[NSRCA-discussion] How I became an expert Snap Judge (TIC)
J N Hiller
jnhiller at earthlink.net
Wed Oct 14 08:46:35 AKDT 2009
I think so, resulting from sweep angle presenting less wing span into the
air stream
Jim Hiller
-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of James Oddino
Sent: Wednesday, October 14, 2009 9:27 AM
To: General pattern discussion
Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)
Is it possible we don't have a well understood definition of the term
"stalled wing"? I was taught that at the critical angle of attack the
change in the lift coefficient for a given change in alpha decreased. (The
slope became non-linear.) Note that it doesn't say the lift went to zero,
it just didn't increase as fast, stayed constant, or started to decrease as
alpha increased beyond the critical angle.
That would seem to say that both wings could be "stalled" and still be
producing lift and if one was lifting more than the other (one wing stalled
more than the other) while the plane was yawing, we'd say it was spinning or
snapping depending on the flight path.
So the question is, can we get one wing lifting more than the other while
yawing without reaching the critical angle of attack?
Jim
On Oct 14, 2009, at 3:12 AM, Budd Engineering wrote:
Chris, Vicente, et al.,
Angle of attack, alpha or AOA, for the aircraft is the difference between
the flight path angle, gamma, and the aircraft attitude, theta (assuming the
airfoil zero lift angle is essentially aligned with the aircraft reference
datum, which for all practical purposes on our designs, it is). Reference:
http://www.aerospaceweb.org/question/aerodynamics/q0165.shtml. Flight path
angle is the aircraft trajectory, or in pattern speak, "track" but along the
pitch axis, i.e. climbing or descending. In level, steady state flight
(~1-g), the flight path angle is zero. Which means that the angle of attack
is equal to the aircraft pitch attitude. If you run the numbers using a
reasonable airfoil lift curve slope at a representative level flight speed
for our planes you'll find that our planes trim out around ~ 0.5 degrees
alpha (Lift = Weight = CL * Qbar * S where CL is the lift coefficient, Qbar
is the dynamic pressure, and S is the reference wing area). This is because
of our extremely low wing loading, it simply doesn't take a lot of angle of
attack to generate 1-g of lift when your airplane only weighs 10 or 11 lbs.
Here's the other part: with the exception of velocity (or airspeed), the
equation is linear, which means that if you double the aircraft weight, for
the same flight speed, you get twice the alpha (again, for 1-g trim). Or if
you kept the weight at say 10 lbs, but cut the wing area in half, the angle
of attack would then double to ~ 1 degree (again, for level, steady, 1-g
flight). Velocity is a little trickier to account for because it's a
non-linear second order function in the lift equation (remember Qbar?
Reference: http://en.wikipedia.org/wiki/Dynamic_pressure, Qbar = 1/2 * rho *
V**2) where rho is air density and V is the velocity). Essentially, the
lift doubles every time we increase Qbar by 2, or velocity by the square
root of 2 (or 1.414). So if you're flying along in level flight at 60 mph
(88 feet per second), and you speed up to 85 mph mph (124.45 feet per
second), you've doubled your dynamic pressure (Qbar) and to stay at level
1-g flight, you'd have to retrim your plane in pitch to 1/2 of what your AOA
was before (or you'll start climbing). In this case the AOA would be ~ 0.25
degrees (as would the pitch attitude). One last bit of info for the point
I'm about to make is that the lift curve slope for our airfoils at the
Reynolds Numbers we are operating at is linear out to around ~15-16 degrees
alpha, with separation of lift occurring above that, closer to 18-20 degrees
alpha (Reference:
http://www.aerospaceweb.org/question/aerodynamics/q0136.shtml). Note that
the onset of separation is independent of airspeed, it's purely a flow angle
phenomenon, i.e. you can stall an airfoil at any airspeed, or attitude (if
you can get to a high enough angle of attack). That's essentially what
Chris is saying below.
So what does this all mean?
