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Va and turbulent air penetration speed.



 
 
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  #1  
Old January 9th 04, 01:04 PM
Dave S
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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.


  #2  
Old January 9th 04, 01:46 PM
Roy Smith
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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.
  #3  
Old January 9th 04, 02:01 PM
Dave S
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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.


  #4  
Old January 9th 04, 03:01 PM
Gary Drescher
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Default

"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


  #5  
Old January 9th 04, 03:56 PM
Dave S
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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



  #6  
Old January 9th 04, 05:10 PM
Gary Drescher
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Posts: n/a
Default

"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





  #7  
Old January 9th 04, 10:23 PM
Dan Thomas
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Default

"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
  #8  
Old January 9th 04, 11:24 PM
Gary Drescher
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Posts: n/a
Default

"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



  #9  
Old January 10th 04, 06:01 PM
Dan Thomas
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Posts: n/a
Default

"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
  #10  
Old January 9th 04, 06:42 PM
Roy Smith
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Default

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.
 




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