Nimbus 4 Accident

Bert Willing wrote:

A Nimbus 4 has airbrakes.
And as for different animals: A glider certified under JAR22 needs to have
means which limit a dive to 45 deg at vne.

Just nitpicking: JAR22 asks for a dive angle of 30 degrees. 45 degrees
are only needed to be certificated for cloud flying or aerobatics.

Stefan

So it seems to be right that Scotch affects the memory...

--
Bert Willing

ASW20 "TW"


"Stefan" a écrit dans le message de news:
...
Bert Willing wrote:

A Nimbus 4 has airbrakes.
And as for different animals: A glider certified under JAR22 needs to
have means which limit a dive to 45 deg at vne.

Just nitpicking: JAR22 asks for a dive angle of 30 degrees. 45 degrees are
only needed to be certificated for cloud flying or aerobatics.

Stefan

At 21:48 10 July 2005, Denis posted the message ' Nimbus 4 Accident'
I don't understand your choice ! if there is a risk at high speeds, the
best choice to avoid it is to avoid these speeds, and that's what the
airbrakes are for, aren't it ? Pulling g's after loosing control is the
best way to break any aircraft...
------------------------------------------
Denis
The point that I was trying to make was that exceeding the positive flap
limiting speed and then opening the airbrakes is also likely to cause damage
to the wing structure. Gliders are certified to withstand +5.3g with the
brakes shut, but only +3.5g with them open.

Derek Copeland



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Derek,

you're mixing up things. Speed limits for positive flap settings have
nothing to do with g-load; they are to limit the forces on the flap.

With a constant speed, a wing produces always the same lift - disregarding
speed or deployed/undeployed airbrakes. If you pull the airbreaks at any
given speed, the load factor of the wing will momentarily be less than 1 g
(you just destroyed a part of the lift and are vertically accelerating), and
then return to 1 g, but now with a different load distribution accross the
wing. Pulling the airbrakes alone will _never_ get you more than 1 g load
factor.

Only when you start to pull up, you have to think about max load factor with
airbrakes deployed.

--
Bert Willing

ASW20 "TW"


"Derek Copeland" a écrit dans le
message de news: ...
At 21:48 10 July 2005, Denis posted the message ' Nimbus 4 Accident'
I don't understand your choice ! if there is a risk at high speeds, the
best choice to avoid it is to avoid these speeds, and that's what the
airbrakes are for, aren't it ? Pulling g's after loosing control is the
best way to break any aircraft...
------------------------------------------
Denis
The point that I was trying to make was that exceeding the positive flap
limiting speed and then opening the airbrakes is also likely to cause
damage
to the wing structure. Gliders are certified to withstand +5.3g with the
brakes shut, but only +3.5g with them open.

Derek Copeland



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I think what Bert says is technically correct (wing load doesn't change
when you open the air brakes) - but the distribution does change a lot -
especially on a ship like the N3 & N4 where the brakes are located inboard
on the inner panels. Stated differently, when the brakes are opened the
outer panels are being asked to do more work supporting the fuselage (and
non flying portions or the inner panels) than before the dive brakes were
opened. The Nimbus 3 and 4 are placarded against carrying water ballast
in the inner panel tanks with the outer panel tanks empty for structural
reasons. You also must dump the inner tanks first. The same structural
problem occurs when the dive brakes are open and that part of the inner
panel becomes "dead weight". So - while the brakes should be used to
prevent the glider getting to extreme speeds - we need to be cautious about
suggesting that nothing bad is going to happen if you open them at extreme
speeds.

Roy B. (Nimbus 3 # 65)

I'll try again.

Positive flap limiting speeds have two justifications:
1) To prevent damage to the flaps themselves or more likely to their hinges.
2) To stop the wings producing more lift than is good for them at high
speeds, remembering that lift increases with the square of the speed for a
given angle of attack, and that flaps increase the camber and therefore lift
produced by the wings.

Airbrakes (when open) reduce the amount of lift produced over the portion of
the wing they occupy and increase the drag. The rest of the wing (mostly
outboard of the brakes) has to produce more lift to support the weight of
the glider and hence the bending moment on the wingspar is increased. The
fact that some stressing is taking place in the region of the airbrakes is
often witnessed by the gel coat cracks that tend to appear in this area.
Please also re-read my account of my test flight to check the double paddle
airbrake mod!

There have been two N4D break-up accidents where the common factor is that
control has been lost in a thermal, either a spin followed by a spiral dive,
or just a straight spiral dive. It is likely that in both cases that
thermalling (ie positive) flap was selected and speed and g. increased very
rapidly. In at least one case the pilot tried opening the airbrakes, but the
glider still broke up.

We had a fatality in the UK involving an ASW20 that dived vertically at very
high speed into the ground after a similar thermalling upset. The flaps also
to some extent act as elevators and it was thought that the pilot was unable
to overcome the nose down pitch tendency by pulling back on the stick.

My main original point was that the first action in any sort of loss of
control situation in a flapped glider must be to select neutral or negative
flap. If you have to open the brakes, do so before Vne is reached.

Derek Copeland
--------------------------------------------------------------------------
----

Bert Willing wrote:
you're mixing up things. Speed limits for positive flap settings have
nothing to do with g-load; they are to limit the forces on the flap.




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--

At 13:30 12 July 2005, Derek Copeland wrote:

We had a fatality in the UK involving an ASW20 that
dived vertically at very
high speed into the ground after a similar thermalling
upset. The flaps also
to some extent act as elevators and it was thought
that the pilot was unable
to overcome the nose down pitch tendency by pulling
back on the stick.