If you look at the numbers, for our planes, you can't get to stall from
steady level flight (at any reasonable cruise speed) without inducing a
significant G-loading on the aircraft (which will cause a noticeable, and
very observable, change in the flight path angle), BEFORE the airfoil
stalls.
Here's some more numbers to help you connect the dots: from level 1-g trim
flight at 1/2 degree AOA, you'd have to induce nearly 30 g's to get to ~ 15
alpha, not likely you'd be able to do that without seeing it. Even starting
at 2 degrees AOA (which is a lot for our models), you have to generate
nearly 5-6 g's to reach stall (think you'd notice that?). Don't believe me?
OK answer this: Have you ever quickly but smoothly from level upright 1-g
flight at a normal cruise speed input full aft stick for a second or two and
then release it but no lateral input? What happens? Unless something is
grossly wrong with your airplane you're likely to see a rapid pitch up and a
corresponding change in flight path angle, probably to something approaching
a near vertical attitude, but not much else. Why? Our planes are so
lightly loaded that only at spin entry and landing speeds can we induce
enough angle of attack to approach stall on the airfoil on the plane without
inducing significant g's and grossly altering the flight path angle (and
flight path angle is what we really see when we're flying at cruise speeds
BTW, not so much the pitch attitude until we're at much lower speeds.
That's because we mentally integrate the velocity vector in our minds but
that's a topic for another time/day). Full scale aerobatic planes (and to a
lessor degree IMAC planes) don't suffer this problem nearly so badly since
their wing loadings and inertia's are much higher (dynamic stability is
somewhat more complex than static stability so I'm not going to go much
further than this on this topic). The bottom line is this: we're not
stalling the wing when we do our snaps, not even a portion of it (unless
you're VERY low on airspeed at entry such as a spin). We're too lightly
loaded to get to stall at any reasonable airspeed, the airplane will respond
too quickly in the pitch axis resulting in a rapid change in flight path
angle, effectively unloading the AOA during the response.
So what are we doing to make the plane present what appears to be a snap
roll when we can't actually be stalling the wing asymmetrically to induce
autorotation like many claim? Lots of control power in pitch and roll
coupled with additional rolling moment induced by dihedral effect (sideslip
driven by rudder input). Pretty much everyone knows that at higher AOA you
can command/control roll with rudder, well that's due to dihedral effect
(roll with rudder), it gets more powerful with a little AOA. That's where
you get the part of the dynamic that visually emulates a full scale snap
roll but physically is quite different (you can make it look like a
full-scale snap, but it really isn't).
So the bigger question is should emulating a full-scale snap roll be a
pattern judging criterion or do we even care? (we know what the answer is
for full scale aerobatics and probably IMAC too but we are neither of
these). Until we decide the answer to THAT question, we're really just
debating "how many angels can dance on the head of a pin"...
OK, it's really, really late out here on the left coast, I've gotta get to
bed. Shoot away.
Thx, Jerry
Budd Engineering
jerry at buddengineering.com <mailto:jerry at buddengineering.com>
http://www.buddengineering.com <http://www.buddengineering.com/>
On Oct 13, 2009, at 8:54 PM, Chris Moon wrote:
Ahhh. angle of attack is not the same as the aircraft attitude.Websters
defines angle of attack as "the acute angle between the chord of an airfoil
and the line of relative air flow". The relative wind flows parallel and
opposite the direction of the wing's movement through the air.The wing's
attitude as we see it from the ground is NOT the same as angle of attack. If
you are in a vertical climb is the wing stalled? After all it is 90 degrees
ATTUIUDE from your perspective on the ground, right? But the wing is not
stalled because the relative wind is coming parallel and opposite the
direction of flight. You can be in a nose low descent and stall a wing. Now
take a straight and level pass and give it an instantaneous large amount of
up elevator. What happens? First, the plane continues in the direction it
was going (straight ahead) for a short time, but what is important is that
the angle between the relative wind (straight ahead) and the wing which is
now pivoting up increases until it reaches the critical angle of attack and
it stalls. Without knowing the specifics of that wing design, we can't know
when exactly this will occur, but it can be extremely fast and at a
relatively low ATTITUDE in relation to the ground. Hence, you do not need a
high nose ATTITUDE in order to have a high angle of attack. There are too
many variables for one to say that they need to see a nose high attitude in
order to define a high angle of attack and thus a stall. Remember also,
that different wings have a different critical angle of attack where a wing
will stall. How does anyone know where that angle is without a wind tunnel
and testing? and who am I to say it did not pitch up enough to stall
therefore I giveth the pilot a 5 or zero even though I cannot possibly know
the particulars of the wing that I am watching.
