Avoiding Vne

Eric Greenwell wrote:

I believe that with the airbrakes open your safe positive
G-limit reduces to +2.5G.

For my ASH 26 E at Vne: 4 G load factor (down from 5.3 at Va); 3.5 G
load factor airbrakes extended. It looks like the G limits at Vne are
fairly close together.

Airbrakes out at Va it's 3.5 g instead of 5.3, thus the difference is
significant. I don't know why there is almost no difference at VNE, butI
suspect that in that case the margin is higher without airbrakes than with.

I am curious about why they decrease.

There is a loss of lift at the airbrakes, thus for the same G the outer
wing is more loaded, and the bending momentat wing root increases.

Perhaps the loss of lift is lesser at high speeds (lower angle of attack) ?

--
Denis

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

Denis wrote:

For my ASH 26 E at Vne: 4 G load factor (down from 5.3 at Va); 3.5 G
load factor airbrakes extended. It looks like the G limits at Vne are
fairly close together.


Airbrakes out at Va it's 3.5 g instead of 5.3, thus the difference is
significant. I don't know why there is almost no difference at VNE, butI
suspect that in that case the margin is higher without airbrakes than with.

I am curious about why they decrease.

There is a loss of lift at the airbrakes, thus for the same G the outer
wing is more loaded, and the bending momentat wing root increases.

I should have said "I'm curious about why the G loading (airbrakes
closed) is lower at Vne than at Va". Perhaps it is to absorb gusts, or
to allow greater control deflections.

--
-----
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Eric Greenwell
Washington State
USA

Eric Greenwell wrote:

I should have said "I'm curious about why the G loading (airbrakes
closed) is lower at Vne than at Va". Perhaps it is to absorb gusts, or
to allow greater control deflections.

The only physical reason I see is that there is a torsional stress on
the wing, increasing with the square of the speed, and which might add
somewhere to the bending stress due to G loading.

Another reason is that the certification standards don't require the
same limitation in G-loads at VNE that at Va, and the flight manual
reflects that...

--
Denis

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

Eric Greenwell wrote:

I should have said "I'm curious about why the G loading (airbrakes
closed) is lower at Vne than at Va". Perhaps it is to absorb gusts, or
to allow greater control deflections.

The only physical reason I see is that there is a torsional stress on
the wing, increasing with the square of the speed, and which might add
somewhere to the bending stress due to G loading.

Another reason is that the certification standards don't require the
same limitation in G-loads at VNE that at Va, and the flight manual
reflects that...

--
Denis

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







I think (and my memory is hazy on this, so I stand to be corrected) it might
possibly have something to do with the gust case. Va I understand to be the
speed at which any one control can be fully deflected and not exceed the design
specifications; at Vne this is reduced to 1/3 of any one control.

'Rough air speed', normally the same as Va (but not always) is the highest
speed at which the glider can encounter an 'OSTIV Strong Gust' (10m/s
instantaneous shear) and stay within the flight envelope. At Vne the airframe
should be able to withstand a 'OSTIV Weak Gust' (2m/s anybody?) without
reverting to kit form. (As an aside, just think about that next time you
redline it across the airfield during the soaring day...)

I'm still chewing over why 'g' limits reduce between Va & Vne. I can picture a
typical flight envelope and this does indeed happen: whether this is a real
(structural/aerodynamic) limitation or just something the manufacturers put in
there to stop us going mad I don't know (probably the former).

Any budding aerodynamicists care to comment?