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Aviation History
1958
1958 - 0011.PDF
FLIGHT, 3 January 1958 11 "Flight" photograph The English' Electric P.IB prototype has two Kelvin Hughes high-speed pitots. The lower is standard and serves the pilot's instruments; the instal- lation projecting from the intake centre-body serves test instrumentation. Supersonic Pitots Measuring Static Pressure with Accuracy ONE of the prime instrumentation difficulties at very highspeed is the accurate measurement of static pressure. Thepitot/static head, which is used for "probing" the airflow and as a convenient point for measuring static pressure, has toproject into the undisturbed "free-stream" air around the aircraft. The distance to which the region of disturbed air extends fromthe parent body depends on the speed at which it is moving and its shape. This means that a particular pitot/ static location onone 300 m.p.h. aeroplane may be far from the optimum on a different aircraft with the same performance, and quite uselesson a high-speed aircraft. An example is the Mosquito, which had its pitot head mounted high up on the fin. On fast jet aero-planes this position would normally be subject to very large errors. Accurate measurement of static pressure is very much moredifficult than the measurement of dynamic pressure. For example, during the computation of position-error on an airspeed indi-cator, by far the greater proportion of the final inaccuracy can be ascribed to the static-pressure circuit. As the region of disturbedair gradually extends farther out from the aircraft so must ihe pitot/static head be lengthened to reach the undisturbed airbeyond it. This explains the very long pressure tubes—fre- quently as much as 10ft—carried by modern high-speed aircraft. Airflow around an aircraft has more influence on the choice oflocation for the pitot/static head than any other one factor. On most aircraft of conventional plan-form the most suitable posi-tion for the pitot head is usually to be found either near the wing tips or on the nose of the fuselage. As a basic rule, the tip of thepressure head on high-speed aircraft should be about 25 per cent of the local chord ahead of the leading edge, to ensure that thestatic slots are in an area of undisturbed air. This is at best only a very general figure, and one which has to be modifiedfor individual types of aircraft. More accurately, the distance of the pitot-head tip from the part of the aircraft to which it isattached is governed by the aircraft's limiting Mach number, the shape of the local surfaces, and the angle of sweep of the wing. Numerous instruments rely on an accurate knowledge ofstatic pressure for their functioning. As all pilots know, the static pressure is usually obtained by cutting narrow slots allaround the side of the casing of the dynamic tube, so that the pressure in the casing equals the undisturbed outside air pressure.A false static pressure could be obtained if some or all of these slots were in disturbed air, for the recorded pressure would thenbe higher or lower than the true value. In designing a pitot/static head some of the adverse effectsexperienced at high Mach numbers by the airframe are en- countered with equal or greater severity by the pressure head.For example, although the tube may be well out in the undis- turbed air ahead of the main structure, as the aircraft approachesMach 1 the pitot/static head will build up its own private shock wave. This shock and its associated discontinuity can have anappreciable effect on the static pressure, but one way of over- coming its influence is to decrease the outside diameter of thecasing and increase the distance of the static slots from the nose of the pitot head. On a pressure head the effect of a shockwave extends for a distance equal to a given number of casing- diameters from the nose; the actual figure is related to the Machnumber. If the diameter is reduced, it follows that the length of tube affected by the shock wave decreases. As some part of the tube will be affected by turbulence, anumber of slots leading to the static air chamber are cut along the length of the casing. This means that, whatever the speed,some static slots will always be in a region of smooth airflow and the pressure head will never give a completely false static readingover the critical speed range (which, in any case, is normally very limited). A compromise must always be accepted in the choice of thediameter and length of the pitot/static head, for these two dimensions are directly related. As the length increases, somust the diameter also increase to maintain stiffness in the tube. A thin tube may be the aerodynamic ideal, but at high speed itmight begin to flutter—probably more severely than the part of the airframe carrying it—and could well suffer major damage asa result. Another reason why pitot heads have to be larger in diameterthan the ideal is that the higher an aircraft goes the greater is the heat required to warm the pressure head. This naturallycalls for the use of bigger heaters. Until a few years ago it was thought that icing of the pitot head was unlikely to occur above35,OOOft but recent experience has shown that fine ice crystals present in the air up to at least 45,000ft are liable to affect itsfunctioning. Very high speeds would seem to call for a modification to theshape of the entry end of the pitot head. Tests so far show that very little is gained, and some useful attributes lost, by changingthe familiar spherical end of the tube for a more pointed one. One of the more obvious problems associated with a really sharp-lipped, pointed head is that it is very difficult to de-ice.
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