[NSRCA-discussion] Judging-snap & spin

[J N Hiller]

Earl, I am following you so far. Your comment regarding CG was interesting.
I moved the CG back on my primary airplane to soften the nose drop when
pushing to horizontal and adjusted things to get an acceptable avalanche. In
test flying my back-up airplane prior to selling it I found the avalanche
snap to actually be better, at least in dense air. Interesting!
The question I have is, what effect dose leading edge sweep have on the wing
stall characteristics? My current design has 12.5 degrees and is 22.1-oz/sq
ft. In preparing design changes for a new airplane I am planning 10.5 degree
and 22.8 wing loading. Would it be beneficial to reduce the leading edge
sweep more?
Thanks
Jim Hiller
-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of Earl Haury
Sent: Monday, October 22, 2007 5:19 AM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Judging-snap & spin

The term "wing stalls" may be a bit confusing when related to snaps
(especially those who fly barrel rolls) and spins. Generally "wing stall" is
considered to be total loss of lift created by high AOA and/or slow speed.
Terms like "asymmetrical stall" or "partial air flow separation" may be more
descriptive for spins and snaps. The trick is to increase the wing AOA to a
point just before stall, then change the forward speed of each wing panel
with yaw. The decrease in speed of the rearward traveling panel completes
the imminent flow separation (set up by being near stall) while the forward
traveling panel maintains lift. The effect is near that of a wing panel
falling off one side with the remaining panel providing the lift for rapid
rotation.

For spins the airplane is generally smoothly slowed to a near stall while
increasing (pitch) AOA , rudder induced yaw then completes the process by
"stalling" one wing. As the total lift is insufficient to support the
airplane, the airplane falls vertically while the asymmetric lift creates
the rotation. As the entry pitch increase is gradual, the AOA will be high
as the yaw application point is reached.

Snaps are flown at relatively high speed. The goal again is to get the air
flow separation on one wing and not the other. The trick is to apply a pitch
increase very rapidly so as to move the wing quicker than the air can
follow, producing near or actual flow separation. (The rapid pitch change
mitigates the need for a large pitch AOA change). While the airplane is in
this state, rapidly following with rudder induced yaw produces the
asymmetric lift and the "autorotation". Note that the term "autorotation"
generally relates to aircraft rotation without input from the roll controls
(ailerons). However, the ailerons play a legitimate roll in true snap rolls.
Primarily by overcoming aircraft inertial resistance to starting and
stopping rotation - makes for more predictable starts / stops. A secondary
benefit is that the aileron on the high lift wing side is acting as a flap
while the aileron on the low lift wing side is acting as a spoiler. The
latter effect may also play a part in quickly exiting a snap as neutralizing
the "flap/spoiler" configuration can allow the wing to return to symmetrical
lift quickly.

Some things to consider when setting up snap control parameters. The ability
to rapidly pitch the wing to the necessary AOA (varies with designs)
requires fast elevator servo(s), enough travel, and a quick thumb. Also,
upon establishing the snap, most designs will benefit from the relaxing of
some percentage of elevator input so as to not over pitch (bury) the snap
and allow for rapid recovery. The rudder input needs to be rapid and
slightly delayed from the elevator. A  fast powerful servo (remember -
airspeed is high making rudder load high) is needed to yank the wing into
the yaw attitude to create the asymmetrical lift. Also note that the rudder
travel will be the controlling factor (given equal pitch) for snap rotation
rate. Ailerons are the least important control - enough (applied at the same
time as rudder) to overcome inertia is all that's needed. If you find
aileron travel is controlling snap rotation rate - the airplane probably
isn't doing a snap roll, but rather a wobbly aileron roll. Different CG
locations will also play a role, some believe the more rearward the better -
but that really just reduces demand for servo power / speed, while making
exits difficult.

Different designs will snap differently for a given set of control
parameters. It's a challenge to "get it right" for snaps, but no more so
than trimming for a straight knife edge of vertical line. Take the time to
trim the airplane for snaps and you will enjoy no longer being "a part of
the problem".

