FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1921
1921 - 0015.PDF
JANUARY 6, 1921 sq. ft.rfor instance, the wing weight is -712 lbs. persq. ft. and the wing loading therefore 7-812 lbs. per sq. ft., whilst when the wing area is 1,600 sq. ft. the wing weight has gone up to 1-41 lbs. per sq. ft. and consequently the wing loading to 8-51 lbs. per sq. ft. The differences are fairly slight, but would make a difference to speed ; on this power loading (of 20 lbs. per h.p.), the speed would be about 100 miles per hour for 7-8 lbs. loading and about 102\ miles per hour for 8-5 lbs. " In the curve of ton-miles per gallon, T, here given, I have (for simplicity) neglected this slight speed variation and considered the speed as constant and as 100 m.p.h. The numerical value for ton-miles per gallon then becomes "It is interesting to note that ton-miles per gallon is a maximum for a machine of about 800 sq. ft. wing area. Obviously it falls off more and more rapidly below this optimum size, because the constant weight of pilot (180 lbs.) + the other previously tabulated constant weight of 81 lbs., represents a large proportion of the total weight as this decreases. It falls off, though much less rapidly, above this optimum size, because the increasing structural weight per square foot is becoming of greater moment than is the de- creasing ratio of constant weight to total weight. If the slight variation in speed were taken into account, the curve of ton-miles would be slightly modified, being increased more and more as the size increased, but the alteration would be fairly small—at 1,600 sq. ft., for instance, it would be about 5 18 instead of 5-05, at 800 sq. ft., 5-51, instead of 5-44, whilst at 200 sq. ft. it would remain the same. " Sufficient for*r>ur present purpose, however, to say that for a duration of 4 hours and a ' crew ' of one, an aeroplane of about 800 sq. ft. wing surface seems about the most economical size, and can accomplish 5-51 ton-miles per gallon. " A point of interest is the amount of improvement due to decreasing duration of flight, and thereby increasing the possible useful load. For this 800 sq. ft. aeroplane, if we decrease the duration to 3 hours, the ton-miles go up to 5 • 86, and if we decrease the duration to 2 hours, the ton-miles go up to 6-15. However, it is probable that a range of 300 miles is about the minimum that will be required for com- mercial aeroplanes in the near future, so fuel, oil and water for 4 hours at 100 m.p.h., air speed does not leave excessive margin for adverse conditions, and we shall say that our typical machine must carry this. . " On Improvements ": ': " Let us now consider what can be done in the way of improvement. The aeroplane we have arrived at is a biplane of 800 sq. ft. wing surface, of 6,545 lbs. total loaded weight, °f 327'3 maximum normal horse-power, with a speed of 101 -3 m.p.h., and a duration of 4 hours at this power. The useful load is 2,737 lbs., §pd the ton-miles per gallon 5-51. " The weights consist of :— lbs. - Total wing weight, ww .. .. 866 Fuselage weighty wr ..'.,, -.. ' 315 Tail unit weight, wr '.. : .. .. 147 Total landing gear weight, wv .. 216 ' Constant' weights and control cables 99 Giving a total structural weight of 1.643 Total power plant (dry) .. .. 1,080 Airscrew .. .. .. .. 61 Petrol, oil and water for 4 hours, i.e., 90 gallons petrol, 3 • 8 gallons oil and 9 • 4 gallons water .. .. .. 780 Petrol tank .. ... . .. .. 56 Oil tank .. .. "-?' L' ::'•'... ,:-•.-.-. 8 " Giving a total power-plant-for-4-hours "weight of .. .. .. . ." 1,985 " (A) Firstly to consider the effect of fuel economy:— The engine is absorbing -55 pint of petrol per b.h.p. per hour, so its thermal efficiency is only about 23 per cent. If it were possible to double the thermal efficiency, we should be able to scrap 325 lbs. of petrol and 20lbs. of petrol tank, meaning that we could add this 345 lbs. to the useful load, thereby bringing this up to 3,082 lbs. and the ton-miles up to 6-21. (B) Assuming the efficiency of the engine, in weight per b.p.h. could be doubled, we should be able to scrap one half of the engine (dry) weight, or about 435 lbs., and we could add this weight to the useful load, thereby bringing this up to 3,172 lbs. and the ton-miles up to 6-39. (C) Assuming that, to a small extent by improved disposition of parts, to a small (but not so small) extent by improved detail design of parts, to a very large