Va and turbulent air penetration speed.

At or below Vo, the criteria that are generally (and erroneously)
thought to apply below Va, do apply, i.e. the wing will stall
before the positive limit maneuvering load factor is exceeded.

People should make sure they see the word "POSITIVE". At Vo you can pull as
hard as you want without exceeding the limit load factor (3.8 for normal
category), but since the limit is much lower for negative load factor (0.4 x
3.8 = 1.52) you can't push as hard as you want.

Barry

"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:wKGLb.9047$8H.23200@attbi_s03...
"Dan Thomas" wrote in message
om...
"Gary Drescher" wrote in message
news:bnzLb.6520$8H.20195@attbi_s03...
"Dave S" wrote in message
. net...
Now... a question about realities.. The POH nazi's will say that the
Word as written is good, praise be to the POH... if I base flight
decisions and speeds on MY calculated numbers rather than the max
weight
sea level standard day numbers published in the almighty POH.. am I
going to be asking for trouble here?

It depends on what you mean by 'trouble'. The laws of physics prevail
over
the POH in determining whether your engine mount will break...

Why do folks worry about engine mounts breaking? They are far
stronger, in most cases, than the rest of the structure. For
production airplanes, the legal standards for certification include a
9G strength for fuselage/cabin structure for crashworthiness, and I
have seen other specs calling for the same 9Gs specifically on engine
mounts.

Are those regulatory specs?

Yes, they are. being a Canadian, I can quote the CARs but the
FARs are a different matter. I'l see what they have to say. In any
case, when have you ever heard of an engine departing an airplane in
turbulence or during violent maneuvering? Our Citabria has a G-meter
in it, and we have seen some pretty big numbers when students get
clumsy on landing.
Landing forces don't affect wings much, since they're still
generating lift and the landing forces on the structure tend to be
negative, and if the engine mount was a 5G structure like the rest of
the airplane it would have fallen off long ago. A missing 300 pounds
or so during a hard landing would be disastrous: CG way back near the
trailing edge, an airplane suddenly much lighter, and airspeed still
sufficient to flip the whole works over into a crash and burn
scenario, all for the lack of another pound or so of tubing.
The only times I have heard of engine mounts failing on light
airplanes is when a prop throws part of a blade, or maybe the whole
blade on a constant-speed prop. The imbalance is more than enough to
rip the engine off the airplane. Blades will fail when propeller nicks
are left untreated and cracks develop. The prop is the most highly
stressed bit of metal on the whole airplane, and THAT'S what pilots
should be concerned about, not engine mounts.

Dan

"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/

"Dan Thomas" wrote in message
om...

The only times I have heard of engine mounts failing on light
airplanes is when a prop throws part of a blade, or maybe the whole
blade on a constant-speed prop. The imbalance is more than enough to
rip the engine off the airplane. Blades will fail when propeller nicks
are left untreated and cracks develop. The prop is the most highly
stressed bit of metal on the whole airplane, and THAT'S what pilots
should be concerned about, not engine mounts.


Nah. Properly designed engine mounts would never let that happen.

"Julian Scarfe" wrote in message
...

The "maneuvering speed" placarded in the cockpit is not Va. It is Vo.
It
is
defined by 23.1507 and the placard is mandated by 23.1563. "Vs is a
selected speed that is not *greater* than Vsvn". My *s. At or below
Vo,
the criteria that are generally (and erroneously) thought to apply below
Va,
do apply, i.e. the wing will stall before the positive limit maneuvering
load factor is exceeded.

"Tony Cox" wrote in message
news

Indeed. Vo was discussed in the thread in rec.aviation.piloting
that I referred to earlier. As I understand it, Vo is a more recent
certification requirement. Not sure then it came in, but 23.1507 is
dated 1993. My 1966 182 doesn't come with a Vo.

FWIW, here's the extract from AC23-19 (which is entirely consistent with
what you've said)

---
(c) The design maneuvering speed is a value chosen by the applicant. It may
not be less than Vs*sqrt(n) and need not be greater than Vc, but could be
greater if the applicant chose the higher value. The loads resulting from
full control surface deflections at Va
are used to design the empennage and ailerons in 14 CFR part 23, §§ 23.423,
23.441, and 23.455.

Va should not be interpreted as a speed that would permit the pilot
unrestricted flightcontrol 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*sqrt(n), the pilot would
have to check the maneuver; otherwise the airplane would exceed the limit
load factor.

Amendment 23-45 added the operating maneuvering speed, Vo in § 23.1507. Vo
is established not greater than Vs*sqrt(n), and is a speed where the
airplane will stall in a nose-up pitching maneuver before exceeding the
airplane structural limits.
---

Julian Scarfe

"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.. let me break this down simply for you..

Im not that familiar with Excel and PROGRAMMING it. I dont use Excel
except other than to plug numbers into existing apps and that is rare. I
just dont have that familiarity with it. Blance gave me what I needed.
It was about saving my time, not about my lack of understanding concepts
and practical application. Remember.. I'm not the guy who though you
couldnt stall at Va without some monster tailwind gusts..

