Glider angle of attack indicator by SafeFlight

On Dec 8, 12:49 pm, Eric Greenwell wrote:

Does anyone know of documentation that supports the idea showing the
pilot the AOA will actually improve a glider pilot's thermalling? Or
even that the range of AOA needed to be "efficient" is too small for a
pilot to obtain it easily by using airspeed, or by just looking out the
canopy, once he's flown the glider enough to be familiar with it?

Eric, when we fly airspeed while thermalling we are actually trying to
fly AOA. We start with the minimum sink speed (specifically, the
point on the polar we want to thermal at), add speed for ballast, then
add speed for bank angle, then come up with an adjusted airspeed that
approximates our ideal AOA for the selected gross weight and bank
angle.

Using AOA directly (once one has chosen where on the polar one wants
to thermal at) eliminates the need to make all those guesses. The
wing does it all, automatically.

Guess what - when you fly attitude - "what feels right" - in a
thermal, glancing at the airspeed to see what it is - you are flying
AOA!

For example, I couldn't even find a mention of AOA in "Fundamentals of
Sailplane Design" when discussing thermalling. Circling efficiency is
discussed (page 63-65), but without mention of AOA, which suggests to me
that it's not the important factor. Climb performance, which is what we
really are after, is very dependent on the thermal shape (pages 65-66).
Circling at the best AOA doesn't give you the best rate of climb;
instead, the circling radius is the most important factor.

I disagree. Thermalling at the most efficient bank angle/AOA for the
size of the thermal is the most important factor. Waddling around a
knot above the stall with landing flaps down will give me the smallest
circling radius, but a horrible climb rate.

Look at the "rate of sink versus turn radius" table like the one on page
64 of "Fundamentals...". Does anyone know if the optimum is always at
the same AOA? And if not, what the range of AOA is for the table?

My guess it that the optimum AOA may vary based on turbulence, but
only a very small about - probably less than can be accurately flown
by the average pilot in a typical thermal. And this would only be for
airfoils that are susceptible to turbulent flows. In most cases, the
AOA range for effective Cl max (which I assume is close to the optimum
for min sink and thermalling) is probably big enough to be measured
and flown accurately.

Regardless of the answer is to the question above, what would be useful
would be two additional tables "rate of sink versus turn radius". One
table would use an AOA greater (say, 3 degrees) than optimum; the other
table would use an AOA smaller by the same amount from optimum. This
would give us an idea of how sensitive circling efficiency is to AOA
errors.

If performance is not sensitive to the AOA, there is no need to look for
an indicator of it. A stall warning device would still be useful, but it
doesn't have to be based on AOA: it just needs to tell you when the wing
is getting close to a stall.

If performance is not sensitive to the AOA, we wouldn't need an
airspeed indicator! At low speeds, that old ASI is at best a poor
compromise - the only good thing about it is that is doesn't fail
often (although, the only instrument I've ever had fail in a glider
was the airspeed indicator). And how can anything tell you the wing
is getting close to the stall without measuring AOA? Excessive AOA is
what defines a stall. Airspeed is just an approximation - and can
easily trick you. Try landing back after a low altitude rope break
full of ballast, if you haven't flown wet in a while. Slow to the
airspeed you are used to using to turn back and you will get a big
surprise! In the same situation, slow to the same AOA, and you have
the same margin over the stall you had dry. This isn't opinion, it's
basic aerodynamics.

I think the lack of references to angle of attack in gliding
publications is largely due to the fact that AOA is still mainly
limited to military jets and expensive airliners/biz jets. Most
general aviation pilots never have a chance to be exposed to the joys
of knowing exactly what their wing is doing. Or not doing, as the
case may be! Funny thing is, the common Cezzna uses a crude AOA
sensor for it's stall warning (the little paddle on the leading edge).

Kinda like audio varios - once you try it, you'll never want to go
back to airspeed as a low speed control instrument.

