Airspeed Indication and Relative Wind

[Scott Lowrey]

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

[Peter Duniho]

"Scott Lowrey" > wrote in message
news:35Cmc.33788$TD4.5609844@attbi_s01...
> [...] Is indicated airspeed affected by high AOA?

Yes. And a variety of other errors related to the position and alignment of
the pitot tube relative to the airflow. This is why we have "indicated" and
"calibrated" airspeed. Use the table in the POH to correct the indicated
airspeed to get the calibrated airspeed.

Of course, which to use depends on what is specified. If you're using a
table that shows indicated airspeed for specific performance, then you don't
need to correct. If the table shows calibrated airspeed, you do.

Pete

[Rod Madsen]

A normal glide slope is about 3 degrees. If you were doing a real steep
decent you might have a glide path of 6 degrees. Then, let's say your nose
is 4 degrees above the horizon giving us an angle of attack for the pitot
tube of 10 degrees. The cosine of 10 degrees is about .985 meaning your
airspeed indication might be 1.5% low. At 65 knots you ain't gonna notice a
half knot.

Rod
"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

[Bob Moore]

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.)

Yes, and this difference can be found in the flight manual. In early
C-172s, the difference can be 10 mph or more. The difference is the
difference between Indicated Air Speed (IAS) and Calibrated Air Speed
(CAS). At a CAS of 52 mph, a C-172B will indicate only 40 mph. In the
pilot's handbook for this early C-172, CAS was called "true IAS".
I certainly hope that you aren't already a pilot asking such a basic
question!

Bob Moore

[Scott Lowrey]

Yep, I thought about this after sending. I didn't pick up on the "CAS
vs. IAS" concept too well during my early training - I thought of the
error as more of a machine-design-thing than an aerodynamic issue.
Makes good sense.

Thanks!

[Andrew Sarangan]

At high angles of attack the air flow pattern around the pitot tube is
modified by the wing and the fuselage. This is the biggest source of
error. Of course, the horizontal component of the free flowing air also
gets smaller (cosine of the AOA), but this is a minor effect as you just
demonstrated.


"Rod Madsen" > wrote in
:

> A normal glide slope is about 3 degrees. If you were doing a real
> steep decent you might have a glide path of 6 degrees. Then, let's
> say your nose is 4 degrees above the horizon giving us an angle of
> attack for the pitot tube of 10 degrees. The cosine of 10 degrees is
> about .985 meaning your airspeed indication might be 1.5% low. At 65
> knots you ain't gonna notice a half knot.
>
> Rod
> "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
>
>

[jer]

Scott, you got 100 percent bad advice in the previous responders.

If you look at the data on pitot tubes and their sensitivity to angle you
will find that they are very tolerant of angles even in excess of stall
angles.

The airspeed calibration is very sensitive to the static system, almost
anything will work for the pitot side. Get the pitot out of the boundary
layer and out from behind anything that gives turbulent air and
it will work fine.

In article <HDCmc.32805$Ia6.5547422@attbi_s03>, Scott Lowrey
> wrote:
>Yep, I thought about this after sending. I didn't pick up on the "CAS
>vs. IAS" concept too well during my early training - I thought of the
>error as more of a machine-design-thing than an aerodynamic issue.
>Makes good sense.
>
>Thanks!

[Greg Esres]

<<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.)>>

First, descending or climbing is irrelevant. Only your airspeed
affects the RW.

Second, according to the most authoritative sources, the static source
is the primary culprit behind the difference between IAS and CAS. The
pitot is very accurate until AOA's much greater than our little
aircraft can achieve. I know that popular literature suggests
otherwise.

[Peter Duniho]

"jer" > wrote in message
.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

[Cub Driver]

>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?

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?

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

[David Megginson]

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

[jer]

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
.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
>
>

[Scott Lowrey]

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

[Otis Winslow]

"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.

[Trent Moorehead]

"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

[Peter Duniho]

"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

[Roger Long]

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

[Newps]

"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]

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

[Casey Wilson]

"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.

[Cub Driver]

>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

[David Megginson]

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

[Bob Moore]

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

[Dan Thomas]

"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

[Casey Wilson]

"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.