REFLECTOR: 65VT/ angle-of-attack system

[Jim Sower]

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
>
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