Avoiding Vne

Finally, someone bothered to get the regs out.

I still believe that the G-limit is better understood
in most designs than the Vne limit, just due to the
difference in testing approach. G-loads are tested
to destruction, Vne is not.

In either case it's good to know the demonstrated margins
in excess of certified limits - just in case.


At 13:12 04 April 2004, Bruce Greeff wrote:
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)

Andy Blackburn wrote:

Finally, someone bothered to get the regs out.

I still believe that the G-limit is better understood
in most designs than the Vne limit, just due to the
difference in testing approach. G-loads are tested
to destruction, Vne is not.

Another difference: if *you* survived to overspeed (i.e. flutter did not
occur), your glider is still safe for you or *other pilots*

If you survived overloading (i.e. over limit G-load but the wings did
not break) your glider may be *unsafe* and next time might break well
under extreme G-load limit...


--
Denis

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