FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1955
1955 - 0967.PDF
FLIGHT, 15 July 1955 79 THE ANGLO-AMERICAN CONFERENCE . . . that individual "black boxes" were invariably connected togetherby heavy, bulky wiring, which must be kept to a minimum. Mr- Joy concluded with a plea to equipment manufacturersto make every effort to reduce the weight and bulk of their pro- ducts, pointing out that a 10 per cent reduction in equipmentand payload made possible a corresponding reduction in structure weight. Further benefit resulted from reducing stowage volumerequirements and, hence, fuselage diameter; and Fig. 6 illus- trated the effect of changes in equipment weight and fuselage dia-meter on the all-up weight of a hypothetical future aircraft de- signed to cruise at Mach 2 for 4,000 n.m. It showed that savingswhich were helpful today may be vital in the years ahead. The Discussion Lt-Cdr. Bloch (French Navy) diicussed electric-power generationin the simplified aircraft attempted in France. In addition to the four common solutions, the French had established a fifth—only a smallproportion of the total power was required by constant-frequency electronics, and the bulk could be supplied by direct engine-drivengenerators, light inverters being used to provide constant-frequency supplies. Mr. T. P. Wright (Cornell) spoke of the problems inherent in achiev-ing high intake efficiencies, particularly as sonic speed was approached. Hunsaker had said in 1920 (Wright lecture) that "the success orfailure of any aircraft depends on what it first weighs on the scales." Equipment and systems had to be determined prior to determining even the layout of the aircraft. For example, a 1,000-lb saving inan existing aircraft could offer an ultimate gain of 1,500 to 2,000 lb; but the inherent gain (applying the full growth factor) was nearer 10,000 lb.The excellent work by Ivan S. Driggs emphasized this. Mr. Joy replied that much tunnel work had preceded the first Sightof the Victor—since which time no alteration had been made to the intakes. Ram recoveries on the Victor were around 95 per cent inspite of the leading-edge sweep of some 57 deg. Mr. R. L. Lickley (chief engineer, Fairey's) said that it was no usethinking of putting old equipment in new aircraft. Established stocks of standard units should not be drawn upon; modern aircraft had tohave redesigned equipment. Powered controls, also, were a way of making better aircraft, and yet Mr. Joy seemed "to hanker after theold days of servo tabs and similar devices." The solution to constant- frequency a.c. power was to have constant-speed engines—which somemanufacturers had developed. Mr. Joy agreed that one must have specially tailored equipment,and suggested that the equipment manufacturer should not have a free hand in its design. Where an airborne auxiliary power unit was usedduplication was required; it was better to use a main engine if possible. Mr. S. Scott-Hall (B.J.S.M., Washington) emphasized that Serviceaircraft were designed for a specific job, and a better title for Mr. Joy's paper might be The Influence of Aircraft Design on an AirborneWeapons System. It had to be recognized that some of the greatest advances had been made by equipment designers. Mr. Joy did not approve of the suggestion of going to an equipmentdesigner and saying "here is a weapons system, you take charge." It was preferable to develop everything in parallel, although it took along time to develop a combination of new airframe, engines and equipment. DESIGN of LARGE HELICOPTERS A Paper by BelVs Chief Helicopter Engineer IN his conference paper, Mr. Bartram Kelley, chief engineerof the Texas (Helicopter) Division, Bell Aircraft Corporation, gave an account of the recent design philosophy and results ofmodel experiments which had led up to the latest Bell helicopter project, the twin-rotor side-by-side "flying wing" helicopter. Mr. Kelley's remarks were confined throughout the paper totransmission-driven helicopters such as were in practical use at the present time and he began by discussing the problem of discloading. Given a certain gross weight, he said, it was advantageous from a weight and size point of view to have as small a rotor aspossible, with a high disc loading. This meant a greater power requirement and, therefore, fuel load, the basic relationship forhovering flight being Gross Weight 3/2'.••'••• ".' ' Horsepower = Ki x — — r—-.—— — Rotor Diameter x v Air Density where the dimensionless constant Ki depended on the lift co-efficient, the tip speed, blade smoothness and other factors. In the early days, low disc loadings of the order of 2 lb/sq ft were usedbut in more recent designs values as high as 6 to 7 lb/sq ft were not uncommon. This trend was similar to the correspondingincrease in aeroplane wing loadings, which had given rise to the need for longer runways for take-off. The helicopter equivalent of a longer runway was, perhaps,a higher rotor speed and it was of interest to examine what might be the limits to increase of rotor speed. In a recent series ofinvestigations by the Bell Aircraft Corporation, contracting for the U.S. Air Force, a Mach number of 0.92 was reached momen-tarily at the tip of the advancing blade, corresponding to a con- tinuous Mach number of 0.7 in the hovering condition. Thegeneral conclusions of this paper were based arbitrarily on a fixed hovering tip speed of 700ft/sec. The effect of mean blade-lift jlOOO I8OO coefficient on the disc loading had also to be considered, the basicformula being CL= K2 x Gross WeightTip speed2 x Solidity X Air Density Tip speed2 X Blade Area X pK2 x Disc Loading where K2 was another dimensionless constant having a value inthe range from 6 to 7. This equation showed why an increase in tip speed allowed an increase in disc loading without changingCL. In general, C L was chosen as large as possible. Throughout the working range, an increase in CL resulted in increased effi-ciency and a reduction in the power required to hover. However, if this gain were pursued too far, stalling of the blades on theretreating side of the disc became a limiting factor. If the heli- copter were being designed for sea-level operations, a high valuefor CL could be chosen; but if high-altitude flight were required, the attendant reduction in air density necessitated that this valueshould be lower if blade stalling at altitude was to be avoided. One means of delaying the blade-stall phenomenon was toincrease the built-in and dynamic twist of the rotor blades. Rotors with a built-in blade twist of as much as 12 deg had been flown bythe Bell Aircraft Corporation with notable success, although no quantitative data on the actual stall conditions were yet available.Another method of delaying the blade stall was by means of boundary-layer control, which could be still further refined by itsintroduction in a cyclically varying manner to compensate for the changing airflow conditions during the rotation of die blade. Thelast-mentioned method was actually being tested, in spite of the obvious mechanical complications. Even these devices wouldeventually be limited by their weight and complexity and a basic maximum practical value of CL would remain one of the limita-tions of rotors in their present form. [Corn, overleaf 600 W- KRl REFERENCE SLOPE BLADE WEIGHT TRANSMISSIONWEIGHT_ ,200 Fig. 1.—Weights of blades and transmissions plotted logarithmically as a function of rotor diameter. A square-law line is shown tor purposes of comparison. a. 1OO LjJ a.ui £ 80 o 6O 2O 4O ROTOR DIAMETER 6O 8O tOO Fig. 2.—Comparison of w speed/power - require- ment curves^ for three rotor configurations. zu- o UJ O NT A LU orr 4O 2O TANDEM ROTORS SINCLE ROTOR — •- SIDE-BY-SIDE ROTORS 30 6O 9O VELOCITY (kt) 120 . 15O
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
