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#21
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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 |
#22
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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 |
#23
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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 |
#24
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"Casey Wilson" wrote in message .. .
"Scott Lowrey" wrote in message news:35Cmc.33788$TD4.5609844@attbi_s01... If I'm descending slowly with a relatively nose-high attitude - say, in preparation for a short field landing - does the high angle of the pitot tube have any effect on indicated airspeed? (I suppose all designs are different - say this is a 172.) If the descent was steep, the relative wind would be coming from below and forward with respect to the wing, right? Add the pitch angle, and it seems like the pitot tube would be at a fairly angle with respect to the direction of air flow. I suppose this is a simple angle of attack question.... Seems like the air would be passing slightly "over" the pitot tube opening rather than "into" it, thus reducing the measure air pressure. Is this correct? Is indicated airspeed affected by high AOA? -Scott The effective area of the pitot orfice is reduced by the cosine of the angle of the pressure wave impinging on it. Presuming an angle of twelve degrees or less [the stall AOA of most wings] the effective orfice area is reduced to 97.81% of the normal area. Short of a digital ASI, you couldn't tell the difference from the parallax error in reading the instrument in the first place. In any case, the error is on your side, since the result is a lower than normal[real] reading. I think there's more than the cosine at work here. That theory would say that at 90 degrees the pitot would generate no dynamic pressure, but at (or even before) 90 degrees an open tube will generate suction. I have flown airplanes at high AOAs, lots of power on, and seen zero indication on the ASI; I don't think static error is responsible for all of it. Dan |
#25
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"Dan Thomas" wrote in message m... "Casey Wilson" wrote in message .. . "Scott Lowrey" wrote in message news:35Cmc.33788$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. |
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