V-n Diagrams

"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.

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?

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?

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.

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?

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.