"Gary Drescher" wrote in message
news:T_z9d.436094$8_6.136539@attbi_s04...
Minor correction: the wings snap due to excessive lift (force), not due to
excessive Gs (acceleration). Staying below Vne keeps the force from being
excessive; staying below Va keeps the acceleration from being excessive.
Vne does not vary with weight; Va varies in proportion to the square root
of gross weight, becoming a stricter limit when weight is lower. That's
because the less mass you have, the more acceleration is produced by a
given force.

IMHO, you are making things more confusing, not less.

Yes, what will break the structure is force. But when we talk about "G's",
typically we are describing a *factor* multiplied with the weight of the
airplane, not "acceleration due to gravity". In that context, all "G's"
means is "a factor increasing the nominal 1G force experienced by the
airplane".

For example, in a bank, lift needs to be increased in order to maintain
vertical speed (i.e. prevent up or down acceleration), usually at zero. At
a high enough bank angle (much higher than any normal operation would
require), the "G's" exceed the certification limit. Those "G's" would be a
static state, not due to acceleration of the airplane (if allowed to turn,
the airplane would also be accelerating, but this acceleration isn't
directly related to the "G's" experienced...if you could somehow counteract
the lateral force caused by the increase in lift, the wings would still
experience increased "G's", but the airplane would track straight ahead).

Vne is not typically related to lifting force at all, but rather to other
things like flight control flutter and longitudinal strength of airframe
structures (e.g. drag pulling the wings backwards). In level flight, at any
airspeed, the wing is generating exactly as much lift as there is weight of
the airplane. The force (lift) is constant.

Va, on the other hand, is related to a variety of other things. In the
sense that those things have to do with rapid changes in airfoil loading,
they are related to acceleration. But ultimately, those limits still have
to do forces imposed on structure. The acceleration is relevant -- higher
acceleration results in higher G forces on structure -- but the reason that
Va changes due to weight has less to do with the distinction between
"force", "G's", and "acceleration" and more to do with the fact that there
are internal structures carrying loads that don't change with total gross
weight of the airplane.

It's true that "the less mass you have, the more acceleration is produced by
a given force", but the reason this is relevant here is not because
acceleration is the problem per se. Rather, it's because there are
components in the airplane with a fixed load that, with the increase in
total acceleration of the airplane, could experience loads higher than
designed for. Because of these, the acceleration limit of the entire
airframe needs to be held constant. If it weren't for that, increased
acceleration would not be a problem, because the total load (force) at lower
weights would be the same, and it's force that breaks things, not
acceleration.

As this relates to the original statement, Va is the only airspeed related
to "keeping the force from being excessive". You can fly above Va and below
Vne, and still create out-of-limit *force* on the airframe, enough to damage
or even break the structure, simply by exerting too rapid a control movement
(on the elevator, for example). Vne does nothing to prevent this; only Va
does (and it's not perfect either...there exist gusts in nature than can
still exceed the structural limits at or below Va).

IMHO, arguing that Vne is only about force and Va is only about acceleration
is misleading and only serves to confuse the issue. Ultimately, they are
*both* about force imposed on various parts of the airframe structure; they
just happen to be addressing different *force-related* issues.

Pete