Low-speed testing

Since helicopters are typically slower, less reliable and more expensive to operate than aeroplanes, their procurement only makes sense if the mission requires operations in

the hover and at low speed. Thus, whatever the role of a helicopter, it will need to have satisfactory handling qualities in the low-speed regime. The definition of low speed is not consistent throughout the flight test world but it is normally taken to be from zero to 40 or 45 knots airspeed/groundspeed. Any full flight test programme will need to include qualitative tests of the manoeuvres that take place in this low-speed area of flight.

Hovering is clearly one of the most important manoeuvres to evaluate as helicopters may be required to hover for extensive periods and often with considerable accuracy due to the proximity of obstacles. Testing examines the workload to control plan position, heading and height. As well as evaluating hovering in zero wind, tests include operations in winds up to the airspeed limits from all azimuths. Although a pace vehicle can be used to generate relative winds it cannot be used to determine handling qualities in the hover due to the difference in visual cues and in the level of turbulence present. For most purposes it is only necessary to evaluate relative winds from along the longitudinal and lateral axes and the directions at 45° to these axes. In addition to the consideration of pilot workload, other factors such as the vibration level and roll angle at each wind azimuth are evaluated. Not only will these factors affect crew comfort but they may also have implications for on-board systems. A widely deployed missile system which is fitted to a number of battlefield helicopters is a good case in point. The missile will not launch at bank angles above 5° which can be below the aircraft roll angle required to maintain position in the hover with some relative winds. In addition aircraft fitted with winches often have bank angle limitations for operations in the hover. Figure 5.27 shows an example of how to present HQR data for hovering; vibration ratings (VARs) can be presented in a similar way. The stability characteristics of the aircraft and the FCMC will be the major factors that affect hovering characteristics.

Hover turns are made up to the maximum permitted rate starting in calm conditions before moving on to tests in winds up to the lateral and rearwards envelope limits. Tests investigate the ease of controlling yaw rate as well as stopping the turn on selected headings. The accuracy of height and plan position maintenance during the manoeuvre is checked. Torque spikes when initiating and stopping turns with the ‘power’ pedal are a common problem with responsive governor systems: transient droop may be an equivalent problem with less responsive governors. At high yaw rates some aircraft demonstrate significant cross-coupled responses. Other aircraft have suffered from loss of tail rotor effectiveness due to the tail rotor entering the vortex­ring state during high rate turns which has led to accidents or damage to the transmission when arresting the turn.

There are a number of different manoeuvres which the helicopter pilot can use to leave and return to the hover. These transitions can be longitudinal (normal approach and quickstop); lateral (sidestep); and vertical (bob-up/bob-down and towering transi­tion). In each case the manoeuvre can be flown with different levels of aggression by varying the maximum bank or pitch angle used or varying the time to reach the maximum angle. When flown with high levels of aggression these manoeuvres will be affected by any deficiencies in the field of view; control response; cross-coupling and engine and rotor governing system. Tests of these manoeuvres are conducted in a variety of wind conditions with incrementally increasing levels of aggression up to role-relevant values.

Assessing take-offs is relatively straightforward and is mainly concerned with evaluating the control activity required to go from the stationary, on-surface condition to either the hover or forward flight. Large control inputs may not be a problem in good visual conditions but may have more serious implications at other times such as when using night vision goggles (NVG). The available tail clearance to absorb pitching motions on lift-off is one area that is investigated. For running take-offs a particular point of concern is any tendency of the aircraft to pitch nose down once airborne due to changes in the main rotor downwash on the horizontal stabilizer.

Landings are normally split into vertical and running landings. In both cases the ability of the undercarriage to absorb vertical rates that are appropriate for the role has to be proved. Ministry of Defence Standard 00-970 [5.1] defines a minimum vertical velocity of 2 m/s that the undercarriage must be capable of absorbing during landings on flat, non-moving surfaces. Higher velocities are specified for deck landings that depend on the maximum intended sea state. Vertical landings are essentially just an extension of hovering with the added complication that differing downwash effects can cause uncommanded aircraft disturbances. Assessment is also made of the behaviour of the aircraft when in partial contact with the surface to assess the likelihood of ground resonance occurring. Running landings have more considerations such as tail clearance during the deceleration; heading maintenance during the landing run; tendency of the undercarriage to dig into the surface and the effectiveness of the brakes. For skid equipped aircraft the effects of reducing collective pitch during the ground run are investigated cautiously as it can lead to the helicopter ‘nosing over’.

Sloping ground operations are important for tactical rotorcraft where the choice of landing site may be very limited. Particular considerations are the control margins available and any tendency towards dynamic rollover. Dynamic rollover is a phenom­enon where beyond a certain angle of bank when in contact with the ground the rotorcraft develops a rolling moment that exceeds the corrective moment that the pilot is able to generate with the cyclic and the collective. This can lead to the aircraft rolling on to its side very rapidly. It is necessary to separate workload due to the relative wind and that due to the slope by performing a landing on the same heading but on level ground. Similarly the effect of stronger winds on control margins can be predicted if the difference in displacement with increasing wind strength is known from pace vehicle tests. Ideally, the tests should be flown on surveyed slopes with a variety of surface conditions.

If required ground taxying is tested on a variety of surfaces to assess turning circles and aircraft stability on the ground. Aircraft with high CGs and relatively narrow undercarriage tracks have been known to roll over even with full lateral cyclic applied when turning through a strong wind. The ease of operation of devices such as wheel locks and steerable nose or tail wheels is also evaluated. Ground taxying tests up to the maximum permitted speed are conducted prior to attempting evaluations of running take-offs and landings.