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I've noticed that on combat aircraft the forward (nose-mounted) pitot tube is usually very long, on the order of half a meter to well over a meter on some aircraft. Why are they so long? Surely the body of the aircraft is not disturbing the air in front of the craft, neither during subsonic nor supersonic flight.

Example, from an F-16:

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Example, from a Mirage III:

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Note that I couldn't find any good pictures of the Mirage in which the entire pitot tube actually fit in the photo!

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TIL: Close up photos of F-16s make for quick instant StackOverflow karma. –  dotancohen Jun 24 at 14:53
    
Isn't the one on the front of the Mirage III a refueling probe? –  Jon Story Nov 27 at 14:46
    
@JonStory: No. –  dotancohen Nov 27 at 14:51
    
My bad :p I could have sworn I read something about nose refueling probes on a Mirage though –  Jon Story Nov 27 at 16:12

2 Answers 2

up vote 17 down vote accepted

Sorry to contradict, but jwenting's answer is totally wrong. I'm shocked to see that it collected so many upvotes. And the picture shows a condensation collar, which is quite unrelated since it is caused by the displacement effect of a relatively blunt rocket near Mach 1 in humid air.

The "boundary layer" at the nose of an aircraft is very thin. What he probably means is a detached shock, but this happens only on a blunt nose and produces too much drag for supersonic aircraft. A detached shock can be found on the nose of the Space Shuttle, and it was selected to keep temperatures there manageable in hypersonic flow. Mach 2.3 aircraft are better off with a pointed nose which produces an attached shock.

But the length of the pitot tube hasn't anything to do with it. The length is not needed for supersonic flight, but for subsonic flight at high angles of attack. Due to the high wing sweep, a high angle of attack capability just happens to coincide with supersonic configurations.

In subsonic flow the air ahead of the aircraft is influenced by the aircraft's pressure field, and at high angles of attack and high wing loadings this reaches out quite a bit. The pitot tube can only measure total pressure when it points into the flow direction. Ahead of the aircraft, the local flow angle increases the closer you are to the aircraft, and this increases measurement errors, because now the pitot tube sits at an oblique angle to the airflow. A longer pitot tube reaches farther out into still relatively undisturbed flow, so less compensation is needed to arrive at good values for total and static pressure. In early flight test, the pitot tube is much longer again, because the compensation factors are not yet established.

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Thank you Peter! I had noticed as well that early development aircraft have longer pitot tubes. Thank you for taking the time to explain the issue and include so much detail. –  dotancohen Jun 25 at 5:31

Actually, the aircraft is distorting the airflow in front of itself. As speed goes up, the shockwave builds and the aircraft starts to experience ever more sonic drag.
The long pitots will pierce through this thick boundary layer, into the smoother airflow in front, giving correct readings.

Physics Central has some Schlieren photos of the effect. When breaching the "sound barrier" the shockwave separates, which causes the sonic boom.But an area of distorted air remains.
The image below shows the effect on a space launcher, the escape tower sticking out through the separating shockwave.
Shockwave on a space launch vehicle

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Thank you, I did not realize that the disturbance could stick so far out forward. That is the Ares rocket, no? –  dotancohen Jun 24 at 7:21
    
@dotancohen I don't know, just found the image looking for something that'd illustrate the idea –  jwenting Jun 24 at 9:18
    
@dotancohen A quick reverse image search says that it is indeed the Ares I-X rocket. –  Nate Kerkhofs Jun 24 at 11:15
    
Thanks.‏‏‏‏‏‏‏‏ –  dotancohen Jun 24 at 11:30
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@jwenting, your explanation seems fairly sound, but I know for a fact that calculating airspeed with a pitot tube from supersonic flow is a completely different thing than for sub sonic flow, see these lecture notes. So althouh the pitot tube may be protruding through the bulk of the shock wave, there is still a (smaller) shock wave in front of the pito itself. –  Jonny Jun 24 at 16:43

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