2.^0 



NATURE 



[August i8, 1923 



interested in flying, but technical opinion, as voiced 

 in public discussions, considers that in attaining this 

 feature by the helicopter almost every other desirable 

 quality of a flying machine is sacrificed. 



Criticism has turned largely on the lack of efficiency 

 and safety in the helicopter. The airscrew is not a 

 new device and the principles of its operation are well 

 established ; efficiencies of 75 per cent, can be reached 

 and utilised in the aeroplane because it is an aeroplane. 

 This point is of some interest and merits further study ; 

 all heavier-than-aircraft are supported during flight 

 on the sacrificial principle, that is, something else is 

 driven down to keep the aeroplane from falling under 

 the influence of gravity. In the aeroplane the utilisa- 

 tion of power in producing lift is indirect, for the air- 

 screw is made to overcome the resistance of the aero- 

 plane, whilst the wings produce the down current, 

 and by reaction, the lift. This lift may be nine times 

 as great as the thrust of the screw, and is rarely less 

 than three times its amount. 



The arrangement is efficient because the wings are 

 large organs ; it is the momentum generated per 

 second which produces lift, whilst the power required 

 is roughly measured by the energy thrown away in 

 the downwardly moving air. The loss of energy for 

 a given lift decreases progressively as the area of the 

 downward stream is increased, and hence the efficiency 

 of the aeroplane follows, in part, from the use of large 

 wings. If the hehcopter is to compete with the 

 aeroplane on the score of efficiency its lifting screws 

 must be large. 



As the extreme case of large size, consider two 

 aeroplanes flying in a circle and connected by some 

 framework at present undefined, except that it sup- 

 ports a car at its centre. A first problem is immedi- 

 ately indicated — means must be provided for keeping 

 the car free from rotation. If the aeroplanes are far 

 enough apart their efficiency will remain as before, 

 and as supporters of weight are fully effective. As 

 a means of getting from place to place the combina- 

 tion is, of course, useless ; modification of the problem 

 still leaves us with the obvious conclusion that, whereas 

 the wings of an aeroplane travel directly from point 

 to point, those of the helicopter follow sinuous and 

 longer paths. The argument seems to be fundamental 

 and to exclude the helicopter from the degree of 

 efficiency as a means of transport which can be reached 

 with a good aeroplane. 



Returning to our example, it will be found that a 

 new factor enters into the problem as the two aero- 

 planes fly in smaller and smaller circles ; each passes 

 through the downwash produced by the other and 

 by itself on previous passages. If near enough, this 

 interference becomes very important, and it constitutes 



NO. 2807, VOL. 112] 



the only real difference between the helicopter as 

 hitherto projected and the airscrew as commonly used. 

 In looping, pilots frequently feel a bump on closing 

 the loop, although some fifteen seconds has elapsed 

 since the first passage and the distance travelled has 

 been about 1000 ft. A further illustration explained 

 on this principle arises from the observation that an 

 increase of thrust arises from the sideways moving of 

 a stationary airscrew, and therefore may be expected 

 in a helicopter when used for transport. 



All this is known, and the principles were laid down 

 many years ago by the late Lord Rayleigh and others. 

 Combined with modem data, it is possible to use 

 existing knowledge to predict the limits of efficiency 

 of a helicopter and to rely on the results. The design 

 of the structure which holds the wings together presents 

 greater difficulties, and attempts to build helicopters 

 now may react favourably on structural design, but 

 probably at a cost far in excess of that required to 

 produce the same results by research. 



Most of the attempts at helicopter design have led 

 to screws some 40 to 60 ft. in diameter moving on 

 the periphery at speeds of 70 to 100 m.p.h. Devices 

 produced in more than one country have lifted them- 

 selves into the air, but little has been attempted in 

 free flight. The Air Ministry has announced the 

 development of the Brennan helicopter to the stage 

 of lifting itself, and only ten per cent, of the prize 

 money is allocated to the extension of this performance 

 from a few feet to vertical flight up to 2000 ft. in a 

 light breeze. 



The rest of the competition relates to transport and 

 control. One particularly hazardous requirement is 

 that the helicopter "... must descend vertically from 

 a height of not less than 500 ft. without engine. . . ." 

 It is a crucial test which, I believe, would involve 

 certain death to the pilot who attempted it in the 

 helicopters so far devised. In the case of engine 

 failure, the helicopter at best is less effective than 

 a parachute having an area equal to its blade sur- 

 face, and is quite unable to provide an adequately 

 small rate of descent. At its worst it is far inferior 

 to this. In all circumstances the aircraft will require 

 control, and the solutions hitherto proposed do not 

 inspire confidence. It is evident that even the essential 

 principles of a happy solution depend on that stroke 

 of genius for which the Air Ministry is appealing and 

 which it appears to think only needs a monetar}- 

 stimulus to become operative. 



Unlike the helicopter, the aeroplane does not lose 

 its lift when the engine fails. It must perforce descend, 

 but all its controls remain intact and danger comes 

 only if the available alighting ground is unsuitable, j 

 Safety in aeroplanes is a subject for insistent inquiry. 



