Va and turbulent air penetration speed.

"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

"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?

In any case, it's just an example. The crucial point is that Va is a speed
that limits the _acceleration_ that the control surfaces can impose before
the plane stalls, whereas Vno is a speed that limits the _force_ that the
wings can develop before the plane stalls. Therefore, staying below Vno is
what keeps the wings attached and intact, whereas staying below Va is what
keeps _other_ parts of the plane attached and intact (because the plane's
acceleration determines the force exterted upon other structures). This
distinction is key to understanding why Va is proportionate to the square
root of weight, whereas Vno is independent of weight. (Whether or not the
engine mounts are the weak link in the rest of the plane presumably varies
from one aircraft to another.)

--Gary


Dan

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.

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

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!

"Tony Cox" wrote in message
. net...

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!.

Your points are excellent, but I think the issue is over terminology.

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.

Julian Scarfe

"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!

I 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".

Just to clarify, Vsvn is the way that Vs*sqrt (n) pasted from the text. n
is the "positive limit maneuvering load factor".

Julian

"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.


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.

Certainly, terminology caused a lot of confusion the last time
this was discussed, and I expect it will again if the thread
continues!

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

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