FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1912
1912 - 0033.PDF
conceive a case where an increase of the size of the engine might bring more fuel economy, i.e., where too small an engine had previously been raced at an inefficient speed of rotation, but normally this should be a rare occurrence to-day, since the safety of the large margin of engine power is recognised. The fact is only worth mentioning because the result of practical motor car experience is likely to be a temptation to use the smallest engine possible where fuel is to be saved, and to run that at the speed which has previously been determined by experiment on the bench and by adjustment of the carburettor to be the most economical one in petrol. Although this may be justifiable from a limited point of view, it is necessary to remember that even if the competition regulations do not call directly for a large engine by insisting on a minimum rate of climbing, there are atmospheric reasons for keeping a large margin of power. 6. When a smaller engine can be used we can generally save in weight thereby, which raises the question whether the flight speed of greatest economy for one weight is not different from the economical speed with another weight on the same aeroplane. The chart shows by curve I that such is the case, since the effect of a lighter loading is to shift this curve bodily to left across the chart, thereby moving the point Q to the left also. It will be useful for present purposes to keep clear of the con siderations introduced by supposing the competitor to fly to a great height, either for the purpose of coasting as much as possible or to find favouring winds. When a pilot does this, he may effect a certain amount of flying without appreciable use of fuel in exchange for the energy expended in climbing. To discuss this consideration is dis tinct from the matter now in hand, but it presents no particular difficulties when we remember that it is analogous to the " constant acceleration" system for electric railway services (discussed by Parshall, and also in P. Dawson's monumental tome on this subject). 7- Even when the entire flight is supposed to be executed at one level, the best engine to adopt, the best speed to adopt, the best angle of incidence of main planes to adopt, all become more and more difficult to decide unless some such drawing as this chart is used to analyse fairly exhaustively the conditions of working of the particular aeroplane. This apparent complexity might wrongly decide the entrant upon the simple course of trusting to luck, which unfortunately and unavoidably enters into other aspects of all competitions. 8. Having said thus much to.show that the curves may be of use, it is as well to say briefly how they can be got for any aeroplane. 9. To Make the Chart.—As a basis, it is, of course, necessary to have the curve of lift and drift at all angles from zero to 20° of the particular wing shape to be used. Much assistance towards this, the first and most elementary necessity for aeroplane design, is found in the fine series of such curves given in "La Resistance de l'Air et l'Aviation," but the actual wing curve used should itself be submitted to test in the wind tunnel, either at the N.P.L. or at one of the technical colleges. The outcome of this experiment is the curve No. 1, marked "plane resistance" in the * chart. On it are marked the angles of incidence required at the various speeds to secure lift enough to support the aeroplane, supposed to be of constant weight. 10. Body Resistance.—After this, it is necessary to plot the curve 3, obviously a close approximation to a parabola (though it looks straight) giving the body resistance of the actual fuselage to be used at the various possible speeds of flight. It is laborious, but otherwise easy, to get the curve which represents the summation of the resistances of the skids, wheels, struts, wires, rudder, body, pilot's head and shoulders, and any exposed part other than the planes. This is curve " body resistance," and if the tail is a lifting tail it may be separately plotted, just as curve I was plotted and added in with body resistance. n. Horse-power Required and Available,—Next, these two curves, Nos. I and 3, are added to give the curve 4 " total resistance." After that we multiply the speed at each abscissa by the ordinate of " total resistance " at that abscissa and get thereby the curve 5 of " horse-power required." Then, whatever engine is to be used must be tested at a range of speeds to obtain the curve of " horse-power available " (viz., at the number of engine revolutions denoted by the speeds of travel- allowance being made for the slip which the propeller was designed to have at these speeds). This completes the chart, with the exception of the gliding angle curve, which is the same as the "total resistance" curve but re-plotted to make it easier to see the exact point of minimum and the value of the gliding angle at various speeds. 