extent by employing improved materials, we could reduce the whole structural weight by one half. We should then be able to scrap 978 lbs. of aeroplane weight and add this weight to the useful load, bringing it up t° 3,715 lbs. and the ton miles to 7-48. Aerodynamic Improvements " The foregoing are all problems in structures and structural materials, let us now turn to aerodynamic considerations. "The engine power is 327-3, the speed 101-3 m.p.h. and the propeller efficiency about 82 per cent., so the thrust, " The total drag is equal to the thrust, of course, and the following is approximately how this drag would be ap- portioned :•— lbs. Per cent. .333 33 i16-1 6'8 6-9 Wings (alone).. ' ; ,-.' . .s '• fBody .. ... .. .. rR • Landing gear . . ^External struts and wires of wings. »".. Tail unit 6869 363 36-5 "" *"'~ - 995 ioo-o " The drag of the wings (alone) has been fixed on the assumption that their lift over drag is 18, which is probably about correct for this case, namely full-size biplane of aspect ratio 7, with rounded off and fined down tips when the absolute lift coefficient, Lc, is about -156. " First then is it possible to reduce this figure of 363 lbs. of wing drag, rw ? By using a monoplane form, of aspect ratio about 10, and reducing the area till the loading be- came about 11 lbs. per square foot, we could probably achieve i an L/D ratio of about 25. If the wing structure could then be of the same weight (which is certainly untrue), we should reduce the wing drag to 262 lbs., meaning that we should reduce the total drag from 995 to 894 lbs. at 101-3 m.p.h. This reduction, of about 11 per cent., would allow of an increase of speed, for the same power, of about 3-6 per cent., and would therefore put the speed up to about 105 nup.h. and the ton miles up to about 5-71. '' Next to see what can be done about body drag, the biggest item on the list. Now this body has a maximum cross section of about 18 -4 sq. ft. ; 15 -2 sq. ft. of rectangular girder struc- ture and 3-2 sq. ft. of curved top fairing. Thus for this (normal present-day) aeroplane body, with nose radiator, cockpit opening, wind screen, etc., the drag is about 18-2 lbs. per sq. ft. at 101 -3 m.p.h. or about 8-2 lbs. per sq. ft. at ico ft. per second. Now the drag of the best torpedo form, of square section, is only about 1 -6 lbs. per square foot of maxi- mum cross section at 100 ft. per second. So if it were possible to make the body of this optimum torpedo form, whilst maintaining the same maximum cross sectional area (of 18-4 sq. ft.), the drag would be reduced from 335 lbs. to about 65 lbs. But we should have to add radiator drag, and drag of wind screen and cockpit opening, or at any rate of some form of projection, to ensure a good field of view for the pilot. The minimum radiator drag at present practicable, to cool this engine (of 327 h.p.) at this speed (101-3 m.p.h.), is about 50 lbs. Probably we could arrange a fairing, with' suitable sliding windows, over the pilot's head for an added drag of about 10 lbs. " There is no reason why, by using a suitable spinner on the airscrew, the remainder of the body could not be made of torpedo form, so it would appear that we should be able to reduce the body drag from 335 lbs. to 65 -f 50 -f 10 = 125 lbs. This means that total drag would be reduced from 995 lbs. to 785 lbs. at 101-3 m.p.h. This reduction of about 27 per cent, would allow of an increase of speed, for the same power, of about 8-2 per cent., and would therefore put the speed up to about no m.p.h. and the ton miles to about 5-98. " There is no reason why we should not eventually be able to construct a satisfactory retractable under-caniage, and therefore be able to dispense with the whole of the drag of this part in flight. This would reduce total drag from 995 to 835, or by 19 per cent., and would allow of an increase of speed of about 5-9 per cent., putting the speed up to about 107-5 m.p.h. and the ton miles to about 5 '84. " By choosing optimum wing position with respect to C.G. (of whole machine), and using the most efficient shapes for the various members of the tail unit, it is probable that the drag of this could be reduced from 69 lbs. to about 39 lbs. This would reduce total drag by about 3 per cent, and would therefore allow of increase of speed of about 1 per cent., putting speed up to about 102-3 m.p.h. and ton miles toabout 5-56.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
