Va and turbulent air penetration speed. [Archive]

Robert Moore

January 8th 04, 10:28 PM

(Doug) wrote

> 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.

And commonly incorrectly taught at that!

> 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.

Probably taught because of igorance.

Quoted from FAR Part 1:

VA means design maneuvering speed.

VB means design speed for maximum gust intensity.

VC means design cruising speed.

Quoted from Aerodynamics for Naval Aviators:

"The loads imposed on an aircraft in flight are the result of maneuvers and
gusts."
"As a general requirement, all airplanes must be capable of withstanding an
approximate effective +/- 30 foot per second gust when at maximum level
flight speed for normal rated power. Such a gust intensity has relatively
low frequency of occurrence in ordinary flying operations. The highest
reasonable gust velocity that may be anticipated is an actual veritical
velocity, U, of 50 feet per second."

Quoted from FAR 23:

Section 23.333: Flight envelope
(c) Gust envelope. (1) The airplane is assumed to be subjected to symmetrical
vertical gusts in level flight. The resulting limit load factors must
correspond to the conditions determined as follows:

(i) Positive (up) and negative (down) gusts of 50 f.p.s. at VC must be
considered..
(ii) Positive and negative gusts of 25 f.p.s. at VD must be
considered...........

End Quoting......

As can be seen, Vb, the speed for the maximum gust has nothing to do with Va.
How does one find Vb for most of the GA fleet?.....simple...just look
for the top of the green arc on the airspeed indicator.
By flying at Va during turbulence, one is flying un-necessarily close to
stall and a possible upset.
The aircraft can be loaded to a much higher value with the controls than
by a 50 fps gust.
I would suggest a look at the flight envelope picture in FAR Section 23.333.

Bob Moore
ATP CFI

Maule Driver

January 9th 04, 12:47 AM

"Robert Moore" > >
> As can be seen, Vb, the speed for the maximum gust has nothing to do with
Va.
> How does one find Vb for most of the GA fleet?.....simple...just look
> for the top of the green arc on the airspeed indicator.
> By flying at Va during turbulence, one is flying un-necessarily close to
> stall and a possible upset.

At Va, it would take a full, instaneous deflection of the controls to stall
the aircraft. How is that un-necessarily close to stall?

> The aircraft can be loaded to a much higher value with the controls than
> by a 50 fps gust.

Whicih is why it is recommended to fly at Va in turbulent conditions. That
is, 1) because a full deflection of the controls, presumably commanded to
recover from a turbulence induced upset, will result in a stall instead of a
breakup, and 2) incidently because it is conservatively lower than Vb and
therefore well within any expected maximum gust.

I sense this is going to be a long one...

Maule Driver

January 9th 04, 01:00 AM

"Doug" > >
> 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.

There should be no reason to go slower than Va for turbulence penetration.
The case is made later in this thread for using the faster Vb for such
conditions (I disagree but can be convinced).

However, one point that is often overlooked is that the published Va is
usally/always stated for max gross weight. At lesser weights, Va is lower
and can be significantly lower. Va isn't marked on the airspeed indicator
but it is an indicated airspeed.

ArtP

January 9th 04, 01:01 AM

On Thu, 08 Jan 2004 22:28:48 GMT, Robert Moore
> wrote:

>By flying at Va during turbulence, one is flying un-necessarily close to
>stall and a possible upset.

Which in my plane (SR20) is also 60% power at 50 degrees LOP, my
normal cruise (120 IAS) and I am nowhere near stall which is 65 IAS.

Robert Moore

January 9th 04, 01:22 AM

"Maule Driver" > wrote

> At Va, it would take a full, instaneous deflection of the controls to
> stall the aircraft. How is that un-necessarily close to stall?

A gust of sufficient value will also stall the a/c at Va.

> Whicih is why it is recommended to fly at Va in turbulent conditions.

With the FAA saying that Vb is the speed for maximum gust intensity,
who do find suggesting otherwise.

> 2) incidently because it is conservatively lower than Vb and therefore
> well within any expected maximum gust.

This is true, but also closer to a stall

> I sense this is going to be a long one...

Just post the documentation. :-)

Bob Moore

Robert Moore

January 9th 04, 01:29 AM

ArtP > wrote

> Which in my plane (SR20) is also 60% power at 50 degrees LOP, my
> normal cruise (120 IAS) and I am nowhere near stall which is 65 IAS

And I thought that every private pilot was taught that an airplane
can be stalled at any airspeed and any attitude. I assure you that I,
or a gust of sufficient value can stall your SR20 at 120 kts.

Bob Moore

Teacherjh

January 9th 04, 01:37 AM

Even if Vwhatever is "close to stall" (inasmuch as a gust could stall it), when
the gust is over, so is the stall. However, if that gust ripped your wings
off, when the gust is over, you don't recover your wings.

Jose

--
(for Email, make the obvious changes in my address)

ArtP

January 9th 04, 01:49 AM

On Fri, 09 Jan 2004 01:29:05 GMT, Robert Moore
> wrote:

>And I thought that every private pilot was taught that an airplane
>can be stalled at any airspeed and any attitude. I assure you that I,
>or a gust of sufficient value can stall your SR20 at 120 kts.

