FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1921
1921 - 0012.PDF
HT JANUARY 6, 1921 CAMBRIDGE UNIVERSITY AERONAUTICAL SOCIETY (OFFICIAL ORGAN, "FLIGHT") AEROPLANE DESIGN* Some Present and Future Possibilities By Captain F. S. BARNWELL, B.Sc, O.B.E., A.F.C., F.R.Ae.S. Drag.—-"The firs't thing to consider is the load-carrying power of the aeroplane. In horizontal flight at some uniform velocity, propeller thrust is equal to total drag, and total lift is equal to total weight. It is convenient to divide up and consider the total drag into three parts :—Drag of wing surface alone, which I shall call ' Wing Drag ' and denote by >'w; drag of tail plane with elevator flaps, fin, and vertical rudder, which I shall denote by rs ; and drag of all the rest of the machine, which I shall call ' Residual Drag ' and •denote by rR. Probably the optimum lift over drag for wing surface of full-sized aeroplane is about 25 to 1. Therefore we must have a drag at least one-twenty-fifth of the total weight. The Stabilising Drag also has some fairly definite* minimum value, probably we cannot get below about 12 per cent, of the wing drag. Therefore, simply to maintain the aeroplane at a constant height in a stable and controllable condition, we must have a drag of at least one-twenty-second of the total weight. The Residual Drag, however, is a much more variable quantity, and offers far greater possibilities of improvement. Item Weights " The total weight is best considered as made up of : (A) Aeroplane structure; (B) power plant complete (dry) ; (C) fuel, oil and water (if required) and necessary tanks; (D) weights more or less constant, i.e., flying and engine controls, instruments, seating and windscreen; (E) useful load ; (F) the pilot. Of these weights (B) is, at present, the most variable. From the point of view of ton-miles-per- gallon efficiency, the lower the power the higher the efficiency! Probably the low limit for power in commercial aeroplanes is fixed more by considerations of ' get-off ' qualities than by anything else. Probably one can say that, with present conditions, it is not sound to go below j h.p. per 20 lbs. of total loaded weight. An average figure for weight per b.h.p. of modern aero engines is 2.7 lb., and an average figure for weight per b.h.p. of total power plant (dry) is 3.3 lb. This gives us therefore a minimum for total power plant (dry) of about ,165 lb. per lb. of total loaded weight, or i6i per cent. Our best aero engine practice at present gives petrol con- sumption of . 55 pint per b.h.p. per hour, and oil consumption of about .023 pint per b.h.p. per hour. Cooling water may fairly be taken as directly proportional to the b.h.p., an average figure being .023 gallon per b.h.p. This amount is a minimum, and in practice it has been found necessary to add a constant amount of one gallon. Also to add an amount equal to some constant times b.h.p. x hours' duration, an H X Naverage figure being I6QJ in gallons, where H1 s the numerical value of the b.h.p. and N is the numerical value of flight dura- tion in hours. This gives a total weight of fuel, oil and water of 10 + 23 H + .5298 (H X N), in lbs. For a machine with b.h.p. equal to one-twentieth of total loaded weight we get total weight, in lbs. of fuel, oil and water = 10 + .0115 WT + .02649 (WT x N), where WT is the total loaded weight in lbs. and N the duration in hours. Neglecting for the moment the constant weight of 10 lbs., which is of importance in quite small machines only, we see that for a duration of one hovr, total weight of fuel, oil and water — about 3.8 per cent, of WT ; for two hours about 6.45 per cent. ; 3 hours 0.1 percent. ; 4 hours n .75 percent.; 5 hours 14.4 percent. ; 6 hours 17.05 per cent., and so on. The weight of tanks will be considered more fully later, but in the meantime a fair average figure is 11 per cent, of the weight of their contents. Approximately then, weight of fuel, oil and water and weight of necessary tanks for fuel and oil amount to :— 4.1 per cent, of WT for 1 hour's duration. 2 ,, 3 ,, 4 .. ' v~" '• -:"'••*" 5 ,, "" 6 and so on. Airscrew Weight" With regard to the airscrew, assuming that form and material be kept constant, the weight will vary as the cube • Extracts from paper read on December t, 1920. 7-°5 10 12.95 15-918.85 of the diameter. Assuming in addition that the speed of revolution and speed of translation be kept constant, the h.p. absorbed will vary as the fourth power of the diameter. Further, if the speed of translation vary, and the pitch only of the airscrew be varied, so that the ratio of pitch to advance per revolution be kept constant, the h.p. absorbed will vary- approximately directly »s the speed of translation. " The curves in Fig. 1 are for a speed of translation of 100 m.p.h. and for a speed—of revolution of 1,170 r.p.m., and represent good average practice for a 4-Waded screw made of Honduras mahogany* 100 m.p.h. has been chosen as a useful speed for commercial machines, and as being the FK:I. 1 ' • _——• ——• - •! • • lU * ' 140 H 120 a no IO w • cfe • 40 4 Fig. 1 : Curves for diameter and weight of four-bladed mahogany airscrew on base of b.h.p., for constant revs. (1,170 per min.) and constant speed (100 m.p.h.). Weight in lbs. = -0578 D8, where D is diameter in feet. B.h.p. absorbed at 1,170 r.p.m. and 100 m.p.h. is 0307 D«, and varies as V. speed attainable on a power loading of 20 lb. /b.h.p. with a wing loading of 8 Ibs./sq. ft.; 1,170 r.p.m. represents the airscrew revolutions given by the reduction gear of the Rolls-Royce ' Falcon ' when the crankshaft is turning at ^,000 r.p.m., the revolutions for maximum normal power. Constant Weights " The weights which are more or less constant for any aeroplane with single control can be taken as .— lbs. A Heron and elevator controls .., .. ,. 9^7 Rudder control .. ,. .. ' •> -•,.*-, :-">*-, 3-5 Tail plane control ... .. • .; , w. -8*o Throttle control.. •.],'-.' \:i..'».. ...."*""•'"•".• .-• 2-5 Magneto control .. --_ ,, -;."-' . . 1-5 Altitude control.. .. ._• '.?%: >...'. .. 1-5 Radiator shutter control ..-.." ~^ 5. ... 1 • 6 Switches and wiring .. .. " .,, . ,.• 2-7 Starting mag. and mounting .. .. " .. 7-2 Dashboard and all engine and navigating instruments .. .. ., „.,.. .. 16-o Pilot's seating ,. .. ,, ;-vi .. 12-o Engine doping system .. .. .. .. 4-0 Cocks, pumps, filter, etc., of petrol and oil systems .. ,. .. .. .. 8-8 Pilot's windscreen 20 Total 81-o " One other small item, which is not included in above table of controls, is weight of control cables, which can be taken as varying directly as the linear-dimension of the aeroplane. Wing Weights " I nave divided the weights of the structural members up into:—Weight of complete wing structure WK, of complete fuselage wr, of complete tail unit wT, and of complete under- carriage [wv. For convenience I have considered these weights as functions of the wing chord in feet, denoted by C. The various equations for structural weights, which I shall give presently, are based on the detail weights of the ' Bristol Fighter,' whose strength has been calculated and checked particularly fully, and in the structure of which little weight 1 .*£• 12
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
