Va and turbulent air penetration speed.

"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

"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.

"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

On Fri, 09 Jan 2004 18:43:43 GMT, "Gary Drescher"
wrote:

"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.

Right! For a C-172 (most models/years) the error gets pretty large at Vs0
-- 50 KCAS = 33 KIAS. That difference is certainly NOT trivial.


--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!

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

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...
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!

"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/

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!