By my calculations, if I am flying in cruise it would take a tail wind
gust of 56 knots to stall me. I suspect that would fall in the
category of sever turbulence and I don't think there is any airspeed
that would be safe under those conditions in a single engine normal
category aircraft. In any case a stall at cruise altitude should not
be a problem but parts (like the engine or the wings) falling off the
aircraft would be.

ArtP

January 9th 04, 01:59 AM

Robert Moore

January 9th 04, 02:03 AM

ArtP > wrote

> By my calculations, if I am flying in cruise it would take a tail wind
> gust of 56 knots to stall me.

That's not the type of gust being discussed, try "vertical" gust.

Bob Moore

Dave S

January 9th 04, 02:17 AM

Art..

With all due respect.. you really need to go and review the section on
angle of attack and accelerated stalls.. I've been able to intentionally
stall a light (ASEL) aircraft in smooth air at Va.. its called a steep
turn with extra back pressure. It has nothing to do with "tailwind
gusts".. it has EVERYTHING to do with angle of attack.

Dave

ArtP wrote:
> On Fri, 09 Jan 2004 01:29:05 GMT, Robert Moore
> > wrote:
>
>
>
>>And I thought that every private pilot was taught that an airplane
>>can be stalled at any airspeed and any attitude. I assure you that I,
>>or a gust of sufficient value can stall your SR20 at 120 kts.
>
>
> By my calculations, if I am flying in cruise it would take a tail wind
> gust of 56 knots to stall me. I suspect that would fall in the
> category of sever turbulence and I don't think there is any airspeed
> that would be safe under those conditions in a single engine normal
> category aircraft. In any case a stall at cruise altitude should not
> be a problem but parts (like the engine or the wings) falling off the
> aircraft would be.

Maule Driver

January 9th 04, 03:49 AM

"Dave S" > >
> With all due respect.. you really need to go and review the section on
> angle of attack and accelerated stalls.. I've been able to intentionally
> stall a light (ASEL) aircraft in smooth air at Va.. its called a steep
> turn with extra back pressure. It has nothing to do with "tailwind
> gusts".. it has EVERYTHING to do with angle of attack.
>
Agreed. Wings stall at a given AOA, not at a speed.

ArtP

January 9th 04, 03:52 AM

On Fri, 09 Jan 2004 02:03:13 GMT, Robert Moore
> wrote:

>ArtP > wrote
>
>> By my calculations, if I am flying in cruise it would take a tail wind
>> gust of 56 knots to stall me.
>
>That's not the type of gust being discussed, try "vertical" gust.

Since the maximum vertical gust that the plane is designed to handle
is 30 knots (3000 feet per second) based upon a 30 kt vector and a 120
kt vector by my calculations the maximum change in the AOA before
reaching the design limit of the plane should be 14 degrees. An upward
gust would increase the AOA but that would also cause a rapid increase
in altitude which would be naturally corrected by a decrease in AOA
(even if on auto pilot). A downward gust would similarly be corrected
by trying to regain altitude. At cruise altitude in a small plane
neither of these should be particularly exciting as long as you don't
bounce off the ceiling too hard, something that is less likely at Va
than at a higher speed.

Doug

January 9th 04, 04:59 AM

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.

Mike Rapoport

January 9th 04, 04:59 AM

"Robert Moore" > wrote in message
. 6...
> "Maule Driver" > wrote
>
> > At Va, it would take a full, instaneous deflection of the controls to
> > stall the aircraft. How is that un-necessarily close to stall?
>
> A gust of sufficient value will also stall the a/c at Va.
>
> > Whicih is why it is recommended to fly at Va in turbulent conditions.
>
> With the FAA saying that Vb is the speed for maximum gust intensity,
> who do find suggesting otherwise.
>
> > 2) incidently because it is conservatively lower than Vb and therefore
> > well within any expected maximum gust.
>
> This is true, but also closer to a stall
>
> > I sense this is going to be a long one...
>
> Just post the documentation. :-)
>
> Bob Moore

How about you post an example of an airplane with a published Vb that is the
same as Vmo (or Vno)?

Mike
MU-2

Roy Smith

January 9th 04, 05:01 AM

ArtP > wrote:
> By my calculations, if I am flying in cruise it would take a tail wind
> gust of 56 knots to stall me.

Arrrgghhhh! No, that's not what's going on at all.

You stall when you exceed the critial angle of attack. One way to do
that is to slowly reduce airspeed under 1g conditions (i.e. straight and
level flight in non-turbulent air) until you reach Vs. At Vs and 1g,
the wing has to be operating at the critical AOA to generate lift equal
to the plane's weight, so you stall. This is your standard private
pilot power-off stall demo.

But, there are many other ways to reach critical AOA. One way is to
instantaneously change the direction the relative wind is coming from.
Let's say you're in straight and level flight at 120 KTAS. That's just
about 200 ft/s (feet per second). I'm going to pull two plausable
numbers out of my butt for the sake of argument:

1) At 120 KTAS, 1g, and whatever you weigh at the moment, the wing is
operating at an AOA of 5 degrees.

2) The critial AOA for your wing is 18 degrees.