12. We now have a very complete expose of the qualities of the aeroplane and can trace by mere inspection the general effect of varying the things that are usually constant once the aeroplane has been built. For the best performance on a minimum quantity of petrol we require to use a minimum number of horse-power-hours on that per formance. If the test is for duration, then the number of hours must be a maximum, and two reasons in this case conduce to make us desire that the " horse-power required " shall be as small as possible. Reference to the "II.P. Iiequired " curve (5) shows that this occurs at say 70 ft. per second, or about 47 m.p.h. 13. If one were building a new aeroplane and desired to improve on this, one plan is to shift the "total resistance" curve (4) by some device so that it shall be lowered at the flight speeds to be used. There are several means of doing this. The body resistance curve (3), which is one of its components, can be lowered further if possible by the drastic omission of the unnecessary, and by clothing to a fair shape the necessary parts such as struts and wires. To economise on head resistance is a well-known difficulty, and is not to be enlarged upon here. Then it is desirable, as pointed out in paragraph 5, to move the "planes resistance " curve (1) either downwards, or, what comes to the same for present purposes, more to the left. We see that at the point of minimum horse-power, or 70 ft. per sec, the planes are cabre, the angle being over 11°. 14. Angle of Incidence.—If the angle could be brought to 6k" or 6" there would be a clear diminution of the h.p. required at that point. One way of doing this is by increasing the sail area. This, obviously, can be put in elementary terms :—The weight to be sup ported is constant, therefore the " M.V." of the air that is necessary to support it is constant, and since diminishing the angle diminishes the downward component of the air's velocity, we must compensate for that by increasing the mass of air moved. The simple way of doing this is to use larger planes—reciprocally, if we use larger planes the angle will be diminished, and so we arrive by the simplest possible stages not precisely to a confirmation of the view that larger aeroplanes are slower, but that increasing the wing area may help within limits to diminish fuel expenditure. By whatever amount the plane area is increased, by so much will the angle be altered, and accordingly the curve in question will be moved to the left. The user of the method can re-plot for himself. 15. If now we diminish the weight carried, the same reasoning leads to the same conclusion, the curve of " planes resistance " is moved to the left. The effect of both is that the horse-power required for the flight is less. The particular aeroplane, not a bad one, whose chart is here given, by no means represents the universal state of affairs. Instead of flying with the main planes at an angle of n" during the most economical flight, a certain well-known aeroplane will be found at an angle of 8°. The former aeroplane may be good, but it is, at any rate, susceptible of improvement. In such a case as this, a 10 per cent, diminution of the loading (which means a 10 per cent, diminution of lift), may be shown by the " lift to drift" curve spoken of in paragraph 9, to give a diminution of angle of incidence down to 8"5°> and to diminish the drift of the main planes at this point as much as 20 per cent, or 30 per cent. Turning to another matter :— 16. Rate ol Climbing.—If the competition involves climbing at a maximum rate as one of its conditions, this chart is again of use. A speedometer giving speed through the air is imperatively necessary, and such an instrument has been designed at H.M. Aircraft Factory by Mr. F. Short, and used with conspicuous success on many flights. There is another instrument, viz., Mr. Ogilvie's, and also an excellent one by the Cambridge Scientific Co., and doubtless others yet that may not have been produced commercially. Reference to the chart shows that the maximum of horse-power for climbing purposes is to be obtained at the point of speed at which there is the greatest vertical distance between the "H.P. required" curve and the "H.P. available" curve. This may coincide, by a mere chance, with the speed of most economical flight, but it does not necessarily do so. The figured chart shows that the 5'3 h.p. was to spare for climbing purposes in the case of this par ticular aeroplane—and this was checked against the measured rate of climbing and the weight of the aeroplane. It tallied very well, as the rate of climbing was 140 f.p.s., and the weight loaded = 1,250 lbs. 17. It may be well to point out that if the regulations of any future competition were to call for, not the longest flight in duration of time on a limited amount of fuel, but for the longest flight in point of distance, a totally different speed is the best speed, viz., the speed corresponding to the best gliding angle, in the example chosen a speed of 53 m.p.h. (instead of 47 m.p.h.). 18. It would be a mistake at this stage to enter into minor con siderations such as the effect of the increased speed if the air due to the propeller slip passing over a portion of the planes when pro peller is in front. It will be appreciated that such air movement over the planes is not accompanied by a corresponding forward movement over the ground. There is no particular difficulty in taking account of this, and in the present chart it accounts for the maximum speed at the intersection of curves 5 and 6 being in excess of the maximum flying speed by almost \\ miles per hour. The actual flying speed is marked on the chart.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
