Polar with spoilers extended?

On Oct 24, 8:44 pm, Brian wrote:
First I thought that you were pulling our legs, but it seems you're
actually serious.

Every year a couple of pilots die because they are too slow on approach.
Where I fly, a student will fail his checkride big time if he's only one
knot below the yellow triangle on final. *Especially* with a headwind.
If you don't understand this, I *strongly* recommend you talk to a
knowledgeble instructor.

I will be the first to agree that pilots die every year because they
are too slow on the approach. However I have not been able to find any
evidence that any of these were caused by the pilot intentinally
flying the approach slower than normal. In fact in nearly every case
I have examined it appears more likely that pilot was flying by sight
and feel and not paying any attention at all to the airspeed. Often
they are landing or flying downwind which gives the illusion of
airspeed, as does flying close to the ground. If fact the few pilots I
have been able to interview or read their description of the accident
described it as feeling like the controls went limp and had no effect.
The didn't make any mention of airspeed. It is not hard to find this
description in NTSB reports for power aircraft. I believe nearly all
Stall Spin Accidents are caused by the illusion of speed. The pilot
thinks he going fast so he doesn't look at the airspeed indicator and
he is not thinking about a stall or a spin.

Brian

I'm pretty sure that's not a true. I know of at least a couple of
accidents where the pilot was knowingly flying as slow as possible and/
or s-turning on final to get into a tight spot. Both were fatal.

I think the best way to think about this is in terms of energy
dissipation between wherever you start and some fixed touchdown point
(or stopping point if you are willing to plant the glider on the
ground and use the wheel brake too). Frankly if I am looking at a
finite distance to an unpleasent end of the field I will do whatever I
can to burn energy - before and after touchdown.

For starters I took a look at my ASW-27B factory polar. It turns out
that I will fly the same L/D spoilers closed at 37 knots (stall) as at
86 knots. This is basically the breakeven tradeoff between between
high induced and and high parasitic drag manuevers. If I add spoilers
I am adding a parasitic drag device which increases in effectiveness
with the square of velocity. That means that the breakeven speed for
the parasitic maneuver has to be lower than 86 knots. You don't need
to get to Vne to do better by going faster - even 70-75 knots is
probably better.

The ground effect argument has some merit I think, but keep in mind
that you can always stop the manuever before you get into ground
effect and bleeed off airspeed at 100' or so - you will still be
ahead. The altitude you consume slowing form the higher speed will be
roughly equal to the altitude you burned getting to the higher speed.

This is for still air. If I add headwind the breakeven airspeed for
speeding up versus slowing down goes up, but in most cases I'd be hard
pressed to believe that with spoilers out you will get a better energy
dissipation going slow than fast. At some point there will be a
crossover as headwind goes up. Just think of a headwind that is
greater than your stall speed to convince yourself.

Theory aside, I am convinced that as a practical matter making the
glider as draggy as possible and adding speed is almost always a more
practical and safe solution to slowing down and maneuvering at low
altitude. Stall/spin is a buzz-kill.

Lastl;y, I have done the parasitic drag maneuver down to touchdown in
a G-103 with spoilers closed and convinced myself that it is the
preferred method. I 'm guessing a Duo with the boards out is at least
as draggy as a G-103 clean.

My 2c.

9B