Snaps/Spins

[Ron Van Putte]

For a non- aeronautical engineer, Earl has a great explanation about 
what happens during a snap roll.  I tried to find fault and couldn't. 
 Nice job Earl.

Ron Van Putte
aeronautical engineer


EHaury at aol.com wrote:

> Ron
>
> Very good points. The rotational rate changes during snap initiation 
> and stopping should not be downgraded. My point is that major track 
> changes during initiation should be downgraded, as well as returning 
> to an unstalled condition and rolling to finish. The latter is not a 
> zero, but a severe downgrade!
>
> You're the AE, take a look at my view of the snap. First the wing must 
> be quickly (to maintain track) loaded to stall or very near stall. The 
> mechanism is pitch input in the positive or negative direction. The 
> pitch angle needed will vary with the stall characteristics of the 
> wing, the existing load (level, 45, vertical) and speed. Fast pitch 
> change is definitely better!
>
> Just as the wing reaches stall a yaw input In the desired direction of 
> rotation retracts one wing and advances the other. The retracting wing 
> now has less relative airspeed and stalls (or stalls more deeply) than 
> the advancing (higher airspeed) wing and rotation occurs as though a 
> wing fell off. Again fast rudder input is good.
>
> The accelerations and decels of the rotation of snaps done this way 
> will be very dependent upon the roll axis inertial characteristics of 
> the aircraft. The lighter the wings the better, with heavy wings 
> making the snap unmanageable.
>
> Enter ailerons. The ailerons contribute in a couple of ways. The 
> upward traveling aileron on the retracting wing, that we wish to stall 
> more deeply, helps the stall while the downward traveling aileron on 
> the other lowers stall speed of this wing somewhat like a flap. 
> Therefore less pitch and yaw to stall / rotate. The big benefit is 
> that the aileron application helps overcome the roll inertia involved 
> at the start and stop, making both more controllable. (Yes, the full 
> scale folks use ailerons in snaps to help manage inertia also.)
>
> Back to the elevator. The amount of pitch needed to initiate a snap is 
> more than required to maintain it (likewise yaw). If maintained at 
> high angles throughout the snap the aircraft will retain some pitch 
> and yaw upon snap exit. (General term is "buried snap.") If the pitch 
> and yaw inputs are reduced after snap initiation to levels just 
> sufficient to maintain the snap, the snap will be tighter and exits 
> cleaner. Unfortunately, some who have the skills to achieve this level 
> of control are mistakenly accused of performing an axial roll.
>
> Another spin control that can be used to advantage is the throttle, 
> remember that the faster the wing is moving the more pitch angle is 
> needed to effect stall and the more likely the track will change 
> before stall occurs. While idle will get the quickest stall, the drag 
> created during the snap will render the airplane a dead duck on exit. 
> Some middle ground exists for each airplane design, be it faster entry 
> or powering up during the snap to offset drag.
>
> I agree that the snap is probably more difficult to judge than 
> execute. However, it seems that it's here to stay (and it's a fun 
> maneuver). I started this discussion to stimulate thought and 
> encourage folks to think about appropriate judging of the maneuver. 
> "Sticks in the corner" doesn't necessarily result in a good snap and 
> the very skilled folks, who have developed techniques to make the snap 
> a joy to watch, haven't all figured out a way get something that isn't 
> a snap judged as such.
>
> Earl


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