Airspeed control during ground launch?

Speed control is OPPOSITE to that during aerotow and gliding

I'm sure if I asked this of one of our instructors the answer would be "you
don't" You control the attitude, the winch driver controls the speed. Lowering
the nose is the signal to the driver that you are too slow, wagging the rudder
the signal for too fast. If the winch driver doesn't or cannot respond you
abandon the launch.



Roger Worden wrote:
What is the correct relationship between pitch angle and airspeed during
ground launch, either by auto or winch? Different sources make conflicting
statements:

1. Speed control is OPPOSITE to that during aerotow and gliding

* SSA Soaring Flight Manual, page 13-10: "Under most circumstances, the best
way to reduce the airspeed is to raise the nose. During ground launch,
however, the opposite is true ... pulling the nose up results in an increase
in airspeed, assuming the launch vehicle has adequate power."

* FAA Glider Flying Handbook, page 7-16: "The pitch attitude/airspeed
relationship during ground launch is unique. During the launch, pulling back
on the stick tends to increase airspeed, and pushing forward tends to reduce
airspeed. This is the opposite of the normal pitch/airspeed relationship."

* This is what I was taught during autotow training. I can't say that I have
varied the pitch enough in my few launches to have demonstrated it.

2. Speed control is the SAME as during aerotow and gliding

* British Gliding Association Instructors' Manual, page 16-2: "If the launch
speed starts to tail-off, lower the nose." Page 16-3: "monitor the airspeed
trend. If it is falling back towards the minimum safe speed, lower the nose
or relax any back pressure on the stick." BGA does not mention attempting to
slow down: "If the glider is starting to go too fast, maintain the normal
climb attitude, and signal. If it remains too fast, or gets even faster,
release."

* During presentations on winch launch at a recent seminar, two leading
instructors discussed lowering the nose to increase speed and raising the
nose to decrease speed.

* This is what my experience with the Condor flight simulator winch launch
has shown.

It seems to me that this is a critical point on which to be clear, since I
will be winch-launching for the first time soon. I understand that the
vectors involved are different than in aerotowing, because the wings are
converting forward rope travel to angular motion (just like the skeg on a
water ski!). I also understand that a properly balanced glider using a CG
hook for winch launch will tend to nose up initially, and will seek an
optimal climb angle naturally. Altering the climb angle with elevator will
necessarily result in a less-than-optimal altitude gain. But what is the
actual effect of raising or lowering the nose in a stable ground launch
climb?

A. Is it different depending on the angle of climb? In my experience,
autotows tend to achieve a 25- to 30-degree climb angle, winch launch a 40-
to 45-degree climb. Does that cause a difference in the effect of pitch
angle?

B. If #2 above is true, how did the SSA and FFA manuals get it wrong?

C. If #1 is true, is it reversed in Britain? (Just kidding...)

"ucsdcpc" wrote in message
...
Speed control is OPPOSITE to that during aerotow and gliding

I'm sure if I asked this of one of our instructors the answer would be
"you don't" You control the attitude, the winch driver controls the speed.
Lowering the nose is the signal to the driver that you are too slow,
wagging the rudder the signal for too fast. If the winch driver doesn't
or cannot respond you abandon the launch.


And your instructor would be wrong in most cases. Pilots who think they are
controlling attitude are at risk of stalling while on the cable. We are in
a transition period in winch launch where old ideas are giving way to the
new. New gliders launched by new winches operate by somewhat different
rules.

In the modern view, the winch driver controls POWER and the pilot controls
AIRSPEED. The relationship is the same as in airplanes where throttle
controls power and pitch controls airspeed. As the glider leaves the ground
on a winch launch. the pilot smoothly pitches up to hit a target climb
airspeed. In an airplane, this would be Vy. In the typical glider, it
will be between 55 and 65 knots depending somewhat on headwind. It's
helpful to think of the winch driver as your flight engineer with his hands
on the power levers.

A modern trainer such as a ASK-21 would need a very powerful winch indeed
for its pilot to be unable to control airspeed. In most cases, the entire
launch will be flown with neutral elevator leaving considerable up elevator
to control airspeed.

An important exception is with gliders which have hooks too far forward
and/or have too little up elevator authority - for example, most Schweizers.
When flying these gliders the pilot will reach full up elevator before the
airspeed can be controlled. Since the pilot cannot control airspeed, the
winch driver has to. A further complication is that these gliders, which
require the winch driver to control airspeed, will not reach more than 1/2
to 2/3's the height of the modern 2-seater with a properly positioned CG
hook.

This exception case also introduces complications in the event of a cable
break. With the elevator positioned at full up, the glider will pitch up
strongly at the cable break - sometimes beyond the point where a safe
recovery can be made. In any event, a very quick and positive action will
be required of the pilot. This contrasts with gliders like the ASK-21 which
will continue on the same trajectory giving the pilot additional seconds to
handle the cable break.

Bill Daniels