The relative wind is coming from straight ahead and the wing is tilted
up at an angle of 5 degrees from the horizontal. This gives you your 5
degree AOA. NOTE: this doesn't mean your nose is pitched up 5 degrees,
because the wing has a built-in angle of incidence, i.e. the chord line
of the wing is not parallel with the longitudinal axis of the plane.

Now, let's say you hit a vertical gust of 50 ft/s. That means the
relative wind is now 200 ft/sec straight back plus 50 ft/sec up. If I
did my vector sums right, that means it's now 206 ft/sec coming from an
angle of 14 degrees below the horizontal. Since your wing is tilted up
at 5 degrees, the AOA is suddenly 5 + 14 = 19 degrees, which is greater
than the critical AOA. Presto, you are suddenly stalled!

If the gust were just a little bit weaker, you would only reach an AOA
of, say, 17 degrees. This is below the critcial AOA so you're not
stalled. But, the AOA has suddenly gone up by a factor of 3.4. Since
you're still going at the same airspeed (actually, a little bit more,
206 instead of 200 ft/s), you're suddenly generating 3.4 times as much
lift, and you will find yourself being accelerated upward at 3.4 g's,
which is enough to make your socks droop down around your ankles. Well
endowed men or women will be wishing they had worn more supportive
underwear about now. But, since your wings were probably designed to
withstand 3.8 g's (plus 150% safety factor), they should stay on and not
even get bent or wrinkled.

Here's the mantra; repeat after me:

"Stall is when you exceed the critial AOA. You can do this in any
flight attitude, at any airspeed"

Doug

January 9th 04, 05:06 AM

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?

ArtP

January 9th 04, 06:13 AM

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.

Hilton

January 9th 04, 07:49 AM

ArtP wrote:
> By my calculations, if I am flying in cruise it would take a tail wind
> gust of 56 knots to stall me.

Wings don't stall because of a lack of airspeed.

Hilton

Dave S

January 9th 04, 12:51 PM

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.

Dave S

January 9th 04, 01:04 PM

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.

Roy Smith

January 9th 04, 01:46 PM

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

January 9th 04, 02:01 PM

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.

Gary Drescher

January 9th 04, 02:40 PM

"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

Tony Cox

January 9th 04, 02:56 PM

"Orval Fairbairn" > wrote in message
...
> 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/

Gary Drescher

January 9th 04, 03:01 PM

"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

Tony Cox

January 9th 04, 03:16 PM

"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 here:-

http://www.access.gpo.gov/nara/cfr/cfrhtml_00/Title_14/14cfr23_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/

Dave S

January 9th 04, 03:51 PM

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
>
>

Maule Driver

January 9th 04, 03:54 PM

"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.

Dave S

January 9th 04, 03:56 PM

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
>
>

Blanche

January 9th 04, 04:17 PM

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

January 9th 04, 04:50 PM

"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/

Gary Drescher

January 9th 04, 04:52 PM

"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

January 9th 04, 05:10 PM

"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
> >
> >
>

Mike Rapoport

January 9th 04, 05:27 PM

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.

Roy Smith

January 9th 04, 06:36 PM

"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

January 9th 04, 06:42 PM

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.

Gary Drescher

January 9th 04, 06:43 PM

"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

Dan Thomas

January 9th 04, 10:18 PM

(Doug) wrote in message >...
> 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.

Doesn't have to. Pulling full up elevator loads the wings the
same as a strong vertical gust; both increase AOA and the airplane and
occupants both feel increased G loading. At or below Va the airplane
will stall and thereby unload the structure somewhat if the AOA
reaches stall angle, and the load factor won't exceed the designed
structural limits.
Va is lower at lower weights because the airplane is going to
tend to change direction more easily in a gust or sharp pull-up, and
the directional change keeps AOA below stall angle and maintains the
high wing loading. Lowering Va will allow it to stall sooner.
Everybody fears wing failure, but many airplanes will suffer tail
failure first. Bonanzas and 210s are famous for such accidents, with a
VFR pilot entering IMC and losing control. He pops out of the overcast
at 400 feet in a screaming spiral dive, and promptly pulls up hard.
The stabilizer fails downward, then the airplane pitches forward onto
its back and the wings fail downward (negative Gs).

Dana

Dan Thomas

January 9th 04, 10:23 PM

"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

Gary Drescher

January 9th 04, 11:24 PM

"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

Resident Cynic

January 10th 04, 12:45 AM

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.

Resident Cynic

January 10th 04, 12:46 AM

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
>

Dave S

January 10th 04, 05:33 AM

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!
>
>

Julian Scarfe

January 10th 04, 11:29 AM

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

> You'll find Va covered in the FAR's for part 23 certified
> aircraft here:-
>
> http://www.access.gpo.gov/nara/cfr/cfrhtml_00/Title_14/14cfr23_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

Gary Drescher

January 10th 04, 12:56 PM

"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!
> >
> >
>

Julian Scarfe

January 10th 04, 01:05 PM

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

Tony Cox

January 10th 04, 01:19 PM

"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/

Barry

January 10th 04, 01:49 PM

> 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

Tony Cox

January 10th 04, 02:04 PM

"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 Drescher

January 10th 04, 05:07 PM

"Tony Cox" > wrote in message
k.net...
> "Gary Drescher" > wrote in message
> news:HDSLb.15032$I06.94614@attbi_s01...
> > ... 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.

