[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