[NSRCA-discussion] Judging Snaps & spins II

[J N Hiller]

Thank you, that is very interesting especially the comments about snap
offset and spin stopping. Looks like the hardest thing is training the
thumbs to apply a rapid sequence of inputs.
Jim Hiller

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of Jack Keiser
Sent: Tuesday, October 23, 2007 8:42 PM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Judging Snaps & spins II

Earl,

Been following this thread for a while now and felt compelled to offer my
two cents worth.

I have a little problem with your auto analogy. In the case of an abrupt
steering input in a fast moving car, the forces trying to change the car's
path are due to the traction of the tire against the pavement; the force
trying to keep the car going in its original direction is due to the
momentum of the vehicle, according to some guy named Newton. Even the
smallest lateral force will produce some correspondingly small change in
direction. In the real world, a lot of factors come into play: sidewall flex
causing tire slip angle, suspension response, roll stiffness and so on. If
traction were lost immediately, as might be the case when driving on a sheet
of ice, turning the wheel sharply produces no change in direction - momentum
keeps the car going straight ahead. Only after some traction is regained
will the car turn.

Now let me switch gears a bit and talk about snaps and spins. I'm not an
aerodynamicist so I don't pretend to understand all the aerodynamic events
that take place in a snap. But I do have quite a bit of experience in full
scale aerobatics in a variety of aircraft ranging from Cessna Aerobats,
Pitts bipes, Giles 200&202 to the Sukhoi 26. I have also been flying pattern
since the '70's (with a few breaks here and there) and recently, IMAC, so I
appreciate that aspect as well.

The judging criteria for snap rolls is basically the same in all three
venues; particularly with respect to the pitch break. I have received zeros
in all three venues when that was not clearly observed ( I can't recall any
10's even when the break was obvious).  That does not mean the airplane did
not snap (auto-rotation); I just didn't do it according to the prevailing
judging criteria. When you're in the aircraft, its quite obvious when
auto-rotation occurs. Some aircraft, particularly the Giles 200, can roll so
fast (about 540 deg/sec) that its nearly impossible from the ground to tell
a full deflection aileron roll, with a little rudder thrown in, from a true
snap. And you do not need the pitch break to get a true snap - just jerk the
stick toward the right (or left) rear corner and kick hard right (or left)
rudder simultaneously, and I guarantee you'll experience a violent snap with
true auto-rotation. The G-200 is the closest thing you can imagine to a
model; you can fly it with two fingers and it responds about the same as an
IMAC plane.

The other extreme is the Su-26. Its like flying a John Deere. Two hands on a
stick that resembles a baseball bat. If you try the same control inputs for
the snap as the Giles, it just mushes around. Here, the pitch break to load
the wing seems more of a necessity. And (in a positive snap) you only pull
the stick back partially,but smartly,  maybe a third of the way, then shove
it quickly toward the right forward corner to unload the elevator and bring
in the aileron. Full right rudder applied just as the nose pitches up.
Despite the significant mass, and relatively high polar moments, the plane
auto-rotates with surprising speed - not like the Giles, but pretty darn
quick. To stop on heading, you need hard left rudder and some left aileron.
You may stop on heading (with practice) but you will have moved to the right
of your original flight path - that can be readily observed in a snap on the
vertical up line.

Why the Sukhoi and the Giles behave so differently is due to a lot of
factors. I suspect one major factor is the gyroscopic effect of the
extremely large prop on the Sukhoi; the gyro effects are much more
noticeable - that's why it does a Lomcevak so well. The wing loading of the
Sukhoi is much higher than the Giles, and so are the moments of inertia. Of
course, the wing airfoil is different too. Many other differences (including
fuel consumption).

Regarding spins, hardly any full scale pilot does a true spin in
competition. The entry is usually done in the classic way; stick full back
to the stall break, nursing it with subtle aileron and rudder input to get
it to drop off on the side you want, then as soon as the nose falls through,
push the stick forward to break the stall and keep the rotation going with
aileron and rudder. You're really in a tight aileron turn, both wings are
flying, but it looks like a spin. Most important, you can stop on heading
very accurately. The downside is that you probably lose a little more
altitude. You could just hold the stick back, full rudder deflection and
neutral aileron, then stop the spin with opposite rudder and stick forward
but the stop is unpredictable.

How does all this relate to models? Not sure, but I thought a different
perspective might be interesting.

It might also be interesting to know the that the same topics - spins,
snaps, judging competence, etc - receive about the same amount of gripes in
each venue. No surprise there.

Jack



----- Original Message -----
From: Earl Haury <mailto:ejhaury at comcast.net>
To: Discussion List, NSRCA <mailto:nsrca-discussion at lists.nsrca.org>
Sent: Tuesday, October 23, 2007 5:26 AM
Subject: [NSRCA-discussion] Judging Snaps & spins II

Posted this as a reply yesterday but it got snagged as too long with the
ongoing thread attached - started a new thread.

Jim

Don't think that'll work very well. Let's take the analogy of a car moving
in a straight line slowly - turn the steering quickly and the car will
turn - changing "track". Do the same thing at high speed and the car will
skid - track stays mostly the same and only attitude changes. Entering a
snap is similar - establish a "skid". In both cases there will be a minimal
departure from the original line until "traction" is lost and the skid
occurs. Consider that the faster the pitch input the less AOA increase will
be needed before rudder application. This is where we get into trouble
trying to define a "break" into a snap, some visualize this as needing to be
huge while, in reality, it may only be a few degrees.

Also - with regard to the departure from track before the "skid". F3A rules
require a "separation from the flight path" and AMA rules allow it "track
closely maintains the flight path". Numerous things will define the amount
of "separation" including rapidity of pitch / yaw, mass of airplane, wing
loading, etc. The separation may be a few inches to a couple of feet and is
not to be downgraded as long as the aircraft "closely maintains" track. So -
if the snap (skid) progresses more or less parallel to but slightly offset
(maybe in both pitch & yaw) it's not only OK, but a pretty good indicator
that the snap isn't an axial roll.

Use the description to think through set-up and control inputs. Recognize
that a properly done snap entry takes only a fraction of a second (if you
have time to see exactly what's going on you'll be getting downgrades for
track changes). With the proper set-up and practice a snap entry can be
perfect almost every time - that just leaves the exit to deal with. Finish
wings level and enjoy the 10!

Earl
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