Snaps

[David Lockhart]

Interesting discussion RE snaps.

The way the snaps are currently defined (correctly I think) requires the aircraft to be stalled in pitch prior to any rotation in yaw and roll.  The break is a required element of the maneuver (not unlike other maneuvers).  As with any maneuver, I believe it is the pilots responsibility to clearly show/demonstrate all elements of a maneuver.  If a stall is not demonstrated at the point of entry into a spin, the maneuver should be scored zero.  If a snap does not have a break on the entry, it should also be scored zero.

It is not likely that a good break and snap entry will occur using a snap switch/button - unless the elevator servo is much faster than the other servos.  What helps define the break on the entry of the snaps is very fast application of the elevator input and a very fast elevator servo - lots of throw also helps (I have used as much as 30 degrees of throw on some planes for some snaps).  The result is a very rapid change in the pitch attitude of the plane (like the definition says) with very little displacement in track.  If the airplane is truly stalled at the time of rudder (and aileron) application, the change in track in yaw will be minimal, if any.  It is not easy to get a modern day pattern plane to perform a nice snap - as has been noted, they are very stable in pitch, generally have fairly small elevators, very light wing loadings, and generally have control throw setups that lack the authority to initiate a clean break.  Most pattern planes I see on the flightline quite simply won't break cleanly because they lack the elevator throw to do so.

If the plane displaces from the entry track during the break and then resumes a parallel track after the snap, that is ok (and actually required depending on ones interpretation of the FAI book).  If the track of the plane does change slightly during either the break or after the snap, this can be downgraded - and the downgrade could be minimal to the point of not reducing the maneuver score due solely to the change in track.  A "10" is not a flawless maneuver, it is a maneuver which is perfect or contains flaws that are not substantial enough to warrant downgrading to a 9.5 (or 9 in FAI).

The rotation rate of the plane during the snap depends on many factors - including design parameters of the plane (wingspan being a biggy) and the pilot technique.  In general, the greater the break (deeper the stall), the slower the rotation of the plane will be.  If the left/right wings are stalled equally at the break, the rotation rate is largely determined by the amount of rudder applied (which accelerates one wing panel forward and the other aft resulting in asymmetric lift which causes the rotation about the roll axis).  The same airplane that does not break cleanly in pitch (reduced throw, poor technique) will likely break in yaw - meaning the wing panel accelerated forward by application of the rudder never does actually stall, and the wing panel accelerated aft does stall and the asymmetry in lift again results in rotation about the roll axis.  The difference between these two scenarios is that the first contains the element of the break and is a scorable snap.  The second scenario does not contain the element of the break and should be scored zero.

What is interesting to note (and I've many witnesses to this at the practice field), is that the difference in the rotation rate of the snap is not effected noticeably by the amount of aileron used - 10 degrees or 15 degrees of aileron throw result in the same rotation rate - IF the snap was entered from a stalled condition.  If I substantially reduce the elevator throw (and the break becomes very, ahem, questionable), the amount of aileron used has a dramatic effect on the rotation rate. 

The entry speed of the plane is not a judging criteria.  With proper setup and technique, the airplane can be made to break in pitch at virtually any airspeed (with commensurate increases in stress at higher speeds) - but the break is usually slower and easier to judge at lower speeds.

For several years at the US NATs, I received comments from both pilots and judges regarding the crispness of my snap entries and the easily visible cone of the nose/tail during the snap.  Many of the pilots suggested I should "back off" the snaps and exaggerate the break less to make them easier to do (even tho I rarely missed an exit).  In general, my scores on maneuvers containing snaps were not very good (relative to other maneuvers in the flight).  Since that time, I have "toned" down the snap entry and the snap itself - the break is now much less defined and harder to see, and the snap itself is much more axial, and the rotation rate is faster (and after adapting, I miss the same number of exits - none on a good day <G>).  My scores have generally increased - my conclusion is that very few judges are either capable of seeing the break, looking for the break, understand the break, or have the confidence to downgrade (to zero) a snap which does not demonstrate a break.  I believe both styles of snaps I flew are within the rulebook definition - but one scores better than the other - I guess I'll continue to fly the style that scores better, even tho I think it is the style that is harder to judge.

And, if I've offended any of the aerodynamic purists out there with my oversimplification of the dynamics of a snap, my apologies - I didn't think additional detail about the aerodynamics were of help in this discussion.

Regards,

Dave Lockhart
DaveL322 at comcast.net





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