REFLECTOR: 65VT/ angle-of-attack system

[Brian Michalk]

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

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