I got a warning on beginning this post that there had been no post in 120 plus days and a suggestion I should start a new thread. I think though since so many people have referenced this thread that I should include this section as part of the original for completeness. My T-Rex has sustained an inflight wing failure. Fortunately we caught it just before things came apart and modification is being done to solve the issue. What was observed was that the wing began to flex in maneuvers in the air. One inch flex at each wing tip one one flight and two inches on the next. At that point the plane was grounded. Rick Sawicki my flying partner noticed it as did another pilot on the flight it showed up and on the next one when it had increased we decided it had to be addressed. I cancelled my trip to Brodak because this was the Friday before Brodak's and I did not know the nature of the failure of if repair would be possible but I did not want to be limited by time.
Prior to starting any mod I checked with others who have seen the plane fly. Rick Sawicki has seen all but a handful of its flights. No flexing was noted. I checked with the Adamisin Clan because they had all seen its maiden flights. None had noted any wing flex whatsoever. The flex began the day I taped the hinges of both the elevators and the flaps. The issue of major concern is that it progressed and got worse and these were 2 minute flights. It would have separated in a full pattern.
On returning to the shop what I found prior to opening things up was that both wings could be twisted probably 20 degrees by grabbing the tips and applying pressure to the trailing edges. This occurred in both directions and on both wings. When the plane was suspended from the wing tips and not a lot of pressure applied to the fuselage vertically perpendicular to the wing a large wrinkle developed in the covering emanating from the junction of the outboard edge of the centersheeting junction with the trailing edge and progressing outwards and forwards. The leading edge section of the wing flexed as well but the trailing edge flexed badly.
My analysis from MY perspective... and I am not an engineer and will defer to any who have an opinion... was that the trailing edge of the wing was failing in overload. The failure was made more imminent by the added loading from taping the hinge lines. I felt that to correct the problem and allow the plane to return to flying status I had to achieve the following: 1) stiffen the entire wing and eliminate the flexing, 2) Strengthen the trailing edge section, 3) tie the trailing edge to a more rigid structure to eliminate the stress riser that was overcoming the strength of the trailing edge. I think the cause was the wood that was selected for the airplane. It was just not up to the loads.
To correct the problems I did the following: First I opened up the bottom of the wing. Using a scalpel I simply cut the film along the back edge of the leading edge sheeting, the inboard tip rib, the front of the trailing edge and the inboard edge of the center sheeting. This piece was removed in tact and kept for a pattern for the repair film piece. I will enlarge it 1/2 inch in all directions to close up when I'm done. I'll only have to apply the red trim tape along a small portion of the wing. The white section and the silver tape stripes will not be impacted. Secondly on the leading edge I installed vertical grain 1/8 inch balsa spar webs from the top to the bottom spar. I did this from the rib that is at the edge of the center sheeting to one rib outboard of the landing gear and then did the trailing edge mods I'll discuss below and evaluated the result. The leading edge became rigid to the rib beyond the end of the spar webs. But there was still a tad of flex beyond. Granted there is little load there. But on looking at it and evaluating it over the last week I have decided to extend the leading edge spar web one more rib outboard to really lock the leading edge in.
The trailing edge I treated in much the same way but in two stages. First I created the webs placing them between the upper and lower trailing edge sheeting at the front of the trailing edges. The webs do not overlap the edges of the sheeting but set between the sheets. Each one is sanded to fit the contour of the trailing edge sheeting and to fit each rib well. They, like the leading edge webs are CA'd in place. These webs I took out to the next to last bay. The plane is not in my estimation flyable at this point. The stress riser is still there at the junction of the center sheeting and the trailing edge. The wing while now stronger against flex from vertical loads still twists severely. I needed to get rid of the flex stress at the inboard end. The key is already in the wing. The alignment spar box! It is a thick box section tied together from one side of the wing to the other with the alignment spar. The alignment spar box extends out past the rib that the center sheeting ends on and into the rib bay beyond the center sheeting. I started with a piece of balsa essentially a spar web doubler and custom sanded it to fit between the spar web I placed between the trailing edge sheets and the spar box. Once this was fitted and snug I epoxied it in place to both the trailing edge spar web and the alignment spar box. This eliminated the stress riser. There is now no flex from either vertical loads or twisting loads in the inboard section of the wing. I could still detect twist deflection outboard of the spar webs, most notably in the trailing edge. This led me to extend the spar webs to the inboard wingtip rib on the trailing edge and at least one more bay on the leading edge (so two bays outboard of the gear).
I hope to add the additional spar webs tomorrow and will post pictures and results at that point. I'm pretty sure from what I have seen that this will result in an acceptably rigid wing structure.
I should also mention that I reentered the fuselage aft of the cockpit and installed the fuselage doubler to reinforce the area aft of the flap hinge line there. I had noticed flexing occurring there as well. If you will recall however, I had to cut into the structure there to access the flap horn after assembly This doubtless weakened the structure. Not shown in the build thread was a modification I did to the reinforcing member that runs along the top of the fuselage sides and perpendicular to them.To clear the complex flap horn I used I had to relieve these. I think I may have weakened things there as well. To fix this and tie this area and structure into the back of the fuse I fabricated a 1/8 play plate to span the structures I relieved and tie them and the fuse sides together and to the bulkhead at the front of the rear deck. To close up my entry I used replaced the entry cutout and used an epoxy structural fillet with WEST System structural filleting blend added to the epoxy. This tired everything back together there in a very solid manner and the repair was recovered.
One last mod I have done I think was mentioned by someone else, I bent new gear out of music wire. My outboard gear flexed due to wire being soft and I was suffering nose overs on landings due to the gear twisting off. This no longer occurs.
My final power system turned out to be a 4S system. I am currently using 4S Rhino 3700 mah packs with the Axi 2826/12 and turning an APC 12X6EP prop. I found the Rhino batteries to handle the loads much better than the 3000 mah Turnigy packs. 4 cell packs this size were inadequate. 5 cell packs were very borderline. The 3700 mah Rhinos have performed very well and are able to deal with the loads much better. The weight difference does not appear to be an issue... as long as the wing issue is solved.
Again, I apologize for adding this so late on but since so many folks have told me they have referenced this thread I thought this post should be a part of it.
bob branch
Topic: T-Rex electric build thread (Read 26171 times)