My comment of the 1-2 degrees was to say that we do not know how closely any
particular wing is flying from it's critical angle of attack. If it is in
fact close, a change of only 1-2 degrees can cause a stall. We are trying
to be aerodynamic engineers from the ground and deciding for ourselves what
the angle "should" look like and downgrading accordingly. Now throw in the
conceptual difference between angle of attack and aircraft attitude, and it
is easy to come to the wrong conclusion about stall or no stall. Are we
fling only 1-2 degrees from the critical angle most of the time, no but the
point it that there is absolutely no aerodynamic requirement for a very nose
high ATTITUDE to be a requirement in order to get a high angle of attack on
the wing.
Chris
Vicente "Vince" Bortone wrote:
Chris,
I am copying from the first sentence FAI rule book: " A snap-roll (or flick
roll/rudder roll) is a rapid autorotative roll where the model aircraft is
in a stalled attitude, with a continuous high angle of attack" The
question: Is 1-2 degrees consider a good amount to define a high angle of
attack?
Vicente "Vince" Bortone
----- Original Message -----
From: "Chris Moon" <cjm767driver at hotmail.com>
<mailto:cjm767driver at hotmail.com>
To: "General pattern discussion" <nsrca-discussion at lists.nsrca.org>
<mailto:nsrca-discussion at lists.nsrca.org>
Sent: Tuesday, October 13, 2009 3:07:01 PM GMT -06:00 US/Canada Central
Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)
I think what this goes back to is the erroneous belief that you need to see
an exaggerated pitch departure or it should be severely downgraded. Keep
reading the maneuver description. It needs "A" pitch break, and depending
on the current aoa (angle of attack) that can be a difference of just a
degree or 2 if you are near the critical aoa. Please don't tell me guys,
you are looking for MORE than a simple exceeding of the critical aoa and
resulting stall. An exaggerated pitch break just to prove to naysayers that
you make a break is wrong, wrong, wrong. It says it needs "A" break.
Please keep re-reading it. Same for spin entry. It needs to stall, not go
30 degrees nose up to "prove" a stall. We are getting wound up over a
misunderstanding of the mechanics of a stalled condition. As Don and Verne
are alluding to, it really is not that complicated.
Chris
verne at twmi.rr.com <mailto:verne at twmi.rr.com> wrote:
Or you could just write in "DNO".... I always seem to see the break as in,
nose up, tail down, and my 56 year old eyes are lousy. Verne ---- "Vicente
"Vince" Bortone" <vicenterc at comcast.net> <mailto:vicenterc at comcast.net>
wrote:
Verne, Following AMA description: if we don't see the break is 5 points
downgrade. That is 1/2 of the snap roll maneuver. Therefore, if we see the
snap roll but don't see the break the judge has the right to write down 5
points score assuming that all other components are perfect. Therefore,
base on the rule book the snap roll without a break has a value of 5 points.
