Tethered hovering is a versatile technique and is widely used. The helicopter is attached to the ground by a cable of known length fitted with a tensiometer. A wide range of effective weights can be evaluated in a short time as the rotor thrust can be varied from a low to a high value without the need for reballasting. Although the minimum and maximum weights that can be studied are usually the same as can be flown using the ground referenced method, the big advantage of this method is that height control ceases to be a problem. Therefore given good weather conditions and access to a range of cable lengths, accurate IGE and OGE performance figures can be obtained in a short time. A further possible advantage is that of safety: a multi-engine helicopter may be flown throughout the test at an actual AUM for which it has OEI hover performance. However, unless a high altitude test facility is available, data at the higher referred weights will be lacking. As with the ground referenced method, tethered hovering tests demand very light winds. Turbulence or wind fluctuations tend to play havoc with hovering accuracy and the cable tension.
Disadvantages of this method are those associated with the items of additional equipment and extra personnel required to conduct a potentially hazardous test technique in reasonable safety. A tensiometer, usually a strain gauge device, is placed in series with the aircraft’s cargo hook. It is obviously undesirable to allow the tensiometer to fall to the ground from any significant height and so test procedures will have to be conducted accordingly. Some installations employ a modified cargo hook and the tensiometer remains attached to the aircraft at all times. The tensiometer typically drives a cable tension indicator that is temporarily mounted in the cockpit where it can be seen by both the observer and the pilot. More sophisticated test installations will be capable of providing the pilot with a ‘cross-hairs’ type indication of cable angle. A single sensitivity setting may be inappropriate for all cable lengths, in particular they are often too sensitive for use with short cable lengths. The cockpit indicator is placed so that the pilot can easily scan to it whilst retaining a good view of external hover references. Ground marshallers may be required in the absence of a cable angle indicator and, possibly, as a back-up to such a device, to assist the pilot in maintaining the cable vertical. A range of cable lengths designed to give the required wheel/skid height will be required. The cables need to be proof loaded and certificated to the maximum likely working tension. The tethering point should be in a clear area well away from structures that could cause recirculation. The surface should be level and smooth and, preferably, hardened to facilitate handling the cables. As with the cables the actual tie-down ring should be proof-loaded and certificated. Well-briefed and practised ground handlers will be required to facilitate cable changes. Ground observers are required to observe the cable angle and indicate corrections to the pilot accordingly. The employment of a site controller, in radio contact with the pilot, is an effective risk reduction measure as is the carriage of a rear crew member to monitor the cable angle and advise the pilot on positioning.
The cable should be as close to vertical as possible when data is taken, the maximum error which can be tolerated is typically +10°. Aircraft plan position must thus be held very accurately, this is particularly important when short cable lengths are used. The optimum method of maintaining plan position varies according to cable length. For short cable lengths (about 20 ft or less) the best hover cues are the normal external references. The exclusive use of a cable angle indicator may well lead to overcontrolling. For very short cables the necessary accuracy may be achieved by observing the position of the aircraft’s shadow relative to a feature on the ground. A cable angle indicator becomes more ‘user-friendly’ with cable lengths greater than about 20 ft. The best technique may be to identify the correct position using the indicator and then to maintain that position using normal hover techniques.
The cable tension, and therefore the effective AUM of the aircraft, can be varied very quickly and so the most expedient test method is to explore a range of tensions for a given cable length, NR and loading and then to change NR, cable or ballast before repeating the process. For a particular condition the cable tension will vary from a maximum that is limited by the effective mass, hook limits or power, to a minimum limited by the requirement to maintain a small nominal tension to aid precise height control.
Since the force applied to the helicopter by the cable tension is below the CG, an excessive cable angle is destabilizing therefore the normal and emergency cargo release mechanisms must be checked and be functional before commencing these tests. After the cable is attached, the cargo master is usually selected to ON to facilitate a rapid emergency release. Usually the after take-off checks are performed in an IGE hover over the tether point before the aircraft is climbed vertically until the cable goes taut.
It is important to avoid snatching the cable since this could cause either structural damage or a premature failure of the cable. Once minimum cable tension has been established, power is progressively applied to achieve the desired value. Longitudinal compensation is often required as the effective CG changes with increases in hook load. Once the test condition has been stabilized it is unwise to chase the cable tension, it being better to leave the collective fixed and take an average tension reading. Data recording will facilitate this process. It may be advantageous to allow the co-pilot/ observer to control the collective while the pilot concentrates on maintaining an accurate plan position. If the aircraft drifts well out of position it is important to allow the cable to go slack immediately before resuming the correct plan position and reapplying tension. Helicopters have been rotated into the ground by a combination of excessive cable angle, high cable tension and low control power. On completion of the tests, it is vital that the pilot receives a verbal assurance from the groundcrew that the cable has been disconnected from the aircraft before being marshalled from the test site.