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Aviation History
1931
1931 - 0058.PDF
FLIGHT, JANUARY 16, 1931 the outlying portions of the British Empire without covering in some parts of the journey considerable portions of open sea. I propose now to describe briefly the method and procedure adopted by my firm when commencing a new design of either a flying-boat or a seaplane. Assuming that all the preliminary details have been settled, the first step is to determine the suitability of the proposed hull or floats for the design under consideration. A model is, therefore, made of a hull or float as accurate as possible to some convenient scale, usually ^th to ^th full size, according to the size of the machine and the range of speeds at which the results are required. These models are usually made of mahogany with a polished finish. By means of weights and a light frame attached to it, the model is balanced about a transverse axis, passing through the point corre- sponding to the centre of gravity of the full-size aircraft, and is attached to a supporting apparatus on the testing tank carriage. The model is towed by means of a long towing rod which is arranged to be approximately at the same height and inclination as the propeller thrust axis. The other end of this rod is attached to some form of recording apparatus, whereby, when the model is being run, the pull required to overcome the water resistance can be measured. At any particular speed at which it is required to run the model, due allowance is made for the equivalent lift obtained from the planes in full size, this being done by the application of weights supported by light wires over pulleys. It is usual in the preliminary runs to assume this air lift propor- tional to the square of the speed. If the angle at which the boat runs departs much from the angle of maximum lift, further runs are made correcting for this want of air lift. A form of parallel link apparatus attached to the model enables the angle of the model, compared with some fixed datum line, to be measured at any particular speed. The datum line usually used is the forebody keel line just forward of the main step. The water resistance is assumed to be wholly due to wave formation, the results being predicted for full size from the model records, on the basis of Froude's law of similarity. The above is a very brief description of the apparatus used, and the method of carrying out the various tests. Those who require further information on this subject are referred to R. & M. 655. 1 ? X"» n 2fiOO' / / FIG. 2 \ \ X ........MM* ' — 22 M. SO M ,56 42 46 DO — SPEED IN KNOTS — FLYING BOAT — DISPLACEMENT 36.200 LBS. —-— —— FIG. 3 22 26 3O 94 36 42 SPEtO IN KNOTS. TWIN FLOAT SEAPLANE - DISPLACEMENT 21,000 LAS. V ^ —-^ •—. ~ — *• FIG3A 22 26 SO 3+ 3S •OS SPEED IN KNOTS \ V FIG.4 FLV1NS BOAT * B5PU>CEMENT 56.20C LBS T**Eorr 1 26 3O 5PCFD IN 4a si) II \ \ FIG.4A TWIN'FLOAT SEAPLANE \ — DISH ACEMtN-r eioooias TAKE OF - 60 KNOT A 22 2© 5O 34 V fiPEffJ Ifsi KNOT S The two main objects of model tests on a testing tank are (1) to determine the resistance of a float or hull during the take-off run ; and (2) to determine the angle at which the float or hull runs relative to the fixed datum line. The planes on the full-size aircraft are attached at some fixed angle relative to the datum line, and consequently the running angle of the hull or floats has an important bearing upon the lift obtained. Thus, by carrying out tests at the tank, curves showing resistance and running angle at any speed may be plotted. It should be noted here that it is not always possible to set planes on a hull at the best angle for taking off the water, because other considerations may determine that the planes should be set at a finer angle. Thus, if the chief consideration in the design of a boat or seaplane is top speed, the wings will definitely be at a finer angle than if the chief consideration is cruising speed. This is one of the differences between military and civil aircraft. Military aircraft are usually designed on top speed, whereas civil aircraft are invariably designed to be as efficient as possible at a definite cruising speed. A typical resistance curve of a hull shows that the resistance increases steadily to a maximum value at a speed approxi- mately 30 per cent, of that at which the machine becomes airborne, and as speed increases beyond this, the resistance decreases more or less uniformly. See Figs. 1 and 1A. Fig. 1 shows the resistance curve of a normal type flyinp boat at an all-up weight of 36,200 lb. Fig. 1A shows the resistance curve of a twin-float seaplane of an all-up weight of 21,000 1b. Cases have arisen where a certain increase of resistance occurs at about 60 to 70 per cent, of take-off speed, but this is usually due to some characteristics of the hull line* and is seldom encountered in present-day design. See Fig - If it is suspected that on a particular model the resistant' will increase at high speeds, then a small model is man'1 so that the resistance at speeds approaching the take-of speed may be investigated. 4 These small models are used as an indication only, a'1'1 are not relied upon, because the scale is too small for a nig" degree of accuracy. The angle taken up by a hull datum line increases gradual!) as speed increases, although very often a slight decrease i> noticed just prior to the machine becoming airborne '^ Fig. 3, which shows the angle curve corresponding to the resistance curve shown in Fig. 1. , Floats angle back rather suddenly as the speed at whicn the hump resistance occurs is reached, and then usual!1 tnw forward slowly as speed increases. Efficient floats can J* expected to angle forward about 1| to 2 deg. from t.- maximum value, by the time take-off speed is obai* See Fig. 3A, which shows the angle curve corresponding the resistance curve shown in Fig. 1A. Load on Water/Resistance - As a means of relative comparison between differen" typ._ of hulls and floats, a curve showing the value of the ra load on water/resistance at any speed, is plotted. 60
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