Nimbus 4 Accident

Er, I'm only a humble Nimbus 4 pilot (not an instructor),
so what do I know about things?

'I also understand that Nimbus 4s have a non-standard
spin recovery procedure, which further complicates
the issue. '

Derek, my Nimbus 4 flight manual says:

'a) Apply opposite rudder.

b) Hold ailerons neutral.

c) Ease control sick forward until roation ceases and
the airflow is restored.

d) Centralise rudder and pull gently out of the dive.

Recovery speeds are between 70 - 113 knots - depending
on the flap setting. If necessary, flaps must therefore
be reset at '0' or '-1' so as to avoid exceeding their
speed limits.'

Sounds to me very similar to what Mr Piggott taught
me all those years ago?


At 17:54 12 July 2005, Derek Copeland wrote:
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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--

Just for general clarification, Section 4.5.6 of my
Nimbus 4T flight manual says:

“Note:

In order to achieve a high maneuverability, a favourable
c/g position (when using the fin tank) and a maximum
in ground clearance of the wing tips on take-off and
landing, it is always recommended to fill the inboard
water tanks first.”

I know it is the opposite for the Nimbus 3, I assume
the wings are just that much stronger on the 4. In
my experience it does make take off less fraught and
handling a bit nicer on the 4 than the 3.

Mike


At 18:30 11 July 2005, Roy Bourgeois wrote:
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)

Michael Clarke wrote:

Derek, my Nimbus 4 flight manual says:

a) Apply opposite rudder.
b) Hold ailerons neutral.
c) Ease control sick forward until roation ceases and
the airflow is restored.
d) Centralise rudder and pull gently out of the dive.

No surprize: JAR22 explicitely demands "standard spin recovery". And it
must demonstrated from a spin of "at least 5 turns" and with "the most
unfavorable configuration". (Cited from memory, so the exact wording may
be different.)

Stefan

At 20:18 13 July 2005, T O D D P A T T I S T wrote:
Don Johnstone
wrote:

If you have to open the brakes, do so before Vne is
reached

Absolutely correct, carve that in stone.

The implication here is that if you find yourself extremely
nose down, but at an initially low speed, opening the
brakes
is a desirable action. I disagree.

Opening the brakes might be advisable if brakes were
able to
produce large amounts of drag - sufficient to limit
you to
speeds below Vne - but that is generally not true.
So, your
only other option is to get the nose back up to stop
the
acceleration. The only way to do that is to apply
the
maximum force possible in a direction perpendicular
to the
downwardly angled path of the aircraft to curve it
back
towards level.

A force perpendicular to the path of the aircraft is
called
'lift' and by opening the brakes you prevent much of
the
wing from producing the lift you desperately need to
bring
you back to level flight. This delays the critical
recovery. Up until you reach Va, you can operate the
wings
at maximum lift coefficient and produce maximum lift
without
risk of structural damage, and that's exactly what
you want
to do to get the nose back up.

In addition, by opening the brakes, you seriously increase
the risk that the pilot will overstress the aircraft
at
higher speeds. With brakes open, the max G load for
many
gliders is so low that the pilot simply does not think
he's
about to break anything. His built in warning system
does
not begin to go off until much higher G loads are felt.

Finally, if you open the brakes, you increase the altitude
loss significantly, a potentially critical factor in
a low
altitude recovery.

The proposal to use a tail chute does not suffer from
these
problems, as a tail chute does not decrease lift or
max G
limits. It also has the advantage of allowing recovery
from
otherwise unrecoverable spin modes.

I accept what you say my original reponse was to the
whole paragraph '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'

I standby that. It is important to select a non positive
flap setting and if the brakes are going to be used
it should be before VNE is reached.

T o d d P a t t i s t a écrit :

The implication here is that if you find yourself extremely
nose down, but at an initially low speed, opening the brakes
is a desirable action. I disagree.

A force perpendicular to the path of the aircraft is called
"lift" and by opening the brakes you prevent much of the
wing from producing the lift you desperately need to bring
you back to level flight. This delays the critical
recovery.

