V-n Diagrams

[john smith]

I have been playing around making V-n diagrams for the various airplanes
I rent from my flying club.
I made one for our P28T-201RT and was looking for the numbers on the
PA32-300.

The one item for the PA32-300 that would not follow the formula for
calculating the lift line was Va. The book shows it as 132 kias, but the
formula generates a speed of 103 kias for the intercept of the 3.8
load limit line.

Can anyone explain this discrepancy?

It is interesting to note that Piper does not allow any negative
maneuvers in the Normal catagory, on 3.8 positive. (Cessna does, -1.52.
Another reason to own a high-wing! ;-)))
Another interesting item was that both have a Vne of 192 kias and Vfe of
108 kias.

[Bob Moore]

john smith > wrote

> The one item for the PA32-300 that would not follow the formula for
> calculating the lift line was Va. The book shows it as 132 kias, but
> the formula generates a speed of 103 kias for the intercept of the
> 3.8 load limit line.

When is 3.8g not 3.8g? When it is 4.5g or some other number.
Remember, 3.8g is a minimum figure for the normal category.
I have seen V-N diagrams for aircraft where the 3.8g line was
extended upward to accomodate the gust loading envelope at Vno.
Observe the gust envelope in the diagram at this web site:
http://adg.stanford.edu/aa241/structures/vn.html


> It is interesting to note that Piper does not allow any negative
> maneuvers in the Normal catagory, on 3.8 positive.

This is contrary to the required limits in FAR 23

Bob Moore

[Mike Rapoport]

The load limit for normal catagory isn't always 3.8G. In a 747 it is around
2G and a MU-2 is 3.25G.

Mike
MU-2


"Bob Moore" > wrote in message
. 6...
> john smith > wrote
>
> > The one item for the PA32-300 that would not follow the formula for
> > calculating the lift line was Va. The book shows it as 132 kias, but
> > the formula generates a speed of 103 kias for the intercept of the
> > 3.8 load limit line.
>
> When is 3.8g not 3.8g? When it is 4.5g or some other number.
> Remember, 3.8g is a minimum figure for the normal category.
> I have seen V-N diagrams for aircraft where the 3.8g line was
> extended upward to accomodate the gust loading envelope at Vno.
> Observe the gust envelope in the diagram at this web site:
> http://adg.stanford.edu/aa241/structures/vn.html
>
>
> > It is interesting to note that Piper does not allow any negative
> > maneuvers in the Normal catagory, on 3.8 positive.
>
> This is contrary to the required limits in FAR 23
>
> Bob Moore

[Bob Moore]

"Mike Rapoport" wrote

> The load limit for normal catagory isn't always 3.8G. In a 747 it is
> around 2G and a MU-2 is 3.25G.

Too early for you Mike? :-) 747s aren't certificated in the
Normal Category...how about the Transport Category?

Bob Moore

[john smith]

Bob Moore wrote:
> john smith > wrote
>>The one item for the PA32-300 that would not follow the formula for
>>calculating the lift line was Va. The book shows it as 132 kias, but
>>the formula generates a speed of 103 kias for the intercept of the
>>3.8 load limit line.

> When is 3.8g not 3.8g? When it is 4.5g or some other number.
> Remember, 3.8g is a minimum figure for the normal category.
> I have seen V-N diagrams for aircraft where the 3.8g line was
> extended upward to accomodate the gust loading envelope at Vno.
> Observe the gust envelope in the diagram at this web site:
> http://adg.stanford.edu/aa241/structures/vn.html

Yes, where the lift line intercepts the 3.8 load limit line is Va. Some
V-n diagrams then show another linear lift line extending from the load
limit line to Vno linearly back down then back down to Vne line.

Va=(33*sqrt(wing loading)).
But for the PA32-300 this calculation yields 145 kias, higher than book
value, 132 kias.
Vno is 149 kias.

For other aircraft the formula and book values coincide, but for some
reason, the PA32-300 numbers do not.