While I understand your earlier point about the certification regulations,
nonetheless Va is explicitly defined in some places as "the maximum speed at
which you may use abrupt control travel" (C172P POH, for example), and that
speed _does_ scale with weight (and the C172P POH, for example, specifies
different values of Va for different weights).

But the more important question concerns the physics, not the terminology.

> 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.

Agreed. More specifically, we're looking for the speed at which the lift
force resulting from an abrupt transition to the maximum coefficient of lift
would not accelerate the plane enough to exceed the force that any of the
plane's components can withstand.

> 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.

Sure. Some component is going to be the weak link, capable of withstanding
less force than the others.

> 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.

Here I don't follow you. If the components have constant mass and each
component has a maximum force that it can withstand, then each component
thereby has a maximum acceleration that it can withstand, does it not? And
for any given acceleration, the maximum airspeed at which abrupt control
deflection would not exceed that acceleration (namely, the maximum speed at
which the maximum coefficient of lift would not provide enough force to
exceed that acceleration) does indeed scale in proportion to the square root
of the plane's weight.

> 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.

Is there any better guideline for a pilot than to use the published
max-gross Va, scaled in proportion to the square root of current gross
weight, as the limiting speed for abrupt control deflections?

--Gary

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

Dan Thomas

January 10th 04, 06:01 PM

"Gary Drescher" > wrote in message news:<wKGLb.9047$8H.23200@attbi_s03>...
> "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?

Yes, they are. being a Canadian, I can quote the CARs but the
FARs are a different matter. I'l see what they have to say. In any
case, when have you ever heard of an engine departing an airplane in
turbulence or during violent maneuvering? Our Citabria has a G-meter
in it, and we have seen some pretty big numbers when students get
clumsy on landing.
Landing forces don't affect wings much, since they're still
generating lift and the landing forces on the structure tend to be
negative, and if the engine mount was a 5G structure like the rest of
the airplane it would have fallen off long ago. A missing 300 pounds
or so during a hard landing would be disastrous: CG way back near the
trailing edge, an airplane suddenly much lighter, and airspeed still
sufficient to flip the whole works over into a crash and burn
scenario, all for the lack of another pound or so of tubing.
The only times I have heard of engine mounts failing on light
airplanes is when a prop throws part of a blade, or maybe the whole
blade on a constant-speed prop. The imbalance is more than enough to
rip the engine off the airplane. Blades will fail when propeller nicks
are left untreated and cracks develop. The prop is the most highly
stressed bit of metal on the whole airplane, and THAT'S what pilots
should be concerned about, not engine mounts.

Dan

Tony Cox

January 10th 04, 06:33 PM

"Gary Drescher" > wrote in message
news:CjWLb.15152$8H.35818@attbi_s03...
> "Tony Cox" > wrote in message
> k.net...
> >
> > 1) Va by definition is just a number and _does not_ scale
> > with weight.
>
> While I understand your earlier point about the certification regulations,
> nonetheless Va is explicitly defined in some places as "the maximum speed
at
> which you may use abrupt control travel" (C172P POH, for example), and
that
> speed _does_ scale with weight (and the C172P POH, for example, specifies
> different values of Va for different weights).

Hi Gary. As Julian pointed out, there may be terminology problems
here. It may well be that the 172 POH defines Va as you say, but in
that case Cessna are telling you something more -- they are telling you
specifically that their Va is defined to meet the equality condition in
23.335. So it is really just their own private definition, applicable to
that plane and model year only. And of course if the equality condition
applies, then scaling proportional to sqrt(w) should be adequate or
better as I pointed out earlier (and discuss further below).

Now my POH just gives me Va. Nothing about abrupt control
inputs or anything. So absent other information, it's not really
much use to me. I could yank the yoke at Va and watch the wings
fall off - all this in a properly certified plane. Odd, eh? Of course,
its not likely to happen because of the additional safety factors
built into the design, but I could still be exceeding the load factor.

>
> > 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.
>
> Here I don't follow you. If the components have constant mass and each
> component has a maximum force that it can withstand, then each component
> thereby has a maximum acceleration that it can withstand, does it not? And
> for any given acceleration, the maximum airspeed at which abrupt control
> deflection would not exceed that acceleration (namely, the maximum speed
at
> which the maximum coefficient of lift would not provide enough force to
> exceed that acceleration) does indeed scale in proportion to the square
root
> of the plane's weight.

Well, the control surfaces don't care how much weight is in the
plane (at least to first order). If you yank them lightly loaded, you'll
stress the cables and hinges just the same as if you were over gross.
So that Va'(w) is flat if you plot it against w.

Things like baggage compartment objects stress should scale like
sqrt(w). Wing bolts (on a Cessna) are more complicated. At less
weight - lets assume this is mostly less weight in the cabin - you'll
be able to withstand greater acceleration; load factor isn't really
the issue here. Va'(w) in this case probably drops off less rapidly
with decreasing w than sqrt(w).

