Airspeed Indication and Relative Wind

"Scott Lowrey" wrote in message
news:35Cmc.33788$TD4.5609844@attbi_s01...
If I'm descending slowly with a relatively nose-high attitude - say, in
preparation for a short field landing - does the high angle of the pitot
tube have any effect on indicated airspeed? (I suppose all designs are
different - say this is a 172.)

How slow will you be going? If I want to do a short field landing I fly at
the bottom of the white arc, 60 mph IAS for my 182. If I want to get in
really short I will fly at 50 mph. There is error at all but a small range
of speeds so who cares? Are you worried about suddenly falling out of the
sky? If so then don't worry about it, won't happen in a 172. When you
really get comfortable with the plane you can fly between the stall speed
and the stall warning speed in no turbulence conditions.



If the descent was steep, the relative wind would be coming from below
and forward with respect to the wing, right?


For a given airspeed the angle of attack will always be the same. It does
not matter what that angle may look like in reference to the ground.

"Scott Lowrey" wrote in message
news:35Cmc.33788$TD4.5609844@attbi_s01...
If I'm descending slowly with a relatively nose-high attitude - say, in
preparation for a short field landing - does the high angle of the pitot
tube have any effect on indicated airspeed? (I suppose all designs are
different - say this is a 172.)

If the descent was steep, the relative wind would be coming from below
and forward with respect to the wing, right? Add the pitch angle, and
it seems like the pitot tube would be at a fairly angle with respect to
the direction of air flow. I suppose this is a simple angle of attack
question....

Seems like the air would be passing slightly "over" the pitot tube
opening rather than "into" it, thus reducing the measure air pressure.
Is this correct? Is indicated airspeed affected by high AOA?

-Scott

The effective area of the pitot orfice is reduced by the cosine of the
angle of the pressure wave impinging on it. Presuming an angle of twelve
degrees or less [the stall AOA of most wings] the effective orfice area is
reduced to 97.81% of the normal area. Short of a digital ASI, you couldn't
tell the difference from the parallax error in reading the instrument in the
first place.
In any case, the error is on your side, since the result is a lower
than normal[real] reading.

Casey Wilson wrote:

In any case, the error is on your side, since the result is a lower
than normal[real] reading.

And even more importantly, it doesn't matter, because the published speeds
and ASI markings are IAS, already taking into account any pitot-static
readings at high angles of attack.


All the best,


David

"Casey Wilson" wrote in message .. .
"Scott Lowrey" wrote in message
news:35Cmc.33788$TD4.5609844@attbi_s01...
If I'm descending slowly with a relatively nose-high attitude - say, in
preparation for a short field landing - does the high angle of the pitot
tube have any effect on indicated airspeed? (I suppose all designs are
different - say this is a 172.)

If the descent was steep, the relative wind would be coming from below
and forward with respect to the wing, right? Add the pitch angle, and
it seems like the pitot tube would be at a fairly angle with respect to
the direction of air flow. I suppose this is a simple angle of attack
question....

Seems like the air would be passing slightly "over" the pitot tube
opening rather than "into" it, thus reducing the measure air pressure.
Is this correct? Is indicated airspeed affected by high AOA?

-Scott

The effective area of the pitot orfice is reduced by the cosine of the
angle of the pressure wave impinging on it. Presuming an angle of twelve
degrees or less [the stall AOA of most wings] the effective orfice area is
reduced to 97.81% of the normal area. Short of a digital ASI, you couldn't
tell the difference from the parallax error in reading the instrument in the
first place.
In any case, the error is on your side, since the result is a lower
than normal[real] reading.

I think there's more than the cosine at work here. That
theory would say that at 90 degrees the pitot would generate no
dynamic pressure, but at (or even before) 90 degrees an open tube will
generate suction. I have flown airplanes at high AOAs, lots of power
on, and seen zero indication on the ASI; I don't think static error is
responsible for all of it.

Dan

"Dan Thomas" wrote in message
m...
"Casey Wilson" wrote in message
.. .
"Scott Lowrey" wrote in message
news:35Cmc.33788$TD4.5609844@attbi_s01...
If I'm descending slowly with a relatively nose-high attitude - say,
in
preparation for a short field landing - does the high angle of the
pitot
tube have any effect on indicated airspeed? (I suppose all designs
are
different - say this is a 172.)

The effective area of the pitot orfice is reduced by the cosine of
the
angle of the pressure wave impinging on it. Presuming an angle of twelve
degrees or less [the stall AOA of most wings] the effective orfice area
is
reduced to 97.81% of the normal area. Short of a digital ASI, you
couldn't
tell the difference from the parallax error in reading the instrument in
the
first place.
In any case, the error is on your side, since the result is a lower
than normal[real] reading.

I think there's more than the cosine at work here. That
theory would say that at 90 degrees the pitot would generate no
dynamic pressure, but at (or even before) 90 degrees an open tube will
generate suction. I have flown airplanes at high AOAs, lots of power
on, and seen zero indication on the ASI; I don't think static error is
responsible for all of it.

Put the cigar back in the box, you don't earn it with your response. In
the conditions set by the original post [see top paragraph above] the cosine
function will be predominate. Extreme alpha is not relevant.