"Greg Esres" wrote in message
...
In my view, the most correct definition of Va will be it's the speed
above which you cannot make full or abrupt control movements, due to
control surface integrity.
This is way interesting & I've got the FAR's in front of me
now to get to the bottom of this.
First, I can't find a specific definition of "Design maneuvering speed" in
the FAR's, but my personal working definition is almost like yours.
I'd substitute "without risk of structural failure" for talk of control
surface integrity. Since control surface failure is indeed structural
failure, my definition would seem more restrictive than yours.
It looks like Va is mentioned twice in pt 23.
In 23.335 we get Va must be = Vs sqrt(n), with n the load
factor. We also get "Va need not exceed Vc" which
makes no sense to me, at least as far as a regulation goes.
Then, in 23.423 we see Va used in establishing the characteristics
of the (horizontal) control surfaces. Note that this doesn't
say this is how you calculate Va, it says you must use this speed
in the design of control surfaces to achieve certain rates of
response when they are used and/or to make sure you don't
break anything..............I suppose that manufacturers
do such a poor job of designing control surfaces that
they have to restrict Va just to meet this certification
requirement.....Well, bugger me Greg, looks like you're right!
New airplanes are supposed to come with a new Vo speed, which DOES
require that the airplane stall before exceeding the load factor.
Since control surfaces seem to be the limiting factor, I'd assume
that manufactures would design them for as low a Va as possible,
consistent with 23.335. So they'd choose Va = Vs.sqrt(n).
Vo does differs a little from pt 23 certification requirements, in that
Va isn't exactly Vo, because Va calculations assume that airfoil
lift does scale linearly with AOA and as the square of airspeed
when in fact these are only approximately true.
I'd bet that Vo and Va are pretty close. Allowing for the 1.5 safety
factor, I bet they're indistinguishable.
Here's a copy from a draft copy of an AC 23.something that I found.
The AC was intended to make this clear to test pilots, but I don't
think the draft was ever finished:
------------snip-----------------
VA should not be interpreted as a speed that would permit the pilot
unrestricted flight-control movement without exceeding airplane
structural limits nor should it be interpreted as a gust penetration
speed. Only if VA = Vs sqrt(n) , will the airplane stall in a nose-up
pitching maneuver at, or near, limit load factor. For maneuvers where
VAVS n , the pilot would have to check the maneuver; otherwise the
airplane would exceed the limit load factor.
Isn't this just a warning that Va "may not be less than Vs.sqrt(n)", and
so could be higher?
Va would be the same at any aircraft weight, which it certainly
isn't.
It is in some airplanes. My Piper arrow doesn't scale it with weight.
Moreover, Part 23 says that Va is *only* defined at max gross. Some
manufacturers do publish Va's at lower weight, but that appears to be
at their option. As written, it doesn't match Part 23 definition.
I don't see that in pt 23. I see it being defined as 'may not
be less than' some expression involving gross weight parameters,
but there is nothing to say that this applies only to gross
weight (to be pedantic). Nor does 23.423 - which we both
agree partially defines Va - say anything about the weight
of the plane during the certification maneuver.
I'd remind you how we got here. The suggestion was that
Va, should be scaled upward in an overloaded airplane. We
both claim that it should not. I'd also scale my maneuvering
speed downwards if underweight just to stay within load
factor limits, and I bet you would too. To my mind, the laws
of physics trump the FAR's. (and my Va is indeed pretty close to
Vs.sqrt(3.5)). After all, pt 23 just tells me how to certify a
plane, not how to fly it.
I'd claim that Va shouldn't be increased because it is really
the minimum of a number of different speeds where things
start to fall apart, and without further data we don't know
which one does the limiting.
Interesting discussion.
--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/