A aviation & planes forum. AviationBanter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » AviationBanter forum » rec.aviation newsgroups » Piloting
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

Airspeed Indication and Relative Wind



 
 
Thread Tools Display Modes
  #1  
Old May 7th 04, 03:00 AM
Scott Lowrey
external usenet poster
 
Posts: n/a
Default Airspeed Indication and Relative Wind

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
  #2  
Old May 7th 04, 03:24 AM
Peter Duniho
external usenet poster
 
Posts: n/a
Default

"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


  #3  
Old May 7th 04, 03:30 AM
Rod Madsen
external usenet poster
 
Posts: n/a
Default

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



  #4  
Old May 7th 04, 03:30 AM
Bob Moore
external usenet poster
 
Posts: n/a
Default

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

  #5  
Old May 7th 04, 03:37 AM
Scott Lowrey
external usenet poster
 
Posts: n/a
Default

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!
  #6  
Old May 7th 04, 03:44 AM
Andrew Sarangan
external usenet poster
 
Posts: n/a
Default

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




  #7  
Old May 7th 04, 04:49 AM
jer
external usenet poster
 
Posts: n/a
Default

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!

  #8  
Old May 7th 04, 05:22 AM
Greg Esres
external usenet poster
 
Posts: n/a
Default

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.

  #9  
Old May 7th 04, 07:14 AM
Peter Duniho
external usenet poster
 
Posts: n/a
Default

"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


  #10  
Old May 7th 04, 10:33 AM
Cub Driver
external usenet poster
 
Posts: n/a
Default


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
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
USAF = US Amphetamine Fools RT Military Aviation 104 September 25th 03 03:17 PM


All times are GMT +1. The time now is 12:31 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 AviationBanter.
The comments are property of their posters.