Avoiding Vne

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)

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 ?

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 ?

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 ?