Ground-reference vertical performance test techniques are primarily used to determine IGE performance although a limited amount of OGE data may also be gathered. The range of referred weights will be restricted by the elevations of the available test sites.
The method is often very time-consuming since frequent ballast adjustments are required to both establish and maintain a range of referred weights.
The method obviously requires extremely light wind conditions, typically less than 3 kts. A mast-mounted anemometer located at the hover height is often used to ensure that this condition prevails. A pace vehicle may be used in light to moderate winds to achieve zero airspeed provided the wind strength and direction are steady and an into – wind track can be found. Caution must be exercised to avoid downwash interference as the test point is established. It is often best to give the pace vehicle driver the opportunity to determine what ground speed gives the best average still-air condition with the helicopter well clear. This ground speed can then be maintained as the helicopter approaches to establish a good relative position reference. The most usual height reference is the aircraft’s radar altimeter. Zero-errors are eliminated by subtracting the residual reading obtained when the aircraft is on the ground from any height datum. An alternative method is the use of a weighted rope whereby a crew member voice-marshals the aircraft in the vertical axis to keep the end of the rope just brushing the ground. External references may also be used but these will be subject to parallax errors.
A series of ground-referenced hovers are flown at different heights to document ground effect and to determine the IGE/OGE boundary. Once this has been accomplished, testing will concentrate on role-relatable hover heights (particularly in the case of rotorcraft destined to conduct roles requiring a load lifting or winching capability). A test session will normally be commenced at maximum AUM. If desired, a rotor speed sweep can be conducted at each hover. The AUM should be maintained within about 1% of the nominal value and so frequent reballasting (perhaps once every 10 minutes) will be required to compensate for fuel burn. Once the range of heights has been completed at a particular weight, ballast is reduced and the process repeated. Accurate data requires the minimum of pilot interference during the test point itself. Consequently the collective is not moved during data gathering and the aircraft is allowed to drift up and down gently as necessary. If height excursions exceed about +10% of the datum the weather will generally be unsuitable for accurate testing.