I just finished a letter to a friend and I told him why I've been so busy lately. I thought I share this with others.
Let me tell you a story... A couple of my friends bought a couple of Pulse 60 XT's.
Both installed OS 95's in them. The factory says 60 to 75 two stroke, so yes they're a bit overpowered, the OS 95 is lighter than the 75 so overpowered it is, you can always fly at reduced throttle.
Dick was flying when the elevator and then the stab started fluttering, it ended with the elevator ripped off and the stab broke. The plane didn't survive.
We found the elevator joining wire wasn't glued from the factory.
The next one, they removed the balance tabs on the elevator and used a 1/4 inch solid sheet stab with large DuBro pinned hinges and a heavier elevator joining wire.
That worked good for a while, but the flutter would re appear every once in a while.
Finally the stab broke, that one didn't survive either.
Next a heavy duty servo was used , a Futaba 3010, that had twice the torque of the 3004 it replaced. That one fluttered also. A pattern was starting to appear, the flutter started in level flight but only in high speed.
The next one had the elevator join wire reinforced and this one did it too, and the next. There was never a clear cause for the flutter A little play in the pushrod, or the threads on the clevis had a little play or this or that. Nothing definite just a bunch of maybes. . Always the stab broke in high speed level flight.
I repaired one of the earlier crashed planes and am flying it with an E Flite 60. The plane flies great. and no flutter, and I'm using one of the stock stab assemblies. I only removed the balance tabs. The glow planes are a bit faster, but not significantly so. I do fly at partial throttle and I reduce power whenever I go vertically down, but that's just the way I fly.
I got to fly the next one for a few minutes It flew differently than mine did, it needed a lot of down elevator when going inverted. Maybe nose heavy?
Less than 5 minutes after I handed the transmitter back to the owner, you guessed it, the tail fluttered. I watched it vibrate so badly it looked like a flag waving. This was one with the solid stabs too. Nothing was left, it hit the road, even the motor mount lugs on the engine got ripped off.
I told him to let me put the next one together for him and see what I can do.
I looked at 4 or 5 stabs that had failed and they all failed in the same place and all failed in a downward motion.
I decided I'd build a new stab and it would be stronger and a little thicker.
I added a 1/4 square spruce spar and some 1/32 ply vertically ahead of the trailing edge and behind the leading edge. I sheeted the whole stab with 1/16 balsa. the stab is now 3/8 thick instead of the 1/4 inch stock.
I used 4 DuBro hinges per elevator half and I increased the push rod wire diameter to .095 instead of the stock set up.
I used hardwood dowels in the push rod and clevises with jam nuts on them to take out any slop in the threads.
We went to two elevator servos too. If you wiggle the elevator, the servo moves in it's rubber grommet mounts. There is no give anywhere.
Dick flew this one this weekend, no flutter. He even tried to go faster and faster to be sure the tail wouldn't flutter. He finally dived it from as high as you could see it, full power, and no flutter.
I don't know exactly what caused the flutter, but it's now gone. I'm glad I didn't have to pay for all those planes that got destroyed. I ended up with enough reparable parts to make two more Pulse 60's for myself. I don't have the exact number of planes they crashed but I have parts from at least 5.
Sunday night , on the last flight of the day, Dick hit a tree. The fuselage is OK, I'm making him a new wing. It will be back in the air this weekend coming.
Generally flutter is caused by a resonance at a certain airspeed. It doesn't have to be at the very highest speed you reach but in this case the motor was probably taking the plane outside it's design envelope. Mechanical resonances are nasty because once they start, they very quickly amplify until failure and it can happen very fast.
By adding a stiffener you weren't just making it stronger you were making the resonant frequency higher. Stiffening an object will almost always make it's resonant frequency go higher. I have a very light P-51 that the horizontal stab vibrates a lot when the plane is just being tested on the ground just from the prop wash. However, in the air it doesn't.
Flutter is hard to predict and by the time you detect it in the air it is often too late.
Dave, Thanks. The spar was to increase overall strength but the ply and the sheeting were to increase overall torsional rigidity and hopefully bring the tail out of the range of sympathetic vibration.
You are right, it's very hard to predict when flutter will strike, and once it strikes, it's all over but collecting the pieces.
Thanks for your comments.
Mike, that's a very interesting link. Thanks for that.
mclarkson, I think we all would have given up before the problem was solved, but from all the encouragement from the club members who provided the desire to go on.
The plane flew with a new wing today. No problems.