Discus polar curve at high speeds

The polars of most gliders will be similar after the wings depart the
fuselage.

Andy

Hello Everybody, and many thanks for your most useful answers!

Let me tell you the strange story behind my strange request:

The aviation accident investigation establishment of an "old" european
country is preparing to present the result of their investigation
regarding a glider accident with a sailplane having Discus-like
performance (the pilot is a good friend of mine, and got away
unscathed).

The investigators of this establishment have observed that some metal
parts of the air brakes were bent in the wreck. Bench testing of these
parts indicate that an abrubt extension of the airbrakes at 460 km/h
(250 kts) or above can indeed cause the observed bending. For your
information, the Vne of the concerned glider is less than 270 km/h.
Surprisingly enough, this accident investigation establishment is now
using this fact as a proof that the pilot did indeed fully extend the
airbrakes at 460 km/h (250 kts).

I am now trying to help my friend present another proof, which is to
show that a speed of 460 km/h simply was unreachable the given day,
considering weak thermals and fairly low cloud base. However, to
prepare this proof I need some high speed polar data for the Discus,
which has a performance similar to the given glider type.

Best regards

Karl

PS: I would be very interested in having your opinon about the
probability a standard class glider has to survive an abrubt airbrake
extension at 460 km/h (250 kts).

wrote:

The investigators of this establishment have observed that some metal
parts of the air brakes were bent in the wreck. Bench testing of
these parts indicate that an abrubt extension of the airbrakes at 460
km/h (250 kts) or above can indeed cause the observed bending. For
your information, the Vne of the concerned glider is less than 270
km/h. Surprisingly enough, this accident investigation establishment
is now using this fact as a proof that the pilot did indeed fully
extend the airbrakes at 460 km/h (250 kts).

Have investigators given an opinion about how the pilot was able to
achieve 460 km/h and still had any wings attached before opening the
spoilers? And why he would wait until then to open the spoilers?

I am now trying to help my friend present another proof, which is to
show that a speed of 460 km/h simply was unreachable the given day,
considering weak thermals and fairly low cloud base. However, to
prepare this proof I need some high speed polar data for the Discus,
which has a performance similar to the given glider type.

Best regards

Karl

PS: I would be very interested in having your opinon about the
probability a standard class glider has to survive an abrubt airbrake
extension at 460 km/h (250 kts).

I think most of us are still trying to imagine how a standard class
glider can even get to 460 km/h and be in only one piece.


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

I'm taking a wild guess here, a hypothesis only...

If you exceed VNE can you divide the failure causes into two families?
Structural and Flutter?

Ok, discount Flutter for the moment and consider only a structural failure.

If you are in moderate dive, the wings still have to support the weight
of the glider and therefore you have higher drag because the wing is at
some positive angle of attack.
However, if you are in a near zero G pushover, the wings are unloaded
vertically and therefore you are at a minimum drag condition from an AoA
standpoint. Then, if there is no flutter, you might get well beyond VNE
before something goes bang..

pondering out loud...



Eric Greenwell wrote:
wrote:

The investigators of this establishment have observed that some metal
parts of the air brakes were bent in the wreck. Bench testing of
these parts indicate that an abrubt extension of the airbrakes at 460
km/h (250 kts) or above can indeed cause the observed bending. For
your information, the Vne of the concerned glider is less than 270
km/h. Surprisingly enough, this accident investigation establishment
is now using this fact as a proof that the pilot did indeed fully
extend the airbrakes at 460 km/h (250 kts).


Have investigators given an opinion about how the pilot was able to
achieve 460 km/h and still had any wings attached before opening the
spoilers? And why he would wait until then to open the spoilers?


I am now trying to help my friend present another proof, which is to
show that a speed of 460 km/h simply was unreachable the given day,
considering weak thermals and fairly low cloud base. However, to
prepare this proof I need some high speed polar data for the Discus,
which has a performance similar to the given glider type.

Best regards

Karl

PS: I would be very interested in having your opinon about the
probability a standard class glider has to survive an abrubt airbrake
extension at 460 km/h (250 kts).


I think most of us are still trying to imagine how a standard class
glider can even get to 460 km/h and be in only one piece.

If "something goes bang" it is either because of a) flutter or b)
extreme drag forces when extending airbrakes at extreme speeds.

Which are the known incidents of gliders fluttering to bits?
(apparently Dirks parachuted from the DG-600 prototype after wing loss,
also almost loosing his eyeballs in the process...)

Karl

Gary Emerson wrote:

I'm taking a wild guess here, a hypothesis only...

If you exceed VNE can you divide the failure causes into two families?
Structural and Flutter?

Ok, discount Flutter for the moment and consider only a structural failure.

If you are in moderate dive, the wings still have to support the weight
of the glider and therefore you have higher drag because the wing is at
some positive angle of attack.
However, if you are in a near zero G pushover, the wings are unloaded
vertically and therefore you are at a minimum drag condition from an AoA
standpoint. Then, if there is no flutter, you might get well beyond VNE
before something goes bang..

To go from level flight at 125 knots to 250 knots would require losing
over 2000 feet in this zero lift flight, much of which would vertical. I
have to wonder how a pilot could resist pulling back on the stick or
opening the spoilers for such a long distance! And how carefully he'd
have to fly to avoid G forces that would remove the wings; after all,
this maneuver isn't in the usual glider training syllabus.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

In article , Eric Greenwell wrote:
Gary Emerson wrote:

To go from level flight at 125 knots to 250 knots would require losing
over 2000 feet in this zero lift flight, much of which would vertical. I
have to wonder how a pilot could resist pulling back on the stick or
opening the spoilers for such a long distance! And how carefully he'd
have to fly to avoid G forces that would remove the wings; after all,
this maneuver isn't in the usual glider training syllabus.

You might also want to consider the twist in the wings.
Ever looked along your wing at high speed and noticed the tips bending down?

At really high speed you can twist your wings off. A blanik did that in
NZ a few years ago. One wing twisted off leading edge up, the other went
leading edge down.

--
Philip Plane _____
|
---------------( )---------------
Glider pilots have no visible means of support