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Jan 19, 2013, 04:58 PM
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Build Log

FMS T28 Part 6: wing assembly

Part 6: wing assembly

The morning Santa was supposed to bring me something from Winterland, the postman rang with one of the Hobby King packs. On an order of various small things for 47 dollars, I had to pay 25 euro (about 33dollar) for import, taxes and administration, no wonder our hobby is expensive. Included in that order were the long awaited heavy duty flap hinges.
After cutting slots in wing and flaps to install them, I was able to play with front and back alignment, and elevation versus each other, to find the ideal position where the top curve of the flap would hug the still to be made cover plate, and a gap would only start between take-off and land position. I now could measure how deep that cover plate had to be to get best visual and aerodynamic results. Two centimeters seemed fine and I used an iron saw to cut the flap cover strips out of a 1mm white styrene sheet. The pictures below show the cutouts and how much wing area I lost at the back compared to the original wing root fillet.

Those flap hinges are shown here at the various positions, demonstrating their shape is ideal for deployments with the extrados hugging the same top portion (of the future cover plate).

After much sanding I dry fitted the flaps on their loose hinges and observed everything went as thought of. Dry fitting of the FMS flap actuator horns showed they wouldn’t work. Trying to pull (even below the hinge pivot point) on a flap that moves backwards during extension is nearly impossible. Either the horn has to be very long, requiring extra-long servo arms, or a way has to be found to push the flap out on its top. But how to make a (hidden) strong pushpoint in the flap with precut horn and opposed plate recesses already molded in the foam. With the servo cutout already in the bottom of the wing modifications had to be made. I came up with the idea of mounting the flap horn upside down on its designed position. Cutting a slot in the foam allowed me the insert the horn in the foam instead of out of the foam. I now had 4 attach holes possible from flap center to above the flap. After installation of the original pushrod I found out the two extreme holes wouldn’t allow sufficient flap deflection and I cut the horn halfway so it wouldn’t protrude outside the flap anymore. In order to get a straight line between servo arm and new flap attachment I had to cut a serious gorge in the bottom of the wing, but that would result in most of the actuation system to be completely hidden (as in the real aircraft). I then even deepened the factory recesses for the horn and opposition plate, used the provided screws to mount everything tight, dremeled off the protruding lengths and filled and sanded everything to obtain as flush a flap as possible (at least on the top side). I then vinyled the flaps in one go, but because when extended their front would be visible, I started from the back then around their front and joining at the back again so no joint would be visible. The ailerons had been vinyled the other way around, starting in the middle of the front rounding, because that would always be hidden from view into the wing recess.

The deep cut over half the depth of the wing seriously weakened it so I studied possibilities to reinforce that area. With positive g’s it would have been better to just install lateral carbon strips through the bottom, but the flap actuator rod and flap hinges prevented using that ideal position along the total flap width. I thus was forced inserting the long reinforcement strip on the extrados. The white styrene not having been glued yet provided me the opportunity to cut a 5mm deep one mm wide channel for the carbon to be glued in. The cut was made by carefully letting a straight saw glide at the extreme end of the already carved out surface. Following picture illustrates everything together from top to bottom: inverted actuator horn embedded in vinyl covered flap, deep gorge cutout for actuator pushrod passage, flap hinges and their slots, saw in cut carbon reinforcement slot, styrene strip to be glued on top, carbon strip to be glued into newly made slot.

I glued both carbon and styrene strips together in their fabricated wing recessions (visible on the first picture in part 6) ensuring they were perfectly in the prolongation of the wing extrados and bonding together for a strong L profile. It then required a few layers of filler to get it completely smooth on top, and even more filler at the bottom to make a smooth transition from the hollow flap receiving area. It was important to get that right because it not only made it stronger, but would guide smoother airflow over the leading edge of the flap (through the gap) to get extra lift when deployed.

Trial fitting demonstrated this setup worked like a charm, but I started getting doubts about the ability of the FMS installed (slow) servo to push out this rather large flap surface into the airstream. The thin rod also wasn’t up to par and I took no risks. I glued a hollow carbon tube around the metal rod and then cut out the factory servos. Problem was I already had used filler to integrate them into the wings, and also to fill the servo wire gutters flush. I had no envy to get all of it out, so I cut the wires at the servo, freed about 3 inches of servo wire from the filler, soldered female servo plugs on those, and connected 2,3kg torque 9gr German analog servo’s I had in stock. I also took the opportunity to install those servo’s just a few millimeters deeper so they would not show. After some fiddling, filling and sanding, everything looked good and testing revealed I would not fall short of power extending the flaps, and the pushrod would not bend either. Disregard the reverse installed near right flap hinge in following test picture setup.

