Our club's DG1000T has a stall warning device that sounds when a certain
angle of attack is reached. However I find the thing a distaction, as it
goes off every time you hit a gust (thermals tend to be gusty) and I can
comfortably circle at a speed where it is sounding all the time.

If Noel Wade came to visit the UK, he would probably not be able to climb
at all in our often tiny little thermals using the techniques he suggests.
The necessary skill is to be able to fly in accurate well banked turns at
not more than about 5 knots over the turning stall speed. Otherwise he
will be going round in the sink surrounding the thermal.

However I agree that you should not fly so slowly that the glider is
buffeting, as that is inefficient and dangerous.

By the way, my favourite spin entry for annual checks on our members is
out of a well banked turn. All you have to is slow the glider down in a
typical thermalling turn until it is just starting to buffet and then feed
in a bit of bottom rudder. The glider will depart instantly into a fully
developed spin. This demonstrates the need for accurate flying and for not
using too much rudder into the turn. If you put in a bit of top rudder it
is almost impossible to make the glider spin, however slowly you fly it.

Del Copeland


At 22:27 02 July 2009, bildan wrote:
On Jul 2, 3:27=A0pm, "noel.wade" wrote:
I personally am of the opinion that a lot of people thermal too slowly
and put themselves in a position to have an inadvertent stall/spin
accident.

Most of us are aware that stall speed goes up in a banked turn. =A0But
the desire to fly slowly and stay in the thermal keeps us down in the
"danger-zone", just above the stall speed. =A0This is unnecessary
and
can actually be a big hinderance! =A0Why?

1) There's no reason to fly below min-sink speed, as any good glider
handbook will tell you. =A0What they don't usually mention is that
your
min-sink speed is higher in a banked turn than straight-and-level
flight! =A0The same G-loading that affects your stall speed also
affects
your whole glide polar - just as added wing-loading does when you use
water ballast. =A0So in a thermal you should already be flying a bit
faster than the POH indicates for min-sink. =A0How much faster depends
on your bank angle, and the same calculations for stall speed in a
banked turn should be applicable to your aircraft.

2) The slower you go, the less control authority you have. =A0This
means
upsets are tougher to avoid, recovery from an unusual attitude is
harder to achieve, and control deflections have to be bigger in order
to make normal corrections to maintain your bank & pitch angles.
Remember - bigger control deflections equal more drag!

3) Remember that your airframe and wings start to experience localized
areas of separated airflow long before you get down to true stall
speed. =A0_Any_ dirty or separated airflow is extra drag and can
result
in a slower overall climb-rate (as you burn some of your energy
maintaining speed and countering the drag forces). =A0You want the
entire aircraft to be sliding through the air as cleanly as possible
to maximize the lift in the thermal!

4) Thermals contain unstable air - turbulence and wind gusts and the
shear around the edges of the thermal can all create assymetrical lift
conditions across the wings; or cause airflow separation on parts of
the wing (if the boundary layer is already close to seaparating).
=A0The
closer you are to stall speed, the more likely this is to suddenly
occur due to gusts or shear effects.

(For those of you who are trying to fly fast, add in the effects of
your water-ballast to all of this, too)

Just some food for thought,

--Noel

For this reason and for safety is why I advocate an angle of attack
indicator. The AOA indicator will help determine the exact airspeed
for minimum sink for your wing loading and bank angle. Then, you can
use either instrument as your guide.

There's actually quite a large range of AOA between min sink and stall
so flying min sink AOA will keep you a safe distance from a stall.

I have to think that an AOA indicator 'might' have saved at least one
life this summer.