"Gary Drescher" wrote in message
news:j8YLb.15333$na.12586@attbi_s04...
"Tony Cox" wrote in message
ink.net...
Hi Gary. As Julian pointed out, there may be terminology problems
here. It may well be that the 172 POH defines Va as you say, but in
that case Cessna are telling you something more -- they are telling you
specifically that their Va is defined to meet the equality condition in
23.335. So it is really just their own private definition, applicable to
that plane and model year only.

Yeah, except that the POH (or rather aircraft manual) acquires regulatory
force from the FARs, so it's not just a private definition; rather, as
usual
with the FAA, it's one of several mutually inconsistent definitions that's
in official use. (For what it's worth, the Piper Arrow POH gives
essentially the same definition as the C172P POH.)

I suppose its that old terminology problem again. What can
I say? The FAR's are quite explicit on how Va is defined, and
that is most definitely _not_ what is in the POH. Thank heavens
for the 50% safety factor, or we'd have planes falling out of the
sky all over.

But thinking again, I don't see the problem even if the POH inherits
regulatory authority. It is, after all, only true in the context of that
particular make and model (which is consistent with the FAR
definition when 23.335 takes the equality). It's only when you
extend that definition to cover other planes that it doesn't ring true.


Well, the control surfaces don't care how much weight is in the
plane (at least to first order). If you yank them lightly loaded, you'll
stress the cables and hinges just the same as if you were over gross.
So that Va'(w) is flat if you plot it against w.

Right, but aren't the wings and control surfaces protected by Vno (a
weight-invariant force limit) rather than by Va (a weight-dependent
acceleration limit)? That's how I think about it anyway, even if it
doesn't
match (some of) the official definitions.

Vno doesn't say anything about control input. I've always wondered
how it is established. Seems like a test pilot would earn his or
her money finding out. I've always assumed that the windshield
would be the first thing to go...


Hope you've found this rant more informative than pedantic!

Sure, and I don't mind pedantry anyway. :-)


Hey thanks! A strangely interesting subject, don't you think?

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