I wasnt looking for approximations, pnemonics or handy dandy quick tools
that serve as memory aids for people who have trouble with math.

Please dont try to judge my comprehension and abilities because I made a
simple request for a simple answer that you werent able to fulfil.

paste
If anyone HAS or KNOWS (or has the formulas)how to do this in Excel
snip
thats exactly what I asked for.

So.. please dont render me "unsafe" because you cant pay attention to
detail.. ok?

And once again.. thank you Blanche and the others who were constructive.
Dave

Gary Drescher wrote:
"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. If you use the
Excel expression without understanding how to derive it yourself or why it's
correct, you're essentially choosing a speed to keep your plane intact by
delegating the decision to someone on Usenet whom you don't even know. And
since you were also uninterested in a very close approximation (within 2%)
that lets you do the same calculation in your head, how are you going to
check whether your implementation of the formula contains a typo or other
problem that results in the wrong answer?

I don't mean to be critical, but I implore you to be sure you understand
exactly why and how some of the V-speeds (Vs, Vs1, Vx, Vy, Va, Vl/d) vary
with weight, and why others (Vfe, Vle, Vlo, Vno, Vne) do not, and how the
relation translates into a mathematical expression. (The reference I
pointed to earlier contains a full explanation using nothing more advanced
than high-school physics.)

Fly safely,
Gary


Blanche wrote:

For the type of aircraft your club will be flying, the formula in
Kershner will be adequate.

The formula in Excel is

full_va*SQRT(A6/full_weight)

where

full_va printed weight in the POH (usually at gross weight)
full_weight gross weight for aircraft (again, most recent W&B)
A6 column with weight for calculation

I fly a cherokee, so I have weights from 1800 (lightest load with
fuel and me and gear) to 2400 (gross weight) in column A.

And while you're calculating Va, the Glide speed can be done at
the same time since it's also weight-based:

full_glide*SQRT(A6/full_weight)

have fun!

"Dave S" wrote in message
.net...
Gary.. let me break this down simply for you..

Im not that familiar with Excel and PROGRAMMING it. I dont use Excel
except other than to plug numbers into existing apps and that is rare. I
just dont have that familiarity with it. Blance gave me what I needed.
It was about saving my time, not about my lack of understanding concepts
and practical application.

Dave, I apologize if I misinterpreted your request. If you knew that the
formula is Va times the square root of weight-divided-by-max-gross, but you
didn't know that the corresponding Excel expression is
Va*SQRT(weight/max_gross), then you did indeed understand the concepts, and
just needed a hand with the spreadsheet itself. (Note though that you did
incorrectly classify Vx and Vy among the "static", weight-independent
speeds, which contributed to my impression that the concepts themselves were
in question.)

I wasnt looking for approximations, pnemonics or handy dandy quick tools
that serve as memory aids for people who have trouble with math.

Well, here I think _you've_ misinterpreted _me_. The approximation I
suggested was not a mnemonic, and had nothing to do with having trouble with
math--unless you consider the inability to mentally calculate square roots
while flying a plane to consititute a math deficiency. :-) I regard that
approximation as an important tool that should be familiar to any good
pilot, so my suggesting it was in no way a put-down.

Anyway, I was honestly trying to be constructive. I apologize again if I
did not succeed.

--Gary

Please dont try to judge my comprehension and abilities because I made a
simple request for a simple answer that you werent able to fulfil.

paste
If anyone HAS or KNOWS (or has the formulas)how to do this in Excel
snip
thats exactly what I asked for.

So.. please dont render me "unsafe" because you cant pay attention to
detail.. ok?

And once again.. thank you Blanche and the others who were constructive.
Dave

Gary Drescher wrote:
"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. If you use the
Excel expression without understanding how to derive it yourself or why
it's
correct, you're essentially choosing a speed to keep your plane intact
by
delegating the decision to someone on Usenet whom you don't even know.
And
since you were also uninterested in a very close approximation (within
2%)
that lets you do the same calculation in your head, how are you going to
check whether your implementation of the formula contains a typo or
other
problem that results in the wrong answer?

I don't mean to be critical, but I implore you to be sure you understand
exactly why and how some of the V-speeds (Vs, Vs1, Vx, Vy, Va, Vl/d)
vary
with weight, and why others (Vfe, Vle, Vlo, Vno, Vne) do not, and how
the
relation translates into a mathematical expression. (The reference I
pointed to earlier contains a full explanation using nothing more
advanced
than high-school physics.)

Fly safely,
Gary


Blanche wrote:

For the type of aircraft your club will be flying, the formula in
Kershner will be adequate.

The formula in Excel is

full_va*SQRT(A6/full_weight)

where

full_va printed weight in the POH (usually at gross weight)
full_weight gross weight for aircraft (again, most recent W&B)
A6 column with weight for calculation

I fly a cherokee, so I have weights from 1800 (lightest load with
fuel and me and gear) to 2400 (gross weight) in column A.

And while you're calculating Va, the Glide speed can be done at
the same time since it's also weight-based:

full_glide*SQRT(A6/full_weight)

have fun!