Disc Loading

The choice of the main rotor disc loading will be influenced by the following considerations:

Advantages of Low Disc Loading Advantages of High Disc Loadings

Low induced velocities Compact size

Low autorotative rate of descent Low empty weight

Low power required in hover Low hub drag in forward flight

In the early days of helicopter development, designers used low disc loadings because engines were heavy. With the development of the turbine engine, the consideration of engine weight became less important and designers chose higher disc loadings to take advantage of the potentials of smaller overall aircraft size and low empty weight. A possible limit to this trend, however, may be a specific customer requirement not to exceed a given disc loading. The total amount of air displaced by a hovering aircraft is a function of its gross weight, but the ability to ‘placer mine” the landing surface is a function of its disc loading. Thus interference with pilot visibility or the difficulty of concealment in snow or dust, or the ability to tumble equipment at some distance from the hover spot are all related to gross weight, but the ability to entrain gravel, dirt clods, or bushes into the recirculation pattern is related to the disc loading. The limits for operation are not yet well defined. Hovering aircraft with very high disc loadings (20 to 50 lb/ ft2) such as the Vertol Model 76 and the Ling-Temco-Vought XC-142, were proved to be quite satisfactory for operation from grass lawns or asphalt but completely unsuitable for operation above sand or gravel surfaces. It is evident that the intended use of the aircraft will have a bearing on the maximum "environmental” disc loading.

The autorotative rate of descent is a function of the disc loading, as shown in Chapters 2 and 3. The ability to autorotate is recognized as one of the inherent and desirable features of helicopters. Good autorotative capability is extremely important for single-engine helicopters since it is practiced extensively during pilot training, but even multiengine helicopters are required to demonstrate full power- off autorotations and landings by both the military and the FA A. Any rotor will autorotate if the rate of descent is high enough and, in theory at least, a successful landing can be made from any rate of descent if the stored energy in the rotor is sufficient. In practice, however, the pilot’s chances of making a successful landing at high rates of descent are limited by his reaction time and his ability to judge the

precise altitude at which to initiate the landing flare. The same problem applies to airplanes in deadstick landings. Tests made on a B-25 bomber showed that good landings could consistently be made by average pilots if the steady rate of descent in the glide was less than 2,500 ft/min, but that at higher rates of descent, increased pilot skill was required. Although these tests have not been repeated on helicopters, it is felt that the rate of descent of 2,500 ft/min is a valid division line between satisfactory and unsatisfactory for single-engine helicopters in which students will practice autorotations. For multiengined helicopters in which the autorotation capability will be demonstrated by skilled pilots, it is suggested that the boundary be raised to 3,500 ft/min. The energy methods of Chapter 3 can be used to establish the maximum disc loadings that correspond to these limits.