Any real aero majors lurking out there, please join in!

Cheers,

Kirk

Ever since I got a copy of the book Ruder and Stick by Wolfgang L. I
think about ways to implement an effective AoA indicator for my flapped
glider.

I have the AoA string on the right side of my canopy, but it is not very
helpful, as it has to be calibrated for every flap setting.

"kirk.stant" wrote in message
...
On Dec 8, 12:49 pm, Eric Greenwell wrote:

Does anyone know of documentation that supports the idea showing the
pilot the AOA will actually improve a glider pilot's thermalling? Or
even that the range of AOA needed to be "efficient" is too small for a
pilot to obtain it easily by using airspeed, or by just looking out the
canopy, once he's flown the glider enough to be familiar with it?

Eric, when we fly airspeed while thermalling we are actually trying to
fly AOA. We start with the minimum sink speed (specifically, the
point on the polar we want to thermal at), add speed for ballast, then
add speed for bank angle, then come up with an adjusted airspeed that
approximates our ideal AOA for the selected gross weight and bank
angle.

Using AOA directly (once one has chosen where on the polar one wants
to thermal at) eliminates the need to make all those guesses. The
wing does it all, automatically.

Guess what - when you fly attitude - "what feels right" - in a
thermal, glancing at the airspeed to see what it is - you are flying
AOA!

For example, I couldn't even find a mention of AOA in "Fundamentals of
Sailplane Design" when discussing thermalling. Circling efficiency is
discussed (page 63-65), but without mention of AOA, which suggests to me
that it's not the important factor. Climb performance, which is what we
really are after, is very dependent on the thermal shape (pages 65-66).
Circling at the best AOA doesn't give you the best rate of climb;
instead, the circling radius is the most important factor.

I disagree. Thermalling at the most efficient bank angle/AOA for the
size of the thermal is the most important factor. Waddling around a
knot above the stall with landing flaps down will give me the smallest
circling radius, but a horrible climb rate.

Look at the "rate of sink versus turn radius" table like the one on page
64 of "Fundamentals...". Does anyone know if the optimum is always at
the same AOA? And if not, what the range of AOA is for the table?

My guess it that the optimum AOA may vary based on turbulence, but
only a very small about - probably less than can be accurately flown
by the average pilot in a typical thermal. And this would only be for
airfoils that are susceptible to turbulent flows. In most cases, the
AOA range for effective Cl max (which I assume is close to the optimum
for min sink and thermalling) is probably big enough to be measured
and flown accurately.

Regardless of the answer is to the question above, what would be useful
would be two additional tables "rate of sink versus turn radius". One
table would use an AOA greater (say, 3 degrees) than optimum; the other
table would use an AOA smaller by the same amount from optimum. This
would give us an idea of how sensitive circling efficiency is to AOA
errors.

If performance is not sensitive to the AOA, there is no need to look for
an indicator of it. A stall warning device would still be useful, but it
doesn't have to be based on AOA: it just needs to tell you when the wing
is getting close to a stall.

If performance is not sensitive to the AOA, we wouldn't need an
airspeed indicator! At low speeds, that old ASI is at best a poor
compromise - the only good thing about it is that is doesn't fail
often (although, the only instrument I've ever had fail in a glider
was the airspeed indicator). And how can anything tell you the wing
is getting close to the stall without measuring AOA? Excessive AOA is
what defines a stall. Airspeed is just an approximation - and can
easily trick you. Try landing back after a low altitude rope break
full of ballast, if you haven't flown wet in a while. Slow to the
airspeed you are used to using to turn back and you will get a big
surprise! In the same situation, slow to the same AOA, and you have
the same margin over the stall you had dry. This isn't opinion, it's
basic aerodynamics.

I think the lack of references to angle of attack in gliding
publications is largely due to the fact that AOA is still mainly
limited to military jets and expensive airliners/biz jets. Most
general aviation pilots never have a chance to be exposed to the joys
of knowing exactly what their wing is doing. Or not doing, as the
case may be! Funny thing is, the common Cezzna uses a crude AOA
sensor for it's stall warning (the little paddle on the leading edge).