Now I think it is true that there is no component for which
Va'(w) falls _faster_ than sqrt(w) with decreasing w. In this case,
scaling an overall Va' at gross by sqrt(w) should mean that
it doesn't matter which Va' is the limiting component, you'll
always be _at or below_ it's corresponding Va'(w).

But caution two things. If you're certified over gross
(91.323), you shouldn't use the relationship to computer a higher
Va. Control surfaces might be the limiting factor, for example,
and they don't scale at all.

Second, if the equality in 21.335 isn't met, control surfaces
are the limiting factor. You can scale them by sqrt(w) if you
like, but it'll be meaningless -- the answer you get will _still_
be above the Va' for things that are load-factor limited.

>
> > 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.
>
> Is there any better guideline for a pilot than to use the published
> max-gross Va, scaled in proportion to the square root of current gross
> weight, as the limiting speed for abrupt control deflections?
>

That's the key question isn't it?

As Julian pointed out, if you have Vo, then you should use
that. But my plane doesn't, and I don't think most of the fleet
does either. Vo is, I believe, a recent certification requirement.

Here's how I approach it. And of course, YMMV, so I hope
no one does the same without thinking about it first.

I think that for most GA planes, the equality in 21.335 applies.
Why overbuild control surfaces? The FAA says that you
*can* set Va above Vs*sqrt(lf), but to do so costs $$$'s and
lowers the useful load. So I think it is a reasonable assumption,
at least for my 182. This means, of course, that the sqrt(w)
relationship ought to be adequate to protect me, which is indeed
how I fly. But I realize I may be flying outside the load-factor
safety zone. After all, there should be a 50% margin to play with!

When I used to fly 172's, I noticed that some were certified in
the utility category at certain light weights. This suggests (but doesn't
guarantee) that the limiting factor in Va' is the wings, rather than
cabin load. Since this scales better than sqrt(w), I think you can
probably fly faster than the scaled Va without issue.

My 182 has no such certification -- it's all normal category.
It might mean that no one could be bothered to certify it in
the utility category, but it might also mean that the wings are
not the limiting factor. This I find comforting & I'm more careful
to maintain Va'(w) in rough air. (why in rough air is, I suppose,
where we came in).

BTW, that 'sqrt(w)' business is quite an approximation in itself,
and relies on quite a few assumptions which are probably not
that supportable over a wide range of w's...

Hope you've found this rant more informative than pedantic!

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

Tony Cox

January 10th 04, 06:35 PM

"Dan Thomas" > wrote in message
om...

> The only times I have heard of engine mounts failing on light
> airplanes is when a prop throws part of a blade, or maybe the whole
> blade on a constant-speed prop. The imbalance is more than enough to
> rip the engine off the airplane. Blades will fail when propeller nicks
> are left untreated and cracks develop. The prop is the most highly
> stressed bit of metal on the whole airplane, and THAT'S what pilots
> should be concerned about, not engine mounts.
>

Nah. Properly designed engine mounts would never let that happen.

Julian Scarfe

January 10th 04, 06:54 PM

> "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.

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

FWIW, here's the extract from AC23-19 (which is entirely consistent with
what you've said)

---
(c) The design maneuvering speed is a value chosen by the applicant. It may
not be less than Vs*sqrt(n) and need not be greater than Vc, but could be
greater if the applicant chose the higher value. The loads resulting from
full control surface deflections at Va
are used to design the empennage and ailerons in 14 CFR part 23, §§ 23.423,
23.441, and 23.455.

Va should not be interpreted as a speed that would permit the pilot
unrestricted flightcontrol movement without exceeding airplane structural
limits nor should it be interpreted as a gust penetration speed. Only if Va
= Vs*sqrt(n), will the airplane stall in a nose-up pitching maneuver at, or
near, limit load factor. For maneuvers where Va>Vs*sqrt(n), the pilot would
have to check the maneuver; otherwise the airplane would exceed the limit
load factor.

Amendment 23-45 added the operating maneuvering speed, Vo in § 23.1507. Vo
is established not greater than Vs*sqrt(n), and is a speed where the
airplane will stall in a nose-up pitching maneuver before exceeding the
airplane structural limits.
---

Julian Scarfe

Gary Drescher

January 10th 04, 07:12 PM

"Tony Cox" > wrote in message
ink.net...
> Hi Gary. As Julian pointed out, there may be terminology problems
> here. It may well be that the 172 POH defines Va as you say, but in
> that case Cessna are telling you something more -- they are telling you
> specifically that their Va is defined to meet the equality condition in
> 23.335. So it is really just their own private definition, applicable to
> that plane and model year only.

Yeah, except that the POH (or rather aircraft manual) acquires regulatory
force from the FARs, so it's not just a private definition; rather, as usual
with the FAA, it's one of several mutually inconsistent definitions that's
in official use. (For what it's worth, the Piper Arrow POH gives
essentially the same definition as the C172P POH.)

> Well, the control surfaces don't care how much weight is in the
> plane (at least to first order). If you yank them lightly loaded, you'll
> stress the cables and hinges just the same as if you were over gross.
> So that Va'(w) is flat if you plot it against w.

