What are the effects on airspeed measurements, if the pitot tubes were installed right on the engine covers of an aircraft instead of being on the nose. Would the reading be false? If so, would the reading be underestimated or overestimated and why?
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Related, maybe duplicate: Which air stream pressure / speed is measured by the pitot? – mins Dec 17 '17 at 18:57
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There is a very good reason to install pitot tubes in a place with relatively constant environmental circumstances: calibration.
When installed at around the nose of the aircraft, the pitot always have a good view of the airflow, at all angles of attack and of sideslip. These parameters do distort the pressure reading, that is why during aircraft development the prototype tows a flying pitot tube and compares measurements. These measurement sets are used for calibration.
If it was installed on the engine covers, not only the aircraft state but also the engine state would have influence on the reading. A nightmare to calibrate, and prone to unforeseen errors for instance if a fan blade breaks off and the wobbling fan causes an airstream situation that was not accounted for.
Best to use the prime location: the nose.
Koyovis
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1On a related note, all of the GA aircraft that I am familiar with have the pitot tube located about the midpoint of the wing. If you look at the calibration tables you can see they provide an accurate reading in normal flight, but give a a misleading reading when flaps and slower speeds (resulting in higher angles of attack) upset the airflow. – JScarry Dec 17 '17 at 18:05
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There was a picture that showed pressure differences along the length of an aircraft posted somewhere on this site, which showed there is a couple of good locations for the sensors where the differences are small and one is a bit behind the nose. I have unfortunately forgotten suitable search terms that would find it. – Jan Hudec Dec 17 '17 at 19:09
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@JanHudec Well, it might not be the same total as another spot, due to a different local velocity. – Dec 17 '17 at 19:31
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@jjack, local velocity changes the dynamic pressure ($p_d = ½\varrho v^2$), but $p_t = p_d + p_s = ½\varrho v^2 + p_s$ and that is just the Bernoulli's equation (without potential term as we assume horizontal flow), so it is constant! So as the velocity, and thus dynamic pressure, increase, the static pressure decreases. Around the nose the speed is lower, but around the fuselage it is higher, so there is a point in between where it matches the free flow speed and that is the best place for the probes. – Jan Hudec Dec 17 '17 at 21:25
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@JanHudec I know, but if you measure the total pressure in a bad place on the aircraft, the local total pressure is not the same as the freestream value which you need. Or am I wrong? – Dec 17 '17 at 21:43
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@jjack, well, the local total pressure is the same everywhere, but the pitot tube can't recover it perfectly and the error depends on the dynamic pressure. Plus the pitot tube also needs the static pressure and that does change quite significantly with position. – Jan Hudec Dec 17 '17 at 21:55
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@JanHudec The freestream total pressure ahead of the plane is the same everywhere. But if you don't measure freestream, but at some aircraft-induced velocity instead, the value will be different. – Dec 17 '17 at 22:13
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@jjack, no, total pressure is the same everywhere, except behind the engines, at least until compressibility kicks in, because total pressure is just energy and only engines change the energy of the stream. It is dynamic and static pressure that change. But their sum does not and cannot. When the stream accelerates, increasing dynamic pressure, its static pressure decreases and vice versa. – Jan Hudec Dec 18 '17 at 13:08
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