Kinda like audio varios - once you try it, you'll never want to go
back to airspeed as a low speed control instrument.

Any real aero majors lurking out there, please join in!

Cheers,

Kirk

No need. That was a damn good explanation.

Bill Daniels

On Dec 8, 5:21 pm, "kirk.stant" wrote:

We start with the minimum sink speed (specifically, the
point on the polar we want to thermal at), add speed for ballast, then
add speed for bank angle, then come up with an adjusted airspeed that
approximates our ideal AOA for the selected gross weight and bank
angle.

Using AOA directly (once one has chosen where on the polar one wants
to thermal at) eliminates the need to make all those guesses. The
wing does it all, automatically.

I realized a bit late that what I really meant to say is that when
using airspeed, we find the performance point on the polar we want (L/
D max, min sink, stall, whatever), then move the polar for ballast and
bank angle, and use the resulting adjusted airspeed. This is made
necessary when using the common sink rate vs airspeed polars for all
the desired conditions of ballast and bank angle. Using AOA directly
(which would require the polar in sink rate vs AOA and L/D vs AOA)
eliminates the need to move the polar (and refigure the resulting
airspeed), as the AOA for a specific flight condition is not affected
by ballast or bank angle.

Has anyone seen glider polars with sinkrate plotted against angle of
attack? That would be interesting. I've seen plots for aircraft of
Cl vs AOA, and L/D vs AOA, neither of which is very useful in this
discussion.

I should have listened up more in aero classes, long time ago...

Kirk

A few years ago I got curious about the use of the word "polar" to
describe the sink vs airspeed performance curve. As far as I could
determine, it's aerodynamic's tribute to Lilienthal. His original Cl
and Cd curves for a bird's wing were plotted only against AOA using
polar coordinates, and the curves were called Lilienthal's Polar. As
aerodynamics developed it became easier to drop the polar plotting and
only use cartesian coordinates. Perhaps this came about because it was
easier to measure airspeed rather than AOA, and thus speeds were of
more practical use?

The AOA is of course embedded into the common cartesian performance
curve. It's theoretically possible to identify specific AOA points
along a performance curve, starting at high AOA values at the stall
and progressing to low AOA values at high airspeeds. I've never seen
AOA angles superimposed onto a performance curve, but I imagine that
the spacing between a AOA degree is closer near the stall end than it
is near the high speed end. If a practical AOA meter were to be
developed, I imagine such curves would be published.

I've never flown a plane with an AOA meter, so I don't understand how
it would respond during changing conditions:

(1) In landing through a wind gradient, I assume the AOA suddenly
increases as you drop into the slower wind? I assume this because I
know you're closer to a stall when the wind suddenly slows and the ASI
decreases, but I don't understand the relative wind change needed to
cause an AOA change in this situation.

(2) What happens in a turbulent thermal? When you fly a thermal using
a constant attitude it's common to see fairly definite ASI changes -
what would an AOA meter show during this situation? I assume the AOA
would change as the relative wind changes, and that you would still
fly a constant attitude much as you do with an ASI.

In both of these situations the ultimate question is: why would an
AOA meter be better than an ASI? I'm guessing the AOA would be good
upon entry so you would know what attitude to maintain, but I think it
wouldn't necessarily be any better than an ASI once you were into the
changing situations described in (1) and (2).

Can someone enlighten me? Thanks!

-John

On Dec 9, 1:03 am, "kirk.stant" wrote:
Has anyone seen glider polars with sinkrate plotted against angle of
attack? That would be interesting. I've seen plots for aircraft of
Cl vs AOA, and L/D vs AOA, neither of which is very useful in this
discussion.