I have been using this formula to generate the lift line:
n=(V/Vs)**2
then plotting n (y-axis, rise) against V (x-axis, run)

>>It is interesting to note that Piper does not allow any negative
>>maneuvers in the Normal catagory, on 3.8 positive.

Should read "... only 3.8 positive."

[john smith]

Bob Moore wrote:
>>The load limit for normal catagory isn't always 3.8G. In a 747 it is
>>around 2G and a MU-2 is 3.25G.

> Too early for you Mike? :-) 747s aren't certificated in the
> Normal Category...how about the Transport Category?

Must not have to know that for the ATP rating. :-))

[Mike Rapoport]

Well it is pretty early here...MU-2 is normal catagory though

Mike
MU-2


"Bob Moore" > wrote in message
...
> "Mike Rapoport" wrote
>
> > The load limit for normal catagory isn't always 3.8G. In a 747 it is
> > around 2G and a MU-2 is 3.25G.
>
> Too early for you Mike? :-) 747s aren't certificated in the
> Normal Category...how about the Transport Category?
>
> Bob Moore

[Mike Rapoport]

"john smith" > wrote in message
...
> Bob Moore wrote:
> >>The load limit for normal catagory isn't always 3.8G. In a 747 it is
> >>around 2G and a MU-2 is 3.25G.
>
> > Too early for you Mike? :-) 747s aren't certificated in the
> > Normal Category...how about the Transport Category?
>
> Must not have to know that for the ATP rating. :-))
>

Fortunately for me!

Mike
MU-2

[john smith]

Mike Rapoport wrote:
>>>>The load limit for normal catagory isn't always 3.8G. In a 747 it is
>>>>around 2G and a MU-2 is 3.25G.

>>>Too early for you Mike? :-) 747s aren't certificated in the
>>>Normal Category...how about the Transport Category?

>>Must not have to know that for the ATP rating. :-))

> Fortunately for me!

Sorry Mike, I hit the send key too quickly.

I am reading a paper about "Radial G" and the next paragraph is:
"By the same logic a large aircraft certified under FAR 25.337(b) with
the minimum allowed limit load of 2.5 would be restricted to a 1.65-g
rolling pull up. To keep things in round numbers, the "rolling" Va, as
calculated for the aircraft weight, would be about twenty percent less
than the symmetrical Va."

[Tom S.]

"Mike Rapoport" > wrote in message
k.net...
> The load limit for normal catagory isn't always 3.8G. In a 747 it is
around
> 2G and a MU-2 is 3.25G.
>
>

3.28g for the TC 690/695 series as well.

Tom

[Tony Cox]

"john smith" > wrote in message
...
> I have been playing around making V-n diagrams for the various airplanes
> I rent from my flying club.
> I made one for our P28T-201RT and was looking for the numbers on the
> PA32-300.
>
> The one item for the PA32-300 that would not follow the formula for
> calculating the lift line was Va. The book shows it as 132 kias, but the
> formula generates a speed of 103 kias for the intercept of the 3.8
> load limit line.
>
> Can anyone explain this discrepancy?


Va is defined by what control surfaces can handle, not the
3.8g load limit line. See FAR 23.423 and 23.335. It doesn't
have to be on the lift line; it can be above it, but not below.

[Tony Cox]

"john smith" > wrote in message
...
>
> For other aircraft the formula and book values coincide, but for some
> reason, the PA32-300 numbers do not.

Perhaps for those that work, Va matches the equality condition
in FAR 23.335. This would imply that the control surfaces in
the PA32-300 are beefier than those in the other planes (or
perhaps their certification tests were more rigourous).

Va doesn't guarantee you protection against exceeding the
load factor, I'm afraid.

[Greg Esres]

<<The book shows it as 132 kias, but the formula generates a speed of
103 kias for the intercept of the 3.8 load limit line.>>

Are you using calibrated airspeeds and then converting to IAS, as you
should?

[john smith]

Greg Esres wrote:
> <<The book shows it as 132 kias, but the formula generates a speed of
> 103 kias for the intercept of the 3.8 load limit line.>>
> Are you using calibrated airspeeds and then converting to IAS, as you
> should?