Vicente "Vince" Bortone ----- Original Message ----- From: verne @ twmi . rr
.com To: "General pattern discussion" < nsrca -discussion at lists. nsrca .org>
Cc: "Don Ramsey" <don. ramsey @ suddenlink .net> Sent: Tuesday, October 13,
2009 2:14:02 PM GMT -06:00 US/Canada Central Subject: Re: [NSRCA-discussion]
How I became an expert Snap Judge (TIC) Is there anybody involved in this
discussion that honestly can't recognize a snap when they see one? I'm just
asking..... Verne ---- Don Ramsey <don. ramsey @ suddenlink .net> wrote:
Vince, What about the next sentence in the FAI definition, “If the
stall/break does not occur and the model aircraft barrel rolls around, the
manoeuvre must be severely downgraded (more than 5 points).” How about if
the break does not show and the model does NOT barrel roll around. Do you
still downgrade by 5 or more points? I don’t know what the intent of the
rule was but I can tell you for a fact that the judges that only score FAI
in Europe do not downgrade it by 5 or more points. I believe they use the
“If it’s not a barrel and not an axial roll then it’s probably a snap, so
judge it that way” because they have been instructed in the past to do it
that way. Don rom: nsrca -discussion-bounces at lists. nsrca .org
[mailto: nsrca -discussion-bounces at lists. nsrca .org] On Behalf Of Vicente
"Vince" Bortone Sent: Tuesday, October 13, 2009 12:10 PM To: General pattern
discussion Subject: Re: [NSRCA-discussion] How I became an expe
rt Snap Judge (TIC) Matt, I am copying the snap description from the
current FAI and AMA manuals. I don't see the AND you mention in the FAI
rule book. See the important portion in bold. I see that the AMA
description is better in this respect. You are correct in regard the
downgrade in FAI . 5 or more points if you don't see the break and the
model barrel rolls. Therefore, what is the downgrade in FAI if the judge
does not see the break and there is autorotation? I will say 5 points since
it says 5 or more points if the model barrel rolls. Again, it appears that
AMA down grad descriptions are better. FAI : SNAP-ROLLS A snap-roll (or
flick roll/rudder roll) is a rapid autorotative roll where the model
aircraft is in a stalled attitude, with a continuous high angle of attack
Snap-rolls have the same judging criteria as axial rolls as far as start and
stop of the rotation, and constant flight path through the manoeuvre is
concerned. At the start of a
snap-roll, the fuselage attitude must show a definite break and separation
from the flight path, before the rotation is started, since the model
aircraft is supposed to be in a stalled condition throughout the manoeuvre,
If the stall/break does not occur and the model aircraft barrelrolls around,
the manoeuvre must be severely downgraded (more than 5 points). Similarly,
axial rolls disguised as snap-rolls must be severely downgraded (more than 5
points). Snap-rolls can be flown both positive and negative, and the same
criteria apply. The attitude (positive or negative) is at the competitor’s
discretion. If the model aircraft returns to an unstalled condition during
the snap-roll, the manoeuvre is severely downgraded using the 1 point/15
degree rule. AMA: Snaps: A Snap roll is a simultaneous, rapid autorotation
in the pitch, yaw and roll axes of flight in a stalled wing attitude. The
following criteria apply: 1. Since the maneuver is defined as a stalled
maneuver, initiat
ed by a stall of the wing induced by a rapid change in pitch attitude, the
nose of the fuselage must show a definite break in pitch attitude from the
flight path in the direction of the snap (positive or negative) while the
track closely maintains the flight path. The lack of a discernable pitch
break is downgraded by 5 points. Large deviations from the flight path,
indicative of a delayed stall, are to be downgraded using the 1 point per
15-degree rule for each axis of the excursion before stall. For example, it
the model pitches 15 degrees nose up and the wings rotate 15 degrees before
the stall, the maneuver should be downgraded 1 point for pitch and 1 point
for roll. 2. The track visualized as the path of the Center of Gravity (CG)
should closely follow the geometric flight path of the maneuver while the
nose and tail auto rotate through opposite helical arcs around the flight
path. Lack of these helical arcs (or coning) is indicative of an axial roll
and is scored zero.
