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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
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"Scott Lowrey" wrote in message
news:35Cmc.33788$TD4.5609844@attbi_s01... [...] Is indicated airspeed affected by high AOA? Yes. And a variety of other errors related to the position and alignment of the pitot tube relative to the airflow. This is why we have "indicated" and "calibrated" airspeed. Use the table in the POH to correct the indicated airspeed to get the calibrated airspeed. Of course, which to use depends on what is specified. If you're using a table that shows indicated airspeed for specific performance, then you don't need to correct. If the table shows calibrated airspeed, you do. Pete |
#3
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A normal glide slope is about 3 degrees. If you were doing a real steep
decent you might have a glide path of 6 degrees. Then, let's say your nose is 4 degrees above the horizon giving us an angle of attack for the pitot tube of 10 degrees. The cosine of 10 degrees is about .985 meaning your airspeed indication might be 1.5% low. At 65 knots you ain't gonna notice a half knot. Rod "Scott Lowrey" wrote in message news:35Cmc.33788$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 |
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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 |
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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
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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$TD4.5609844@attbi_s01... If I'm descending slowly with a relatively nose-high attitude - say, in preparation for a short field landing - does the high angle of the pitot tube have any effect on indicated airspeed? (I suppose all designs are different - say this is a 172.) If the descent was steep, the relative wind would be coming from below and forward with respect to the wing, right? Add the pitch angle, and it seems like the pitot tube would be at a fairly angle with respect to the direction of air flow. I suppose this is a simple angle of attack question.... Seems like the air would be passing slightly "over" the pitot tube opening rather than "into" it, thus reducing the measure air pressure. Is this correct? Is indicated airspeed affected by high AOA? -Scott |
#7
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Scott, you got 100 percent bad advice in the previous responders.
If you look at the data on pitot tubes and their sensitivity to angle you will find that they are very tolerant of angles even in excess of stall angles. The airspeed calibration is very sensitive to the static system, almost anything will work for the pitot side. Get the pitot out of the boundary layer and out from behind anything that gives turbulent air and it will work fine. In article HDCmc.32805$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
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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
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"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
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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 |
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