Right, but aren't the wings and control surfaces protected by Vno (a
weight-invariant force limit) rather than by Va (a weight-dependent
acceleration limit)? That's how I think about it anyway, even if it doesn't
match (some of) the official definitions.

> Hope you've found this rant more informative than pedantic!

Sure, and I don't mind pedantry anyway. :-)

--Gary

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

Dave S

January 10th 04, 11:07 PM

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!
>>>
>>>
>>
>
>

Gary Drescher

January 11th 04, 12:00 AM

"Dave S" > wrote in message
.net...
> 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.

Dave, I apologize if I misinterpreted your request. If you knew that the
formula is Va times the square root of weight-divided-by-max-gross, but you
didn't know that the corresponding Excel expression is
Va*SQRT(weight/max_gross), then you did indeed understand the concepts, and
just needed a hand with the spreadsheet itself. (Note though that you did
incorrectly classify Vx and Vy among the "static", weight-independent
speeds, which contributed to my impression that the concepts themselves were
in question.)

> I wasnt looking for approximations, pnemonics or handy dandy quick tools
> that serve as memory aids for people who have trouble with math.

Well, here I think _you've_ misinterpreted _me_. The approximation I
suggested was not a mnemonic, and had nothing to do with having trouble with
math--unless you consider the inability to mentally calculate square roots
while flying a plane to consititute a math deficiency. :-) I regard that
approximation as an important tool that should be familiar to any good
pilot, so my suggesting it was in no way a put-down.

Anyway, I was honestly trying to be constructive. I apologize again if I
did not succeed.

--Gary

> 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!
> >>>
> >>>
> >>
> >
> >
>

Dan Thomas

January 11th 04, 12:45 AM

"Tony Cox" > wrote in message et>...
> "Dan Thomas" > wrote in message
> om...
>
> > The only times I have heard of engine mounts failing on light
> > airplanes is when a prop throws part of a blade, or maybe the whole
> > blade on a constant-speed prop. The imbalance is more than enough to
> > rip the engine off the airplane. Blades will fail when propeller nicks
> > are left untreated and cracks develop. The prop is the most highly
> > stressed bit of metal on the whole airplane, and THAT'S what pilots
> > should be concerned about, not engine mounts.
> >
>
> Nah. Properly designed engine mounts would never let that happen.

Cessna 185 operated by JAARS Inc, South America, about ten years ago.
Threw a blade and the engine tore off the mount before the pilot could
shut it down. It turned sideways in the cowl, and the cowl was the
only thing keeping it from departing entirely. The O-520 mount is a
bed mount; if it had been the usual rear mounting the engine would
have fallen off.

Dan

karl

January 11th 04, 12:50 AM

*****The O-520 mount is a
bed mount; if it had been the usual rear mounting the engine would
have fallen off.****

No, it isn't a bed mount in a Cessna 185. The mount attaches to fourpoints
on the firewall.

A cessna 206, however, has a bed mount.

Karl
"curator" N185KG

Tony Cox

January 11th 04, 01:51 AM

"Gary Drescher" > wrote in message
news:j8YLb.15333$na.12586@attbi_s04...
> "Tony Cox" > wrote in message
> ink.net...
> > Hi Gary. As Julian pointed out, there may be terminology problems
> > here. It may well be that the 172 POH defines Va as you say, but in
> > that case Cessna are telling you something more -- they are telling you
> > specifically that their Va is defined to meet the equality condition in
> > 23.335. So it is really just their own private definition, applicable to
> > that plane and model year only.
>
> Yeah, except that the POH (or rather aircraft manual) acquires regulatory
> force from the FARs, so it's not just a private definition; rather, as
usual
> with the FAA, it's one of several mutually inconsistent definitions that's
> in official use. (For what it's worth, the Piper Arrow POH gives
> essentially the same definition as the C172P POH.)

I suppose its that old terminology problem again. What can
I say? The FAR's are quite explicit on how Va is defined, and
that is most definitely _not_ what is in the POH. Thank heavens
for the 50% safety factor, or we'd have planes falling out of the
sky all over.

But thinking again, I don't see the problem even if the POH inherits
regulatory authority. It is, after all, only true in the context of that
particular make and model (which is consistent with the FAR
definition when 23.335 takes the equality). It's only when you
extend that definition to cover other planes that it doesn't ring true.

>
> > Well, the control surfaces don't care how much weight is in the
> > plane (at least to first order). If you yank them lightly loaded, you'll
> > stress the cables and hinges just the same as if you were over gross.
> > So that Va'(w) is flat if you plot it against w.
>
> Right, but aren't the wings and control surfaces protected by Vno (a
> weight-invariant force limit) rather than by Va (a weight-dependent
> acceleration limit)? That's how I think about it anyway, even if it
doesn't
> match (some of) the official definitions.

Vno doesn't say anything about control input. I've always wondered
how it is established. Seems like a test pilot would earn his or
her money finding out. I've always assumed that the windshield
would be the first thing to go...

>
> > Hope you've found this rant more informative than pedantic!
>
> Sure, and I don't mind pedantry anyway. :-)
>

Hey thanks! A strangely interesting subject, don't you think?

