Va and turbulent air penetration speed.

Dave S wrote:
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

Easy. Va as published is for max gross weight, and goes down with the
square root of weight. So:

MGW = Max Gross Weight
W = Weight of the aircraft at a given moment
Va = Maneuvering speed as published in the POH
Va,w = Maneuvering speed for a given weight

Va,w = Va * sqrt (W / MGW)

Stall speeds (Vs0 and Vs1) both follow the same formula, and so does
your final approach speed, which is usually calculated as 1.3 * Vs0.
So, if you really want to do landings right, you should calculate your
weight at the end of the flight (taking into account fuel burn),
calculate a Vs0 based on that, and multiply by 1.3 to get your proper
final approach speed (keeping in mind that the multiplication needs to
be done in CAS, not IAS).

It turns out that for the majority of light airplanes, the difference
between max gross and a reasonable minimum landing weight (pilot and
minimum fuel) is a small enough percentage of max gross that stall speed
only varies a few knots between the upper and lower limits. As a
result, most people don't bother with this (nor is it often taught in a
private pilot course), and they never have a problem. On a bigger plane
where half the takeoff weight can be fuel, it's a much more significant
issue and these calculations are done for every takeoff and landing.

If you were really paranoid, you could calculate Vfinal and Va for three
loadings: pilot and minimum fuel, max gross, and halfway in between,
then keep these on your cheat sheet. In flight, just take a WAG which
of those you are closest to and use the appropriate number.

Thanks Roy...

And it didnt take me long to find more than I ever wanted to know about
this, too.. Stuff that brought me back flashbacks of my physics and
calculus classes..

Your formula breaks it down a little more simply than what I did stumble
across.. http://142.26.194.131/ and
http://142.26.194.131/aerodynamics1/Lift/index.htm

I will be puttering around with this and will see what I can come up
with, and if its useful.

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?

This originally was to come up with weight specific Va for the crib
sheet.. but I see (or was reminded of the basics) that Vs is weight
dependent too... even if the difference is negligible in the small spam
cans with only 300 pounds of fuel.

Dave

Roy Smith wrote:

Dave S wrote:

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


Easy. Va as published is for max gross weight, and goes down with the
square root of weight. So:

MGW = Max Gross Weight
W = Weight of the aircraft at a given moment
Va = Maneuvering speed as published in the POH
Va,w = Maneuvering speed for a given weight

Va,w = Va * sqrt (W / MGW)

Stall speeds (Vs0 and Vs1) both follow the same formula, and so does
your final approach speed, which is usually calculated as 1.3 * Vs0.
So, if you really want to do landings right, you should calculate your
weight at the end of the flight (taking into account fuel burn),
calculate a Vs0 based on that, and multiply by 1.3 to get your proper
final approach speed (keeping in mind that the multiplication needs to
be done in CAS, not IAS).

It turns out that for the majority of light airplanes, the difference
between max gross and a reasonable minimum landing weight (pilot and
minimum fuel) is a small enough percentage of max gross that stall speed
only varies a few knots between the upper and lower limits. As a
result, most people don't bother with this (nor is it often taught in a
private pilot course), and they never have a problem. On a bigger plane
where half the takeoff weight can be fuel, it's a much more significant
issue and these calculations are done for every takeoff and landing.

If you were really paranoid, you could calculate Vfinal and Va for three
loadings: pilot and minimum fuel, max gross, and halfway in between,
then keep these on your cheat sheet. In flight, just take a WAG which
of those you are closest to and use the appropriate number.

"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

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

"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.

Dan

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.

"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

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.

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...

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/

On Fri, 09 Jan 2004 16:50:14 GMT, "Tony Cox" wrote:

"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.

Check your reactions. You were given the EXACT mathematical relationship,
and a very close approximation to make in-flight calculations easier. I'm
sure you didn't mean to sound ungrateful, but that is the way it came
across!



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.