Speculation on the Really Low-Drag Helicopter

The ideas in this section come mainly from M. V. Lowson [4]. It is of interest to consider, at least in a hypothetical manner, the lowest level of cruising power that might be envisaged for a really low-drag helicopter of the future, by comparison with levels typically achieved in current design. The demand for fuel-efficient operation is likely to increase with time, as more range­flying movements are undertaken, whether in an industrial or a passenger-carrying context. Any increase in fuel costs will narrow the operating cost differential between helicopters (currently dominated by maintenance costs) and fixed-wing aircraft, and the possibility of the helicopter achieving comparability is an intriguing one.

Reference to Figure 7.3 shows that at high forward speed, while all the power components need to be considered, the concept of a really low-drag (RLD) helicopter stands or falls on the possibility of a major reduction in parasite drag being achieved. This is not a priori an impossible task, since current helicopters have from four to six times the parasite drag of an aerodynamically clean fixed-wing aircraft. For the present exercise let us take as the data case a 4500 kg (10 000 lb) helicopter, the parasite drag of which, in terms of equivalent flat plate area, is broken down in Table 7.1. All calculations were made in imperial units and for simplicity

these are used in the presentation. The total, 14.05 ft2, is somewhat higher than the best values currently achievable but is closely in line with the value of 19.1 ft2 for an 18 000 lb helicopter used by Stepniewski and Keys (Vol. II) for their typical case.

In setting target values for the RLD helicopter, as given in Table 7.1, the arguments used are as follows. Minimum fuselage drag, inferred from standard texts such as Hoerner [2] and Goldstein [5], would be based on a frontal area drag coefficient of 0.05. This corresponds to 2 ft2 flat plate area in our case, which is not strictly the lowest possible because helicopters traditionally have spacious cabins with higher frontal areas, weight for weight, than fixed-wing aircraft. A target value of 2.3 ft2 is therefore entirely reasonable and might even be bettered. The reductions in nacelle and tail unit drags may be expected to come in time and with special effort. A large reduction in rotor head drag is targeted but the figure suggested corresponds to a frontal area drag coefficient about double that of a smooth ellipsoidal body, so while much work would be involved in reshaping and fairing the head, the target seems not impossible of attainment. Landing-gear drag is assumed to be eliminated by retraction or other means. In the miscella­neous item of the data helicopter, a substantial portion is engine-cooling loss, on which much research could be done. Tail rotor head drag can presumably be reduced in much the same proportion as that of the main rotor head. Roughness and protuberance losses will of course have to be minimized.

In total the improvement envisaged is a 64% reduction in parasite drag. Achievement of this target would leave the helicopter still somewhat inferior to an equivalent clean fixed – wing aircraft.

Such a major reduction in parasite drag will leave the profile power as the largest component of RLD power at cruise. The best prospect for reducing blade profile drag below current levels probably lies in following the lead given by fixed-wing technology in the development of supercritical aerofoil sections. Using such sections in the tip region postpones the compressibility drag rise to higher Mach number: thus a higher tip speed can be used which, by Equation 6.45, reduces the blade area required and thereby the profile drag. Advances have already been made in this direction, but whereas in the rotor design discussed in Chapter 6 a tip Mach number 0.88 was assumed, in fixed-wing research drag-rise Mach numbers as high as

0. 95 were described by Haines [6]. Making up this kind of deficiency would reduce blade profile drag by about 15%. If it is supposed that in addition advances will be made in the use of thinner sections, a target of 20% lower profile power for the RLD helicopter seems reasonable.

Reduction in induced power will involve the use of rotors of larger diameter and lower disc loading than in current practice. Developments in blade materials and construction techniques will be needed for the higher aspect ratios involved. These can be expected, as can also the relaxation of some operational requirements framed in a military context, for example that of take-off in a high wind from a ship. A 10% reduction of induced power at cruise is therefore anticipated. The same proportion is assumed for the small residual power requirement of the miscellaneous items.

Table 7.2 shows the make-up of cruise power at 160 knots from Figure 7.3, representing the data aircraft, and compares this with the values for the RLD helicopter according to the foregoing analysis.

The overall reduction for the RLD helicopter is 41% of the power requirement of the data aircraft. An improvement of this magnitude would put the RLD helicopter into a competitive position with certain types of small, fixed-wing, propeller-driven business aircraft for low- altitude operation. Qualitatively it may be said that the RLD helicopter has a slightly higher parasite drag than the fixed-wing aircraft, about the same profile drag or slightly less (since the fixed-wing aircraft normally carries a greater wing area than is needed for cruise, while the

helicopter blade area can be made to suit, provided that adverse Mach effects are avoided) and a lower induced drag if the rotor diameter is greater than the fixed-wing span. The helicopter, however, has no ready answer to the ability of the fixed-wing aircraft to reduce drag by flying at high altitude. Equally of course, the fixed-wing aircraft cannot match the low-speed and hover capability of the helicopter.