The difference between calibrated and indicated are within only 1-2
knots. I thought it was true airspeed that mattered?

[Greg Esres]

On Sat, 28 Aug 2004 12:44:06 GMT, john smith > wrote:

<<The difference between calibrated and indicated are within only 1-2
knots. >>

Around the stall speed, the error is normally much higher. When you
take sqrt(3.8) * Vs1 to get the hypothetical Va, you will normally get
a very different number if you use IAS vs. CAS.

<<I thought it was true airspeed that mattered?>>

No. Aerodynamic forces depend on CAS, rather than TAS.

As others have pointed out, it is permissible for a manufacturer to
set Va at a higher speed than what you will calculate using this
method. Most do not do so, however, but it's unclear what is guiding
Piper. Some of their POH's show stall speed as linear with weight,
which is blatantly untrue. If you start with this premise, Va is
likely to be off too.

[john smith]

Greg Esres wrote:
> <<The difference between calibrated and indicated are within only 1-2
> knots. >>

> Around the stall speed, the error is normally much higher. When you
> take sqrt(3.8) * Vs1 to get the hypothetical Va, you will normally get
> a very different number if you use IAS vs. CAS.

I am using Vs (stall, clean) since the configuration is the same as at Va.

> <<I thought it was true airspeed that mattered?>>

> No. Aerodynamic forces depend on CAS, rather than TAS.
> As others have pointed out, it is permissible for a manufacturer to
> set Va at a higher speed than what you will calculate using this
> method. Most do not do so, however, but it's unclear what is guiding
> Piper. Some of their POH's show stall speed as linear with weight,
> which is blatantly untrue. If you start with this premise, Va is
> likely to be off too.

How does CAS account for density altitude?

[Greg Esres]

<<I am using Vs (stall, clean) since the configuration is the same as
at Va.>>

Vs1 is normally taken to be the clean stall speed. The meaning of Vs
seems a bit more ambiguous. But we're talking the same thing here.

<<How does CAS account for density altitude?>>

With a given CAS, the density altitude is irrelevant. That's the
beauty of it. ;-)

For a given DA, you could use TAS and get the same results, as long as
you're consistent in using TAS for each figure. However, Vs1 is only
published as CAS. And there's no point in doing that anyway.

[john smith]

Greg Esres wrote:
> <<I am using Vs (stall, clean) since the configuration is the same as
> at Va.>>
>
> Vs1 is normally taken to be the clean stall speed. The meaning of Vs
> seems a bit more ambiguous. But we're talking the same thing here.
>
> <<How does CAS account for density altitude?>>
>
> With a given CAS, the density altitude is irrelevant. That's the
> beauty of it. ;-)
>
> For a given DA, you could use TAS and get the same results, as long as
> you're consistent in using TAS for each figure. However, Vs1 is only
> published as CAS. And there's no point in doing that anyway.

Thanks, Greg.
The whole purpose of my exercise is to determine the minimum turn radius
and maximum rate of turn for a given weight. This is dependent upon air
density (hence, density altitude) and true airspeed.
What is really interesting to me is, I learned all this stuff 25 years
ago, but didn't have the experience to fully understand it as I do now.

[Greg Esres]

<<The whole purpose of my exercise is to determine the minimum turn
radius and maximum rate of turn for a given weight. This is dependent
upon air density (hence, density altitude) and true airspeed.>>

Hmmm. Air density doesn't appear in the formulas that I have. Load
factor and true airspeed are the only variables. And load factor
should be independent of density altitude.

What are you doing with air density?

[Jay Smith]

Greg Esres wrote:
> <<The whole purpose of my exercise is to determine the minimum turn
> radius and maximum rate of turn for a given weight. This is dependent
> upon air density (hence, density altitude) and true airspeed.>>

> Hmmm. Air density doesn't appear in the formulas that I have. Load
> factor and true airspeed are the only variables. And load factor
> should be independent of density altitude.
> What are you doing with air density?

Trying to figure minimum safe altitudes for recovery from vertical dives
at different times of the year under different temperature/humidity
conditions.