3. If a stall does not occur and the model barrel rolls, the score is zero.
A barrel roll can be identified when the CG, the nose, and tails scribe the
same helical path through the required rotation of the maneuver 4. Snap
rolls have the same judging criteria as axial rolls as far as start and stop
of rotation, constant flight path through the maneuver and centering on
lines. 5. If the model returns to an unstalled condition during the
maneuver, such that the autorotation is not visible and the model rolls or
barrel rolls to complete the maneuver, it would be downgraded using the 1
point per 15 degree rule. 6. Airspeed is not a criteria which should be used
to judge this maneuver. The wing of the model is stalled during this
maneuver; therefore a significant decrease in speed may occur and is not a
cause for downgrade. Vicente "Vince" Bortone ----- Original Message -----
From: "Matthew Frederick" <mjfrederick at cox.net>
<mailto:mjfrederick at cox.net>
To: "General pattern discussion" < nsrca -discussion at lists. nsrca .org>
Sent: Tuesday, October 13, 2009 9:47:30 AM GMT -06:00 US/Canada Central
Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC) ?
While speaking with Don Ramsey about the nuances of judging snaps at a
recent contest I found that he agreed with my interpretation of the FAI snap
rule. The severe downgrade should only be applied if there is no break AND
there is no autorotation (this is exactly what the rule says). Basically,
lack of a break is not substantial grounds for the severe downgrade in FAI .
If the break is not seen and autorotation still occurs at some point during
the roll the one point per 15 degree rule applies. Since the snaps happen so
fast, for me it's usually not more than 1 or 2 points unless it was
blatantly obvious that the plane rotated a while before the snap truly
began. It's the same as if you stop the snap before completing the rotation
and do an axial roll to
finish. This nonsense of people being so quick to apply a severe downgrade
has gone too far. One element of a maneuver (because I can't think of any
sequence that has just a snap roll) should not ruin a whole flight, or eve n
that one maneuver unless it just wasn't a snap. I like the idea of "if it's
not a barrell roll and not an axial roll, it's probably a snap."
Matt ----- Original Message ----- From: Vicente
<mailto:vicenterc at comcast.net> <mailto:vicenterc at comcast.net> "Vince"
Bortone To: General pattern discussion <mailto: nsrca -discussion at lists.
nsrca .org> Sent: Monday, October 12, 2009 5:12 PM Subject: Re:
[NSRCA-discussion] How I became an expert Snap Judge (TIC) I believe that
the current downgrade is severe. AMA 5 points. FAI 5 or more points if my
memory is correct. In local contest I have been using 3 points
downgrade. I know that is wrong but it has been my best way for me to take
into account the break issue. It used to be zero and it was changed to 5
points (IMAC still a 10 points downgrade or nada). Therefore, Ron is
correct. Probably makes sense to go 2-3 points downgrade if the judge can
not see the break before rotation. Vicente "Vince" Bortone -----
Original Message ----- From: "John Fuqua" <johnfuqua at embarqmail.com>
<mailto:johnfuqua at embarqmail.com>
To: "General pattern discussion" < nsrca -discussion at lists. nsrca .org>
Sent: Monday, October 12, 2009 1:51:00 PM GMT -06:00 US/Canada Central
Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC) Ron
makes valid observation which I came to many years ago at the TOC when Mr.
Bill graciously funded for full scale pilots like Patty Wagstaff do demo
flights to entertain us. The one thing that I came away with in comparing
full scale to our airplanes is the speed of the snap/rotation. In the full
size aerobatics types that I observed there was plenty of time to see the
nose pitch and then after somewhat of a hesitation yaw and rotate. In our
pattern planes, especially when using a snap switch, it all gets to be a
blur due to sheer speed. I have no solution to this issue but to MAKE the
pilots show a break by having severe downgrades. Otherwise the concept of a
snap will be ignored. Yes it's hard to see which makes it incumbent on the
pilot to
present it to the judges. -----Original Message----- From:
nsrca -discussion-bounces at lists. nsrca .org [mailto:
nsrca -discussion-bounces at lists. nsrca .org] On Behalf Of
ronlock at comcast.net <mailto:ronlock at comcast.net>
Sent: Monday, October 12, 2009 1:26 PM To: General pattern discussion
Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC) Here
is a description that shows technically correct snap execution, and valid,
consistent judging is possible. (Half of the District One guy need not
read this, they have already heard it) <G> At a small airport airshow,
one of demos was an in-trail formation of four full scale AT-6 Texans. As
each plane got to stage center, it did a single positive snap roll.
