FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1934
1934 - 1411.PDF
MARCH 29, 1934 23 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT type*, fan cooling may become necessary and does notappear to be inconsistent with the use of ring cowline of the Townend type. Assuming a mean temperature difference of 250 deg. C. between cylinders and cooling air, McKinnon Wood estimates that, allowing for fan losses, 4 pea- cent, of the b.h.p. will be used in providing cylinder cooling alone. A mean temperature of 250 deg. C. between cylinders and air is certainly not available in the majority of engines to-day in service. The maximum permitted temperature at a position on the cylinder heads, which is certainly above the mean temperature of the cylinders as a whole, is normally between 215 deg. and 235 deg. C. For a mean temperature difference of 200 deg. C, McKinnon Wood gives the power absorbed in cooling cylinders as 10 per cent, of the b.h.p. Even 200 deg. C. is almost certainly higher than the mean difference which can be permitted with engines of exist- ing type, but we may take this figure of 10 per cent, as representing about the figure which might be achieved in practice with a cooling system of the type suggested. The best result within my knowledge which has so far been obtained, using a Townend Ring, is that given by 54-dn. diameter nine-cylinder radial engine mounted on a streamline nacelle of 42-in. maximum diameter, fitted with a polygonal Townend Ring, and having a total drag at 100 ft./sec. of 27 lb. This result has already been referred to (Fig. 12). With an engine developing the maximum b.h.p. of 480 at 4,000 ft., the speed of the aircraft fitted with this installation was 140 m.p.h. true (132 i.a.s. at 4,000 ft.), and the power absorbed by a resistance of 27 lb. at 100 ft./sec. at this speed is 43.5 h.p., or 9 per cent, of the engine b.h.p. When an engine of the same overall dimensions and type, but supercharged to give 600 h.p. at 5,500 ft., was fitted in place of the lower-powered engine, the speed of the aircraft in- creased to 160 m.p.h. true (148 m.p.h. indicated at 5,500 ft.), the engine installation drag being unaltered, and the power absorbed in overcoming engine drag became 54 h.p.—still 9 per cent, of the total. Allowing for airscrew efficiency, the drag of the engine installation in this case at top speed is about 11.5 per cent, of the total drag of the aircraft. I understand that Messrs. Armstrong Siddeley Motors, Ltd., have measured a total drag of 35 lb. at 100 ft./ sec. for the fuselage, engine and Townend Ring of a military machine with open cockpit, wind screen and pilot. The engine in this case was a "Jaguar" capable of developing about 500 h.p. at sea level. This is 8 lb. in excess of the nacelle figure above quoted, and it is certainly not to be expected that the resistance of the fuselage alone, engine removed and a faired nose substituted, could reach so small a figure as this. The drag to be attributed to the engine in this case must therefore have been less than in the examples which I have considered. EVAPORATIVE COOLING By R. HALEY* EVAPORATIVE cooling in aircraft engines has undoubtedly made great progress in the past five years, and the attempt to improve the efficiency of the air-cooled engines by the addition of the Townend Ring and others has no doubt hastened the efforts of the water- cooled engine manufacturers to improve their system of cooling. it V I* Uu\* *. -I A v>M5 & \ \I 2«e 752— It" 17a 7 s—' — / — / -—^ \ •—> / \ \ Igo '*O *4f> /SO 'to '70 /3O /Bo SLOO 2/0 Sio \tATCR TEMP T. Fig. 1 : Professor Gibson's Experiment. A practical method has been developed for increasing the capacity of a cooling fluid for heat dissipation by using the latent heat of vaporisation. The greatest difficulty to be overcome on any type of I.C. engine is to keep the working temperature level. Temperatures should be kept high, but within the limits of effective lubrication. The most desirable condition is a uniform high temperature, in all parts of the engine. That is to say, an important feature of the cooling system is to keep the engine hot. In an aircraft installation, fitted with evaporative cooling, the control of temperature, external to the system, is of course impossible, due to varying condi- tions of flight, but some form of control over the cool- ing system fitted is necessary, and the method adopted will be described later. In this country the cooling system is vented to the atmosphere, but in America they are using successfully an unvented system, i.e., a pressure gauge is fitted, set to 5 lb. per sq. in. Before describing the layout of an evaporative cooled system it will be interesting to note the results of certain experiments carried out by Pirof. Gibson in 1910 and reported to the Institute of Engineers and Ship- builders. The tests consisted of measuring the surface tempera- ture of an iron vessel in which water was heated by a gas flame. The water was vigorously stirred during the experiment, and it was found that the temperature of the surface actually decreased as boiling point was reached. Fig. 1 shows graphically, on a base of water tem- peratures, the results he obtained. As the tempera- ture of the water increased, that of the vessel increased Mr. Haley is on the Technical Staff of the Gloster Aircraft Co., Ltd. Fig. 2 : Diagram of the Antoinette cooling system. 306 g
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
