Breaking out of the Loop
 

Hello,
Can someone tell me why my plane breaks out of the loop?

Profile Edge 540t, (BTW Great Plane)
Plane is laterally balanced.
Plane does not break to the same side always.
Plane's fuse is banana shaped due to 4th crash but only by 2 to 3 degrees. I think this may be the cause.
50% expo on elevator and ails.
Does not happen with elevator at 10 degrees. But will happen on elevator set to 20 degrees plus.

This has happened with other planes I own.

Thanks all
Speedbird

RE: Breaking out of the Loop
 

It's called a snap.

It happens because one wing stalls before the other one, so loses its lift first. It happens when the plane slows down at the top of a loop, or, even more disturbing, when slowing down to land!

Normally only speeding up the plane, or using the rudder helps to correct the snap. Using opposite aileron also normally makes it worse as the dropped wing has to be given down aileron, which increases the drag (as lift is inversely proportional to drag), thus making the snap in that direction even worse!

It can also happen at high speeds, for slightly different reasons, but still creating different lift from each wing.

Different planes do it in varying amounts, and there seems to be a correlation between how much the LE is swept back. More sweep = more tendency to snap. I could be wrong there, but it's certainly what I've observed.

The most common cause of a slow speed snap is too much elevator movement.

-David C.

RE: Breaking out of the Loop
 

Any plane will snap out of a loop if the wing is forced deep enough into stall by the elevators, even with perfect lateral balance, and exactly equal incidence and airfoils on each wing. At stall, any wing becomes dynamically unstable in roll, because an increase in angle of attack produces less lift. Once a tiny amount of roll begins, the downgoing wing has a higher induced angle of attack than the upgoing wing, and thus develops less lift and more drag than its partner wing, so the process rapidly accelerates. Use of wing washout will help, but with enough excess elevator travel, even the washout-equipped wing will stall deeply enough to make things get pretty murky.

The best way that I know of to beat this problem is to set maximum elevator travel so that it is just adequate to barely stall the wing. I like to test for correct elevator travel by taking the model safely high, and looping it at full throttle with full "up" elevator. If it flies only a partial loop before careening wildly in yaw and roll, it has too much elevator travel. If it continues to fly smooth consecutive loops, it probably has too little elevator travel. The ideal that I have found is to keep fine-tuning elevator travel until you get at between about three-quarters and one full loop before the model yaws and rolls drunkenly. That way, if you pull full "up" elevator to recover from a vertical dive at low altitude, the model will turn at close to its minimum possible radius, without snapping over on its back and re-kitting itself. I have seen a lot of models that were set up with roughly double the elevator travel that is needed - ideal for 3D flying, but producing a model that has to be flown very carefully to avoid low altitude disasters.

RE: Breaking out of the Loop
 

David Culter and Rotaryphile are both right. You are snapping out of the loop by trying to loop too tightly and stalling the wing. However rather than limit the elevator travel, learn to use the stick gently. You will be a better pilot if you learn to fly through the loop rather than just slap the stick back to the limit. The beauty of proportional control is that you can move the surface only the amount that is required for a particular maneuver.

RE: Breaking out of the Loop
 

Thanks all.

Here is the update.

I went to the field today with the 540t and recheckted the CG.
It turns out it was a little tail heavy.

I corrected this, and the plane flew like a gem.

And yes... there is such a thing as a high speed stall.

Thanks again
Speedbird

RE: Breaking out of the Loop
 

Moving the CG ahead will reduce the angle of attack of the wing for a given amount of elevator travel, and thus reduce the tendency to snap out of loops. As CG is moved back, the elevator travel has to be reduced considerably. The problem could probably have been fixed by simply reducing the elevator travel.

I like to locate the CG as far aft as I can get away with. My only check for tailheavy or noseheavy CG is to trim the model to fly level upright, then roll it inverted. If it takes more than just a touch of down elevator to hold it level while inverted, it is noseheavy. The ideal is, of course, no down elevator, but this far aft CG may make the model a bit too "nervous" in pitch in bumpy air. If it climbs while inverted, it is tailheavy. Locating the CG at close to the aft limit for dynamic pitch stability makes for much easier vertical line holding, since almost no elevator input is required to hold vertical uplines or downlines.