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