REFLECTOR: 65VT/ angle-of-attack system

[Jim Sower]

Max glide, climb and Vx, Vy are usually the bottom of a fairly wide 
bucket, and the departures either side of the POH specs are pretty 
gradual, so if you're in the ballpark, your results will be very close 
to optimum.  I tend to go just a little faster than the specs on account 
of the heavy front seat.  AoA indicators are not typically accurate 
enough (fine enough granularity) to be all that useful for stuff like 
that anyway.  At least that's what I recall from tactical jets in the 
60s and 70s.  The big deal as I see it for GA AoA is to prevent approach 
turn stalls, and that's not a problem with canards.

As I see it .... Jim S.



Douglas Holub wrote:

> So, would you say that the airspeed indicator is just as useful as an 
> AOA indicator for establishing maximum glide or maximum rate of 
> climb?  Or would Vx and Vy vary more with CG and weight than an AOA 
> indicator would?
>
>     ----- Original Message -----
>     *From:* Jim Sower <mailto:canarder at frontiernet.net>
>     *To:* Velocity Aircraft Owners and Builders list
>     <mailto:reflector at tvbf.org>
>     *Sent:* Monday, October 04, 2004 8:54 PM
>     *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
>>
>>__
>>