Earl


----- Original Message -----
From: Ed Alt <mailto:ed_alt at hotmail.com>
To: 'NSRCA Mailing List' <mailto:nsrca-discussion at lists.nsrca.org>
Sent: Monday, October 22, 2007 12:59 AM
Subject: Re: [NSRCA-discussion] Judging-snap & spin

The only problem with simultaneous initiation in all 3 axis is that it is a
guarantee that the beginning of the roll was not in a stalled condition,
i.e. not an autorotation.  Therefore, that part of the roll isn’t a snap and
should be downgraded using 1 point per degree criteria.  Then the trick
becomes determining just when the autorotation actually did commence while
the airplane is already rolling quickly, primarily due to aileron and to a
degree rudder.  Until the wing stalls, it’s not a snap.  The main thing that
you can hope for in a real snap roll that relates to precision is that you
get the correct number of degrees of roll caused by autorotation in a
stalled condition.

Ed


-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Nat Penton
Sent: Monday, October 22, 2007 12:34 AM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Judging-snap & spin

The problem we have is not understanding snap dynamics.

Rudder, elevator and aileron are EQUALLY important to the snap.

Visionalizing the S & L up/rt snap we see that aileron makes a critical
contribution to reaching the lead wing stall AOA ( due to rud ).

To be a thing of beauty ( ahem, 10 ) the break will need to be initiated
simaltaneously on all axis.          Nat
----- Original Message -----
From: Earl Haury <mailto:ejhaury at comcast.net>
To: NSRCA Mailing List <mailto:nsrca-discussion at lists.nsrca.org>
Sent: Sunday, October 21, 2007 2:54 PM
Subject: Re: [NSRCA-discussion] Judging-snap & spin

I also agree with the comments of Jon and John. Chris's observations are
also valid and I'd like to expand on his thoughts a bit.

Remember, we use contestant judging - so it's a pretty good bet that if the
judge isn't capable of judging a snap correctly, it's also unlikely that
person can perform one correctly as a pilot. I'll bet that often the judge
"requiring" a huge "break" is the same person flying them with a snap
switch, or dual rates, set to maximum travels for all axis. They just chop
power and pull the switch or stuff the sticks into the corner. Generally
this results in a three axis track departure of some 15 to 30 degrees (as a
barrel roll), now interpreted as a "break", before flow separation occurs on
the lagging wing and a snap actually occurs. These excessive control inputs
also bury the snap and make it totally ugly. However - these folks are doing
what they think is required for a snap, so it must be right. (A lot of these
never snap and are total barrel rolls.)

So - the education process is better focused on the pilot than the judge,
and both will benefit (as will we all). As John points out - each full scale
will have different snap characteristics. Same with pattern airplanes, and
each situation may vary (45 deg lines, vertical up -down, level, avalanche,
etc.). Snaps take a good set-up and skill to fly well, as well as being a
fun part of pattern. Flyers who take the time to determine just what set-up
up and technique their airplane needs to snap well shouldn't be penalized by
those (pilot judges) who don't. However, this whole issue might slip away if
those who've figured out how to do decent snaps would take the time to share
with those that haven't.

Earl



----- Original Message -----
From: chris moon <mailto:cjm767driver at hotmail.com>
To: nsrca-discussion at lists.nsrca.org
<mailto:nsrca-discussion at lists.nsrca.org>
Sent: Sunday, October 21, 2007 2:01 PM
Subject: Re: [NSRCA-discussion] Judging-snap & spin

Jon and John are correct in their comments.
To add something - I think the problem is that many of us "cater" to the
judges who do not judge these maneuvers correctly. There is no
requirement to over exaggerate the break in order to do the maneuver,
however many do just that as a defensive tactic against the judge who
refuses to judge correctly. The problem is the JUDGE - not the pilot.
I don't advocate changing the schedules or K factors as a work around
for poorly informed judges. We are much too politically correct and
accept the zeros from them if we don't do it "their" way. If you
compete regularly, you know who they are and cater your maneuver to
accommodate their lack of ability in the chair. Sorry for the rant, but
we are talking about fixing things the wrong way for the wrong reasons.

