PRESSURE ERROR MEASUREMENT
7.3.1 Sources of pressure error
The altimeter, airspeed indicator and vertical speed indicator are designed on the assumption that they are fed with pressures from the undisturbed freestream. However, as a helicopter flies it disturbs the air mass and in so doing generates a pressure field around the vehicle. In addition to true airspeed this pressure field is affected by factors such as: the downwash from the main rotor, the carriage of external stores, and the configuration of hatches, doors and movable weapons. Since it is common practice to use either a fixed pitot-static probe or a fixed pitot probe with a fuselage mounted static source then errors in measured air data will also arise as a consequence of sustained or transient changes in the angles of attack and sideslip of the fuselage. These changes can be caused by:
• low-speed out-of-wind manoeuvres, transitions, turns, climbs or descents;
• variations in all-up-mass (AUM) and centre of gravity (CG);
• carriage of underslung loads;
• operation of movable aerodynamic surfaces.
Errors associated with feeding pressures, other than the freestream values, to the pitot-static system are called pressure errors. The action of the main rotor and the large fuselage angles of attack and sideslip that are commonplace in helicopter operation can cause errors in both total and static pressures. Whereas a static pressure error (APS) is applicable to both the ASI and the altimeter, a pitot (or total) pressure error (APP) is applicable to the ASI only.
Although there are several different methods for determining pressure errors they fall into either of two distinct groups. Those that compare the aircraft instrument readings with air data obtained from the actual freestream conditions in the vicinity of the aircraft, and those that make a comparison between aircraft instrument readings and data from independent external sources. Pressure errors are measured for a number of reasons and at various times during the life of an aircraft. During development, tests are made to determine the best position for the pitot head(s) and static vent(s) to minimize the errors over the full flight envelope of the helicopter. Due to the complexity of the flow field around the fuselage of a typical rotorcraft this is usually accomplished by fitting several probes and vents and by comparison selecting the location(s) that generate the smallest pressure errors. Prior to release to service formal trials are conducted to obtain data for publication in operating manuals and aircrew manuals so that the rotorcraft can be operated safely with changes in CG, AUM, rotor speed, power and external configuration. Knowledge of the pressure errors will be required to convert any true airspeed or pressure altitude restrictions into limits based on cockpit indications. Likewise, special operating techniques may be necessary to compensate for excessive pressure errors. Detailed knowledge of the pressure errors is also a pre-requisite for other flight tests such as performance evaluations and stability testing. Later in the service life of the rotorcraft, if external store configurations other than those originally fielded are considered, it may be necessary to re-evaluate the pressure errors.