Trailing pitot-static method
Freestream total and static pressures can be measured by suspending pitot and static sources on a cable attached to a suitable strong point fitted with an emergency jettison facility. Pressures are transmitted through two tubes to the helicopter where they are converted to accurate pressure altitudes and equivalent airspeeds by sensitive calibrated instruments. Since the pressures are transmitted to the helicopter prior to conversion to airspeeds and altitudes, no systematic error is introduced by trailing the sources below the helicopter. The system is usually calibrated in a wind tunnel prior to installation and the cable is around two rotor diameters in length. The trailing pitot – static (TPS) typically comprises a streamlined body equipped with a drag cone or tail fins to align it with the relative airflow. Pitot and static ports are placed at suitable locations on the body. A suspension point over the CG incorporates a swivel so that the cable cannot exert a moment on the body. Test instruments in the aircraft cabin display, once their own instrument errors have been taken into account, calibrated airspeed and true pressure altitude. Tubes connect the trailing sources to the test instruments via breakaway links and the support cable is attached to the underslung load hook or, if none is available, to a special-to-type release bracket. Experience has shown that it is possible to deploy a TPS from the aircraft cabin in low-speed flight if OGE hover performance is not available at take-off.
As before, level flight runs are maintained for approximately one minute with the observer recording both TPS and aircraft IAS and pressure altitude every 15 seconds. For climbs and descents measurements are made over a height band 500 ft either side of the nominal test altitude. The same parameters as for level flight are measured on entering the test band, at the test altitude and when leaving the test band. For accurate data it is important that the run is started at sufficient height above or below the test band to ensure that the helicopter is fully stabilized (speed, power, NR and rate of climb/descent) when the test band is entered.