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

Tony Cox

January 11th 04, 01:58 AM

"Dan Thomas" > wrote in message
om...
> "Tony Cox" > wrote in message
et>...
> > "Dan Thomas" > wrote in message
> > om...
> >
> > > The only times I have heard of engine mounts failing on light
> > > airplanes is when a prop throws part of a blade, or maybe the whole
> > > blade on a constant-speed prop. The imbalance is more than enough to
> > > rip the engine off the airplane. Blades will fail when propeller nicks
> > > are left untreated and cracks develop. The prop is the most highly
> > > stressed bit of metal on the whole airplane, and THAT'S what pilots
> > > should be concerned about, not engine mounts.
> > >
> >
> > Nah. Properly designed engine mounts would never let that happen.
>
> Cessna 185 operated by JAARS Inc, South America, about ten years ago.
> Threw a blade and the engine tore off the mount before the pilot could
> shut it down. It turned sideways in the cowl, and the cowl was the
> only thing keeping it from departing entirely. The O-520 mount is a
> bed mount; if it had been the usual rear mounting the engine would
> have fallen off.
>

What's a bed mount, Dan? I've always thought (hoped) that my
182 cowling would contain the engine. And there is always the
fuel line, throttle cable, and battery cable as a last line of defense ;)

Gary Drescher

January 11th 04, 01:59 AM

"Tony Cox" > wrote in message
ink.net...
> But thinking again, I don't see the problem even if the POH inherits
> regulatory authority. It is, after all, only true in the context of that
> particular make and model (which is consistent with the FAR
> definition when 23.335 takes the equality). It's only when you
> extend that definition to cover other planes that it doesn't ring true.

Agreed. In my short few years as a pilot so far, the planes I've flown
(152s, 172s, Warriors, and Arrows) have all had essentially the same
definition of Va in their POHs, so I didn't realize it wasn't universal.

> Hey thanks! A strangely interesting subject, don't you think?

Yup. Thanks for the discussion!

--Gary

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

Roger Halstead

January 11th 04, 09:14 AM

On Thu, 08 Jan 2004 21:52:48 GMT, Orval Fairbairn
> wrote:

>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.
>>
>> 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.
>
>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.)

Even then there is no gurantee there will be no damage from a vertical
gust exceeding 30 fps, or that is the figure used for Bonanzas. Va,
30 fps = a 4 to a 4 1/2 G load which is the load limit for the utility
category. A vertical gust greater than 30 fps at Va would give a
greater load.

If need be I can quote/copy the paragraph right out of the ABS Pilot
Proficency Training Manual.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

Roger Halstead

January 11th 04, 09:21 AM

On Fri, 09 Jan 2004 02:17:48 GMT, Dave S >
wrote:

>Art..
>
>With all due respect.. you really need to go and review the section on
>angle of attack and accelerated stalls.. I've been able to intentionally
>stall a light (ASEL) aircraft in smooth air at Va.. its called a steep
>turn with extra back pressure. It has nothing to do with "tailwind
>gusts".. it has EVERYTHING to do with angle of attack.

It ends up being a bit slower than Va, but I practice steep turns at
60 degree bank with the stall warning horn blowing. Then add a bit of
extra pull to get the beak.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com
>
>Dave
>
>ArtP wrote:
>> On Fri, 09 Jan 2004 01:29:05 GMT, Robert Moore
>> > wrote:
>>
>>
>>
>>>And I thought that every private pilot was taught that an airplane
>>>can be stalled at any airspeed and any attitude. I assure you that I,
>>>or a gust of sufficient value can stall your SR20 at 120 kts.
>>
>>
>> By my calculations, if I am flying in cruise it would take a tail wind
>> gust of 56 knots to stall me. I suspect that would fall in the
>> category of sever turbulence and I don't think there is any airspeed
>> that would be safe under those conditions in a single engine normal
>> category aircraft. In any case a stall at cruise altitude should not
>> be a problem but parts (like the engine or the wings) falling off the
>> aircraft would be.

Roger Halstead

January 11th 04, 09:29 AM

On 9 Jan 2004 14:18:14 -0800, (Dan Thomas)
wrote:

(Doug) wrote in message >...
>> 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.
>
> Doesn't have to. Pulling full up elevator loads the wings the
>same as a strong vertical gust; both increase AOA and the airplane and
>occupants both feel increased G loading. At or below Va the airplane
>will stall and thereby unload the structure somewhat if the AOA
>reaches stall angle, and the load factor won't exceed the designed
>structural limits.

IF you are cruising at Va and encounter a vertical gust that causes a
stall right at the design limit you survive. What happens when you
hit a vertical gust of twice the velocity of the first? According to
the ABS and Airsafety Foundation, you are going to break your
airplane.

