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Avoiding Vne



 
 
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  #1  
Old March 31st 04, 09:18 PM
Bob Kuykendall
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Earlier, Bruce Greeff wrote:

...Similarly the latest carbon designs
seem to have G limits imposed by
the JAR22 deflection limits rather
than ultimate strength...


I'll certainly agree that composite sailplane structure is bounded
more by stiffness than by strength. However, I've spent my lunch hour
searching JAR22 and I can't find anything that codifies deflection
limits. The closest thing I found seems to be:

: JAR 22.305 Strength and deformation
: (a) The structure must be able to support
: limit loads without permanent deformation. At
: any load up to limit loads, the deformation may
: not interfere with safe operation. This applies in
: particular to the control system.
: with respect to the sailplane.

Do you know of other relevant JARs that specify maximum structure
deflection in quantifiable terms? I'm not trying to nitpick or
anything, I just want to make sure I'm not missing something
important.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com
  #2  
Old March 31st 04, 10:59 PM
Eric Greenwell
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Bob Kuykendall wrote:
Earlier, Bruce Greeff wrote:


...Similarly the latest carbon designs
seem to have G limits imposed by
the JAR22 deflection limits rather
than ultimate strength...



I'll certainly agree that composite sailplane structure is bounded
more by stiffness than by strength.


I've been told that is more likely true for fiberglass construction, but
not so likely to be true for carbon fiber construction, because of the
great differences in material characteristics, such as stiffness. So, it
might correct to argue that a glass fiber sailplane has a "substantial"
G loading margin, but not correct for the carbon fiber sailplane.

And the bounds might be quite different for a 15 meter glider and a 25
meter glider, or a thick wing trainer and a thin wing racer.


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

Eric Greenwell
Washington State
USA

  #3  
Old April 4th 04, 02:06 PM
Bruce Greeff
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HI Bob

That is what I was referring to.

The deformation limit for carbon designs with thin wings appears to be the point
at which it becomes impossible to maintain control movement.

As an example, there are various apocryphal tales of uncommanded airbrake
openings on open class aircraft with thin flexible wings. The Nimbus 4 appears
to be the most common suspect here.

So the deflection limit is not a "x degrees from rest", or a plastic deformation
(although there is a requirement for this in the regulations) but a deflection
beyond which the control actuators do not work correctly or have unacceptably
high resistance.

My point came from published discussions on the construction of the Eta, and the
DG1000 where both constructors commented that the ultimate strength of the
structure was well in excess of the limit load, and that the limit load was
imposed by the deflection of the wing.

There is an interesting test story at:

http://www.dg-flugzeugbau.de/bruchversuch-e.html

The destructive test requirement is that the wing must withstand 1.725* the
limit load for three seconds at a temperature of 54Celsius. The DG1000 wing
withstood this - and eventually failed at 1.95 times the design load limit. This
is one reason why I believe you would probably be able to get away with a brief
overstress load. I am not sure of the limits on older designs, but would expect
there to be less margin of strength.

As I understand it the modern thin section wings are flexible enough that the
load limit is imposed by control freedom limitation, and the wing must withstand
1.725 times this load in test. Flutter is the subject of speed limitation which
give speeds and margins that the designer/manufacturer must demonstrate flying
to. The regulations imply that the glider must be demonstrated safe at a minimum
of 23% margin above the placarded Vne. So your margins for flutter, versus
ultimate strength are 1.23 vs 1.725 in JAR22 (unless I got the math wrong)
  #4  
Old April 5th 04, 06:40 PM
Denis
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Bruce Greeff wrote:

As I understand it the modern thin section wings are flexible enough
that the load limit is imposed by control freedom limitation, and the
wing must withstand 1.725 times this load in test. Flutter is the
subject of speed limitation which give speeds and margins that the
designer/manufacturer must demonstrate flying to. The regulations imply
that the glider must be demonstrated safe at a minimum of 23% margin
above the placarded Vne. So your margins for flutter, versus ultimate
strength are 1.23 vs 1.725 in JAR22 (unless I got the math wrong)


It's perhaps mathematically true but your argument is wrong (if your
conclusion is to say that there is more risk of flutter than
overloading). You cannot compare pourcentages of load and speed !

It takes less tenth of second at any moment to take the 2 or 3 g's that
will exceed your (supposed) 72.5% load margin, whereas it will take
several seconds to take the 60 or 65 km/h of margin in speed (supposing
23% margin), or depending of the dive angle you might never get over the
speed margin...

And although it may be true that some parts of the wing (e.w. center
section) has more stress margin due to deflection limit, it does *not*
guarantee you that all the parts of the wing has the same extra margin:
in the Nimbus 4 accident the central wing did not break, but the outer
wing did, with fatal consequences :-(


--
Denis

R. Parce que ça rompt le cours normal de la conversation !!!
Q. Pourquoi ne faut-il pas répondre au-dessus de la question ?
  #5  
Old April 5th 04, 10:17 PM
K.P. Termaat
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Started this thread (Avoiding Vne) some weeks ago with a kind invitation to
respond to the idea of pulling the airbrakes while still in the rotating
mode of a spin. The idea behind it is when rotation has been stopped with
the glider at a pitch angle of say 60° or more this will be at a lower speed
then when the airbrakes stay closed all the time. Possibly a build up of
speed to over Vne can then be avoided after that. Of course airbrakes should
be closed again in the following pull up manouvre.
Any comments?


