angle-of-attack and airspeed indicators
Posted: Fri Feb 17, 2012 5:47 pm
I've been lurking on the list for some time and thought it was time to join
in...
The talk about landings had me wondering about using angle-of-attack
indicators. Wouldn't a good solution be to use a modern AOA indicator to
setup the desired angle of attack, and use power to control the sink rate.
By specifying desired approach/landing speeds I think you are really trying
to achieve a desired angle of attack, but using airspeed to indirectly
measure the angle. The problem is that aircraft behavior as a function of
airspeed will change with weight and other conditions, whereas it's behavior
with AOA is unchanged. For instance, the stall angle of attack is the same
for all weights, bank angles, density altitudes. And AOA indicators are more
accurate at higher angle of attacks eg. during landing, whereas airspeed
measurements become less accurate. I would expect (from what I read) that an
AOA indicator would give you better and more repeatable landings and also
provide a stall warning.
Anyway, I'm planning on installing an AOA in my SR. If your interested check
out http://www.angle-of-attack.com/. I would be interested if others have an
opinion (I have no experience except what I read, won't get my pilot's
license until the SR is nearing completion).
For airspeed indicators, it seems logical that that the under wing type
pitot tubes would be more accurate as BobP suggests. The air under the wing
is moving more or less parallel to the bottom of the wing. Whereas with the
pitot tube on the leading edge the airflow will make an angle (relative to
the pitot tube) equal to the angle of attack, so airspeed errors have to
increase at lower speeds. Of course if you have have a AOA indicator then
you won't be looking at airspeed while landing, but in general I would want
to have all of the instruments as accurate as is (reasonably) possible. I
have MAM's pitot tube but I think I'll look into the under wing types.
Tim Saxton
SR060 (getting ready to pro-seal the wings)
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in...
The talk about landings had me wondering about using angle-of-attack
indicators. Wouldn't a good solution be to use a modern AOA indicator to
setup the desired angle of attack, and use power to control the sink rate.
By specifying desired approach/landing speeds I think you are really trying
to achieve a desired angle of attack, but using airspeed to indirectly
measure the angle. The problem is that aircraft behavior as a function of
airspeed will change with weight and other conditions, whereas it's behavior
with AOA is unchanged. For instance, the stall angle of attack is the same
for all weights, bank angles, density altitudes. And AOA indicators are more
accurate at higher angle of attacks eg. during landing, whereas airspeed
measurements become less accurate. I would expect (from what I read) that an
AOA indicator would give you better and more repeatable landings and also
provide a stall warning.
Anyway, I'm planning on installing an AOA in my SR. If your interested check
out http://www.angle-of-attack.com/. I would be interested if others have an
opinion (I have no experience except what I read, won't get my pilot's
license until the SR is nearing completion).
For airspeed indicators, it seems logical that that the under wing type
pitot tubes would be more accurate as BobP suggests. The air under the wing
is moving more or less parallel to the bottom of the wing. Whereas with the
pitot tube on the leading edge the airflow will make an angle (relative to
the pitot tube) equal to the angle of attack, so airspeed errors have to
increase at lower speeds. Of course if you have have a AOA indicator then
you won't be looking at airspeed while landing, but in general I would want
to have all of the instruments as accurate as is (reasonably) possible. I
have MAM's pitot tube but I think I'll look into the under wing types.
Tim Saxton
SR060 (getting ready to pro-seal the wings)
*-----------------------------------------------------------------------------------*
The Murphy Rebel Builders List is for the discussion
between builders and owners of Murphy Rebel aircraft.
Archives located at:
http://www.dcsol.com/murphy-rebel/lists/default.htm
*-----------------------------------------------------------------------------------*
-----------------------------------------------------------------
List archives located at: https://mail.dcsol.com/login
username "rebel" password "builder"
Unsubscribe: rebel-builders-unsubscribe@dcsol.com
List administrator: mike.davis@dcsol.com
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