Human-Powered Helicopter

The problem of designing a human-powered helicopter (HPH) is not a new one, with some good theoretical studies being performed by Kendall, (1959), Naylor (1959), and Shenstone & Whitby (1959). In 1981, the American Helicopter Society (AHS) first offered a $20,000 prize [see Sopher (1997)] for the first successful controlled flight of a HPH. As of 2005, the prize remains unclaimed. The AHS’s requirements for the HPH dictate that the machine must lift itself and the pilot off the ground without the use of any stored energy devices, climb to a height of 3 m (9.84 ft) and be in flight for 1 minute while maintaining horizontal flight position within a 10 m (32.81 ft) square. A slow descent is allowed after the maximum height is reached. There have been two notable attempts to do this – one in Japan [see Naito (1990) and Fig. 6.41] and one in the United States at California Polytechnic Institute [see Mouritsen (1990)]. Both of the machines were driven by tip mounted propellers, very much like the early Brennan machine (see Fig. 1.13) so requiring no anti-torque devices, and had rotor diameters of over 100 ft (30.48 m). The pilot delivered power from a chain and sprocket system through a cable transmission to the propellers. The current flight record by the US team is 8 seconds at a maximum altitude of

0. 203 m (8 in) and the Japanese team holds the duration record of 19 seconds, both attempts which are far from meeting the AHS’s requirements. In the late 1990s, there were only two active HPH projects, one at the University of Michigan and the other at the University of British Columbia.

Patterson (1986) gives a good overview of the technical and practical issues in building a HPH. While the choice of strong lightweight materials and the constmction of a suitable HPH is one technical barrier, much of the problem has to do with the limits of human physiology. For an endurance of 1 minute an athletic human can be expected to deliver a power of between 0.67 hp and 0.8 hp (500-600 W) and perhaps up to a peak of 1.34 hp

(1,000 W) for a duration of 10 seconds. This means that a net possible energy expenditure of a human before exhaustion is about 34.1 BTU (36 kJ). Based on these power expectations it is possible to proceed with the conceptual design of a rotor to accomplish the AHS’s requirements. The design problem has been recently considered in some detail by Filippone

(2002) . The minimum power to hover a vehicle of weight W out of ground effect (OGE) will be given by the modified momentum theory (Chapter 2) where

1 Wъ'[22] [23] 1 W3/2

P0GE ~————– ‘ =————————- — – -. (6.52)

FM JlpA

5 vCwiisc the most optimistic average power output is known (Pavaii = 0.8 hp) the foregoing equation can be rearranged to solve for the rotor radius R giving

considered. Fradenburgh (1960) reports on measurements made with rotor height/diameter (h/D) ratios as low as 0.05, and Prouty (1985) gives a summary of other ground effect measurements. Reductions in induced velocity IGE of more than 60% seem possible for a large rotor in very close proximity to the ground. This favorable IGE effect will result in a lower effective disk loading (higher effective disk area) and so a smaller rotor can be built to meet the same requirements. The rapid loss in the benefits of IGE operation, however, are found for h/D > 0.4. Therefore, for the HPH to remain IGE throughout its flight for good aerodynamic efficiency, a balance between the choice of rotor diameter and hovering IGE benefits must be sought.

Proceeding further by assuming such a rotor could be built and hovered in ground effect, it is interesting to evaluate whether sufficient human energy is available to climb a HPH to 3 m and then to hover there (i. e., further out of ground effect) for a period of time. The power to climb vertically can be established from the results in Chapter 2. Assuming low rates of climb then the extra power required to overcome induced losses and also increase potential energy will be

where Vc is the climb velocity and A Preq is the excess power required to climb. The extra energy expended during this process is

if Vc is assumed to be constant. This means that the excess energy available to climb to the required altitude can either be expended quickly by climbing at a higher rate or slowly by climbing at a low rate. If the maximum extra power that is humanly possible is assumed to be 0.25 hp (0.186 kW) for a duration of 10 seconds (equivalent to an energy of 1.768 BTU or 1.86 kJ) then a maximum climb velocity of about 1.85 ft/s (0.56 m/s) is possible, reaching the required altitude in about 5 seconds. A subsequent hover for 55 seconds would consume at least 31 BTU (32.6 kJ), clearly to the point of human exhaustion. Alternatively, a slower climb over 60 seconds will need a climb velocity of only 0.164 ft/s (0.05 m/s) using a total energy of about 34.8 BTU (36.7 kJ), but this again is right at the limits of human endurance. Even if such a flight could be performed by a super-athlete, the final stages of the flight will require the aircraft to descend and possibly autorotate to the ground. Of interest is that because of the extremely low induced velocity through the rotor, even a modest rate of descent may take the rotor into the vortex ring state. This will require the machine to have good control capability, although it would seem that with kinesthetic (weight shifting) alone this may be insufficient.

In conclusion, it would seem that the design of a HPH to meet the AHS’s requirements is feasible but probably not yet practical. This is in part because of the enormous size and structural difficulties in building rotor to give the low power requirements that are necessary to match the physiological capabilities of even the most athletic, super-fit human. Also, it would seem that the assumed aerodynamic efficiencies of such large, slow turning rotors are unlikely to be realizable, even operating IGE. Some means of effective flight control will also be required for such a vehicle to stay within the within the defined 10 m (32.81 ft) box, although it would seem that apart from some allowance for kinesthetic control this aspect of the problem has received little attention from designers. The design of a HPH, however, will continue to be a problem that will fascinate generations of engineers for many years to come.