Angle of attack and sideslip

Unlike conventional aeroplanes with wings rigidly attached to the fuselage the main rotor of a helicopter has pitch and roll degrees of freedom as well as the yaw freedom required by its rotation. As the rotor system generates most of the forces required for flight the fuselage attitude is, to a first approximation, simply governed by the balance of local aerodynamic forces and moments. Since stability axes are most often established relative to the fuselage it is evident that the velocity vector of a helicopter is rarely aligned with its longitudinal axis. The measurement of both angle of attack (a) and sideslip (P) is important for stability, control, performance and structural work. Some helicopter automatic flight control systems also use a direct measurement of sideslip as an aid to turn co-ordination.

The simplest form of instrumentation uses lightweight wooden vanes to measure the airflow direction relative to the boom attaching the vanes to the aircraft. Provided the vane is operating outside the wake from the rotor and any interference field associated with the fuselage or any external stores, the vane will align itself with the freestream airflow. The defection of the vane relative to the boom can be picked off electrically and presented to the crew as well as processed by any on-board data storage system. Depending on the orientation of the vane either angle of attack or sideslip can be sensed. Other systems determine a and P from the pitot-static system. A popular system fitted to test helicopters is the swivelling pitot-static probe which uses fins, like those fitted to a dart, to align the probe with the freestream flow. If correctly articulated and instrumented the angle of attack and sideslip can be inferred from the angular defection of the probe from the boom. An additional advantage of this device, provided there is no rotor wake interference, is that the true total pressure will be measured by the pitot. An alternative approach uses two fixed static pressure sources. If a static source is located on either side of the fuselage then in the presence of a sideslip the ‘into-wind’ source will over-read whilst the other may under-read. The subsequent difference in static pressure can be calibrated to give a measurement of sideslip.

6.2.2.2 Low airspeed

Low airspeed information is required for two reasons: firstly for test purposes to either ensure that the helicopter is in a true zero-wind hover or to provide accurate low airspeed so that its effect on handling and performance can be investigated; secondly for operational reasons to provide wind speed and direction information for improved weapon delivery. Thus there arises a subtle distinction between a ‘hovermeter’ and a true low airspeed sensor. A hovermeter is simply a device that accurately indicates the zero airspeed condition whilst giving the pilot a general indication of airspeed away from this condition so that he can bring the aircraft into the hover. A low airspeed sensor on the other hand delivers accurate airspeed information throughout the whole low speed envelope of the helicopter.

Simple hovermeter systems use a cranked boom or vane attachment that is aligned vertically to sense the direction of the downwash from the main rotor. Appropriate calibration is used to determine the vane angles, relative to the vertical, which are associated with a true zero-wind hover. Alternatively if the vanes are located sufficiently far below the rotor it may be satisfactory to assume that a true hover is indicated by each vane being aligned in a purely vertical direction.

Practical low airspeed systems generally use the rotational energy of the main rotor to boost the total pressure measured by a pitot probe. The Pacer system fitted to the AH-64A Apache consists of two pitot probes attached on top of the main rotor. When the helicopter is situated in a true zero-wind hover both probes sense the same dynamic pressure. However when the helicopter is moving or is hovering out-of-wind there will be a sinusoidal time-variation in total pressure and a phase difference between the readings from each probe. This time variation and phasing can be used along with the rotor speed and the mean pitot reading to determine the speed and direction of the airflow relative to the helicopter. If correctly calibrated a system like Pacer can also be used to measure sideslip in forward flight for test purposes and by the AFCS to enhance turn co-ordination. Another system called HADS (Helicopter Air Data System) which is fitted to the AH1 Cobra and the AH-64D also uses the dynamic pressure associated with the rotor wake but in a different manner. The HADS probe is a gimballed pitot-static probe that is located below the main rotor. It works on the principle that the main rotor wake strength and direction at any point below the rotor disk will vary with differing airspeeds and wind directions. Thus if a matrix of data points are coded into the system it is then possible for a given measurement of HADS probe angle and pitot pressure to be used as an indication of airflow relative to the aircraft. As with Pacer suitably calibrated HADS have been used to gather air data for test purposes. (See Section 3.5.6 for details of other systems.)

6.2.2.3 Temperature

A thermometer works by the transfer of heat energy from the medium under test to the temperature sensing element (bulb or thermo-couple). The local air temperature measured by an airborne probe will be higher than the ambient static temperature since the air will be necessarily slowed around the sensor. If the flow is completely halted and the temperature sensor is ideally screened then it can be assumed that the stagnation or pitot temperature will be measured by the thermometer and the compressible Bernoulli equation can be applied:

T = T [1 + 1-1 M Л

This equation can be re-written to replace Mach number with true airspeed:

Assuming air is a perfect gas (y = 1.4 and R = 277 J/kg K):

T = t + I______________ —________

p A + ‘ 1.4 X 287

Currently helicopters rarely exceed 200 kts and typical cruise speeds are closer to 100 kts (approximately 50 m/s). Thus the temperature measured by a perfect pitot- type probe would be no more than 5 K above the true ambient value and would usually be closer to 1 K greater than TA. Also noting that practical temperature probes do not fully decelerate the flow so that the recorded temperature is less than Tp it is clear why for all practical purposes the temperatures measured by in-service probes are assumed to be equal to the ambient value.