Not sure that is exactly right, the flaps tend to cause
the fuselage and tailplane to 'pitch down'. Ergo the
higher the ias the greater the angle of attack of the
tailplane and elevator. (The AoA of the tailplance
increases as the AoA of the wing reduces as the tailplane
is trying to produce lift 'down' rather than up relative
to the pilot). Pulling back on the stick increases
the AoA of the tailplane which could become stalled
and this would certainly cause it to be ineffective
as you describe.

My main original point was that the first action in
any sort of loss of
control situation in a flapped glider must be to select
neutral or negative
flap. If you have to open the brakes, do so before
Vne is reached

Absolutely correct, carve that in stone.

Derek Copeland
------------------------------------------------------------------
--------
----

Bert Willing wrote:
you're mixing up things. Speed limits for positive
flap settings have
nothing to do with g-load; they are to limit the forces
on the flap.

You're point number two doesn't hold: At constant speed (whichever speed), a
wing of a sailplane will never produce more lift than corresponds to the
weight of the glider. Otherwise you would be climbing.

Your main original point is absolutely right.

--
Bert Willing

ASW20 "TW"


"Derek Copeland" a écrit dans le
message de news: ...
I'll try again.

Positive flap limiting speeds have two justifications:
1) To prevent damage to the flaps themselves or more likely to their
hinges.
2) To stop the wings producing more lift than is good for them at high
speeds, remembering that lift increases with the square of the speed for a
given angle of attack, and that flaps increase the camber and therefore
lift
produced by the wings.

Airbrakes (when open) reduce the amount of lift produced over the portion
of
the wing they occupy and increase the drag. The rest of the wing (mostly
outboard of the brakes) has to produce more lift to support the weight of
the glider and hence the bending moment on the wingspar is increased. The
fact that some stressing is taking place in the region of the airbrakes is
often witnessed by the gel coat cracks that tend to appear in this area.
Please also re-read my account of my test flight to check the double
paddle
airbrake mod!

There have been two N4D break-up accidents where the common factor is that
control has been lost in a thermal, either a spin followed by a spiral
dive,
or just a straight spiral dive. It is likely that in both cases that
thermalling (ie positive) flap was selected and speed and g. increased
very
rapidly. In at least one case the pilot tried opening the airbrakes, but
the
glider still broke up.

We had a fatality in the UK involving an ASW20 that dived vertically at
very
high speed into the ground after a similar thermalling upset. The flaps
also
to some extent act as elevators and it was thought that the pilot was
unable
to overcome the nose down pitch tendency by pulling back on the stick.

My main original point was that the first action in any sort of loss of
control situation in a flapped glider must be to select neutral or
negative
flap. If you have to open the brakes, do so before Vne is reached.

Derek Copeland
--------------------------------------------------------------------------
----

Bert Willing wrote:
you're mixing up things. Speed limits for positive flap settings have
nothing to do with g-load; they are to limit the forces on the flap.




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--

Bert Willing wrote on 12th July.
You're point number two doesn't hold: At constant speed (whichever speed), a

wing of a sailplane will never produce more lift than corresponds to the
weight of the glider. Otherwise you would be climbing.

Your main original point is absolutely right.
--------------------------------------------------------------

Er, I'm only a humble gliding instructor, so what do I know about things?

You are also right in that in steady flight lift must equal the weight of
the glider. However in a spiral dive you are in accelerated flight and the
glider could effectively weigh several times its own weight, and the wings
(if not stalled) have to produce the equivalent extra amount of lift to
balance this. If you are pulling more than 3.5 g at high speeds, opening the
airbrakes could just be enough to finish things off, due to the extra
bending load on the wings this entails. The correct recovery from a spiral
dive is just to carefully reduce the angle of bank while keeping the stick
fairly well back, by the way.

As an instructor I do lots of spins and spiral dives, so can easily
recognise what is going on. Many good cross-country pilots haven't done
either for years, so could be caught out should either occur unexpectedly .
The recovery actions are quite different. I also understand that Nimbus 4s
have a non-standard spin recovery procedure, which further complicates the
issue.

Derek Copeland


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--

Ok - you've got a point :-)

--
Bert Willing

ASW20 "TW"


"Derek Copeland" a écrit dans le
message de news: ...
Bert Willing wrote on 12th July.
You're point number two doesn't hold: At constant speed (whichever speed),
a

wing of a sailplane will never produce more lift than corresponds to the
weight of the glider. Otherwise you would be climbing.

Your main original point is absolutely right.
--------------------------------------------------------------

Er, I'm only a humble gliding instructor, so what do I know about things?

You are also right in that in steady flight lift must equal the weight of
the glider. However in a spiral dive you are in accelerated flight and the
glider could effectively weigh several times its own weight, and the wings
(if not stalled) have to produce the equivalent extra amount of lift to
balance this. If you are pulling more than 3.5 g at high speeds, opening
the
airbrakes could just be enough to finish things off, due to the extra
bending load on the wings this entails. The correct recovery from a
spiral
dive is just to carefully reduce the angle of bank while keeping the stick
fairly well back, by the way.

As an instructor I do lots of spins and spiral dives, so can easily
recognise what is going on. Many good cross-country pilots haven't done
either for years, so could be caught out should either occur unexpectedly
.
The recovery actions are quite different. I also understand that Nimbus 4s
have a non-standard spin recovery procedure, which further complicates the
issue.

Derek Copeland


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Any opinions expressed in this e-mail are those of the individual and not
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from ADT Fire and Security are confidential and solely for the use of the
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If you are not the intended recipient or the person responsible for
delivery
to the intended recipient, be advised that you have received this e-mail
in
error and that any use is strictly prohibited. In this event, please
notify
us via e-mail at ' or telephone on 0121 255 6499 and
then
delete the e-mail and any copies of it. WebSite:
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--