Va and turbulent air penetration speed.

"Gary Drescher" wrote in message
news:HDSLb.15032$I06.94614@attbi_s01...
"Dave S" wrote in message
.net...
Wonderful.. Thankyou Blanche... I only have to tweak the name of the
variable A6 to plug this in..

This was exactly what I was lookin for.
Dave

Dave, 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.

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.

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.

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.

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.

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

"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/

"Gary Drescher" wrote in message
news:CjWLb.15152$8H.35818@attbi_s03...
"Tony Cox" wrote in message
k.net...

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).

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. And of course if the equality condition
applies, then scaling proportional to sqrt(w) should be adequate or
better as I pointed out earlier (and discuss further below).

Now my POH just gives me Va. Nothing about abrupt control
inputs or anything. So absent other information, it's not really
much use to me. I could yank the yoke at Va and watch the wings
fall off - all this in a properly certified plane. Odd, eh? Of course,
its not likely to happen because of the additional safety factors
built into the design, but I could still be exceeding the load factor.


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.


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.

Things like baggage compartment objects stress should scale like
sqrt(w). Wing bolts (on a Cessna) are more complicated. At less
weight - lets assume this is mostly less weight in the cabin - you'll
be able to withstand greater acceleration; load factor isn't really
the issue here. Va'(w) in this case probably drops off less rapidly
with decreasing w than sqrt(w).

Now I think it is true that there is no component for which
Va'(w) falls _faster_ than sqrt(w) with decreasing w. In this case,
scaling an overall Va' at gross by sqrt(w) should mean that
it doesn't matter which Va' is the limiting component, you'll
always be _at or below_ it's corresponding Va'(w).

But caution two things. If you're certified over gross
(91.323), you shouldn't use the relationship to computer a higher
Va. Control surfaces might be the limiting factor, for example,
and they don't scale at all.

Second, if the equality in 21.335 isn't met, control surfaces
are the limiting factor. You can scale them by sqrt(w) if you
like, but it'll be meaningless -- the answer you get will _still_
be above the Va' for things that are load-factor limited.


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?


That's the key question isn't it?

As Julian pointed out, if you have Vo, then you should use
that. But my plane doesn't, and I don't think most of the fleet
does either. Vo is, I believe, a recent certification requirement.

Here's how I approach it. And of course, YMMV, so I hope
no one does the same without thinking about it first.

I think that for most GA planes, the equality in 21.335 applies.
Why overbuild control surfaces? The FAA says that you
*can* set Va above Vs*sqrt(lf), but to do so costs $$$'s and
lowers the useful load. So I think it is a reasonable assumption,
at least for my 182. This means, of course, that the sqrt(w)
relationship ought to be adequate to protect me, which is indeed
how I fly. But I realize I may be flying outside the load-factor
safety zone. After all, there should be a 50% margin to play with!

When I used to fly 172's, I noticed that some were certified in
the utility category at certain light weights. This suggests (but doesn't
guarantee) that the limiting factor in Va' is the wings, rather than
cabin load. Since this scales better than sqrt(w), I think you can
probably fly faster than the scaled Va without issue.

My 182 has no such certification -- it's all normal category.
It might mean that no one could be bothered to certify it in
the utility category, but it might also mean that the wings are
not the limiting factor. This I find comforting & I'm more careful
to maintain Va'(w) in rough air. (why in rough air is, I suppose,
where we came in).

BTW, that 'sqrt(w)' business is quite an approximation in itself,
and relies on quite a few assumptions which are probably not
that supportable over a wide range of w's...

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

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

"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.)

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.

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

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

--Gary

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

"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/

"Tony Cox" wrote in message
ink.net...
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.

Agreed. In my short few years as a pilot so far, the planes I've flown
(152s, 172s, Warriors, and Arrows) have all had essentially the same
definition of Va in their POHs, so I didn't realize it wasn't universal.

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

Yup. Thanks for the discussion!

--Gary

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