That's correct. If you kill the lift over part of the
wing by opening the spoilers, the rest of the wing
needs to produce a higher coefficient of lift to keep
the glider from accelerating downward. This can only
happen through an increase in angle of attack. This
will cause the wing to flex more because the center
of lift has moved outboard (imagine lifting your glider
on the ground by the wingtips versus the wing roots
to appreciate the point).

If you don't increase the angle of attack to compensate
then liftweight and the glider will accelerate downward
(accelerating sink rate). In this case the outer panels
are producing no more lift (same aoa, same airspeed)
and the inner portion of the wing is spoiled so net-net
there is less lift, less bending moment and less flex
in the wings. However, this is a transient state since
we all naturally compensate for the loss of lifting
surface by pulling back ever so slightly on the stick
and loading up the tips - hence the extra flex.

It's hard to notice the aoa shift in flight because
you need a lower pitch attitude to maintain airspeed
(due to higher drag) at the same time that aoa goes
up to produce enough lift to hold the glider up. Remember
pitch attitude and aoa are not the same.

Try going from dive brakes closed and locked to fully
open on landing rollout - AFTER you're sure you have
the tailwheel down. In this case aoa should be held
pretty constant. See what the wings do then.

9B


At 07:54 30 March 2004, Bert Willing wrote:
You're both partially right. The moment you deploy
the airbrakes and
maintain a constant angle of attack, you loose lift
which means the glider
accelerates it's sink rate. G load decreases, bending
decreases. Once the
sink rate is stabilized, the initial lift must have
been restored but it's
different distribution along the wing span increases
the bending.

BTW, Denis is fairly well know in France (an Morocco...)

--
Bert Willing

ASW20 'TW'


'W.J. (Bill) Dean (U.K.).' a écrit dans le message
de ...
You are just plain wrong. The immediate effect of
opening Schempp-Hirth
type airbrakes, if nothing else is done, is to make
the wings bend more.

Have you ever tried opening the airbrakes with a bendy
wing and watched
what
actually happens?

When I flew an ASW20L I always used some landing flap
when approaching to
land. Sometimes I would keep the brakes closed until
near the ground.
I
always opened them as I rounded out, because as soon
as I did this the
wings
bent up, to give me better ground clearance!

I remember flying a Skylark 3 at about 75 knots (fast
for the type), at
this
speed the tips bent down a bit, because of the washout.
If I then opened
the airbrakes, the wings bent up.

Your theory is wrong, it does not work! Don't try
to argue that I did
not
see what I know I did see, get in something with bendy
wings such as a
Pegasus, and try it.

I also remember seeing an article in 'Technical Soaring'
with a photo of a
Jantar 1 at Vne, and at 1 g., with the brakes fully
out. The wing bend,
at
1 g. remember, was horrendous. Don't try and give
us some theoretical
reason why this cannot happen, it does!

You also say:
'all I want is to give my opinion when I think something
is said here
that
may lead to dangerous flying - such as sentences like
'don't exceed VNE,
but
no problem if you exceed permitted G-loading' '.
Who said that, which posting?

This whole discussion has been around the point, if
you look as if you are
going to exceed Vne, what should you do?

Exceeding Vne is outside limits and dangerous, so
are any of the
alternatives - the discussion is about which of the
alternatives is the
least worst.

With the Minden accident on 13th July 1999, it is
clear from the report
that
the glider was pitched down to well beyond a 45 degree
dive, so the
airbrakes would not have been speed limiting.

You say 'I never experienced a spin recovery', presumably
you mean in a
large span glider. I hope you have done plenty in
training and short
span
machines. An essential part of stall/spin recovery
training is to be
able
to distinguish at once the difference between a spin
and a spiral dive.
If
you treat a spiral dive as if it is still a spin,
this is very likely to
lead to excessive speed, as well as using more height
for the recovery.

I still think that the advice I gave in my first posting
to this thread is
correct:
'If you exceed Vne you are taking a risk, if you pull
too hard above
manoeuvring speed you are taking a risk, and if you
pull hard and roll at
the same time you are taking a risk. If you pull
the brakes you are
increasing the bending load on the wings.

'If you get it wrong and have to take one of the risks,
I am told that you
should centralise the ailerons, then pull however
hard is necessary not to
exceed Vne, and make sure the brakes stay shut.'

Denis (Denis who and from where?), if you still feel
like answering,
please
answer what I have actually written.

W.J. (Bill) Dean (U.K.).
Remove 'ic' to reply.


'Denis' wrote in message
...


W.J. (Bill) Dean (U.K.). wrote:

There were postings to Rec. Aviation Soaring when
the report was
published, from pilots with experience of the Nimbus
4 and similar
models who had experience of inadvertent deployment
of the airbrakes.
If the brakes deployed inadvertently while the pilots
were recovering
from the dive, this surely may have been the reason
for the amount of
bending seen; and for the overload which led to
failure. Presumably
those investigating the accident were not aware
of these incidents
when
writing the report.


If airbrakes deploy inadvertently, the first effect
(along with the very
high drag) will be a *decrease* in G-loading *and*
bending moment), both
due to the loss of lift near the airbrakes. The increase
of bending
would happen only after the angle of attack has been
further increased
(voluntarily or not) to restore the initial G-loading
with more lift on
the outer panels (instead of the airbrakes section),
hence the higher
bending.


Denis, you are very scathing.


That is not my intention... all I want is to give
my opinion when I
think something is said here that may lead to dangerous
flying - such as
sentences like 'don't exceed VNE, but no problem
if you exceed permitted
G-loading'.


What do you think went wrong?

What would you have done?

Do you have any experience in the Nimbus 3 & 4 series?
I don't.

Are you more experienced or better than the pilots
who did not make
it?


I don't know them and I would not pretend to be better
(there are no
good pilots, only old pilots...). And although I
have some experience in
Nimbus 4D (more on ASH 25) I never experienced a
spin recovery and I
hope I never will have to. Therefore I don't know
what I would do in
such a situation. All I can say is what I think (sitting
comfortably in
my chair) is the better thing to do, as I said in
a previous post :

'If your speed is going to exceed VNE within this
manoeuvre [pulling
up], you should stop or reduce pulling and apply
full airbrakes. At any
dive angle up to 45° this prevents the glider to
exceeding VNE, and you
have time to recover pulling gently (under 2 g's).
This of course
supposes that there is sufficient ground clearance...
'

Denis