyl behind the CG was the culprit, but the 190gram of the MrRcSound system well behind the wing and extra elevator servo certainly did. Substract the 200gr lead, sound sys and servo from the 3700gr takeoff weight, and you'll come to very acceptable vinyl figures. I agree with you some of my changes are not visible when it's flying, but I usually take about 5 airplanes along in the camper and they gather a lot of attention just sitting on the flight line.
Happy Landings, Laurence
|Jan 21, 2013, 05:21 AM|
Joined Mar 2010
I forgot to mention my engine mods. Both of my T-28's had the nose sliced off at the line where the cowl meets the fuse. This is exactly behind the existing plastic firewall. I then put a piece of thin ply between the two sections and compress while setting with gorilla glue. This provides a very solid base for any motor and screw gripping points for the mount. I manufacture my own mounts from aluminium plate to match the stock screw configuration but the length as needed by the motor. Getting the down and right thrust just right can be a bugger though, but taking it slow and steady usually works.
I have several flights with my powerful system and the prop handles it well so far. I did balance the prop well though. The motor I used was the exact fit for the prop and prop adapter -- 8mm I think, so no hassles here. The only issue was that the Scorpion was a lot longer than the FMS motor, so as noted, I had to make a shorter motor mount. The cowl opening also needed to be widened by about 2mm to accept the prop adapter. The adaptor screw thread pattern meant that I had to tap a new pattern into that prop 'nut', but this has held very well. No one has noted any failures regarding this FMS system to date. I reported a failure of the other FMS system as used on the P-40 and P-51 though.
I would upgrade to an 80A esc if going 5s, but I always do a watt meter test and with the standard FMS G+C motor, the 65A esc should be fine as far as I recall.
I also always use the CC 10A BEC -- CC because this is what is available here and has a good reputation.
Yes, the sound system is probably the main CG culprit. My glassing added surprisingly little weight -- about 110grams. The firewall mod only about 12gms. I also insert 5mm wide x 2mm thick carbon stiffeners along the inside of the front wheel well (butted up against the wooden firewall) and another set along the top edge of the cockpit opening. This seems to increase rigidity and also crash resistance significantly. Again the 4 rods and glue only add about 15 grams.
I totally agree with you about the pure good looks on the flight line. It's like food, tastes so much better when nicely presented.
I think I have covered some of your further observations.
|Jan 22, 2013, 05:35 AM|
Thank you Brian, you covered it all, but a lot of that stuff is way beyond my mechanical capabilities and available tools, and too late to modify on my bird now that it is ready to fly. Happy landings, Laurence
|Jan 22, 2013, 07:50 PM|
Hey Baf that is one pretty paint job. Could you point me to the full size aircraft that you used for reference. I would like to use that paint scheme on my composite version. With all the panel and rivet detail that I have along with that paint scheme would be amazing. Thanks if you can.
|Jan 23, 2013, 02:30 PM|
Hi acesimmer, I had the advantage of having access to the real one now stationed at our local airfield, but a Google search with just its US registration N1328B gives you tons of pictures on https://www.google.be/search?q=N1328...w=1280&bih=884
I also used pictures of the various cockpit interior panels cropped to be printed on photo paper to be afixed in the model. If interested, send me a PM with your e-mail so I can send them as attachments.
If that T28 had not been stationed on our airfield I would have chosen another very attractive scheme, that of NX289RD still flying on the US airshow circuit.
If you want to make mine, just contact Callie, she made my graphics and sure must have the patterns still on her computer, they are an almost perfect fit for the FMS model.
Anxious to see how yours will turn out , and congratulations on all the efforts and good info you posted so far.
Happy landings, Laurence