> Va is lower at lower weights because the airplane is going to
>tend to change direction more easily in a gust or sharp pull-up, and
>the directional change keeps AOA below stall angle and maintains the
>high wing loading. Lowering Va will allow it to stall sooner.
> Everybody fears wing failure, but many airplanes will suffer tail
>failure first. Bonanzas and 210s are famous for such accidents, with a

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

>VFR pilot entering IMC and losing control. He pops out of the overcast
>at 400 feet in a screaming spiral dive, and promptly pulls up hard.
>The stabilizer fails downward, then the airplane pitches forward onto
>its back and the wings fail downward (negative Gs).
>
> Dana

Robert Moore

January 11th 04, 02:38 PM

Roger Halstead > wrote
>
> IF you are cruising at Va and encounter a vertical gust that causes a
> stall right at the design limit you survive. What happens when you
> hit a vertical gust of twice the velocity of the first? According to
> the ABS and Airsafety Foundation, you are going to break your
> airplane.

Again referencing a couple of previous posts:
-------------------------------------------------------
Quoted from Aerodynamics for Naval Aviators:

"As a general requirement, all airplanes must be capable of withstanding an
approximate effective +/- 30 foot per second gust when at maximum level
flight speed for normal rated power. Such a gust intensity has relatively
low frequency of occurrence in ordinary flying operations. The highest
reasonable gust velocity that may be anticipated is an actual veritical
velocity, U, of 50 feet per second."
-------------------------------------------------------
And from FAR 23

Section 23.333: Flight envelope
(c) Gust envelope. (1) The airplane is assumed to be subjected to
symmetrical vertical gusts in level flight. The resulting limit load
factors must correspond to the conditions determined as follows:

(i) Positive (up) and negative (down) gusts of 50 f.p.s. at VC must be
considered.........

(ii) Positive and negative gusts of 25 f.p.s. at VD must be
considered...........
--------------------------------------------------------

Now since 50 fps is the highest reasonable gust that may be anticipated,
and all aircraft are designed to withstand this gust all the way up to
Vc, what causes the wings to come off? As Dana has posted:

>>VFR pilot entering IMC and losing control. He pops out of the overcast
>>at 400 feet in a screaming spiral dive, and promptly pulls up hard.
>>The stabilizer fails downward, then the airplane pitches forward onto
>>its back and the wings fail downward (negative Gs).

Yes, I understand that older aircraft may have been certificated to only
a 30 fps gust value, but as pointed out in AFNA above, that will be
encountered very infrequently and in my opinion, never outside of a
thunderstorm. In fact, the aiframe must withstand the 25 fps gust (not
far from 30 fps) all the way to the maximum demonstrated dive speed.

The aircraft is already designed for the maximum anticipated gust. There
is no gust "twice the velocity" for which the aircraft is designed.

Is there some reason that you don't copy/paste or at least provide a url
for the material that you have referenced? Who is (are) the ABS and what
engineering credentials are possesed by members of the AOPA's Airsafety
Foundation?

Bob Moore

Dan Thomas

January 11th 04, 08:30 PM

"karl" > wrote in message >...
> *****The O-520 mount is a
> bed mount; if it had been the usual rear mounting the engine would
> have fallen off.****
>
> No, it isn't a bed mount in a Cessna 185. The mount attaches to fourpoints
> on the firewall.
>
> A cessna 206, however, has a bed mount.
>
> Karl
> "curator" N185KG

The mount attaches to the firewall, but extends underneath the
engine and attaches to four mounts on the *bottom* of the case. These
are the mounts that failed, not the tubing. The engine in question was
resting on that structure when the noise was finished.

Dan Thomas

January 11th 04, 08:35 PM

"Tony Cox" > wrote in message t>...
> "Dan Thomas" > wrote in message
> om...
> > "Tony Cox" > wrote in message
> et>...
> > > "Dan Thomas" > wrote in message
> > > om...
> > >
> > > > The only times I have heard of engine mounts failing on light
> > > > airplanes is when a prop throws part of a blade, or maybe the whole
> > > > blade on a constant-speed prop. The imbalance is more than enough to
> > > > rip the engine off the airplane. Blades will fail when propeller nicks
> > > > are left untreated and cracks develop. The prop is the most highly
> > > > stressed bit of metal on the whole airplane, and THAT'S what pilots
> > > > should be concerned about, not engine mounts.
> > > >
> > >
> > > Nah. Properly designed engine mounts would never let that happen.
> >
> > Cessna 185 operated by JAARS Inc, South America, about ten years ago.
> > Threw a blade and the engine tore off the mount before the pilot could
> > shut it down. It turned sideways in the cowl, and the cowl was the
> > only thing keeping it from departing entirely. The O-520 mount is a
> > bed mount; if it had been the usual rear mounting the engine would
> > have fallen off.
> >
>
> What's a bed mount, Dan? I've always thought (hoped) that my
> 182 cowling would contain the engine. And there is always the
> fuel line, throttle cable, and battery cable as a last line of defense ;)

I couldn't trust a few wires and cables to keep the weight of the
engine attached to the firewall.
I spoke with a former 747 pilot who was into Formula 1 racing. He
had a single-seat (Cassutt?) racer that used an O-200 turning a tiny
prop at 4400 RPM to get the flat-out speed he wanted. I asked him
about the prop failure/engine departure scenario, and he told me that
he (and his buddies, in their airplanes) had a stout cable wrapped
around the engine and bolted to the firewall to cover this
eventuality. Apparently it had happened more than once before to other
unfortunate racers.

Dan