[NSRCA-discussion] How I became an expert Snap Judge (TIC)
James Oddino
joddino at socal.rr.com
Wed Oct 14 08:26:46 AKDT 2009
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
> 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>
>>> To: "General pattern discussion" <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 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> 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>
>>> 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>
>>> "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>
>>> 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
>>> 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. nsrca .org/mailman/listinfo/ nsrca -discussion
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