I once more tested all servo’s and lights, and with all the mechanics of ailerons and flaps ready (but not glued or attached) I now vinyled the wings, starting with the bottom because the wing shape did not allow a one piece top and bottom covering in one go. I extended the gear and opened the gear doors in order to permit the vinyl to follow the contours into the wheel wells. The gear now stood into the way to cover everything with one huge vinyl panel so I started with the part behind the gear, with the edge a straight line between gear pivot corner and middle flap hinge cutout. It was a tedious and time consuming job to get the vinyl to follow all the complex contours of the wing fuselage fillets and the complex cutouts of the gear well and gear door recesses, not to speak of the recesses to accept the wing fuselage mounting plates. I then got bold and decided to have a go at all the rest of the bottom in one sheet, starting from a shoulder to shoulder with the red wingtip vinyl. It worked well, without a single crease or bubble, but getting around the leading edge extension and fuselage filet was a real pain in the ass. It took me 2-1/2 hours to do just that one (large panel), but the result was marvelous.

Still having the sequence (of trimming, stretching, initial spine application etc) fresh in mind I then did the bottom of the other wing before tackling both tops. After gear and door retraction I could work flush again and spent a few hours making the pattern of the top part. This was huge, because at the complex wing root it measured a whopping 30cm and required extra length to wrap it around the edges. Only after a good night rest and thinking about where to start and slowly unroll while applying pressure on what I call the spine of the vinyl application, did I attempt this operation (with much apprehension). The part over the leading edge would overlap the first bottom cm so the joint wouldn’t be visible from a normal viewing angle (invisible shoulder to shoulder would be almost impossible for such a length. At the back of the wing the vinyl had to be applied over the styrene flap plate, around it, then on the bottom and into the concave flap leading edge cutout till the end of the bottom wing. It might be called a miracle after a few hours everything was perfect, but it was nerve wrecking and I often had sweat bursts, not knowing what to do next, how to hold everything with only two hands, while the very sticking vinyl caught on places I didn’t want yet.

Vinyl sticking to foam can be easily removed. Vinyl sticking to other vinyl top is almost impossible to break free without lifting the bottom vinyl from the foam, stretching both in the process. Vinyl to itself on the glue side is deadly and inevitably causes excessive stretching to break free, which make those parts impossible to apply flat later on. It might be called a wonder the only unrepairable mishap I got was when I had to pull it from the aft part of the wing filet (extending behind the flap to the fuselage), I found out there still was factory paint that came along, neglecting further use of that part because it didn’t adhere anymore. Luckily it only was about 2-1/2 inches long and one inch high that had to be cut out and replaced with a fresh patch, that was shoulder joined almost invisibly. It took me two full days, from morning till past midnight to cover both wings, but the end result was worth the effort and looks magnificent from every angle. After trimming off little things and cutting out the spaces for flap and aileron hinges it became time for the final assembly. Ailerons hinges were carefully glued in, because they fit in so well it would be impossible to try to remove excess expanding glue through the narrow slots, but after drying they were solid and moved freely over the desired angles. The partially exposed flap hinges were easier to glue and after painting them white everything looks very realistic and pleasing.

When all was dry I looked at the pictures of the original aircraft and noticed the front of the flap top portion wasn’t painted but left in bare (polished) aluminum. Even with flaps fully up a small strip still was visible so when I cut out and applied adhesive mirror foil over my flap leading edge, I made sure the effect at the back of the styrene cover plate was symmetrical and realistic. Following pictures show the flap up, takeoff flap, land flap positions and aileron angles to a good effect, but actuators were not installed yet.

Having no possibilities anymore to change the servo arm angle, and so little space to adjust the flap actuator length, I had no other choice as to use two separate channels for flap operation adjustments. I assembled both wings and connected all servo leads to the 10 channel AR1000 receiver. The grossly insufficient Spectrum DX10t manual forces you to use trial and error, and much experimentation to get the flaps to operate perfectly symmetrically. Servo operation first was reversed (remember I push the flaps out instead of pull), then the central takeoff point mirrored in approximately the same servo arm angle (by adjusting the subtrims). With travel adjust I then got both flaps in the correct up position, and also full down position got adjusted the same way (in fact by looking flat along one of the flap surfaces and aligning the other one on it). With the 4 second servo speed selected it all looks very realistic.

The flap actuator rods remained deep enough to allow 4 mm carbon strips to be glued in without hampering the movements. I definitely wanted reinforcements to take care of G stresses along the deep cuts I had made. Besides the all width CF strip previously installed on top of the wing, I choose to glue in two strips across each bottom cut, mainly because there was so little area for the carbon strips to hold on (in between the flap hinges). 4mm also was the maximum seize because on one side they had to remain flush with the intrados, on the other they couldn’t rub against the flap rods during their operation. With the carbon strips in place, there is no possibility anymore to take the pushrod out of its new cave, and when dry, a vinyl piece will be used to cover the modification and most of the gorge. This picture was intentionally taken before that filling and covering, and also shows one flap up and the other fully down at what I consider a reasonable angle to start with.

With the ailerons being incrusted deeper in the wing, the original aileron pushrods were now too long to correct by turning the quick links. The factory Z bend was cut off, and with the special Z pliers a new bend was made. The neutral point was further refined by turning the quick links, but it wasn’t too critical because I also use two separate aileron channels so I could create a differential function by limiting the down travel to 50 percent on the travel adjust.

Click here to read part 7 of the T28 build log
Last edited by BAF23; Jan 25, 2013 at 04:15 AM.
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