Kirk

jcarlyle wrote:
I've never flown a plane with an AOA meter, so I don't understand how
it would respond during changing conditions:

I haven't either, so the below is all theoretical, take it with a grain of
salt.

(1) In landing through a wind gradient, I assume the AOA suddenly
increases as you drop into the slower wind? I assume this because I
know you're closer to a stall when the wind suddenly slows and the ASI
decreases, but I don't understand the relative wind change needed to
cause an AOA change in this situation.

Right, the AOA increases. On a very short timescale, when you drop into
slower wind, your airspeed decreases. This then decreases the amount of
lift your wings are developing. The lift they create is now less than your
weight, so you begin to drop. As you drop, the relative wind becomes more
vertical, increasing your AOA and the lift created by the wings. Once the
AOA increases to the point where the lift balances out your weight, you
reach a steady state again at a slower airspeed and higher AOA. If the AOA
reaches the stall angle before the lift increases to match your weight,
you'll stall, and this is why you should carry plenty of extra speed in
that situation.

(2) What happens in a turbulent thermal? When you fly a thermal using
a constant attitude it's common to see fairly definite ASI changes -
what would an AOA meter show during this situation? I assume the AOA
would change as the relative wind changes, and that you would still
fly a constant attitude much as you do with an ASI.

Turbulence would bounce it around similar; if you get hit with a gust from
below the AOA will momentarily become larger, increasing the lift from the
wings. You then begin to climb until your vertical speed matches the gust
and your AOA goes back to what it was. Similarly with a gust from above,
where you start to drop.

In both of these situations the ultimate question is: why would an
AOA meter be better than an ASI? I'm guessing the AOA would be good
upon entry so you would know what attitude to maintain, but I think it
wouldn't necessarily be any better than an ASI once you were into the
changing situations described in (1) and (2).

I think the advantage in a thermal is that the optimal airspeed changes
with bank, whereas the optimal AOA doesn't change. Instead of trying to
figure out the best airspeed to track as you keep altering your bank to
center the thermal, you can just track a single AOA all the time.

For landing I think the advantage is just that you can stick a warning
signal around the stall AOA to remind you to stop hauling back on the
stick when you get too close to a stall.

--
Michael Ash
Rogue Amoeba Software

Thanks for a very clear explanation, Michael. I don't think any salt
is needed!

The only thing I'm left unsure about is AOA behavior vs. ASI behavior
in a turbulent thermal. I buy that AOA eliminates worrying about bank
angle and ballast - just set the attitude for best min sink AOA and
try to keep it there. But I wonder how much an AOA meter (say one
based upon pressure ports) would vary in a thermal vis a vis an ASI.
I've seen plus/minus 5 kts in ASI. If this translated in several
degrees in AOA, it might make the AOA as useless as the ASI.

-John

Michael Ash wrote:
Right, the AOA increases. On a very short timescale, when you drop into
slower wind, your airspeed decreases. This then decreases the amount of
lift your wings are developing. The lift they create is now less than your
weight, so you begin to drop. As you drop, the relative wind becomes more
vertical, increasing your AOA and the lift created by the wings. Once the
AOA increases to the point where the lift balances out your weight, you
reach a steady state again at a slower airspeed and higher AOA. If the AOA
reaches the stall angle before the lift increases to match your weight,
you'll stall, and this is why you should carry plenty of extra speed in
that situation.

Turbulence would bounce it around similar; if you get hit with a gust from
below the AOA will momentarily become larger, increasing the lift from the
wings. You then begin to climb until your vertical speed matches the gust
and your AOA goes back to what it was. Similarly with a gust from above,
where you start to drop.

I think the advantage in a thermal is that the optimal airspeed changes
with bank, whereas the optimal AOA doesn't change. Instead of trying to
figure out the best airspeed to track as you keep altering your bank to
center the thermal, you can just track a single AOA all the time.

For landing I think the advantage is just that you can stick a warning
signal around the stall AOA to remind you to stop hauling back on the
stick when you get too close to a stall.

