Polar with spoilers extended?

On Oct 23, 2:00 pm, Marc Ramsey wrote:
John Galloway wrote:
as an ex Duo owner I agree with Dave Nadler and Bill
Daniels; the Duo airbrakes are better than their reputation:

Well, clearly, not all current and former Duo owners agree.

My 2c: (Our club has a duo, so based on some experience.) The actual
glide angle of the duo, with full spoilers out and at a stable
approach speed, is decently steep. Looking at this angle at altitude
is instructive. The duo seems not to lose speed as quickly as other
gliders when you open the spoilers, especially in ground effect. "Too
high" really often means "too fast". I think a lot of the perception
that the duo has poor divebreakes is realy that it does not slow down
fast, rather than the actual steady state glide angle is shallow.

This all makes some aerodynamic sense. The duo is heavier than basic
trainers, and much heavier than the single seaters we are used to.
"Spoilers" work as much by "spoiling lift" as by "increasing drag",
and much of the latter is induced drag due to the gap in the lift
distribution anyway.


John Cochrane

BB wrote:
On Oct 23, 2:00 pm, Marc Ramsey wrote:
John Galloway wrote:
as an ex Duo owner I agree with Dave Nadler and Bill
Daniels; the Duo airbrakes are better than their reputation:
Well, clearly, not all current and former Duo owners agree.

My 2c: (Our club has a duo, so based on some experience.) The actual
glide angle of the duo, with full spoilers out and at a stable
approach speed, is decently steep. Looking at this angle at altitude
is instructive. The duo seems not to lose speed as quickly as other
gliders when you open the spoilers, especially in ground effect. "Too
high" really often means "too fast". I think a lot of the perception
that the duo has poor divebreakes is realy that it does not slow down
fast, rather than the actual steady state glide angle is shallow.

This all makes some aerodynamic sense. The duo is heavier than basic
trainers, and much heavier than the single seaters we are used to.
"Spoilers" work as much by "spoiling lift" as by "increasing drag",
and much of the latter is induced drag due to the gap in the lift
distribution anyway.

That's how this whole discussion got started. Someone suggested that
the best thing to do when high on final is to dive with full spoilers,
pull up above ground effect and wait for the speed to bleed off. I said
that won't work too well with a Duo, as with full spoilers it isn't all
that draggy, will accelerate relatively quickly, and bleed off speed
slowly. Others said nonsense, the Duo has wonderful spoilers. And so
on, and so on...

Marc

That's how this whole discussion got started.

What, RAS going around in circles (in sink!) ? Unheard of!

I haven't tried the high parasitic drag maneuver in a duo yet. When
demonstrated by Marty Eiler in an ASK 21, it consisted of a near VNE
dive to the ground well short of the intended landing area, and then
bleeding off the speed quite low. The key is that you lose so much
energy near VNE with spoilers out, you can afford now to bleed off
speed, even in ground effect. Most of our duo discussions have not
invovlved such high speeds -- I'm curious how it might work. I know
that being high, 80 knots and aiming at the spot in a duo is a bad
combination, but that's not what we're taling about!

As fun as the high parasitic drag maneuver is, I wonder if anyone has
ever actually used it in combat. Has anyone been so flustered and out
of synch to get monstrously high in an off field landing, then had the
presence of mind and skill left to dive to the ground at near VNE,
aiming several hundreds of yards short of the intended small paddock
with fence at the far end, and had it work? The mental attitude that
gets to the problem seems incompatible with the attitude needed to
pull this one off. If you don't aim short enough in the dive, you just
crash into the far fence at really high speed. But I'd be curious to
hear a "it worked for me" story.

John Cochrane

"BB" wrote in message
ups.com...
That's how this whole discussion got started.

What, RAS going around in circles (in sink!) ? Unheard of!

I haven't tried the high parasitic drag maneuver in a duo yet. When
demonstrated by Marty Eiler in an ASK 21, it consisted of a near VNE
dive to the ground well short of the intended landing area, and then
bleeding off the speed quite low. The key is that you lose so much
energy near VNE with spoilers out, you can afford now to bleed off
speed, even in ground effect. Most of our duo discussions have not
invovlved such high speeds -- I'm curious how it might work. I know
that being high, 80 knots and aiming at the spot in a duo is a bad
combination, but that's not what we're taling about!

