Angle of attack

On Dec 13, 7:10 am, Scott wrote:
This is an interesting topic. How exactly does the horizon give you
angle of attack reference?

It does NOT give an angle of attack reference.

The pitch attitude (horizon) differs from AOA by the climb angle. See
http://www.av8n.com/how/htm/aoa.html for a good explanation.


While I agree that understanding of AOA is crucial for a pilot. I
don't think that a AOA indicator would be very useful in gliders.
(except for flight testing of a new design)

I think that stall accidents are caused by lack of awareness and lack
of attention. Letting the airspeed "creep away" and incorrectly
responding to the loss of airspeed. Substituting an AOA dial for the
airspeed dial, won't change that.


AOA gauges seem best suited for heavy aircraft flown by reference to
instruments. Where you might have to wait a couple of minutes after
setting the AOA and power lever to get the airspeed to respond.
Gliders will change their airspeed much quicker than a 747 or F-14.
We also only really fly at 2 or 3 different weights. No ballast, half
or full. So we only really have to remember the best L/D speed for
these conditions.

Todd Smith
3S

toad wrote:


While I agree that understanding of AOA is crucial for a pilot. I
don't think that a AOA indicator would be very useful in gliders


Why would the best reference for the actual performance of the aircraft
be less desirable than an error-prone and very approximate indication
which, like a stopped clock, is exactly right about twice a day?



I think that stall accidents are caused by lack of awareness and lack
of attention. Letting the airspeed "creep away" and incorrectly
responding to the loss of airspeed. Substituting an AOA dial for the
airspeed dial, won't change that.


It is my understanding that one is supposed to be able to fly a glider
safely without reference to the ASI, and I suppose that should also then
apply to AOA as well. Obviously, not everyone can do so. And there are a
few each year that manage to demonstrate the inability to fly safely in
spite of the availability of a working ASI. It is also possible that the
fact that the stall speed of a given sailplane on a given flight could
be +/- 15%, or more, from book speed _in one-g flight_ should be given
more thorough consideration. Do we know then what it might be in a 30-60
degree banked final turn, or in a wind shear condition where the ASI
reads considerably below the book stalling speed? "More" is an answer,
in one case, and "Less" in the other. How much more or less we find by
trial and error, guesswork and luck--or not. An AOA would tell us
instantly and throughout the normal operating range of the glider, and
beyond, just how close to the stall we are operating.



AOA gauges seem best suited for heavy aircraft flown by reference to
instruments. Where you might have to wait a couple of minutes after
setting the AOA and power lever to get the airspeed to respond.
Gliders will change their airspeed much quicker than a 747 or F-14.


I can't use the F-14 or the 747 as a reference, because I have not flown
either. My understanding is that the F-14 can accelerate rather rapidly
and slow down pretty quickly as well. High performance military aircraft
which I have flown certainly exhibited that capability. To say that any
of them, or even a 747, could not accelerate more rapidly than an
unpowered glider in a specified attitude seems an incompletely
considered statement, but perhaps you can provide some evidence to
support your claim.



We also only really fly at 2 or 3 different weights. So we only
really have to remember the best L/D speed for these conditions.

Be my guest--memorize as many speeds as you like. You'll still be
guessing at how close your ASI is to the actual stalling speed at any
particular moment. AOA never lies: any g-load; any altitude; any
attitude; any airspeed.



Jack

Be my guest--memorize as many speeds as you like. You'll still be
guessing at how close your ASI is to the actual stalling speed at any
particular moment. AOA never lies: any g-load; any altitude; any
attitude; any airspeed.

While I think an AOA indicator might be useful, isn't this over selling
it's ability? How about with ice, rain, or bugs on the wing - is the
stalling AOA still the same? Does the AOA indicator read correctly in
slip? Does it read the AOA of the inboard tip and the outboard tip in a
turn? What if it freezes, or gets water it?

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

Eric Greenwell wrote:

Be my guest--memorize as many speeds as you like. You'll still be
guessing at how close your ASI is to the actual stalling speed at any
particular moment. AOA never lies: any g-load; any altitude; any
attitude; any airspeed.

While I think an AOA indicator might be useful, isn't this over selling
it's ability? How about with ice, rain, or bugs on the wing - is the
stalling AOA still the same? Does the AOA indicator read correctly in
slip? Does it read the AOA of the inboard tip and the outboard tip in a
turn? What if it freezes, or gets water it?


See both Daniels' and Pfeiffer's recent comments in the nearby "AoA Keep
it Going" thread.


Jack

On Dec 16, 8:06 pm, J a c k wrote:
toad wrote:
While I agree that understanding of AOA is crucial for a pilot. I
don't think that a AOA indicator would be very useful in gliders

Why would the best reference for the actual performance of the aircraft
be less desirable than an error-prone and very approximate indication
which, like a stopped clock, is exactly right about twice a day?

I think that stall accidents are caused by lack of awareness and lack
of attention. Letting the airspeed "creep away" and incorrectly
responding to the loss of airspeed. Substituting an AOA dial for the
airspeed dial, won't change that.

It is my understanding that one is supposed to be able to fly a glider
safely without reference to the ASI, and I suppose that should also then
apply to AOA as well. Obviously, not everyone can do so. And there are a
few each year that manage to demonstrate the inability to fly safely in
spite of the availability of a working ASI. It is also possible that the
fact that the stall speed of a given sailplane on a given flight could
be +/- 15%, or more, from book speed _in one-g flight_ should be given
more thorough consideration. Do we know then what it might be in a 30-60
degree banked final turn, or in a wind shear condition where the ASI
reads considerably below the book stalling speed? "More" is an answer,
in one case, and "Less" in the other. How much more or less we find by
trial and error, guesswork and luck--or not. An AOA would tell us
instantly and throughout the normal operating range of the glider, and
beyond, just how close to the stall we are operating.

AOA gauges seem best suited for heavy aircraft flown by reference to
instruments. Where you might have to wait a couple of minutes after
setting the AOA and power lever to get the airspeed to respond.
Gliders will change their airspeed much quicker than a 747 or F-14.

I can't use the F-14 or the 747 as a reference, because I have not flown
either. My understanding is that the F-14 can accelerate rather rapidly
and slow down pretty quickly as well. High performance military aircraft
which I have flown certainly exhibited that capability. To say that any
of them, or even a 747, could not accelerate more rapidly than an
unpowered glider in a specified attitude seems an incompletely
considered statement, but perhaps you can provide some evidence to
support your claim.

We also only really fly at 2 or 3 different weights. So we only
really have to remember the best L/D speed for these conditions.

Be my guest--memorize as many speeds as you like. You'll still be
guessing at how close your ASI is to the actual stalling speed at any
particular moment. AOA never lies: any g-load; any altitude; any
attitude; any airspeed.

Jack

AOA is my favorite subject to teach. It is not easy but is a life
saver. Most pilots really don't understand relative wind. Ask them to
explain it. What happens when a gust is encountered + or -? Near a
stall the asi pitot tube is at an angle to the wind and is less
accurate. I use a pieceof yarn taped to the side od the canopy with a
"normal" air flow line marked. A the moment of the stall, the string
rises as you approach the stall. It is only a cheap teaching aid. I
probe my students understanding of why and where does the AOA COME
FROM AND WHY. Lift depends on speed of the air and angle of
attack...and it varies in many ways. At altitude, my students get a
lot of slow flight experience. They know! Fred