[NSRCA-discussion] How I became an expert Snap Judge (TIC)
Vicente "Vince" Bortone
vicenterc at comcast.net
Thu Oct 15 11:27:11 AKDT 2009
Yes, you deleted many post without reading. I erased most but kept one because I need more time to read it and digest it. This is by Jerry Budd.
Here is a copy. Happy reading:
>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,
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"...
>
Vicente "Vince" Bortone
----- Original Message -----
From: "J Shu" <jshulman at cfl.rr.com>
To: "General pattern discussion" <nsrca-discussion at lists.nsrca.org>
Sent: Thursday, October 15, 2009 1:46:52 PM GMT -06:00 US/Canada Central
Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)
But they left NO doubt about pitch break... isn't that what everyone needs to see, or did I delete too many posts without reading them?
Regards,
Jason
www.shulmanaviation.com
www.composite-arf.com
----- Original Message -----
From: Vicente "Vince" Bortone
To: General pattern discussion
Sent: Thursday, October 15, 2009 2:42 PM
Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)
and SEBANDREW SNAP for the not normal one.
Vicente "Vince" Bortone
----- Original Message -----
From: "Vicente \"Vince\" Bortone" < vicenterc at comcast.net >
To: "General pattern discussion" < nsrca-discussion at lists.nsrca.org >
Sent: Thursday, October 15, 2009 1:40:03 PM GMT -06:00 US/Canada Central
Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)
Let's called NORMAL SNAP.
Vicente "Vince" Bortone
----- Original Message -----
From: "J Shu" <jshulman at cfl.rr.com>
To: "General pattern discussion" <nsrca-discussion at lists.nsrca.org>
Sent: Thursday, October 15, 2009 11:45:13 AM GMT -06:00 US/Canada Central
Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)
I don't remember anyone saying they received a 0 at the Worlds. Andrew and Seba had the 'IMAC' pitch break, most of the the rest of us had the 'normal' snap.
One thing I found funny was some of the other pilots that would 'show pitch break' would actually do mostly a roll afterwards. I guess if you 'show pitch break' the plane 'must be stalled' right...lol.
Regards,
Jason
www.shulmanaviation.com
www.composite-arf.com
----- Original Message -----
From: Vicente "Vince" Bortone
To: General pattern discussion
Sent: Thursday, October 15, 2009 12:35 PM
Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)
Jason,
It will be interesting to know at the WC in Portugal. I never got one in local contests this year. I don't think at local contest we are zeroing even when there is not a clear snap. At the Nats I zeroed some years ago. I never got a complain. I have been downgrading 3 points for not presenting the break in pitch trying to use the current snap description.
Vicente "Vince" Bortone
----- Original Message -----
From: "J Shu" <jshulman at cfl.rr.com>
To: "General pattern discussion" <nsrca-discussion at lists.nsrca.org>
Sent: Thursday, October 15, 2009 11:17:46 AM GMT -06:00 US/Canada Central
Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)
Just curious... how many are getting zero'd for doing snaps?
Regards,
Jason
www.shulmanaviation.com
www.composite-arf.com
_______________________________________________ NSRCA-discussion mailing list NSRCA-discussion at lists.nsrca.org http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
_______________________________________________
NSRCA-discussion mailing list
NSRCA-discussion at lists.nsrca.org
http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
_______________________________________________ NSRCA-discussion mailing list NSRCA-discussion at lists.nsrca.org http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
_______________________________________________ NSRCA-discussion mailing list NSRCA-discussion at lists.nsrca.org http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
_______________________________________________
NSRCA-discussion mailing list
NSRCA-discussion at lists.nsrca.org
http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
_______________________________________________ NSRCA-discussion mailing list 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/20091015/b1f51184/attachment.html>
More information about the NSRCA-discussion
mailing list