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Snap Rolls - Managing the Risk - What Can I Do?

Question: I've heard some horror stories about snap rolls and seen some spectacular sudden crashes that the pilot stated were caused by a snap roll. What can I do to prevent this happening to me? Is the risk manageable?

Answer: Yes the risk is manageable. Keep your airspeed up and limit the amount of elevator throw at lower air speeds. Be aware that low wing aerobatic models with smaller horizontal stabilizers generally will snap roll more easily than others.

Better Answer: First of all it is important to say what we mean by the term "snap roll". It is frequently misused and it is not always a bad thing. A snap roll is like a roll induced by using the ailerons but it occurs with little or no warning, is usually extreme or violent and usually does not involve using the ailerons. Snap rolls are often messier... they are not a clean aileron induced roll and often look like the rudder is being put into play as well. This scares the dickens out of pilots who have become used to seeing an airplane roll only when it is asked to do so by deflecting the ailerons.

In fact most aerobatic aircraft use snap rolls as part of their repertoire of maneuvers. So one way to reduce but not eliminate the probability of snap rolls is to stay away from aerobatic models! Unfortunately this approach takes a heck of a lot of fun out of one whole category of RC Flying!

Rather than give up on aerobatic models, manage the risk with the following tips:

1) Watch your Altitude: Snap rolls are not dangerous themselves unless they are so violent that they tear the airframe apart. This can happen in lightweight 3D type models but other than that most model airframes are built to withstand a snap roll. It's not the snap roll that wrecks most models... it's impacting the ground that causes the problem! Granite Congestus is hard stuff! Altitude is everything. If you are going to experiment, do so with lots of air between the model and the ground.

2) Watch your Air Speed: Many unexpected snap rolls are caused by insufficient air speed. Keep your flying speed up. If in doubt, a bit too much air speed is better than too little.

3) Watch your Angle of Attack: All wings have an angle of attack beyond which they will not fly. If you force a wing beyond it's critical angle of attack, it stops flying and your model becomes a brick.

4) Watch your Elevator Throw: This is probably the most critical single thing you can do. Too much elevator throw rapidly and often uncontrollably increases the angle of attack and reduces your air speed. High angles of attack and reduced air speeds are precursors for the almost immediate onset of a snap roll.

5) Watch your Landings and Take Offs: Things gang up on you when you are near the ground. Your altitude is low, your air speed is low, your angle of attack is higher (particularly when landing... slowing down, preliminary flair setting up) and you are nervous on the sticks with a tendency to overcompensate on elevator every time the model takes a bit of a lurch downward.

6) If your radio supports dual rates, set up dual rates for the elevator and limit the throws at the low rate to not more than 65% of full aileron deflection and use the low rates during take offs and particularly during landing. You might want to experiment with exponential as well to reduce sensitivity of the servos to movements of the sticks near center.

Remember that Snap Rolls can be fun if you've got altitude and time on your side. Stay out of low altitude situations when one or more of the other contributing factors start to come into play and you will be managing the risk of a snap roll induced crash!.

 

Article ID: 2690