No. The radius of curvature of your recovery trajectory is proportionnal
to lift, but inversly proportionnal to the square of speed. Thus, even
with less lift, the radius may be smaller with airbrakes out.


Finally, if you open the brakes, you increase the altitude
loss significantly, a potentially critical factor in a low
altitude recovery.

No. Smaller radius means smaller altitude loss (though smaller speed
means less altitude gain after the low point, but this is not likely to
be a problem)


--
Denis

R. Parce que ça rompt le cours normal de la conversation !!!
Q. Pourquoi ne faut-il pas répondre au-dessus de la question ?

On Mon, 15 Aug 2005 10:15:22 -0400, T o d d P a t t i s t wrote:

are trying to decide what to do at the point when brakes are
closed and speed is fixed at some speed V1. If pilot 1
leaves the brakes closed and pulls to max G for brakes
closed, and pilot 2 opens the brakes and pulls to max G for
brakes open, pilot 1 will have a smaller radius of
curvature, since both are at speed V1.

If you are in a situation (perhaps recovering from a spin) with your nose
pointing 60 deg down below the horizontal and the ASI reading 180 km/h and
accelerating rapidly. You could try:

- Pull full airbrakes, and raise the nose by 15 degrees keeping the G
force below 2G. In this new attitude (45 degrees nose down) many gliders
airbrakes will be speed limiting below VNE. The acceleration will stop and
you can pull out of the dive being careful to limit the G force to 2G.

- Or, you could leave the dive brakes closed, and attempt to raise the
nose by 55 degrees to 5 degrees below the horizon without pulling
more than 4G.

I have not worked out the maths of the two options but I think that in
a clean glass ship, you would have a better chanced completing the first
manoeuvre without exceeding VNE than the second.

Ian

In article ,
Ian wrote:

On Mon, 15 Aug 2005 10:15:22 -0400, T o d d P a t t i s t wrote:

are trying to decide what to do at the point when brakes are
closed and speed is fixed at some speed V1. If pilot 1
leaves the brakes closed and pulls to max G for brakes
closed, and pilot 2 opens the brakes and pulls to max G for
brakes open, pilot 1 will have a smaller radius of
curvature, since both are at speed V1.

If you are in a situation (perhaps recovering from a spin) with your nose
pointing 60 deg down below the horizontal and the ASI reading 180 km/h and
accelerating rapidly. You could try:

- Pull full airbrakes, and raise the nose by 15 degrees keeping the G
force below 2G. In this new attitude (45 degrees nose down) many gliders
airbrakes will be speed limiting below VNE. The acceleration will stop and
you can pull out of the dive being careful to limit the G force to 2G.

- Or, you could leave the dive brakes closed, and attempt to raise the
nose by 55 degrees to 5 degrees below the horizon without pulling
more than 4G.

I have not worked out the maths of the two options but I think that in
a clean glass ship, you would have a better chanced completing the first
manoeuvre without exceeding VNE than the second.

Another option is to raise the nose 15 degrees with the airbrakes
closed, keeping below 4G, and *then* pull the airbrakes.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

Ian wrote:
If you are in a situation (perhaps recovering from a spin) with your nose
pointing 60 deg down below the horizontal and the ASI reading 180 km/h and
accelerating rapidly. You could try:

[ leave A/B closed and pull 4G ...]

I have not worked out the maths of the two options but I think that in
a clean glass ship, you would have a better chanced completing the first
manoeuvre without exceeding VNE than the second.

I couldn't resist the temptation and did the maths. I kind of lost the
habit, so it took me 20 minutes. The result is: My intuition was
right: Do *not* use the airbrakes.

Even in an ideal ship (zero drag) and with the nose pointing vertically
down initially, the peak speed is only 240km/h under your assumptions
(4g pull-up and 180km/h initially). In a real ship and with only 60
degrees down, the peak speed would of course be lower.

