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Aviation History
1934
1934 - 1415.PDF
APRIL 26, 1934 27 THE AIRCRAFT ENGINEER SUPFLEM£NT TO FLIGHT between these edges. The ordinates at any point gives the total cooling obtainable out of a surface extending from the L.E. up to that point. The scale is such that the cooling obtainable from the standard surface is represented by a total ordinate of 100. Thus, for the climb case, from 25.1 per cent, of the upper surface the coaling is 70.8 units, and from 15.2 per cent, of the lower surface it is 29.2 units. Such a scale makes the calculation of the actual surface from the standard surface an easy matter. Suppose the standard surface required in a particular condition of climb to be 110 sq. ft., while the standard area available on the aircraft is only 88 sq. ft., then the cooling required on the wing surface per unit length of the L.E. will be 125 pea' cent, that of the standard. From the curve, one combination which will meet this requirement is 41 per cent, of the upper surface and 15.2 per cent, of the lower surface measured from the L.E. The actual area then used will be: 41 + 15-2 X 88 or 122-8 sq. ft. 25-1+ 15-2 Of course, there ace infinite numbers of positions of the profile which will combine to give the required effect, and the position chosen will be largely governed by the position of the front spar relative to the L.E. and the angle of climb which governs the angle of the drain pipe from the T.E. of the condenser. Retractable Condenser Experiments carried out at R.A.E. show that the triangular honeycomb radiator is more efficient than the square type. Readers who are familiar with marine condensers will have noticed that the steam is intro- duced into a large opening at the top of the condenser, and also at the hottest point of the condenser. We may see something very similar in aircraft in the near future, complete with air pump, etc. To return to the present model, if steam is supplied above the heat dissipating capacity of the radiator some must flow out of the air vent and be lost, see Fig. III. The efficiency of the radiator partly depends on its capacity to adjust its heat dissipation by drawing in air or expelling air as the heat supply is altered. The design of the radiator should be such that no air pockets can be formed, preventing the easy flow of steam. The steam should be introduced at as low a point as possible with the air vent at the top of the radiator, when the full efficiency of the cooling surface of the tubes will be made use of. Here it would be as well to mention that the tubes are of the standard hexagonal ended type either 360 mm. or 400 mm. long by 7 mm. or 10 mm. dia., according to the design in hand. The working weight of the condenser must include the film of water round the tubes and is in the range of 0.38 lb. to 0.43 lb. Tilting the radiator relative to the horizontal plane of flight has no decided advantage, owing to increased drag, but tapering the tubes has a decided advantage. Arrangements can be made in the design for the con- densate to be collected at the lowest point of the sump. There is a definite increase in efficiency when the steam is introduced at the front end of the radiator. The problem of selecting a criterion for the cooling efficiency of a radiator is an intricate one. It is neces- sary to compare, in given temperature conditions, the rate of heat dissipation for the radiator with the maxi- mum rate of heat dissipation it is capable of, i.e., at the boiling point of the liquid, when the maximum demand is made of it. Maximum demand implies that the engine is at full throttle and that the air velocity through the radiator is a minimum at maximum radiator exposure. A practical example to maximum demand is obtained in flight at maximum rate of climb. This condition makes it difficult to measure steady heat conditions, since the atmospheric conditions anl the 410 factors governing the flow of heat are continually changing. Assuming that the lieat entering the radiator per second is a constant fraction of the b.h.p. (1 b.h.p. = 2,545 B.T.U./hr.) at a height "h" (in a standard atmosphere) ; let p and <r be the relative pressure and density respectively, A the air temperature, B the foiling point of the liquid in the radiator, M the mean radiator temperature, V the forward velocity. The heat to be dissipated per second is proportional to p, and the heat discharge from the radiator per second is proportional to (M-A)o-V; and in a climb " V " is pro- portional to a power of a- between o and £, so that the rate of heat dissipation is -proportional to (M-A)cri. The maximum rate oi heat dissipation possible is (B-AVi. To enable the pilot to know when the radiator is too far within the fairing, i.e., when the radiator is not con- densing all the steam received from the wing condensers, a warning device is fitted to the bottom of the air vent, which, when steam passes to atmosphere, makes elec- trical contact with a small lamp fitted on the instru- ment board, giving the pilot warning that steam is being lost. The writer can foresee the day when some electrical device will operate the radiator in and out, functioning by the emission of steam through the air vent. This air vent must protrude clear of the fairing when the radiator is in the full-up position. In this position also, the sump must be in the slipstream. Header Tanks The design of header tank is largely in the hands of the engine manufacturers, who should be consulted be- fore any drawings are issued to the shops. As this unit T CRANKSHAFT oc = ANGLE OF TAIL DOWN Fig. V. is still in its experimental stages, it would be useless to describe any particular type in this article. The material used in construction can be either aluminium with welded ends, or Dural with riveted ends. As most of the internal piping is of intricate Bhape, they can be made up in aluminium and welded. Certain features are common to all header tanks, viz., the sump must have baffles to prevent the water swirling, a pressure gauge, temperature thermometer, and filler neck must be fitted; also a fitting to accom- modate the low water level warning device. This is operated on the same principle as the steam loss device fitted to the retractable radiator. On certain type of aircraft it is necessary to fit a valve for inverted flight to prevent the water from pass- ing up the steam pipe to the wing condensers. To fix the position of the filler neck it is necessary to find out the cylinder jacket volume, plus the volume of all the piping containing cooling water, i.e., the main feed pipe from the tank sump to engine pump, the pipe leading from the cylinder head to tank, and any internal piping. About 50 per cent, of the tank's total capacity
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