3D ONdaEDGE

Hey everyone. I got a brain tickler for you. I had a thunder tiger lazy cub. It is a 25 size plane. I put a wankle rotary .30 on it. It was one of the best flying little planes i had. Well I lost signal one flight and it looped into the ground. The only thing hurt was it knoked the rear section of the plane off. I give it to a buddy of mine and he glued it back together. Went together no problem as it was a clean break. He put some triangle stock for reinforcement but you could not even tell it was damaged. Anyway he put a OS 40 la on it. It flew almost the same excpet everytime you would get the nose pointed down, the plane would continue to rotate in a downward motion. In other words the dive would get steeper and steeper with no elevator input. It was like you were holding down elevator. The oddest part about it is that if you pitched the nose up, it would continue upward and the verticle line would get steeper and steeper. I didnt know why it was doing this. I suggested a smaller engine, to knock some weight off the plane. He got a TT 25 and threw on it. The CG is now 30% of the wing chord. It is still doing the same thing. I wouldnt think that this would be a stabilizer incidenc problem because it pitches up AND down. I also wouldnt think that it is a thrust problem. Could somebody shed some light on this subject, as he is about ready to trash the airplane. And I would hate to see that as I know how good it used to fly.

Tall Paul

The control horn on the servo could be slipping under load.
The control system could be too loose... increasing speed adds download to the horizontal. If the elevator can move down because of a loose system, it will, and the plane will dive.
The same thing going up..might be a loose servo mount itself.

Scar

My Feedback: (3)

How well does the servo return to center? Sometimes they have a dead band; move 'em one way, they stay a little deflected - move the other way, stays deflected. Try swapping Throttle servo with elevator servo?

Good luck,
Dave Olson

3D ONdaEDGE

Thanks I will try both of those, but they are brand new servos. They center pretty good, but I am sure it is not the servos. Say for instance you cut the power and let the nose drop. It does the same thing that way to. This has got me completely stumpped. They only thing I can think of is that the wing incidence is chance with the different forces being applied to it, and that seems unlikely also as we strapped it down with about 12 rubber bands. I will check what you said but I am open to any other suggestions as well.

BMatthews

I had a glider that did that too. It was a combination of slop and stiffness all in the cable and sleeve pushrod to the elevator.

The result was the same as what Scar described but from a different source. With the model on put in some up elevator and slowly return to neutral and see where neutral is with a ruler against the rear of the elevator. Do the same with down and slowly back to neutral. If there's any difference that's more than about 1/16 inch that may be your problem. It may be radio (dirty pots in the servos or even the transmitter sticks) or it may be the control system.

Beyond that try moving the balance forward a little and retrim the elevator. That should fix it. Where was the CG before it crashed?

Other than that I suspect Paul is right about the mechanical slipping thing. Sometimes the simple things bite you like a stripped thread in a nylon clevis that slips when you least expect... Yep, had that one too.

Tall Paul

The instance I had with a slipping servo was flying a new plane which was using parts from a crashed airplane. The arm had split opposite the side with the pushrod holes. When the surface moved under load, it would open up.. then grip the serrations at a different point on the servo output shaft. This made for an amusing shifting neutral.
Fortunately it wasn't my plane, so my concern was minimal, but I was able to get the thing on the ground and determine the reason for the bizarre behavior.

Rotaryphile

I was teaching a beginner on a Sig Kadet Senior, which did the same thing. It turned out to be caused by a really sticky elevator control plastic coaxial pushrod that had far too sharp a bend. The problem was clearly visible when I did the test that B Matthews mentioned. He fixed up the pushrod, and the problem went away.

mulligan

Where was the CG before the crash? 30% wing chord sounds ok, but did you try adding some nose weight?

It's probably a control or linkage issue, but I thought I'd throw that out just in case.

