Airspeed Indication and Relative Wind

Scott Lowrey wrote:

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 your plane has a POH, there should be an airspeed calibration table or
chart in Section 5 "Performance."

My Warrior II's ASI will underread by about 7 kt at a high angle of attack,
and overread by about 7 kt at a very low angle of attack. The ASI in a
Cessna 172P will underread by 6 kt at a low angle of attack (7 kt with full
flaps), or overread by 6 kt at a high angle of attack.

In theory, I think, the ASI could be recalibrated to be more accurate at
high and low settings, but why would the manufacturers bother? The
calibration errors make the plane look like it has a much slower stall speed
and a slightly higher cruise speed than it really does.


All the best,


David

Read my post again. I did not claim that AOA does not affect IAS.
I said that AOA does not affect the pitot to any appreciable degree.
The flow around the static source is another story.


In article , "Peter Duniho"
wrote:
"jer" wrote in message
k.net...
Scott, you got 100 percent bad advice in the previous responders.

And you are 100% idiot.

The bottom line is that the indicated airspeed DOES have errors depending on
the angle of attack.

You may well be correct that the error comes mostly from the static port and
not the pitot tube, but a) this is not relevant to the question asked (only
to part of some of the answers), b) hyperbole just makes you look as stupid
as you claim other people are, and c) much of the response the original
poster got was correct (in other words, the advice was NOT "100 percent
bad").

Welcome to the newgroup. You made quite an entrance.

Pete

jer wrote:
Read my post again. I did not claim that AOA does not affect IAS.
I said that AOA does not affect the pitot to any appreciable degree.
The flow around the static source is another story.



Easy now.

I'd forgotten about the static port and it's position with regard to air
flow. Although my original question focused on the pitot tube, it's
undoubtedly better to consider the entire pitot-static system when
considering the effect of different angles of attack on IAS.

So, if I slip to the right during the aforementioned steep approach and
the static port is on the forward left side of the fuselage, the static
pressure goes up a little bit, I suppose? Then, combining a slightly
lowered pressure at the pitot (due to AOA) plus a slightly higher
pressure at the static port, I get a lower IAS, right?

I realize the total error is still probably negligible. Just armchair
flying.

-Scott

"Bob Moore" wrote in message
. 7...
I certainly hope that you aren't already a pilot asking such a basic
question!

Bob Moore


I'd say that the fact that he's asking a question, even at the risk
of comments like this, shows that he's a responsible pilot who
wants to understand it. In my opinion that shows a pretty
good attitude toward his flying.

"Cub Driver" wrote in message
...
Early in my training, I was warned not to rely on the airspeed
indicator while the plane was in a slip. So I don't even look at it. I
slip pretty hard, too. ("High and hot and slipping like crazy," as
they used to say of Tom Buck.)

Is that dangerous?

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.

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 think Rod Machado did an article about
flying not so much by the numbers, but by the feel of the plane. AI's can
lie and if they lie the wrong way (i.e. indicating too fast) and you really
rely on them, they can bite you.

Don't for a minute believe that I am saying that students shouldn't fly by
the numbers, but I believe their instructors should sometimes slap the ol'
rubber sticky over the AI towards the end of their training.

-Trent
PP-ASEL

"jer" wrote in message
nk.net...
Read my post again. I did not claim that AOA does not affect IAS.

Sure you did. When you wrote "you got 100 percent bad advice".

The only way that the responses to Scott's posts could have been "100
percent bad advice" would have been if there was no effect on the IAS due to
AOA.

Do you actually know what "100 percent" means?

Pete

During the very nose high, full elevator mush, I describe in the BRS an
Descent thread, my Airspeed needle was pegged at the bottom, just like I was
sitting on the ramp. I know damn well I was moving forward but the low
speed in combination with the extreme angle of the pitot tube was preventing
the ASI from registering anything.

With a lot of power and heavy right foot, you can maneuver a 172 around
gingerly in level flight with nothing showing on the airspeed and the stall
horn shrieking like a demented banshee. Very good practice.
--
Roger Long

"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.

Otis Winslow wrote:


I'd say that the fact that he's asking a question, even at the risk
of comments like this, shows that he's a responsible pilot who
wants to understand it. In my opinion that shows a pretty
good attitude toward his flying.


Thanks, Otis. I *am* a responsible pilot. I'm a responsible 100 hour
pilot who took the winter off. But I passed the renter check ride
at my new FBO the other day and I didn't have to be an expert in
aerodynamic minutia to do it.

Having said that, I enjoy learning everything I can about the flight
environment. If that includes the occasional bass-ackward thought and a
hip-shot question to the boys and girls in rec.aviation, so be it.
Nobody else in my new neighborhood knows jack about airplanes, so I come
here for a little social interaction. It's a great group and I'm sure
Bob didn't mean to sound presumptuous. Right, Bob?

-Scott

"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.