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#11
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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.) 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 |
#12
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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 k.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 |
#13
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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 |
#14
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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. |
#15
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"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 |
#16
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"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 |
#17
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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 |
#18
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"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. |
#19
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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 |
#20
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"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. |
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