John,

Check your gmail account for an offline note.

Wayne
http://www.soaridaho.com/Schreder

"jcarlyle" wrote in message
...
Thanks for a very clear explanation, Michael. I don't think any salt
is needed!

The only thing I'm left unsure about is AOA behavior vs. ASI behavior
in a turbulent thermal. I buy that AOA eliminates worrying about bank
angle and ballast - just set the attitude for best min sink AOA and
try to keep it there. But I wonder how much an AOA meter (say one
based upon pressure ports) would vary in a thermal vis a vis an ASI.
I've seen plus/minus 5 kts in ASI. If this translated in several
degrees in AOA, it might make the AOA as useless as the ASI.

-John

Michael Ash wrote:
Right, the AOA increases. On a very short timescale, when you drop into
slower wind, your airspeed decreases. This then decreases the amount of
lift your wings are developing. The lift they create is now less than
your
weight, so you begin to drop. As you drop, the relative wind becomes more
vertical, increasing your AOA and the lift created by the wings. Once the
AOA increases to the point where the lift balances out your weight, you
reach a steady state again at a slower airspeed and higher AOA. If the
AOA
reaches the stall angle before the lift increases to match your weight,
you'll stall, and this is why you should carry plenty of extra speed in
that situation.

Turbulence would bounce it around similar; if you get hit with a gust
from
below the AOA will momentarily become larger, increasing the lift from
the
wings. You then begin to climb until your vertical speed matches the gust
and your AOA goes back to what it was. Similarly with a gust from above,
where you start to drop.

I think the advantage in a thermal is that the optimal airspeed changes
with bank, whereas the optimal AOA doesn't change. Instead of trying to
figure out the best airspeed to track as you keep altering your bank to
center the thermal, you can just track a single AOA all the time.

For landing I think the advantage is just that you can stick a warning
signal around the stall AOA to remind you to stop hauling back on the
stick when you get too close to a stall.

"jcarlyle" wrote in message
...
Thanks for a very clear explanation, Michael. I don't think any salt
is needed!

The only thing I'm left unsure about is AOA behavior vs. ASI behavior
in a turbulent thermal. I buy that AOA eliminates worrying about bank
angle and ballast - just set the attitude for best min sink AOA and
try to keep it there. But I wonder how much an AOA meter (say one
based upon pressure ports) would vary in a thermal vis a vis an ASI.
I've seen plus/minus 5 kts in ASI. If this translated in several
degrees in AOA, it might make the AOA as useless as the ASI.

-John


Many pilots who have tried the "pitch strings" report that they give advance
warning when entering an area of lift. When you enter the edge of a
thermal, the strings show a sharp increase in AOA several seconds before the
vario shows lift. This improves the efficiency of "dolphin flying" by
providing an earlier signal of when to start a zoom. One pilot on a
marginal final glide told me, "Without them, I wouldn't have made it home."
An ASI gives the same signal but it's weaker and harder to interpret.

Most AOA sensors have some damping to smooth the signal in rough air.

Bill Daniels

Bill Daniels wrote:

Many pilots who have tried the "pitch strings" report that they give advance
warning when entering an area of lift. When you enter the edge of a
thermal, the strings show a sharp increase in AOA several seconds before the
vario shows lift. This improves the efficiency of "dolphin flying" by
providing an earlier signal of when to start a zoom. One pilot on a
marginal final glide told me, "Without them, I wouldn't have made it home."
An ASI gives the same signal but it's weaker and harder to interpret.

And yet, I never see any gliders with "pitch strings" on them, not even
on contest winners' gliders, presumably the pilots most interested in
getting the most from their efforts. If it's effective, why hasn't
something so cheap and easy spread throughout the fleet?

I did try these on a Std. Cirrus 30 years ago, and eventually concluded
they didn't help in any way. Maybe it's time to repeat the experiment,
but I'm skeptical about the string's value.

--
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