Chris


John Ford wrote:
> Jon,
> Hear, hear.
> Couldn't have said it better!
> I also share the opinion that in the case of the snap (or the spin
> entry for that matter), our judging standards don't judge actual
> flight characteristics of the particular plane, and we are asking
> pilots to exagerate the break because that is what we agreed we wanted
> to see all the time, not because every plane should show it naturally.
> Maybe we are sitting on this bed of nails because for many people, the
> mystery and controversy of the break is more attractive than
> aerodynamic reality?
> I've done lots of snaps in full-sized planes and there are as many
> break styles as there are airplane designs. Some older/larger planes
> require that you slow up and reef back almost to the buffet before
> mashing the rudder, others are so touchy that a modest tap on the
> rudder with only a hint of pitchup will send the beast thru 150
> degrees of autorotation before you can think about it. In both cases,
> believe me, it was a true snap roll, but in the first case, you may
> have seen some break, but in the latter, it would have looked like
> everything happened at the same time around all 3 axes. I'm sitting in
> the thing, and I can't tell!
> Essentially the same comments for spin entry, in agreement with Jon's
> comments.
> I'll judge by the rules of the CD, but I do it with a bit of a
> shoulder shrug, I suppose.
> John
>
> */JonLowe at aol.com/* wrote:
>
> The age old problem of what a "break" is in a snap was solved at
> the Don Lowe Masters a couple of years ago. They defined it as a
> "simultaneous departure in all three axis". There you saw graceful
> snap entries, clearly defineable as a snap. At the IMAC Tuscon
> shootout, they had had the pitch departure requirement, and most
> were pitching what looked like 45 degrees (was probably 25
> degrees), before they entered the snap. Break, pause, enter snap.
> Ugly as hell. At a pattern judging seminar I went to this year, we
> sat around for half an our trying to decide what a "pitch break"
> was. We finally decided that if you saw anything at all, it was
> ok. But beware of IMAC judges crossing over, unless they have been
> retrained. I got some 5's this year this year, because they didn't
> see a large break.
> As regards spin entries, there are too many spin entry nazis IMHO.
> The rule book clearly defines downgrades for entries. In my book,
> if they don't break any of those rules, (wing coming over before
> the nose passes thru horizontal, not stalled, weathervaning,
> etc.), I don't downgrade for the entry. Too many people want to
> add their own definition to the rules about how an entry "should"
> look, and tell you they downgraded or zeroed you. When you ask
> them what specific rule you violated, they say it "didn't look
> right". Some of these same people will coach you to "cheat" at the
> entry to get a pretty one, dumping up elevator to get the nose to
> fall thru, which really breaks the stall. Unfortunately, all
> airplanes do not enter the same way, and some entries are not
> pretty, but they don't break the rules. Maybe, as well as teaching
> what isn't correct, we ought to teach what ISN'T downgradeable in
> some of these manuevers.
> Jon
> In a message dated 10/21/2007 8:50:52 AM Central Daylight Time,
> patterndude at tx.rr.com writes:
>
> Ron,
> Your idea caused me to stop and think. I'm wondering if it
> would really help, however. If a pilot "in the hunt" screws
> the landing (K=1) he's now "out of the hunt" on that round.
> Scores are often very compressed at local contests so even if
> we reduce the KF, a bad score on any manuver is usually enough
> to do mortal damage.
> --Lance
>
> ----- Original Message -----
> *From:* Ron Lockhart
> *To:* NSRCA Mailing List
>
> *Sent:* Saturday, October 20, 2007 11:34 AM
> *Subject:* Re: [NSRCA-discussion] Judging-snap & spin
>
> Eliminating is one solution - a price that comes with that
> solution is lack of practice doing and judging snaps-
> which is desirable for some in AMA classes, and for sure
> for those looking ahead to F3A.
> An in between thought - reduce the K factor considerable
> for snap and spin maneuvers.
> That leaves them in the schedules, provides flying and
> judging practice on them, but reduces the
> impact of the imperfect judging of them on round scores.
> Ron Lockhart
>
> ----- Original Message -----
> *From:* BUDDYonRC at aol.com
> *To:* nsrca-discussion at lists.nsrca.org
>
> *Sent:* Saturday, October 20, 2007 11:44 AM
> *Subject:* Re: [NSRCA-discussion] Judging
>
> My cents worth on the subject.
> Snaps and Spin entry seem to cause much of the problem.
> Why do we continue to repeat trying to solve a problem
> that most agree is controversial at best and
> impossible to judge consistently on an equal basis?
> Seems that the best solution is to eliminate these
> from the schedules and pick maneuvers that more suit
> Precision Aerobatics and their ability to be judged
> correctly by everyone not just those who have advanced
> to the top of the super judge platform.
>
> Buddy
>
> Jon Lowe
>
>
>
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