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Aviation History
1927
1927 - 0918.PDF
EdM DECEMBER" 1, 1927 MODEL AIRSHIPS JUST recently we referred in FLIGHT to a suggestion put forward at a meeting of the Society of Model Aeronautical Engineers that a model airship section should be formed, a suggestion that was favourably received and supported by Sir Sefton Brancker, Director of Civil Aviation, who was presiding at the meeting in question. As we remarked at the time, there seem to be great possibilities in model airships, not only from the sporting or competition point of view, but also as a means of practical research. In the latter connection, it will be obvious that with a model air- ship it would be possible to fly a model airship under control, and so observe its behaviour with considerably greater facility than would be the case with a model aeroplane. Quite apart from these points, however, the construction and flying of model airships should open up a comparatively new field of activity for modelists and others, one which we feel certain will provide much interest, instruction and amusement—to say nothing of the incidental support it may give to the lighter-than-air movement itself. So far, however, very little has been done with model air- ships, and as a result very little data or information is avail- able for the would-be model airship enthusiast, so that while there are many who are only too ready to set about it, they are at a loss to know where and how to begin. Fortunately, practical interest in the model airship scheme is not lacking, and several of those concerned in airship design and con- struction have looked favourably upon it and have promised their support and assistance. As a start, we give below some useful data on model air- ships—which should enable the lighter-than-air modelist to make a beginning—which has been compiled by Mr. R. H. Scholtel, of British Airships, Ltd. The Technical Aspect of Model Airships LIFT is the most important factor to be considered, so let us therefore form some idea of the lift we are likely to obtain. By the principle of Archimedes, we know that when a body is immersed in a fluid, an upward force is exerted upon it equal to the weight of the fluid displaced. For the body to be buoyant, this upward force must be greater than the weight of the body itself. In the case of the airship the fluid is air, the weight of which may, for practical purposes, be taken as 0-075 lb. cub. foot; hence, for the airship to have lift, its weight per cub. foot must be less than this figure. The method employed is to fill as light an evelope as possible with some gas having a less density, or weight per unit volume, than air. Suitable gases are hydrogen and coal gas. The weight of hydrogen gas is 0-005 lb. per cub. foot, so that the difference of the weight of hydrogen and that of air is 0-07 lb./cub. foot. Thus we find that the lifting force of hydrogen is 0-07 lb./cub. foot, or 70 lbs. per 1,000 cub. feet. Coal gas is heavier than hydrogen and weighs 0-032 lb./cub. foot, and hence it only gives a lifting force of 0-043 Ib./cub. foot. The total lifting force exerted by the gas contained in the envelope is known as the gross lift of the ship, and if the ship itself has an exactly equal weight, then the system is in equilibrium and the ship will neither rise nor fall. Such a state is required for steady flight. Commercial hydrogen will give a lift of between 60 and 68 lb./cub. foot. " We have found then that— Gross Lift. lbs. = Total volume of hydrogen in cub. ft. x 0-07. Shape of Envelope. This is of great importance, because upon it depends the lift, speed and manoeuvrability of the ship. In order that the weight of the envelope shall be as low as possible for a maximum enclosed volume, the shape of the envelope should be as nearly spherical as possible. Such a shape is unpractical, because of the large area of cross section offered to resistance to forward motion. It is for this reason that the airship envelope is elongated to what has been found to be a streamline shape, giving least resistance. Broadly speaking, the longer and thinner the ship, the faster she will be, so long as the shape is kept a good stream- line. Hence, a mean must be struck between these two cases, and the shape must be such that the ship shall be as fast as possible for the necessary lift. The usual values of fineness ratio of length -f- max. diameter, are from 3i to 6 for a good streamline shape. Any parallel portion of envelope is found to increase resistance. A good streamline shape is given by an ellipsoidal nose having its semi-minor axis two-fifths of the length of the airship, from the nose, the tail being outlined by parabolic or circular, arcs tangential to the ellipse at the maximum diameter. It is clear that lift is proportional to gas volume, whilst the weight of the ship may be taken as proportional to the surface area. Linear dimensions are proportional to (volume)1/3. Weight is proportional to (volume)2'3. From which it may be seen that for an increase in gross lift the weight of the ship is not increased in the same proportion. It is for this reason that airships in small sizes are of no practical value. The table below shows overall dimensions and gross lift for sizes of model ship, based upon a fineness ratio of 4 -6 : 1 Model Ships. VolumeCubic feet. 1,000 750 500 250 100 50 25 20 10 5 2* LengthFeet. 35-4 32-2 28-3 22-3 16-413-2 10-3 9-6 7-6 6-0 4-8 Max. Diam.Feet. 7-7 7-0 6-2 4-8 3-7 2-8 2-2 21 1-7 1-3 1-0 Gross Lift.lbs. 70 52-5 35 17-5 7 3-5 2-75 1-4 0-7 = 11-2 ozs 0-35 = 5-6 0-175 = 2-8 „ In the non-rigid or semi-rigid type of ship it is necessary that the gas contained in the envelope shall be under pressure, so that the shape of the envelope shall be maintained under all conditions. This internal pressure is usually between 3 and 5 1b. per square ft., according to the shape, and type of rigging employed. Now it is a practical advantage to use the lightest fabric which will stand the necessary internal pressure. If p = Internal pressure in lbs. per square inch. D = Maximum diameter in ins. F = Factor of Safety. / = Tensile strength of fabric in lbs. per inch width of material. 2/Then 2/ = F.p. D. .-.Din. = FP Thus the minimum practical diameter of envelope for any strength of fabric is fixed. It is necessary in the case of full size ships to fit air ballonets within the envelope, that may be pumped up to maintain the necessary internal pressure, after gas has been lost on account of its expansion on ascent. Such ballonets will probably not be necessary in models as the height obtained will not endanger the envelope to bursting. Power required to propel the Ship.—The resistance to for- ward motion depends upon the shape of the envelope and its additions, such as cars, fins, rudders and elevators, etc., the size of the ship, its velocity through the air, and the density of the air. If R be the resistance in lbs. V the velocity in ft./sec. p the density of the air in lbs. per cubic foot. g the value of gravity in feet/ sec.2 / the cube root of the volume of the shape in cubic feet. /'(ft.)'= vol.*. R lbs. ' = C p/g V2 I2. Where C = a constant depending on the streamline shape, and varying between 0-015 and 0-007. Work = force x distance, and as power is rate of working. Power = resistance X velocity. R lbs. X V ft./sec. R X V.•. Horse-power = —p^-—. Thus h.p. increases as (speed)3 If 'la % = the airscrew efficiency, then the power to be supplied by the motor is given by :— X V 100Motor H.P. = X 826
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