REFLECTOR: 65VT/ angle-of-attack system

[Jim Sower]

IIRC in the airplanes I flew, there was one prominent index for landing 
AoA and a smaller (lower) one for takeoff.  In ACM we maneuvered in 
light buffet and the TO AoA was right close to the onset of buffet.  
Aside from TO and landing, we didn't use it much.  We cruised at a Mach 
nbr, and max endurance was at an IAS that we massaged upward for heavier 
weights because endurance didn't vary much with modest departures from 
optimum AoA.

Don't we have an airline pilot on the list to help us with this? ... Jim S

Ronnie Brown wrote:

> As I understand it, the flap position is used in the Proprietary 
> Systems AOA along with some other stuff to make the AOA more accurate 
> under different conditions - and probably adds to the complexity of 
> getting it to work properly.  See 
> http://home.hiwaay.net/~sbuc/journal/liftreserve-pg2.htm 
> <http://home.hiwaay.net/%7Esbuc/journal/liftreserve-pg2.htm> for a 
> discussion of the different indication when the flaps are out.
>  
> IMOH the AOA would be most useful in final approach to landing and 
> just before touch down.  If you don't use the speed brake,  then 
> everything pretty much is constant and the AOA should be more useful 
> (and less complex). 
>  
> Brian, will you be using differential pressure sensors for AOA?  It 
> might be difficult to get enough accuracy if you just use pressure 
> sensors.  Deserves some more thought(?).
>  
> And, my understanding is that stall occurs at the same angle of attack 
> (assuming no flaps are involved) no matter what the aircraft weight - 
> but CG would have some affect - not major though? 
>
>     ----- Original Message -----
>     *From:* Brian Michalk <mailto:michalk at awpi.com>
>     *To:* Velocity Aircraft Owners and Builders list
>     <mailto:reflector at tvbf.org>
>     *Sent:* Tuesday, October 05, 2004 11:48 AM
>     *Subject:* RE: REFLECTOR: 65VT/ angle-of-attack system
>
>     Okay, Jim.
>      
>     Your argument makes sense.  So does this mean for a standard
>     (non-canard) aircraft, that an AOA gauge will show different
>     values for stall speeds depending on flap extension?  Since the
>     main wing is fixed relative to the fuselage, and assuming no
>     flaps, I would see where the AOA gauge would perform.  When people
>     install those gauges, is it only for a single flap configuration?
>      
>     Okay, now back to canards.  Since the airfoil that stalls is the
>     one that the elevator is mounted, we no longer have a fixed
>     chord.  My AOA sensor that I am planning to grow myself uses a
>     handfull of air pressure sensors feeding into a very small
>     microprocessor.  I assume I would also need to feed it an aileron
>     deflection reading.
>      
>     However, this poses a problem.  I am a big fan of neural
>     networks.  I'm not smart enough to figure out the non-linear
>     equations (NASA said so) that would return the AOA.  I could wash
>     it through Excel, and come up with something, but ... well, it's a
>     lot of work, and I do some of this _at_ work, and its no fun.  So,
>     run the data through the NN.  I was going to use a traditional
>     vane as my reference, but now, that's not even the true AOA.
>      
>     I could run the plane through various speeds and loads at stall to
>     gather data, but I like feeding a NN with lots of data ... I'm
>     talking thousands of data points.
>      
>     You made a remark that the canard does not stall at full elevator
>     deflection.  Okay, but wouldn't it always stall at the same
>     deflection?  If no, why not?
>
>         -----Original Message-----
>         *From:* reflector-bounces at tvbf.org
>         [mailto:reflector-bounces at tvbf.org]*On Behalf Of *Jim Sower
>         *Sent:* Monday, October 04, 2004 8:55 PM
>         *To:* Velocity Aircraft Owners and Builders list
>         *Subject:* Re: REFLECTOR: 65VT/ angle-of-attack system
>
>         Al,
>         <...The relative AOA of the two airfoils is fixed. ...>
>         Actually, they're not.  AoA / incidence are respectively the
>         angle(s) between the chord of the airfoil and the airstream /
>         fuselage centerline respectively.  The airfoil chord line is
>         the line between the "stagnation point" on the nose of the
>         airfoil to the tip of the trailing edge.  Wing flap or canard
>         elevator deflection increase the camber of the airfoil and
>         increase both the incidence angle and the AoA.  An AoA
>         indicator measures airflow relative to a *fixed* line (the
>         fuselage centerline?) and assumes *fixed *incidence angle
>         (which is not the case with a canard unless you never deflect
>         the elevator).
>
>         <...could be useful for determining canard stall at one
>         configuration; full elevator deflection ...>
>         Actually, the canard stall does not always occur at full
>         elevator deflection either. 
>
>         With aft CG, relatively little elevator deflection rotates the
>         fuselage considerably, and the the canard will stall at an AoA
>         that consists of the largish fuselage AoA plus the relatively
>         small increase in incidence caused by relatively small
>         elevator deflection.
>
>         With forward CG, it takes much more (full?) elevator
>         deflection to rotate the fuselage less, for the canard to
>         stall.  Here, the fuselage AoA is much smaller, but the canard
>         incidence increase is much larger with full elevator deflection. 
>
>         The canard stalls in both instances, but the "AoA" that the
>         transducer sees is a good bit lower at forward CG than at aft CG.
>
>         You may recall from your Cessna days that the airplane had to
>         be pretty cocked up to stall clean, and much less so to stall
>         with the flaps down.  The AoA of the *airfoil* was not that
>         much different clean or dirty, but the incidence of the chord
>         line increased radically when you lowered the flaps.  By the
>         same token, cruising at, say 70 kts the airplane had
>         relatively level attitude, but drop the flaps and maintain 70
>         kts straight and level and you were looking at the trees.  The
>         AoA of the airfoil didn't change radically, but the angle
>         between the wing chord line and the fuselage did.
>
>         So for a canard, AoA is a little like teats on a bull .... Jim S.
>
>
>         Al Gietzen wrote:
>
>>Angle of Attack is a very useful tool for conventional plan form 
>>airplanes.  It is of little value for canard type aircraft since the 
>>wing you're trying to observe is the canard, and the incidence and 
>>aerodynamic properties (like stall AoA) are constantly changing (with 
>>elevator deflection). 
>>
>>It just doesn't tell you anything particularly useful ... Jim S.
>>
>>Good point, Jim.  However, I do think that it could be useful for
>>determining canard stall at one configuration; full elevator deflection.
>>Using the pressure port system, I'd expect installation on the main wing
>>will work fine; you just need to calibrate it the canard stall point.  The
>>relative AOA of the two airfoils is fixed.
>>
>>Al
>>        
>>