Fw: Judging Snaps

[David Lockhart]

David,

The key to your question is in the rules you quoted - the rotation of the
snap is from a stalled condition - the stall is generated by pitch.  If the
rotation is not preceded by pitch, the plane is not entering the snap from a
stalled condition.  It is quite easy to fake a snap with aileron and rudder
only - the nose and tail will both cone - and the maneuver should be a zero.

With all manuevers, it is the pilots responsibility to clearly show all
elements of the manuever - and if one or more elements of a manuever are
poorly defined or completely omitted, then it is the judges responsibility
to apply the downgrades (including a zero) from the judging guide.  AMA now
allows severe downgrades for questionable snap entries.  In FAI, it is
basically all or nothing.

Below is a slightly edited/snipped post from 31 Dec 04 - you can check the
archives for the full post.  Several other good posts were made about that
time.

Regards,

Dave

----------------------------------------

Airplane setup -
This is critical with any design, and when a snap is attempted, a couple
things can happen -
- a good snap.
- a twinkle roll (not a snap).
- a barrel roll (not a snap).

The majority of the setups I see are not sufficient to consistently perform
nice snaps.  Consistently meaning the setup is not very tolerant of
differing weather conditions (hot vs cold, snapping in/out of relative
wind), pilot techniques, or entry airspeeds.  I have on more than one
occasion been asked to check a setup for snaps when the owner of the plane
was having problems with snaps.  In some instances, I've been able to
consistently snap a plane with no changes to the setup, but improvements to
the setup made it easier for the owner to consistently snap the plane.  The
below descriptions are generalizations based on my experience flying a
variety of planes and observing a variety of planes and pilots from a
variety of perspectives (amazing how much differently snaps look from
different perspectives).  And, the below does not take into acount the
effects of wind on track and offer relative descriptions of control throws
(exact control throws are influenced by design, weight, pilot technique,
etc).  The most critical setup element is pitch - if enough pitch authority
is not available, the break can not be achieved - the adjustment to make is
more elevator throw, or a more rearward CG (and if neither work, the design
has too much stability in pitch to snap properly).

- The "twinkle" roll - The plane that has too little elevator, a little bit
of rudder, and a boatload of aileron.  Very common.  The plane has little or
no displacement from track (or the path is an extremely tight barrel) during
the "twinkle" and returns to the original track with zero displacement or
deviation in track.  The rotation rate is probably equivalent to a full
stick deflection aileron roll (might be marginally higher due to a slight
assymetry in lift between wing panels caused by the minor amount of rudder
used).  The tail follows the nose exactly and no "cone" is inscribed.  The
advantage of flying a "twinkle" roll is that there is very little if any
loss in track, the entry is non-critical, airspeed loss in the "twinkle" is
minimal, and the only variable the pilot needs to concentrate on is getting
the wings level at exit.  Why risk doing a real snap if the "twinkle" is
getting scored?  The
other benefit of the setup for a "twinkle" is that less elevator throw is
used, and that makes the plane smoother in all other manuevers.

- The "barrel" roll - this one can be easy to see, or hard to distinguish
from a snap, and the setup that yields this result is often variable.  A
very tight diameter barrel roll is hard to distinguish from a good snap
because it will exhibit a pitching element on entry and the plane will
displace from track in pitch and yaw during the snap, but will generally
return to track for full snaps (1, 2, 3....rotations) and generally remain
displaced of the track (in pitch and yaw) by the diameter of the barrel for
1/2 snaps (.5, 1.5, 2.5 ....rotations).  Very little if any cone inscription
is visible after the entry to the barrel.  The large diameter barrel has the
same characteristics of tight diameter barrel, but easy to see, simply
because of the large diameter alone.  Any change in the control inputs
during the "barrel" are generally very easy to see - because the plane is
not stalled and responds readily to any change in surface deflection.
Insufficient elevator to initiate the break is generally the cause of
barrels.  Adding aileron usually increases the rotation rate, and often
reduces the diameter.  Adding rudder may reduce the diameter of barrel,
increase the rotation rate, give the appearance of the inscription of a cone
(but it is only in yaw, and not in pitch, and the airplane is not in a
stalled condition), and may also add the look of a "cartwheel".  The benefit
of the "barrel" setup is that less control throw is used (smoother in other
maneuvers), the rotation rate is slow (easier to hit exit) and the track is
unchanged (but might be dispaced).

- The "good" snap - I hope we know what it is in writing by now (actually
seeing it takes practice), so just some notes.  Theoretically, the break can
occur without displacement in pitch - but this never happens in practice
(angular acceleration, mass, and other physics properties are
real.........blame Newton).  A good clean break (very fast elev servo and
fingers) will reduce the amount of track deviation and displacement in both
pitch and yaw.  The rotation rate of the snap will be marginally effected
(if at all) by low / high rate aileron - ailerons don't work very well when
the plane is stalled.