As fun as the high parasitic drag maneuver is, I wonder if anyone has
ever actually used it in combat. Has anyone been so flustered and out
of synch to get monstrously high in an off field landing, then had the
presence of mind and skill left to dive to the ground at near VNE,
aiming several hundreds of yards short of the intended small paddock
with fence at the far end, and had it work? The mental attitude that
gets to the problem seems incompatible with the attitude needed to
pull this one off. If you don't aim short enough in the dive, you just
crash into the far fence at really high speed. But I'd be curious to
hear a "it worked for me" story.

John Cochrane


I have tried it with my big wing glider and for me the 'high parasite drag'
approach doesn't work unless you shift to a airspeed stabilized approach no
lower than 100 feet AGL.

My reasoning is that the ground effect starts at about a wingspan above the
ground so the bigger the wing the higher it starts. Just above the runway,
ground effect roughly doubles the L/D, (i.e. ~7:1 with full spoilers becomes
14:1) so in ground effect is a bad place to try to scrub off energy.

The key to the Duo spoilers seems to be a stabilized approach. It's a
really slippery glider and it's easy to let the airspeed creep up once your
eyes are on the aim point. In low wind/low turbulence conditions, nailing
the airspeed right on the yellow triangle while holding the glideslope to
the aim point can result in a fairly short landing. You can fly much higher
airspeed in the patern and on long final as long as the airspeed is reduced
to the calculated reference airspeed on short final.

Bill Daniels

BB wrote:
As fun as the high parasitic drag maneuver is, I wonder if anyone has
ever actually used it in combat. Has anyone been so flustered and out
of synch to get monstrously high in an off field landing, then had the
presence of mind and skill left to dive to the ground at near VNE,
aiming several hundreds of yards short of the intended small paddock
with fence at the far end, and had it work? The mental attitude that
gets to the problem seems incompatible with the attitude needed to
pull this one off. If you don't aim short enough in the dive, you just
crash into the far fence at really high speed. But I'd be curious to
hear a "it worked for me" story.

In reality, I almost always have too little altitude rather than too
much when I'm trying to sneek into a field. I did use this sort of
technique with my Ventus a couple of times to get over trees and into
short fields, but it had trailing edge dive brakes that would allow me
to hold 60 to 65 knots in a dive, then round out, slow down, and plop it
in. There is no way I could make that work in a Duo, or most other
gliders...

Marc

BB wrote:

As fun as the high parasitic drag maneuver is, I wonder if anyone has
ever actually used it in combat. Has anyone been so flustered and out
of synch to get monstrously high in an off field landing, then had the
presence of mind and skill left to dive to the ground at near VNE,
aiming several hundreds of yards short of the intended small paddock
with fence at the far end, and had it work? The mental attitude that
gets to the problem seems incompatible with the attitude needed to
pull this one off.


In "combat"? In a Duo?

There are a number of real combat stories about people keeping the speed
high in damaged aircraft to get down out of the fight, through clouds,
etc. to find a safer area for a forced landing or bailout--so as not to
be hosed while under canopy, or to avoid setting up a big slow pattern
that would make them a sitting duck--but I think that diverges from what
you are asking.

I don't think fighter pilots get a chance to practice engine out
landings much, so yes, they adapt. The best adaptation is that for which
the mental groundwork is laid _on_ the ground.

Sailplane pilots have the incomparable advantage of doing their
preparation on the ground _and_ in the air by planning and practicing
both low- and high-energy approaches, as has been outlined here in
previous postings by several contributors.


Jack

On Oct 23, 6:29 pm, Marc Ramsey wrote:
BB wrote:
[snip]
That's how this whole discussion got started. Someone suggested that
the best thing to do when high on final is to dive with full spoilers,
pull up above ground effect and wait for the speed to bleed off. I said
that won't work too well with a Duo, as with full spoilers it isn't all
that draggy, will accelerate relatively quickly, and bleed off speed
slowly. Others said nonsense, the Duo has wonderful spoilers. And so
on, and so on...

Marc

Then why don't you slip it in? The Duo slips quite well.

Darryl


(Sorry Marc I could not resist :-) )