Spectators saw four snap rolls in a row, about 5 seconds apart. The flight
of four went around, and repeated the maneuver. Some spectators are getting
bored - even a pattern guy could get bored with a string of 8 nearly
identical maneuvers. And then, they did it yet again!! What's in this
for us? The snap maneuver by each AT-6 appeared to take a second or so,
from initiation to completion. By the time the fourth plane did a snap, you
could start s
eeing.... - there is a nose pitch up, - then a yaw, - then plane rolled
in direction of yaw, - plane returned to straight and level flight. By
the time the flight came around for another four snaps, you could see more
details.. - there is a nose pitch up, (somewhat sudden, at least sudden
for an AT-6) - then a large amount of yaw, - then rapid roll in direction
of yaw, (rolling faster than it could with ailerons) - plane returned to
fairly close straight and level, nose slightly high. By the time the
flight positioned for yet another four snaps, (Yawn, spectators headed for
cotton candy) the four distinct elements of the snap roll maneuver were easy
to see, and there was time to evaluate (judge) each element. 1. there is
a nose pitch up, (somewhat sudden, at least sudden for an AT-6, with little
rise in altitude) 2. then large amount of yaw, (the yaw proceeds the
upcoming roll) 3. then autorotation at rate faster than it could do an ail
eron roll) 4. plane returns to level flight track, with nose lowering to
level flight attitude. We can all be expert Snap Roll Judges! Ahhh, at
least for AT-6 snaps. What I take from all of this- The problem is not
snap descriptions. It's the application of them; observation,
discrimination and judging of elements in the split second observation time
we have. Is the task beyond reasonable expectations of most of us as a
judging community? I suppose we will continue work started over 10 years
ago to improve in these areas. In the meantime, shall we reduce the impact
of inconsistent judging of snaps by limiting the downgrade of the snap
portion of a maneuver to say..two points2? Ron Lockhart
_______________________________________________ NSRCA-discussion mailing
list NSRCA-discussion at lists. nsrca .org
http://lists <http://lists/> . nsrca .org/mailman/listinfo/
nsrca -discussion _____ _______________________________________________
NSRCA-discussion mailing list NSRCA-discussion at lists. nsrca .org
http://lists <http://lists/> . nsrca .org/mailman/listinfo/
nsrca -discussion _______________________________________________
NSRCA-discussion mailing list NSRCA-discussion at lists. nsrca .org
http://lists <http://lists/> . nsrca .org/mailman/listinfo/
nsrca -discussion No virus found in this incoming message. Checked by AVG -
www.avg.com <http://www.avg.com/>
Version: 8.5.421 / Virus Database: 270.14.9/2428 - Release Date: 10/13/09
06:35:00
_______________________________________________ NSRCA-discussion mailing
list NSRCA-discussion at lists. nsrca .org
http://lists <http://lists/> . nsrca .org/mailman/listinfo/
nsrca -discussion
_______________________________________________ NSRCA-discussion mailing
list
NSRCA-discussion at lists.nsrca.org <mailto:NSRCA-discussion at lists.nsrca.org>
http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
_______________________________________________ NSRCA-discussion mailing
list NSRCA-discussion at lists.nsrca.org
<mailto:NSRCA-discussion at lists.nsrca.org>
http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
_____
_______________________________________________
NSRCA-discussion mailing list
NSRCA-discussion at lists.nsrca.org <mailto:NSRCA-discussion at lists.nsrca.org>
http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
_______________________________________________
NSRCA-discussion mailing list
NSRCA-discussion at lists.nsrca.org <mailto:NSRCA-discussion at lists.nsrca.org>
http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
_______________________________________________
NSRCA-discussion mailing list
NSRCA-discussion at lists.nsrca.org <mailto:NSRCA-discussion at lists.nsrca.org>
http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.nsrca.org/pipermail/nsrca-discussion/attachments/20091014/d9c890ed/attachment.html>
More information about the NSRCA-discussion
mailing list