Snaps

[David Lockhart]

Steve,

My mistake for using the word "prior" - I did not have the Comp Regs in
front of me when I wrote the original post.  The full quote from the AMA
Comp Regs is -
"A Snap roll is a simultaneous, rapid autorotation in the pitch, yaw and
roll axes of flight in a stalled wing attitude."

The "in a stalled wing attitude" is a critical part of the quote.   As
others have noted in more detail, the wing is stalled when the critical
angle of attack is reached - ailerons and rudder don't do this (tho they
certainly can exaggerate a stall).  Elevator is used to stall the wing.  The
wing cannot stall instantaneously (no matter how fast the AOA is changed).
The simultaneous, rapid autorotation in the pitch, yaw, and roll axes of
flight cannot commence prior to the wing stalling and meet the requirements
of the definition.

Granted, the above definition may not be perfect - and anything is subject
to interpretation.  My interpretation is that the rotation component should
not commence prior to the stall (initiated by the break).  As you've noted,
the following paragraphs beyond the initial definition provide additional
detail and clarity - none of which do I see as conflicting to the initial
definition.


Also worth noting -
"A snap that does not show a break and stall to initiate the snap, but does
enter a stalled attitude during the maneuver is severely downgraded."

This is cause for downgrading a maneuver that in all respects appears to be
a snap but does not demonstrate a break on the entry.  As for myself, I've
rarely seen a good snap roll develop if it did not have a break at the
entry.  In my prior post, I stated "If a snap does not have a break on the
entry, it should also be scored zero."  More correctly, the omission of a
break could be a severe downgrade, but in practice, I've found if the break
is missing, the resulting rotation is not a snap and a zero is the proper
score.

Dave


----- Original Message -----
From: RC Steve Sterling
To: discussion at nsrca.org
Sent: Tuesday, April 13, 2004 9:06 PM
Subject: RE: Snaps


Whoa Whoa-- I just got out my book and looked. NOWHERE does it use the word
"before" or "prior". It says
"Snap Roll is the simultaneous, rapid autorotation in the pitch, yaw and
roll axes of flight...."

I'll grant that it then goes on to talk about an "initiating break", I don't
think that precludes that break in pitch occuring simultaneously with rudder
induced yaw and roll. Especially since the opening definition uses the word
simultaneous.

Just because the nose breaks up doesn't mean the airplane is stalling
either. I don't think my Focus will stall if I just pull back hard on the
elevator, it just does a tight loop. But kick full elevator and rudder, it
certainly does stall (and snap) then!
-----Original Message-----
From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]On
Behalf Of David Lockhart
Sent: Tuesday, April 13, 2004 1:20 PM
To: discussion at nsrca.org
Subject: Snaps


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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