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Aviation History
1945
1945 - 0706.PDF
394 FLIGHT APRIL I2TH, I943 ON JETS AND TURBINES the theory of jet propulsion is this: Let's suppose we have a metal sphere filled with common illuminat- ing gas. By means of a sparkplug we ignite the gas. The result is an ex- plosion with tremendous pressure exerted equally against all points on the sphere. But because the pressure is equal at all points on the sphere, the sphere itself will not move as a result of the explosion. The pressure on one side of the sphere is cancelled out by opposite pressure on the other side. Now let's make a hole in the sphere and repeat the same performance. There will be pressure at all points on the sphere's interior surface except at the hole, where the gases will have nothing to press dn and will escape. Thus you have all of the forces created cancelled, as before, except those acting on the surface of the sphere directly opposite the hole area. Here there is positive pressure on one side of the sphere where there is no open- ing. On the other side, where the gases are able to escape, the pressure is zero. The result is obvious, the sphere will move in the direction of the positive pressure—away from the opening. This is simply a matter of internal pressures in the sphere. Nothing out- side the sphere has any bearing on what happens to the sphere. It works as well in a vacuum as in the earth's atmosphere. This is the principle of jet propulsion. How the Unit Operates A jet engine, such as that used in the Lockheed P-80 Shooting Star, is really very simple. Streamlined ducts in the leading-edge of the wings lead air to the engine, where a high-speed fan draws the air into the engine at high compression. From the com- pressor the air passes into a combus- tion chamber, where it is mixed with fuel injected at high pressure. A continuous explosion occurs in this combustion chamber, heating the gases to a very high temperature and causing them to expand violently. Now we have a condition similar to that created in the sphere with the opening on one side. The exploded mixture under great pressure can only move in one direction, toward the back of the aircraft. Thus we have zero pressure towards the rear, equal pressures cancelled out on the sides, and a positive forward pressure at the front of the combustion chamber. No valves are needed as the new air being piled in by the fan prevents air escaping towards the front. You will remember from your high school physics the formula for determining force (F = MA or Force equals Mass times Acceleration). This law applies to jet propulsion. "F" ILLUSTRATING. THRUST POWER IN JET PROPULSION : Note that the combus-tion creates equal forces at all points on a sphere's inner surface. By making an opening A the gases escape and sphere moves toward uncancelled pressure at B. In thelower illustration, the expanding gases in combustion chamber flow through turbine to exhaust. At the exit nozzle the gases have dropped to atmospheric pressure,whilst the positive forward pressure provides thrust. (the force driving the aircraft forward) is equal to "M" (the mass of the expanding gas) times " A" (the acceleration given the gas as it ex- pands and passes to the rear). In jet engines we speak of thrust rather than horse-power and the power of an engine is expressed in pounds of thrust. Once the engine is started no ignition system is necessary. A small metal plug protruding into the com- bustion chamber heats red hot in the first few seconds of operation. This serves to ignite the mixture of air and fuel as it is introduced. For the initial start sparkplugs are used.. From the combustion chamber the hot gases pass rearward through a gas turbine. Here is the heart of your jet engine. This turbine turns very rapidly under the impact of the rapidly expanding heated gases. A shaft connects the turbine to the com- pressor fan in front of the engine sup- plying the power necessary for the intake and compression of the air. In effect you are lifting yourself by your own boot-straps. A ' Safe ' Fuel Used For fuel, the Shooting Star's General Electric engine uses kerosene, which is preferable to gasoline be- cause it provides somewhat more B.T.U. per pound and per gallon. Thus the dangers of handling high octane gas are eliminated. Absence of an airscrew removes another hazard, although the hot gases issuing from the tail at about 1,200 deg. could be dangerous. However, these gases cool rapidly, and 100ft. behind the aircraft only a strong blast would be felt." Looking to the future Mr. Hibbardsays: — "It is hard to forecast the limits of future developments in this field. In my opinion jet engines can be used successfully by large aircraft as by small. I can foresee tremendous in- creases in transport speeds in a matter of a very few years. There will be new facets to our lives in the period following this war, and one of the most important developments, as far / as transportation is concerned, will be ' the introduction of jet-propelled air- craft. Hitherto unheard-of speeds will be common in large jet-propelled trans- ports ten or maybe fifteen years from now, and every capital on the globe will find itself within one day's flying time of every other capital. Jet engines remove the barriers formerly limiting the top speed of aircraft, and we should see aircraft speeds passing through the compressibility range (approximately 725-775 miles per hour at sea level) in a relatively short time. " The basic problems have been pretty well worked out. Now we are entering a period of development that ^ should be at least as spectacular as the development period of the air- craft in the twenty years after the first world war."
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