Va and turbulent air penetration speed.

Ok.. you win.. because ANY speed can be "dangerously close" to stall
speed depending on your angle of attack.

Of course... stalls in and of themselves are not dangerous.. its the
unrecognized ones in uncoordinated flight close to the ground that seem
to present the most problems..

ArtP wrote:
On Fri, 09 Jan 2004 03:49:19 GMT, "Maule Driver"
wrote:


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.

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..

If anyone HAS or KNOWS (or has the formulas)how to do this in Excel,
please feel free to pass it on.. I'm sure I will figure out or find what
I need sooner or later, but I'm not wanting to reinvent the wheel if I
dont have to.. this is for myself and some flying club members (and for
anyone else who happens to see it on here).. not a school project or
work assignment of any kind.

Dave PP-ASEL

Doug wrote:

Kershner's "The Advanced Pilot's Flight Manual" has the following
definition for Va.

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

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.

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...
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

"Orval Fairbairn" wrote in message
news
In article ,
(Doug) wrote:

Kershner's "The Advanced Pilot's Flight Manual" has the following
definition for Va.

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


Va is the MAXIMUM speed (at max gross weight) at which the aircraft can
be stalled without exceeding its max designed load factor. The lower
the gross weight, the lower the effective Va (due to engine mounts, etc.)

This is incorrect. In fact, *both* definitions are incorrect (with
all due respect to Kershner). For planes certified under Pt 23,
flying at Va *does not* guarantee that you'll stall before exceeding
max design load factor, nor that you'll not overstress the airplane.

I know this will be controversial & generate a lot of heat, but
check out the thread "Overweight takeoff / flight" from last
November in rec.aviation.piloting. I used to believe this about
Va too, but it is wrong.

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

"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

"Doug" wrote in message
om...
Orval Fairbairn wrote in message
.. .

Va is the MAXIMUM speed (at max gross weight) at which the aircraft can
be stalled without exceeding its max designed load factor. The lower
the gross weight, the lower the effective Va (due to engine mounts,
etc.)

Where does the above definition come from?

You'll not find it anywhere, because it is incorrect.

You'll find Va covered in the FAR's for part 23 certified
aircraft he-

http://www.access.gpo.gov/nara/cfr/c...4cfr23_00.html

The relevant sections are 23.335 (where you see that Va
can be *no less than* Vs*sqrt (load factor) -- which means
that _it can be greater than this_, and 23.423 where its
relationship to control surfaces is discussed.

Short answer: Va is defined in terms of what the *control
surfaces* can handle, not what the plane can handle.
However, Va must be sufficiently high to satisfy 23.335,
which means (oddly) that there is no regulatory requirement
that stops you from flying at Va without exceeding the load
factor. Well fancy that!.

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

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

"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.