Ben Lanterman

I test flew a model (his own design with a really big really far forward horizontal tail to get it in the strong downwash) for a friend and found this happened during the flight. It was impossible to keep the airplane level and the flight looked like a smooth up and down motion (or a sine wave for the more math inclined) but large in amplitude. I was dreading trying to land it because he was at the time a really good friend and I hated to crash the monster on the first flight. Gross.

When I throttled back the airplane stopped the motion and started to fly like a stable airplane, touchy but stable. It turned out that it was just a hair on the unstable side when I took off with full power and removal of the power and it's destabilizing effect made the airplane OK.

See if the Cub has the same effect in glide as power on. If no change then the CG is probably OK, If the effect goes away in glide then the CG is just too far back. Also just be sure it isn't the good thumbs of the pilot that is making the plane OK in normal flight.

If the CG is OK then look for a mechanical issue, pushrods, servos, etrc as noted by the excellent writers above and or flexing of the horizontal tail with elevator deflection at high speeds. That will cause a twist that will give a load opposite to the elevator deflection. I have some foamys that act this way.

An also had foamys that the pushrod flex cause the same effect at high speeds. It is one of those things that you can find out by just loading up the elevator by hand and seeing what happens to the normally straight line that the pushrod should be.

Montague

The above posts probibly are right.

However, I've seen this effect in some of my combat planes, exaclty like you describe. It's usually a tail heavy condition. You are basically every so slightly unstable. Move the CG forward 1/8" - 1/4" should do it. I've also fixed the problem by raising the ailerons (reflex) a turn or three on the clevis. I don't know why this works (maybe changing the pitching moment?), but I do know that it works in some cases.

3D ONdaEDGE

All sound good, but it does it with power on or power off. It also pitches down with no elevator input needed. Say for instance you are flying along straigh and level and you cut the power. The nose will drop slightly when it gets slow enough. When the nose does ease down it just keeps on till it is pointed straight down. All of the contol linkages are good and strong and the servo is fine. Everthing is brand new except for the airframe which was the only thing involved in the crash. Before my CG was about 20% chord so we will try to move the CG forward a bit, but i dont think that will help as it did the same thing with the OS 40 LA on the nose. With that the CG was about 15%. That is why I kinda ruled out the CG. I am completely stumped on this one. Any more suggestions?

mulligan

Maybe you accidentally set a mix from your throttle to your elevator [sm=bananahead.gif]

Just kidding, you have a strange one. Here are some more things to look at:

- Did you wind up with any upthrust? Check and add some washers to point the engine down a bit to see if this helps.

- Is the main wing seated well. If rubber banded, add a few extra bands.

- Was there any chance the main wing and/or stabilizer incidence was changed in your crash and repair. If so, check.


Also, what is the wing loading and the wing span? I also wonder if you have a combination of a heavy plane and a rearward CG. If the plane is currently tail-heavy, then adding weight to the nose or putting the 40LA in it might make the wing loading too high for the plane- just another shot in the dark.

britbrat

Is there washout in the wing-tips? If so, & if the wing was weakened torsionally in the initial crash, the tips may be washing out more with increasing speed, causing the tuck-under.

I encountered this on a student's LT-40 -- normally a docile airplane. He had a crash involving a low-speed cartwheel in which the elastic-retained wings popped off. No visible damage. On the next flight, the plane lifted off & climbed out just fine, but when it levelled off and began to accelerate, it tucked under. Closing the throttle & applying full up elevator eventually resulted in recovery, which was abrupt. Many fixes were tried with no success, until I was handling the wings one time & I noticed that they seemed to be more torsionally flexible than I thought was normal. On speculation, the wings were stripped, joints re-glued & the wings re-covered. Voila!! Problem gone.

3D ONdaEDGE

HEY I BET THATS IT. We inspected the plane and found the spar broken. We glued it back but apparently the glue didnt hold good or something. And the wind currently will twist with the slightest amount of pressure. We will reglue it and see if that helps. Thanks very much everyone for the helpful suggestions. If we have any more problems, ill be back. Thanks again.