NEW PROBLEMS

Otago Daily Times, Issue 26028, 17 December 1945, Page 6

NEW PROBLEMS

HIGH SPEED FLIGHT MOVING AS FAST AS SOUND Technical and scientific issues of very great significance provide the background of the recent attempt at Herne Bay on the world's air speed record, writes a special correspondent of The Times, London. Aviation development has now reached the stage of groping forward hesitantly into the unknown. Aeronautical experts have known for years that new problefns would arise when the speed of sound was apj proached, but these difficulties were not very real, since the prospect of flying at very high speeds bnce seemed remote. At sea level sound travels at about 760 m.p.h.; and the speed falls with the temperature at high altitudes to about 660 m.p.h. at 30,000 ft. As the speed of sound is approached a condition known as compressibility is encountered. As the aeroplane flies through the sky the air around it is speeded up by its movement, the extent of the acceleration depending on the thickness of the wing which is being forced forward against the atmosphere by the power of the engine or engines. At what are now regarded as normal level flight speeds—up to 500 m.p.h. or more—the air flows smoothly over the wing surfaces. but at a faster rate the velocity of the air flow over the wings can be increased to the speed of sound, even though the aeroplane itself is travelling at a slower speed. “ Shock Wave ” Occurs When the oncoming aeroplane is travelling at, say, 600 m+.h. or more the steadiness of the airflow over the wing then breaks down, with the result that what is known as a shock wave occurs at the thickest part of the wing. The main effect on the aeroplane at very high speed is to increase the drag of the air. At normal speeds the drag is proportional to the square of the speed, so that if the speed of the aeroplane is doubled the drag is quadrupled. But at these higher speeds, some time before the shock wave forms, the drag on the aeroplane begins to increase at an abnormal rate and a tremendous increase in horsepower would be needed to counteract its effect. In the present stage of aeronautical development there is not a single aeroplane in the world capable of beirig propelled at the speed of souhd in level flight. Under the formula of a German, Herr Ernst Mach, the speed up to which an aeroplane is- capable of flying normally is represented as a fraction of the speed of sound. Each aeroplane is given a Mach number, and there are only eight in existence capable of 80 per cent, the speed of sound. Most of the aeroplanes with high Mach numbers are British, with the Spitfire at the top of the list and the Gloster Meteor IV next. The German Me.262 jet-propelled fighter and the American Shooting Star also have good Mach number qualities. Before supersonic speeds can be attained in level flight the shape of the aeroplane will have to be radically changed. Experts believe that the most promising design is an arrow-head shaped aeroplane of the flying wing type—that is, without a tail and with tne “tail” controls mounted on the wing tips—with the wings swept back to an angle of about 30 degrees. Above the speed of sound the rate of increase of drag falls off again—though it never returns to the original proportion of the square of the speed —so that to some extent the problem is less complicated at supersonic speeds.

Critical Velocities The main difficulty is to design an aeroplane capable of behaving normally at the critical velocities around the speed of sound. When the shock wave (Dccurs, the maximum lifting capabilities of the wing are reduced because of the breakdown of the air flow, and the ideally shaped wing to combat this defect has yet to be designed. When the air flow over the wing is upset the aeroplane tends either to go into a dive or to climb, and the pilot cannot control the tendency. , „ x Up to fairly high speeds the effects can be ascertained by wind tunnel tests, but above 80 per cent, of the speed of sound these are useless, since the walls of the wind tunnel itself experience shock stalls at the same time as the model which is being tested. The only way, therefore, to learn the secrets of travelling at supersonic speeds is by experimental flying.