"Denis" schreef in bericht
...
Bruce Greeff wrote:

As I understand it the modern thin section wings are flexible enough
that the load limit is imposed by control freedom limitation, and the
wing must withstand 1.725 times this load in test. Flutter is the
subject of speed limitation which give speeds and margins that the
designer/manufacturer must demonstrate flying to. The regulations imply
that the glider must be demonstrated safe at a minimum of 23% margin
above the placarded Vne. So your margins for flutter, versus ultimate
strength are 1.23 vs 1.725 in JAR22 (unless I got the math wrong)


It's perhaps mathematically true but your argument is wrong (if your
conclusion is to say that there is more risk of flutter than
overloading). You cannot compare pourcentages of load and speed !

It takes less tenth of second at any moment to take the 2 or 3 g's that
will exceed your (supposed) 72.5% load margin, whereas it will take
several seconds to take the 60 or 65 km/h of margin in speed (supposing
23% margin), or depending of the dive angle you might never get over the
speed margin...

And although it may be true that some parts of the wing (e.w. center
section) has more stress margin due to deflection limit, it does *not*
guarantee you that all the parts of the wing has the same extra margin:
in the Nimbus 4 accident the central wing did not break, but the outer
wing did, with fatal consequences :-(


--
Denis

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



  #6  
Old April 6th 04, 08:13 AM
Denis
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K.P. Termaat wrote:

Started this thread (Avoiding Vne) some weeks ago with a kind invitation to
respond to the idea of pulling the airbrakes while still in the rotating
mode of a spin. The idea behind it is when rotation has been stopped with
the glider at a pitch angle of say 60° or more this will be at a lower speed
then when the airbrakes stay closed all the time. Possibly a build up of
speed to over Vne can then be avoided after that. Of course airbrakes should
be closed again in the following pull up manouvre.
Any comments?


well... after 114 answers, I think you have good specimens of the very
diverse opinions that have been expressed so far ;-)

in short, mine is : apply full airbrakes just after applying the initial
spin recovery control inputs, and keep them out during dive (gentle)
pull out...

--
Denis

R. Parce que ça rompt le cours normal de la conversation !!!
Q. Pourquoi ne faut-il pas répondre au-dessus de la question ?
  #7  
Old April 6th 04, 10:22 AM
K.P. Termaat
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Hi Denis,

If I understand you well you will wait with pulling the airbrakes until the
glider has stopped its rotation and then carefully put some back pressure on
the stick. I was considering the idea of pulling the brakes with the glider
still in its rotation mode in order to keep forward speed as low as possible
at any time. However this may frustrate the spin recovery action; I just
don't know. What's your idea about this. Of course handbooks do not say
anything about this.
B.t.w. my provisional handbook for the Ventus-2cxT forbids spin exercises.
My idea is to avoid spins with this glider any time anyway; however I will
try to get some feeling about the glider's behaviour close to entering this
"acrobatic" flying mode.

Karel, NL

"Denis" schreef in bericht
...
K.P. Termaat wrote:

Started this thread (Avoiding Vne) some weeks ago with a kind invitation

to
respond to the idea of pulling the airbrakes while still in the rotating
mode of a spin. The idea behind it is when rotation has been stopped

with
the glider at a pitch angle of say 60° or more this will be at a lower

speed
then when the airbrakes stay closed all the time. Possibly a build up of
speed to over Vne can then be avoided after that. Of course airbrakes

should
be closed again in the following pull up manouvre.
Any comments?


well... after 114 answers, I think you have good specimens of the very
diverse opinions that have been expressed so far ;-)

in short, mine is : apply full airbrakes just after applying the initial
spin recovery control inputs, and keep them out during dive (gentle)
pull out...

--
Denis

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



  #8  
Old April 6th 04, 11:54 AM
Denis
external usenet poster
 
Posts: n/a
Default

K.P. Termaat wrote:

Hi Denis,

If I understand you well you will wait with pulling the airbrakes until the
glider has stopped its rotation and then carefully put some back pressure on
the stick. I was considering the idea of pulling the brakes with the glider
still in its rotation mode in order to keep forward speed as low as possible
at any time. However this may frustrate the spin recovery action; I just
don't know. What's your idea about this. Of course handbooks do not say
anything about this.


the ASH 26 handbook does say "spinning is not noticeably affected by
extending the airbrakes paddles, but it will increase the height loss
when pulling out, and is therefore less advisable"

I suppose the last sentence refers to loss of total energy (i.e. after
recovery you will re-gain more height if you made it without airbrakes
than with). It is not true of height loss down to lowest point (you will
loose less height with airbrakes because the diving speed is diminished
and the curving radius is reduced by the square of the speed -- even
with 3.5 G allowed w/ airbrakes instead of 4 G w/o the height loss
should be lesser with airbrakes out --

--
Denis

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




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