Piloting Technique and more setup -
To me, a good setup for snaps has a bigger "sweet" spot - the exact entry
airspeed and exact timing of the control inputs can vary and still yield a
good snap.  And the good setups "feel" the same whether snapping left or
right, positive or negative.  I fly with triple rates - normal flying is rud
on high, elev and ail on medium.  For spins, everything is on high.  For
snaps, I use a variety of rates setup to make the variety of snaps feel
balanced and I attempt to get the snaps from different perspectives in
different manuevers to "present" the same.  It takes a lot of time to setup.
The technique for all types (different rotations in different attitudes) of
snaps is similar (lead with elevator, then rudder, then aileron) but the
timing of each input usually varies a bit on both the addition and removal
of the inputs.  And each specific snap is usually a little different - I
"feel" the plane approaching the entry to the snap and try to gauge exact
airspeed, wind condtions, evaluate what the track is and if I want to change
the track, etc.....and then I make very small changes to the addition of the
inputs to get the desired effect (ie, I might use more elevator lead to
change the track in pitch if the plane was diving slightly in pitch track
prior to the snap, or lower than usual on airspeed).  During the snap
itself, I watch the progression of the snap and decide if I got the entry I
wanted (expected), and if I need to make any changes on the timing of the
removal of control inputs on the exit of the snap (ie, maybe hold the same
rudder or imediately go to opposite rudder on the exit to immediately
correct a deviation in yaw track).  The more I fly, the better I can make
these adjustments because I seem to see things better, the snaps seem to
take longer, my fingers seem to move faster, and I better know what the
mental image of the plane should be at any instant during the snap.

"Tells" and "cheats"
One of the easiest snaps to judge is the 45 downline snap (airplane is
upright and executing a single positive snap).  The airplane is front and
center, and the break in pitch is easy to observe - and it is actually easy
to confirm.  Odds are, if you don't see a break, you will see the track of
the line steepen as the airplane passes through 90 degrees of rotation -
because the rudder is now "bottom" rudder (and the plane is still flying, or
if snapping, the break was in yaw, not pitch), and the track of the plane
will be steeper than 45 degrees at the finish of the snap.  Kinda of odd to
see a 45 downline get steeper after a positive snap??  Big tell, and pretty
common to see.  I see the same thing on 1.5 snaps on a 45 downline.  The
same thing also happens on 45 uplines when the line substantially flattens
out after the snap, but with an added variable - the flattening may be
partly due to reduced airspeed and gravity (but the plane will show the
initial loss in track at the 90 degree point in rotation).

Exits - exits are generally judged more critically than entrances - for
snaps, rolls, and radii. A "cheat" to help with the exit of a snap is to
take out elev and rud before the snap is complete - such that all the pilot
is doing is completing a roll to level flight, and completing a roll is
easier than completing a snap - the tell is a substantial change in roll
rate (rotational inertia exists and for our models, the heavy wing monster
that appreciably demonstrates this is rare).  This type of cheat is also
employed on spin exists and on the exit of part and full rolls.  The roll
rate is slightly reduced as the rolling element nears completion to make it
easy to nail wings level.  Most judges will downgrade for the wings not
being level, fewer will catch the change in roll rate.  The same thing
applies to corner radii - they are often softer close to the exit to make it
easier to hit the line.  The reason the cheats are employed is because
downgrades are not being applied - "no whistle, no foul".

With snaps - to hide the lateral displacement and yaw track change in a
snap, the "cheat" is normally opposite the direction of the snap prior to
the snap entry - 5 degrees nose in before a snap and 5 degrees nose out
after the snap is harder to detect than simply being 10 degrees off after
the snap.  And of late, there is no doubt in my mind that being 10 degrees
off before the snap will earn little or no deduction if the exit of the snap
is nailed.  All deviations in roll, pitch, and yaw track should be equally
downgraded whether before or after the snap - it is a goal that needs to be
achieved.  The change in track during a snap can often be hidden (or
mitigated) to a large extent by wind - this is simply smart piloting by
choosing to snap into (usually) the wind.  And in the instance of a snap at
the end of the box, where the displacement or loss of heading in yaw is
particuarly easy to see, a more pronounced and aggresive break in pitch can
be used to minimize the deviation in yaw, as the deviation in pitch is
harder to see (and thus harder to downgrade).

Regards,

Dave
DaveL322 at comcast.net



----- Original Message -----
From: David Flynt
To: discussion at nsrca.org
Sent: Friday, August 05, 2005 12:04 AM
Subject: Judging Snaps


I have always judged snaps with a simple rule -- if it is not a barrel role,
and if the tail describes a helix or cone, then the plane must be stalled,
and therefore it is a snap.  I have never downgraded because the plane is
set up with a lot of aileron, so long as the tail wags, and so long as the
nose and tail describe opposing helixes.  I don't see any reference to how
fast or slow the plane rolls with regard to downgrades.  To me, a pilot is
free (and smart) to set up their plane such that it loses as little heading
as possible in a snap.

Am I completely mistaken on this?  A barrel roll is easy to detect.  An
axial roll is easy to detect.  If it is not a barrel roll, and not an axial
roll, then it must be a valid snap, even if it is subtle and the plane is
not buried deeply in a snap.  At least that is my current understanding.

I'll admit that I don't really understand the degree at which the plane must
"break" in the direction of  snap.  Who came up with that idea?  What really
does that mean, and how do you measure it?  "the nose of the fuselage should
show a definite break in the direction of the snap".  Ok, what is definite?
At what point exactly must the aircraft become stalled?  It takes time for
the control surfaces to deflect.  It does not happen instantaneously.

I think the maneuver is over described and over analyzed.  Its a "rapid
autorotation in the pitch, yaw, and roll axes of flight in a stalled wing
attitude."  That should be good enough to judge it.  If not, maybe use my
definition of judging it.  Until I know what a "definite break" is, that's
what I am going to do.

David

=================================================
To access the email archives for this list, go to
http://lists.f3a.us/pipermail/nsrca-discussion/
To be removed from this list, go to http://www.nsrca.org/discussionA.htm
and follow the instructions.

List members email returned for mailbox full will be removed from the list.