whifferdill
Line Up and Wait
The issue of snap roll technique came up in the 'Mosquito Antics' thread. So I started a new thread.
The ideal snap roll involves getting through it as quickly and with as little energy loss as possible. It should stop like "hitting a brick wall". The snap in this video (11:20) shows full aft elevator applied (and held) during the snap. Also here, the elevator is not brought back forward until it's time to recover. This will produce a snap, but it'll be very slow, it'll kill a LOT of airspeed, and it won't stop very crisply. In acro speak, this would be called an extremely "buried" snap. When you hit a perfect snap, it practically explodes with a roll rate that's much higher than if it's buried.
So rather than pulling the stick fully aft and stalling both wings as the snap is initiated, the cleaner way to perform a snap is to use the minimum amount of aft stick necessary to cause one wing to stall as soon as the rudder is fully deflected. In most airplanes, this is much less than full elevator deflection. This is why I feel a "proper" snap is not simply a horizontal spin. A spin entry involves stalling both wings, with rotation occuring due to the lift differential between the two stalled wings. In a proper snap, both wings are not stalled prior to the snap entry, and during the snap, only one wing is stalled. This is why snaps have such greater potential roll rate. But I guess the hamfisted full aft stick snap could be considered a 'horizontal spin'.
The timing between the movement of the elevator and the rudder is critical to getting a clean snap started. If you apply the rudder too late after you've moved the stick aft, then the angle of attack (AOA) will have dropped off, and the rudder input may not get the airplane to snap at all - the airplane may simply wallow around slowly with an uncomfortable yaw. Ideally, full rudder deflection should be reached concurrently with max AOA. The AOA curve during the stick pull looks like a sine wave. It goes up, peaks, and then drops off. Acheiving full rudder deflection anywhere other than peak AOA will result in reduced efficiency of the snap. Here, "peak AOA" does not mean max possible (critical) AOA. It means the peak of the AOA wave that involves the minimum AOA that will produce a clean snap break. Since the controls are applied very quickly, the rudder timing must be accurate to within a fraction of a second.
The other half of the snap entry involves unloading the stick (moving the stick forward to at least neutral) as soon as the snap starts to break. This reduces AOA and also accelerates the snap due to the physical law of conservation of angular momentum. Figure skaters use this to accelerate their rotations by bringing their mass (arms and legs) closer to the axis of rotation. If continuous aft stick is held as the airplane rotates, there will be excessive AOA during the snap, which will slow the rotation, and cause excessive energy loss. The timing of the elevator unload following the rudder application is also critical to the quality of the snap. If it's unloaded too soon (before the snap breaks), then the snap will not happen. If it's unloaded too late, there will be excessive energy loss, and the roll rate will vary.
Typically, aileron is applied in the direction of the snap as part of the unload movement of the elevator. This has a slight accelerating effect on the snap due to the ailerons streamlining themselves with the snap rotation, which reduces drag slightly. But in any snap roll, the rotation is driven by the one stalled wing, and not by the aileron. The stick movement looks like a "check mark" - a short aft pull straight back, and then a diagonal movement foward. Some snap input video (1:30) I shot in the Pitts.
Stopping the snap involves leading with opposite rudder and applying a slight bit of extra forward stick, much like stopping a spin. How soon rudder must be applied before arriving at your desired degree of rotation varies a little between airplanes.
But if you're just fun flopping around, it really doesn't matter how you do a snap. But for those with interest in precision acro and really working on technique, even the basic maneuvers provide a lot of potential for refinement. In competition, we do anywhere from 1/2 to 2 full snap rotations, with the possibility of 1/4 rotation increments between 1/2 and 2 rotations. These can be flown on level, 45 up/downlines, and vertical up/downlines. Then there are outside (negative) snaps in all the same combinations. Each individual snap possibility requires slightly different elevator and rudder inputs during recovery in order to stop the snap in precise pitch, yaw, and roll attitudes. Half snaps are especially different from full snaps. And ideally you should be able to snap equally well in both directions. Lots to work on...I'm definitely not all the way there yet. Very few in the world are...World Aerobatic Champions come damn close.
