"Tony Cox" wrote in message
k.net...
"Gary Drescher" wrote in message
news:HDSLb.15032$I06.94614@attbi_s01...
... please forgive me for saying so, but if you found the statement "the
speed is
proportionate to the square root of gross weight" to be unhelpful, but
Blanche's "full_va*SQRT(A6/full_weight)" is "exactly what you were
looking
for", then with all due respect, you do not understand the calculation
well
enough to base a life-or-death piloting decision on it.

Especially since both the statement and the equivalent
expression are just plain _wrong_. To clarify this (since
there are safety implications):-

1) Va by definition is just a number and _does not_ scale
with weight.

While I understand your earlier point about the certification regulations,
nonetheless Va is explicitly defined in some places as "the maximum speed at
which you may use abrupt control travel" (C172P POH, for example), and that
speed _does_ scale with weight (and the C172P POH, for example, specifies
different values of Va for different weights).

But the more important question concerns the physics, not the terminology.

2) What you really looking for is some speed (lets call it
Va'(w)), a function of weight, below which you can tug on
the controls and not have things break.

Agreed. More specifically, we're looking for the speed at which the lift
force resulting from an abrupt transition to the maximum coefficient of lift
would not accelerate the plane enough to exceed the force that any of the
plane's components can withstand.

3) Va' is the _lowest_ of several speeds where individual
components might overstress -- controls break, engine mounts
crack, cargo bends the floor, wings fall off, etc.

Sure. Some component is going to be the weak link, capable of withstanding
less force than the others.

4) Some of these component Va' don't scale with weight, some
scale as sqrt(w), and some no doubt scale in other bizarre
ways.

Here I don't follow you. If the components have constant mass and each
component has a maximum force that it can withstand, then each component
thereby has a maximum acceleration that it can withstand, does it not? And
for any given acceleration, the maximum airspeed at which abrupt control
deflection would not exceed that acceleration (namely, the maximum speed at
which the maximum coefficient of lift would not provide enough force to
exceed that acceleration) does indeed scale in proportion to the square root
of the plane's weight.

5) Since you don't know without access to the engineering
design reports what these component Va's are, you can
never be certain how they scale with weight or which of
them is the limiting factor in any configuration.

6) Even at gross, Va' doesn't guarantee you protection
against full control movement. For that you need Vo, which
isn't available for older aircraft anyway.

Is there any better guideline for a pilot than to use the published
max-gross Va, scaled in proportion to the square root of current gross
weight, as the limiting speed for abrupt control deflections?

--Gary


--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/