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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 |
#4
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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 |
#5
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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 |
#6
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"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. |
#7
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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 |
#8
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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 |
#9
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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! |
#10
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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! |
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