Did I get carried away?
The ideal snap roll involves getting through it as quickly and with as little energy loss as possible. It should stop like "hitting a brick wall". The snap in this video (11:20) shows full aft elevator applied (and held) during the snap. Also here, the elevator is not brought back forward until it's time to recover. This will produce a snap, but it'll be very slow, it'll kill a LOT of airspeed, and it won't stop very crisply. In acro speak, this would be called an extremely "buried" snap. When you hit a perfect snap, it practically explodes with a roll rate that's much higher than if it's buried.
So rather than pulling the stick fully aft and stalling both wings as the snap is initiated, the cleaner way to perform a snap is to use the minimum amount of aft stick necessary to cause one wing to stall as soon as the rudder is fully deflected. In most airplanes, this is much less than full elevator deflection. This is why I feel a "proper" snap is not simply a horizontal spin. A spin entry involves stalling both wings, with rotation occuring due to the lift differential between the two stalled wings. In a proper snap, both wings are not stalled prior to the snap entry, and during the snap, only one wing is stalled. This is why snaps have such greater potential roll rate. But I guess the hamfisted full aft stick snap could be considered a 'horizontal spin'.
The timing between the movement of the elevator and the rudder is critical to getting a clean snap started. If you apply the rudder too late after you've moved the stick aft, then the angle of attack (AOA) will have dropped off, and the rudder input may not get the airplane to snap at all - the airplane may simply wallow around slowly with an uncomfortable yaw. Ideally, full rudder deflection should be reached concurrently with max AOA. The AOA curve during the stick pull looks like a sine wave. It goes up, peaks, and then drops off. Acheiving full rudder deflection anywhere other than peak AOA will result in reduced efficiency of the snap. Here, "peak AOA" does not mean max possible (critical) AOA. It means the peak of the AOA wave that involves the minimum AOA that will produce a clean snap break. Since the controls are applied very quickly, the rudder timing must be accurate to within a fraction of a second.
The other half of the snap entry involves unloading the stick (moving the stick forward to at least neutral) as soon as the snap starts to break. This reduces AOA and also accelerates the snap due to the physical law of conservation of angular momentum. Figure skaters use this to accelerate their rotations by bringing their mass (arms and legs) closer to the axis of rotation. If continuous aft stick is held as the airplane rotates, there will be excessive AOA during the snap, which will slow the rotation, and cause excessive energy loss. The timing of the elevator unload following the rudder application is also critical to the quality of the snap. If it's unloaded too soon (before the snap breaks), then the snap will not happen. If it's unloaded too late, there will be excessive energy loss, and the roll rate will vary.
Typically, aileron is applied in the direction of the snap as part of the unload movement of the elevator. This has a slight accelerating effect on the snap due to the ailerons streamlining themselves with the snap rotation, which reduces drag slightly. But in any snap roll, the rotation is driven by the one stalled wing, and not by the aileron. The stick movement looks like a "check mark" - a short aft pull straight back, and then a diagonal movement foward. Some snap input video (1:30) I shot in the Pitts.
Stopping the snap involves leading with opposite rudder and applying a slight bit of extra forward stick, much like stopping a spin. How soon rudder must be applied before arriving at your desired degree of rotation varies a little between airplanes.
But if you're just fun flopping around, it really doesn't matter how you do a snap. But for those with interest in precision acro and really working on technique, even the basic maneuvers provide a lot of potential for refinement. In competition, we do anywhere from 1/2 to 2 full snap rotations, with the possibility of 1/4 rotation increments between 1/2 and 2 rotations. These can be flown on level, 45 up/downlines, and vertical up/downlines. Then there are outside (negative) snaps in all the same combinations. Each individual snap possibility requires slightly different elevator and rudder inputs during recovery in order to stop the snap in precise pitch, yaw, and roll attitudes. Half snaps are especially different from full snaps. And ideally you should be able to snap equally well in both directions. Lots to work on...I'm definitely not all the way there yet. Very few in the world are...World Aerobatic Champions come damn close.
Did I get carried away?
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