(Koopas Ly) wrote in message om...

....Consider that a certain airplane weighed at maximum takeoff weight is
designed to withstand + 3.8 g's (its design load). Actually,
airliners that I am familiar with are tested to ultimate load, or
1.5*design load (+ 5.7 g's before permanent deformation). For now,
we'll assume that at + 3.8 g, the plane's wings break off. That would
equal to a total force on both wings of 3.8 x 2550 lb or almost 10,000
lbs.

The thing that bothers me about Va is that it equates to a number of
g's ("design load") AND that Va is being rescaled for weight. By
doing so, Va becomes more of an acceleration criterion rather than a
structural criterion. It appears as though Va limits positive g
acceleration to + 3.8 g, not load itself.

In other words, Va adjusted for say, a lower weight, tells the pilot
"You will not exceed 3.8 g for your current weight, as you will stall
first". If the current weight was 2,000 lbs, the total load on the
wings would only equate to 7,600 lbs at + 3.8 g's, lower than the
design limit of 10,000 lbs of + 3.8 g's at max. takeoff weight. The
acceleration on the airplane would be a "limit load acceleration" but
would not produce a limit load condition, per se, structurally.

If Va was truly a structural consideration, it would not change
(regardless of weight), since no airspeed below Va coupled with any
non-stalled AOA, could produce limit loads of + 3.8 g as tested at max
takeoff weight of 2,550 lbs.

There may be severe flaws in my reasoning...please no flames and be
nice. It's a holiday.

Alex

Not a dumb question, and many have misunderstood Va. I'm no
expert either, but as I understand it, the 3.8 G limit is predicated
on gross weight, or as you point out for your particular airplane,
almost 10,000 lbs.
Flying at less than gross, the airplane will be less likely to
stall as say, full up elevator is abrubtly applied, and will tend to
climb. Climbing reduces the angle of attack, preventing the stall and
maintaining the load on the wing, and the same 10,000 lbs of force
could be achieved easily. It would be higher than 3.8 G, but as I
said, the G load factor is based on gross weight.
So for less than gross weights, Va must be reduced by the square
of the decrease in weight. As Kirschner calculates it, a 20% weight
reduction should give a 10% Va reduction, and a 40% weight reduction
should give a Va 20% lower.
Don't forget that the wing is not the only structure in the
airplane. Several aircraft have been know to fail the tail first;
airplanes like the 210 and Bonanza, which are slippery and build speed
rapidly in the typical spiral entered after the non-instrument pilot
enters cloud. Upon popping out of the overcast at 400 feet, with the
ground coming up fast, he pulls back hard and the stab fails downward,
the airplane pitches forward over onto its back, and the wings fail
downward. End of story.

Dan