Autorotation in snap rolls
 

Could someone explain what "autorotation" is during a snap roll ? Can the rolling rate during a snap be faster than using ailerons only ? Thx !

RE: Autorotation in snap rolls
 

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RE: Autorotation in snap rolls
 

A good simple way to think about it is like this:

To do a snap roll you need to do two things.

1. Apply lots of elevator to increase the angle of attack of the wings close to stall.
2. Apply lots of rudder to get some yaw motion going. This yawing motion causes one wing to speed up, and one wing to slow down. This difference in speed causes the angle of attack of each wing to change relative to the other. The faster wing gets a decrease in angle of attack, and the slower wing gets an increase in AOA.
3. If you've done it right, slower wing stalls (due to the increased AOA), and the faster wing doesn't stall (due to the lower AOA). When a wing stalls, it stops producing most of its lift. So now you end up with one wing making lots of lift, and the other wing stalled (making almost no lift).
4. Violent rolling maneuver ensues.

That's the "textbook" version. Alot of people also put in aileron at the same time as the rudder in order to help the snap roll look nicer. But some models you don't need any aileron at all.

One thing to remember though... if you're using aileron:
When doing an "inside" snap, (i.e.when using "UP" elevator to stall the wings) you'll move your aileron and rudder in the SAME direction.
When doing an "outside" snap, (i.e. when using "DOWN" elevator to stall the wings) you'll move your aileron and rudder in OPPOSITE directions.
The reason for the direction reversal is kinda hard to explain in text... Try and picture a plane doing a normal "inside" snap roll while inverted. Try and logic it out with a toy plane in your hands. Then you'll see that while inverted the ailerons still operate "correctly" while the rudder is "kindof backwards"... like I said, hard to explain... maybe someone else can do it better...

I prolly made it more complicated than you wanted :P :P oh well.

Tom

RE: Autorotation in snap rolls
 

OH and one other thing...

The term Autorotation is correct here. It comes from the fact that since you have one wing stalled and the other wing unstalled the place wants to roll "automatically" on its own.

The term originates from the spin maneuver. Some aircraft once in a spin, when the pilot releases the controls, will continue to spin until the pilot initiates recovery action. This is the AUTO part of auto-rotation.

A snap roll is simply a spin that's done horizontally. So I think you can still call it autorotation.

The roll CAN be faster than with just ailerons because you're using the whole wing to create the rolling moment. Not just the control surface.

Tom

RE: Autorotation in snap rolls
 

Good points Tom !