Snaps [long]

[Jerry Budd]

>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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