Va and turbulent air penetration speed.

"ArtP"
Agreed. Wings stall at a given AOA, not at a speed.

Which makes the original statement that Va is dangerously close to
stall speed inaccurate.

Yessssss.

I guess what Im getting at is.. if the POH and checklist says one thing,
and a homebrew Vref, Vx, Vy, etc. doesnt match "the book" exactly (but
is scientifically correct).. which would prevail if something went wrong
and my decisionmaking was analyzed after the fact by G-men, insurers,
usenet readers, etc..

I get the feeling (without having done any of the math yet) that this
truly is an academic exercise in the typical 4 seat or less light
spamcan anyways, something akin to a few knots here or there...

Gary Drescher wrote:

"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, whether your
climb angle will clear an obstacle, whether you can stop before the end of
the runway, whether you can glide to a landing spot, etc. And those things
are what the V speeds are all about.

In fact, though, I don't think there's any contradiction between the physics
and the way the POH speeds are supposed to be interpreted. But the question
is a good illustration of why understanding the basic physics helps
understand how to use the POH numbers safely.

--Gary

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... I was looking for actual formalas.. not wild ass guesses or
rough approximations.. Computer spreadsheets use mathematical equations.

There are a few issues before you go off treating any equation
you get here as gospel.

1) Va probably isn't what you think it is. See my other posts.

2) In the case that Va = Vs*sqrt(load-factor) (23.335
equality), don't *ever* be tempted to scale it up if you are
over gross (Alaska, for example). Wings falling off may not
be the limiting factor.

3) If you are under gross (and Va is 23.335 equality), the
scaled Va is probably too conservative. If the 23.335 equality
does not apply, then the adjusted Va may not be conservative
enough.Without further specific analysis, you'll never be sure.

This is something that may be used by others besides myself.

This sounds foolhardy. You might want to ask yourself why
manufacturers don't publish Va vs. weight. And if you go
off telling people they can happily fly at Va without the wings
falling off, you're setting yourself up to be sued.

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

"Dave S" wrote in message
. net...
Gary... I was looking for actual formalas.. not wild ass guesses or
rough approximations.. Computer spreadsheets use mathematical equations.
This is something that may be used by others besides myself. I
understand the concepts.. I DIDNT have the actual calcs on hand when I
posted my request.

Sorry, I didn't mean to be unresponsive--saying that the speed's
proportionate to the square root of gross weight _is_ the actual formula
(just expressed in English, and trivially translatable into mathematical
symbols or spreadsheet expressions--it's like saying "take the sum of the
passengers' weights" instead of saying "Pax1weight+Pax2weight").

As for approximations, the point of the alternate formula I gave is that it
gives a very _close_ approximation, not a rough approximation or wild-ass
guess. Unless you can control your airspeed to a fraction of a knot, the
approximation is just as good as the exact answer. Knowing the
approximation is important so that 1) you can quickly and easily
sanity-check what your spreadsheet tells you; and 2) if you realize someday
that you neglected to pre-calculate your V-speeds, or that you calculated
them based on weight assumptions that later changed, you can then
re-calculate in your head while you're flying (it's a lot easier to divide
by two than to calculate an exact square root).

--Gary

Gary Drescher wrote:
"Dave S" wrote in message
. net...

Ok... all you closet aeronautical engineers... I'm asking for someone to
help do my work for me.. with regards to Va..

I have an Excel Spreadsheet application that does W&B and plots it on a
graph... The form also lists certain speeds that are "static": Vx/Vy,
Vne, etc.. I would like to modify this form to list Va dependent on
the given calculated gross weight, and perhaps even doctor it up to do
density altitude computations..


Va is proportionate to the square root of the plane's gross weight. Vx,
Vy,
Vl/d, Vs, and Vs1 are also proportionate to the square root of the gross
weight.

A handy approximation is that for small percentages below maximum gross
weight (say, up to 30% or so), the weight-dependent speeds diminish by
half
the percentage that the weight diminishes. So, for example, if you're
20%
below max gross weight, reduce the appropriate speeds by 10%. (The
calculations should be made with regard to CAS rather than IAS, but the
difference is usually small.)

