Polar with spoilers extended?

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

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 :-) )

That was probably an overstatement; I meant as compared
to the HPDA. I have not flown a 19 or 28. I fly a
Discus B and a G-103 regularly, along with Blanik L-23s
for instructing. The Blaniks are the worst, and not
glass, of course. A fully locked rudder slip won't
allow much opposite aileron before it starts to turn
off course. Of course, that is not glass, (and I almost
made an anadiplosis there.)

At 20:06 23 October 2007, Andy wrote:
On Oct 22, 9:12 am, Nyal Williams
Forward slip in glass gliders won't get you much descent;

The ASW-19 and ASW-28 have a huge increase in sink
rate in a full
rudder slip compared with airbrakes alone. What glass
gliders are
you flying that do not slip well? Can you maintain
a full rudder
slip, and I mean rudder on the stop and never comes
off it until you
choose to exit the slip?


Andy

On Oct 23, 3:12 pm, Nyal Williams
A fully locked rudder slip won't allow much opposite aileron before it starts to turn off course.

Was that comment related to a specific type or was it a general
comment on characteristics of all gliders?

Specific to the ASW-19B (that I owned and with the CG I flew it at) -
I could sustain a stable slip with full airbrakes, gear down, with
the rudder on the stop and the stick hard in the opposite corner. I
practiced this at the start of every season and it saved a couple of
very tight off airport landings for me.

Andy

Back to Tim's original question ---- it appears that the effectiveness of
the high parasite drag (HPD) approach varies depending on the glider. Even
if we don't have the absolute speed polar of a given glider with the
spoilers out, we should be able to figure out a relative measurement that
gives us some idea of a "good" HPD glider vs a "not so good" HPD glider.
It's been about 26 years since my last aero course, but I think I remember
that the term we care about here is wetted area; however you smarter people
please correct me as needed. The measure I propose would be:

Wetted area clean / wetted area with full spoilers, then multiply by the
wing loading. The bigger the number - the less effective the HPD maneuver
in that glider. Maybe we don't even need the wing loading - I'm not
certain, but it would seem reasonable to me that the HPD maneuver would be
progressively less effective with higher wing loadings.

Alright --- ready to be shot full of holes.

Lou McDonald
"LM"





"Tim Taylor" wrote in message
ps.com...
I am working though some calculations and need the sink rate as a
function of speed with the spoilers fully extended. Does anyone know
of such data for a glider? How do spoilers extended affect sink rate
as a function of speed?

Thanks,

Tim

On Oct 21, 12:23 am, Tim Taylor wrote:
I am working though some calculations and need the sink rate as a
function of speed with the spoilers fully extended. Does anyone know
of such data for a glider? How do spoilers extended affect sink rate
as a function of speed?

I have one data point for the ASW 19B. Gear down, full airbrake, IAS
90kts, sink 3,300fpm. Flight data 4/20/91 EW barograph.


Andy

Author: John Smith Date/Time: 17:40 24 October 2007=
First I thought that you were pulling our legs, but it seems you're actuall=
y serious.Every year a couple of pilots die because they are too slow on ap=
proach. Where I fly, a student will fail his checkride big time if he's onl=
y one knot below the yellow triangle on final.=20
=20
-
=20
Whilst I understand your sentiment, I think this is over the top. The yello=
w triangle is best approach speed at all up weight.
For a glider that takes water ballast, this may well be 2-3 knots higher th=
an the best approach speed at dry weight. ie one knot slower than yellow tr=
iangle may be higher than 1.3x stall speed for particular set-up.
=20
I have found out the hard way about too slow approaches. My SLMG best engin=
e speed is 49 knots which is lower than the approach speed of 55 knots. Whe=
n the engine does not start at low level, with wet wings and an uphill land=
ing, you find out that the extra speed is required to enable a flare, and o=
nly incidentally to prevent a stall/spin.
=20
Rory
__________________________________________________ _______________
Celeb spotting =96 Play CelebMashup and win cool prizes
https://www.celebmashup.com=

Rory O'Conor wrote:

Whilst I understand your sentiment, I think this is over the top. The yello=
w triangle is best approach speed at all up weight.
For a glider that takes water ballast, this may well be 2-3 knots higher th=
an the best approach speed at dry weight. ie one knot slower than yellow tr=
iangle may be higher than 1.3x stall speed for particular set-up.

The point is not the triangle. The point is that the student has to call
out a target speed and then hold that speed. 5 knots more is tolerated,
but 1 knot below is not. Simply not. No way. Period.

Now if the student calls out a target speed lower than the yellow
triangle, the expert will ask him why. If the student can explain, then
it may be ok. On the other hand, if there is a headwind, the student
must compensate for this. Rule of thumb is add 1/2 wind speed. (And
certainly not slower than without wind, as some other hero suggested.)
Again, the expert will ask what wind speed the student estimates and why.

As for water ballast, well, I've yet to see a student who flyes with
ballast on his checkride.

On Oct 22, 7:57 am, wrote: On Oct 21, 11:13 am, T=
im Taylor wrote:[snip] If you are THAT much too high, wouldn't it also be =
prudent to consider a large 360? It may not be pretty, but let's face it, =
if you have
I would join the crowd voting a 360 turn on final, a score of 0/10.
=20
If you are cool about scratching at 400ft AGL then fine, but if you like to=
terminate your thermalling by 800ft AGL then dont consider it. The workloa=
d when turning low down is enormous and you need to be prepared to push the=
nose groundwards against your instincts if the speed shows any sign of ble=
eding off.
=20
Dont do full turns below the height you would be prepared to do them when s=
cratching.
=20
Rory
__________________________________________________ _______________
Celeb spotting =96 Play CelebMashup and win cool prizes
https://www.celebmashup.com=