FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1955
1955 - 1108.PDF
220 FLIGHT, UZAugust 1955 The Design of Small Helicopters —-from the Economical Aspect By P. R. PAYNE "In twenty years' time a small helicopter will cost no more than a light car."—Dr. Heinrich Focke. A LTHOUGH there are many gaps in our knowledge, theA\ design of a small helicopter can now—in theory—be carried •*• •** out in reasonable confidence that the end result will besubstantially up to specification. One would therefore expect a rotating-wing Austin Seven to be successfully produced withinthe next few years or not at all. The protagonists of the "car of the air" give witness to their faith with that complete absenceof any critical faculty which usually distinguishes fervour, whilst perforce earning a living by subscribing to the construction ofmonsters of ever-increasing cost and complexity. The opposing school of thought, still unsettled by the remark-able events at Kittyhawk, do little more than draw attention to the present state of the art, where there are currently displayed morebearings, dashpots, gearboxes, and things that go bump once per rev than in any other form of transport. (It is pleasant to recordthat in this respect the average American design seems even worse than our own, as may be seen by comparing, for example, therotor hubs of the S-55 and the Bristol 171). Various reasons for this complexity have been advanced in thepast, but in the writer's opinion the real reason is a lack of know- ledge on some subjects within the design team concerned. Sincediis is an unorthodox viewpoint (the complexity of a fixed-wing design seems to increase directly with knowledge) it may be aswell to give an example. Some of the early constructors, notably Bell and Hiller, usedsee-saw rotors which enabled them to dispense with drag hinges. Unfortunately, it was not then appreciated that eliminating draghinges from such a rotor reduces its damping derivatives to very low values, making the helicopter difficult to fly. Both firms solvedthis problem by fitting "stabilizing bars", the effect of which was to increase the damping and hence the stability. These dampingbars were fairly elaborate mechanisms, the principle reaching its ultimate complexity in the patents of Dr. Sissingh, who specifiedtwo viscously damped bars, for rate and attitude respectively. Since the effect of drag hinges had not then been investigated itis presumed that neither Bell or Hiller realized that two rubber bushes on the flapping pin would have achieved the same improve-ment as the stabilizing-bar system. In this example the simplification is obviously desirable, butmany cases are not so straightforward. If a simplification reduces both the first cost and the payload, some criterion must be estab-lished which will enable the correct decision to be made. Economics of Small Helicopters. The ultimate measure ofefficiency for any vehicle which has to pay its way is economic efficiency. For large transport helicopters the criterion is operat-ing cost per seat-mile or ton-mile, and all project calculations are eventually expressed in terms of this parameter. An example is 500 1000 1500 ANNUAL UTILIZATION (hr) 2D00 A = Hiller 360. B=S.N.C.A.S.O. Djinn.C=Two-seat gas-turbine heli- copter.D=Three-seat helicopter with sub- sonic ramjets. Fig. 1. Variation of operating costs with annual utilization. E=Hiller Hornet. F=Three-seat helicopter with supersonic rotor and ram- jets. the determination of the optimum fin height for a tandem heli-copter : a low fin brings the rear rotor near the plane of the front one, resulting in a high interference power loss which leads to ahigher rate of fuel consumption in cruising flight. A tall fin reduces the interference and the parasitic drag of the fuselage, but resultsin greater structural and transmission weight. The true optimum can only be found by calculating the variation of pence per seat-mile with fin height. For the small helicopter the basic economic parameter is notso obvious. For such tasks as training, advertising, line surveying, whaling and air observation generally, duration is of more im-portance than speed, so that the assessment should be made on the basis of operating costs per hour. For crop spraying, payloadbecomes of importance as well, leading to operating cost per hour per ton of payload. Cruising speed is only of importance for suchtasks as taxi work and light freight carrying. By tacitly assuming the cruising speeds of all small helicopters to be of the sameorder, the operating cost per ton-hour can be used as a general index of efficiency, and this is probably the best compromise,even though it does not allow for such important factors as take- off performance and hovering ceiling. In assessing general trends the most suitable method of calcu-lating operational costs for small helicopters is probably the one used by Commandant Boris.1 The form which this takes for pro-ject estimates is shown in Table I for the Hiller Hornet and 360, and the S.N.C.A.S.O. Djinn. In addition three design studies areincluded in order to cover the field more thoroughly. The insurance rate of 18 per cent of the first cost is based onthe assumption that parts necessary for replacement after an accident would be invoiced at cost price, together with the labourinvolved in repair. This scheme was first suggested by Mr. Colin Cooper, and where it is not operated premiums are as high as 22per cent per annum, a value which would greatly emphasize the points which will be made in the following paragraphs. In Fig. 1 the operating cost per hour is plotted against utiliza-tion for the aircraft in Table I, clearly showing why it is essential TABLE I: ECONOMIC PERFORMANCE OF SMALL HELICOPTERSAND PROJECTS Helicopter All-up weightPassengers First cost (approx.) Obsolescence (5 years) ...Insurance (18% first cost) Interest on capital (3% first cost)Pilot's salary Mechanics' salaries Fixed annual costs Fuel consumption (gal/hr)Fuel cost (per hour) Airframe spares (per hour)Engine maintenance (per hour) Pilot's ftying bonus Hourly costs ... Hiller 360 2,500 Ibtwo £12,500 £2.500 £2,250 £375 £1,200 £600 £6,925 11 £2.5 (petrol) £2.0 £1.0 £1.0 £6.5 Hiller Hornet 1,0001b one £3,500 £700 £630 £105 £1,200 £400 £3,035 40 £4.0 (kerosine) £1.0 £0.1 £1.0 £6.1 S.N.C.A.S.O. Djinn 1,3001b one £8,000 £1,600 £1,440 £240 £1,200 £600 £5,080 29 £2.9 (kerosine) £2.0 £1.0 £1.0 £6.9 Project All-up weightPassengers First cost (approx.) Obsolescence (5 years) ...Insurance (18% first cost) Interest on capital (3% first cost)Pilot's salary Mechanics' salaries Fixed annual costs Fuel consumption (gal/hr)Fuel cost (per hour) Airframe spares (per hour)Engine maintenance (per hour) Pilot's flying bonus Hourly costs Two-seat (Gas Turbine) 1,2001b one £8,000 £1,600 £1,440 £240 £1,200 £600 £5,080 13 £1.3 (kerosine) £2.0 £1.0 £1.0 £5.3 Three-seat (Subsonic Ramjet) 1,590 1btwo £3,500 £700 £630 £105 £1,200 £400 £3,035 55 £5.5 (kerosine) £1.0 £0.1 £1.0 £7.6 Three-seat (Supersonic Ramjet) 1,650 1btwo £4,500 £900 £810 £135 £1,200 £400 £3,445 10 £1.0 (kerosine) £1.0 £0.3 £1.0 £3.3
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
