Snaps [long]

[JOddino]

Very good.  I buy it.
Jim
----- Original Message ----- 
From: "Jerry Budd" <jbudd at QNET.COM>
To: <discussion at nsrca.org>
Sent: Thursday, April 15, 2004 12:48 AM
Subject: Re: Snaps [long]


> >I would say it is when the slope goes to zero.  Many lift curves go flat
for
> >quite a few degrees of angle of attack before the slope goes negative
> >especially on low aspect ratio wings.  We always said a surface was
stalled
> >when the slope dropped to zero.
> >We always strove for a low time constant meaning a short time between
> >establishing an angle of attack and the vehicle pitching.  To do a snap
as
> >people are describing we'd need a long time constant where we can create
a
> >large angle of attack before generating a pitch acceleration.  Larger
> >control surfaces ?
>
> No, larger control surfaces (beyond that needed to adequately
> maneuver the airplane) would only result in using less elevator
> deflection for a given maneuver element.  I say that as we aren't
> currently limited in pitch control authority with the current designs.
>
> What you need to get a significant excursion in angle of attack
> (alpha) before the airplane displaces a noticeable amount in the
> z-axis is a <significantly> higher wing loading.  A higher wing
> loading would yield fewer G's per alpha as G's per alpha is linearly
> proportional to the lift curve slope (CL-alpha) divided by wing
> loading.  Essentially, the higher the wing loading, the fewer G's you
> can generate for a given amount of lift.
>
> So to generate a 20% larger excursion in alpha (just to pick a
> number) while generating the same lift force as before would require
> a 20% increase in wing loading.  In this example, for a 10 lb
> airplane that means increasing the weight to 12 lbs.  Can't do that
> (weight limit) so the only way to achieve it within the current rules
> is to reduce the wing area as much as practical, which is what the
> current designs are favoring.  There are other advantages as well to
> the smaller wings such as reduced roll inertia and reduced roll
> response to lateral wind gusts (if the span is reduced, or if the
> taper ratio is increased).  In reality, most of the current "trendy"
> designs have only reduced the wing area by around 10% to 15% so you'd
> only see an increase in alpha excursion by the same 10% to 15%.  That
> equates to an increase in angle of attack of "maybe" a degree or two.
> Not very significant.
>
> Think of it as a momentum issue.  A heavier airplane will "dig"
> deeper in response to a pitch command before displacing the same
> amount as a lighter airplane commanded to a lesser angle of attack.
> This is where IMAC with the bigger airplanes and no weight limit has
> a big advantage over a <relatively> lightly loaded pattern plane.
>
> I'll throw out one last thought for everyone to chew on.  I don't
> believe we are stalling the airfoils on our pattern planes when
> executing snap rolls (IMAC either for that matter).  The planes are
> simply too lightly loaded to get to a stalled angle of attack before
> the lateral control inputs take over (consider that we need to reach
> 18-20 degrees alpha to stall most of the airfoils we're using).  The
> pitch break requirement described in the rule book simply means that
> you can't get away with doing what is essentially an aileron roll,
> with very little pitch excursion, and a little bit of rudder (we used
> to call them "flip rolls" back in the late 80's/early 90's).  I'm not
> saying that we're not doing snap rolls, or that you have to stall the
> aircraft to do a proper snap roll, only that I don't believe that the
> plane is anywhere close to being stalled during the maneuver.  It's
> mostly a control surface driven maneuver, not flow separation driven.
>
> That said, fire away!
>
> Jerry
>
> (and no, Robert, I'm not going to translate the above into Redneck -
> it'd probably fry my translator!)
> -- 
> ___________
> Jerry Budd
> mailto:jbudd at qnet.com
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