You can find a good explanation of these speeds' weight-dependency at
http://www.av8n.com/how/.

--Gary

"Dave S" wrote in message
. net...
I guess what Im getting at is.. if the POH and checklist says one thing,
and a homebrew Vref, Vx, Vy, etc. doesnt match "the book" exactly (but
is scientifically correct).. which would prevail if something went wrong
and my decisionmaking was analyzed after the fact by G-men, insurers,
usenet readers, etc..

I don't think there's actually a disparity here between what the POH says
and what physics says. Va, Vx etc. are defined at max gross weight, and are
_intended_ to be scaled down for other weights.

But suppose there _is_ a disparity. If you're executing a high-performance
takeoff from an obstructed short field, would you rather use a speed that
gives you the best climb angle, or one that produces a shallower angle, but
gives you an excuse for the crash investigators? (That's not to say that
your question about the legal consequences isn't still of interest, though.)

I get the feeling (without having done any of the math yet) that this
truly is an academic exercise in the typical 4 seat or less light
spamcan anyways, something akin to a few knots here or there...

Well, there's not much math to do--if you're 30% below gross (quite possible
in a typical 4-seater), then Va, Vx etc. get reduced by about 15%--not a
trivial difference.

--Gary

Gary Drescher wrote:

"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, whether
your
climb angle will clear an obstacle, whether you can stop before the end
of
the runway, whether you can glide to a landing spot, etc. And those
things
are what the V speeds are all about.

In fact, though, I don't think there's any contradiction between the
physics
and the way the POH speeds are supposed to be interpreted. But the
question
is a good illustration of why understanding the basic physics helps
understand how to use the POH numbers safely.

--Gary

The desired effect is to have the airfoil stall before breaking but at the
same time that the pilot not lose control. Gusts may increase indicated
airspeeds and consequently produce more stress on the airplane, so from this
standpoint slower is always better. On the issue of controlablity, faster
is better. Vb is the speed that is supposed to provide the best compromise.
There was an article in Business and Commercial Aviation a few years ago
that had a comprehensive explanation but I no longer have the issue. I seem
to recall that for swept wing jets Vb is greater than Va and may even be
greater than normal cruise. Of course jets are also concerned with mach
exceedances and upsets so the issue is more complicated for them

Mike
MU-2

"Doug" wrote in message
om...
Kershner's "The Advanced Pilot's Flight Manual" has the following
definition for Va.

Va - The maneuvering speed. This is the maxiumum speed at a particular
weight at which the controls may be fully deflected without
overstressing the airplane.

Note that this definition DOES NOT say that the airplane will stall
before it breaks due to turbulence.

Now, Va is commonly taught as turbulent air penetration speed. But
nowhere in the definition does it say that Va will protect the
airframe from damage due to turbulence.

Does slowing down even slower than Va protect the airframe from even
more severe turbulence? Or is Va the best speed for turbulence
penetration? Or is Va just used as a turbulence air penetration speed
becauase of tradition or some other non-technically correct reason.

"Gary Drescher" wrote:
(The calculations should be made with regard to CAS rather than IAS,
but the difference is usually small.)

Careful about that. CAS is usually very close to IAS near cruise, but
at the low end of the scale, they may differ significantly.

In article ,
Dave S wrote:
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?

I'm not sure what it is that you're asking here.

The POH gives you experimentally derived performance numbers under
stated conditions. There are standard formulas to extrapolate those
numbers to other conditions of temperature, altitude, etc. A typical
POH will contains tables or graphs showing these extrapolations for a
number of various combinations.

"Roy Smith" wrote in message
...
"Gary Drescher" wrote:
(The calculations should be made with regard to CAS rather than IAS,
but the difference is usually small.)

Careful about that. CAS is usually very close to IAS near cruise, but
at the low end of the scale, they may differ significantly.

True. It depends a lot on the aircraft. Lately I've been flying Arrows,
for which the discrepancy is tiny even near stall speed. But that's not
always the case.

--Gary