Airspeed Indication and Relative Wind

I don't believe it's dangerous. When I'm doing a hard slip (C-172/C-152),
the AI is jumping all over the place anyway, so I choose not to look at it.

Yesterday I slipped into a private field that normally is one-way from
the west. There was a 12 mph wind from the west, so I approached from
the west. There's a road with 60-foot trees next to the end of the
runway, which is on a fairly steep hill. Slip how you may, it seems
that you never can land shorter than midway on the flat part of the
runway, which in any event is not flat but gently sloping to the west.
This reduces the runway to effectively 1000 feet. It is *very*
exciting, though with a headwind straight down the runway I stopped in
a couple hundred feet or less.

Of course what was in my mind when I went past those trees with my
starboard wing pointing down was this same question: is this
dangerous?

Whether it is dangerous or not, it is certainly exciting, and a whole
lot of fun.

Incidentally, my instructor has pretty much broken me of using the AI much
anyhow (I'm VFR only). I fly mostly by feel now in C-172's, using the AI as
validation for what I am feeling.

I also noticed yesterday that I found myself retarding the throttle
when I hadn't looked at the tach. Evidently I fly now as much by the
sound of the engine as by looking at the engine speed.

(Nor do I fly by the airspeed indicator. I peg the tach at 2150, or
2200 if speed is more important than fuel.)

It was a grand day for flying, if a bit breezy for the Cub.


all the best -- Dan Ford
email: (put Cubdriver in subject line)

The Warbird's Forum www.warbirdforum.com
The Piper Cub Forum www.pipercubforum.com
Viva Bush! blog www.vivabush.org

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

Scott Lowrey wrote

It's a great group and I'm sure
Bob didn't mean to sound presumptuous.
Right, Bob?

Just a little chiding, and I must admit that some of the answers
refreshed my knowledge that most of the difference between CAS
and IAS is in the static port and not the pitot tube. The question
got me back into my personal aeronautical bible, Aerodynamics for
Naval Aviators.

Bob Moore

"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.