Here's the script with the phugoid equation (Mathematica). Feel free to
modify the initial conditions and parameters and play around. The
unknowns are v1, v2, the horizontal and vertical component of the speed
vector.

kmh = 1 / 3.6
Ng = 4
g = 9.81
v0 = 180 kmh
tmax = 8
dt = .1

v =
NDSolve[
{
v1'[ t ] == Ng g * v2[ t ] / Sqrt[ v1[ t ]^2 + v2[ t ]^2 ] ,
v2'[ t ] == Ng g * -v1[ t ] / Sqrt[ v1[ t ]^2 + v2[ t ]^2 ] - g ,
v1[ 0 ] == 0 ,
v2[ 0 ] == -v0
} ,
{ v1 , v2 } ,
{ t , 0 , tmax }
][[ 1 ]]

Table[ ( Sqrt[ v1[ t ]^2 + v2[ t ]^2 ] /. v ) / kmh , { t , 0 , tmax , dt } ]

Regards
-Gerhard
--
o o
Gerhard Wesp | http://www.cosy.sbg.ac.at/~gwesp/
\_/ See homepage for email address!

At 04:48 16 August 2005, Gerhard Wesp wrote:
Ian wrote:
If you are in a situation (perhaps recovering from
a spin) with your nose
pointing 60 deg down below the horizontal and the
ASI reading 180 km/h and
accelerating rapidly. You could try:

[ leave A/B closed and pull 4G ...]

I have not worked out the maths of the two options
but I think that in
a clean glass ship, you would have a better chanced
completing the first
manoeuvre without exceeding VNE than the second.

I couldn't resist the temptation and did the maths.
I kind of lost the
habit, so it took me 20 minutes. The result is: My
intuition was
right: Do *not* use the airbrakes.

Even in an ideal ship (zero drag) and with the nose
pointing vertically
down initially, the peak speed is only 240km/h under
your assumptions
(4g pull-up and 180km/h initially). In a real ship
and with only 60
degrees down, the peak speed would of course be lower.

Here's the script with the phugoid equation (Mathematica).
Feel free to
modify the initial conditions and parameters and play
around. The
unknowns are v1, v2, the horizontal and vertical component
of the speed
vector.

kmh = 1 / 3.6
Ng = 4
g = 9.81
v0 = 180 kmh
tmax = 8
dt = .1

v =
NDSolve[
{
v1'[ t ] == Ng g * v2[ t ] / Sqrt[ v1[ t ]^2 +
v2[ t ]^2 ] ,
v2'[ t ] == Ng g * -v1[ t ] / Sqrt[ v1[ t ]^2 +
v2[ t ]^2 ] - g ,
v1[ 0 ] == 0 ,
v2[ 0 ] == -v0
} ,
{ v1 , v2 } ,
{ t , 0 , tmax }
][[ 1 ]]

Table[ ( Sqrt[ v1[ t ]^2 + v2[ t ]^2 ] /. v ) / kmh
, { t , 0 , tmax , dt } ]

Regards
-Gerhard

I think the ground might get in the way before I have
done the sums :-)

T o d d P a t t i s t a écrit :

Last, I'm relying on my recollection of the physics of a
pull-out which I calculated long ago. It's not peer
reviewed work, and I'm quite far from infallible :-) If I
get some time, I'll pull out my old notes. I'd love to have
someone put this into Mathematica and verify my recollection
of the results.

I have no calculation neither, and it would be great to help us compare
our guesses...

What we need to do so is knowing the airbrakes drag. At 1 g it limits
speed to VNE at 30° dive angle, which means that if you get twice this
drag at 4 g, even at 90° dive angle the speed will be kept under VNE.
Which is not unlikely, since induced drag increase due to airbrakes is
high, but once again I have no precise figure yet to prove it. If
someone has, please give it...

--
Denis

R. Parce que ça rompt le cours normal de la conversation !!!
Q. Pourquoi ne faut-il pas répondre au-dessus de la question ?