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Aviation History
1957
1957 - 0772.PDF
778 FLIGHT STUDENTS' PERENNIAL PROBLEMS By ALAN H. YATES, B.Sc, B.Sc.(Eng.), A.F.R.Ae.S. SLIPSTREAMS AND CAGE BIRDS WHICH way is the air moving after an aeroplane has flownby? Consider a cubic mile of still air through whichan aeroplane passes in steady, level flight. Does the air remain still when the aeroplane has gone? This is a problemon momentum. Let us first consider up and down motions. The weight of theaeroplane is supported by its wings, which deflect downwards the air passing over them. The momentum imparted to the air eachsecond is equal to the lift force on the wing. If the aeroplane is flying rapidly, a lot of air is involved each second and the velocity(downwash, as it is called) is fairly small. A slow-flying aircraft of the same weight will leave the air moving downwards at agreater speed. The hovering helicopter, of course, produces quite a rapid downward slipstream. The required amount of momentumis produced either by moving a lot of air slowly or a smaller amount rapidly. Although the net flow of the air is downwards, some of thedeflected air is moving upwards! This occurs with any wing of limited span. Only the air passing over or below the wing isdeflected downwards. Some escapes sideways from underneath the wing and moves upwards outside the wing tips. These rotarymotions of the au behind the tips are known as the wing-tip vortices, and the rotation of the air extends downstream for agreat distance. The rate of rotation is greatest near the axis (as with bath-water) and in certain weather conditions these trailingvortices can be seen—and even heard. Now let us consider air movement in the direction of flight.Whether the aeroplane is driven by a propeller or by a turbojet, the propulsive force is obtained from air discharged backwards—in the slipstream or jet. The momentum per second added to the slipstream is a measure of the propulsive thrust given by theengine. Why is a thrust needed? To balance the drag (friction) of theair on the surfaces of the aeroplane. The layer of air next to the surfaces is dragged forward, and all the air in a "boundary layer"within about an inch of the surface of a wing is affected in various degrees. This air is left behind and forms the wake of the aircraft—no longer at rest but moving slowly after it. In the direction of flight we have, therefore, a balance of forces (thrust equals drag)and a balance of momentum—forward momentum in the wake equals backward momentum in the jet. The fact that air velocitiesare not difficult to measure enables the drag and thrust to be determined in flight. The normal aeroplane in straight flight has no sideways forceon it and does not, therefore, move the air laterally. The total motions are thus as shown in this representation of the "cubicmile of air" mentioned above:— AT a Ministry of Education summer school held at the College ofAeronautics, Cranfield, last year, much discussion was evoked by a talk by Alan Yates to a class of teachers of Aeronautics concerningstudents' problems. In the belief that the basic factors raised are of interest not only to students but also to more-experienced readers, weare printing two of Mr. Yates' "problems," the first of which appears on this page. The author was an aerodynamics lecturer at the Collegeof Aeronautics for several years before taking up his present position as Principal of Bath Technical College. What happens to this air which has been set in motion by thepassage of the aeroplane? It represents kinetic energy going to waste, originally supplied from the fuel and eventually dissipatedas heat in the atmosphere. This waste can be minimized by good design: careful streamlining will reduce the size of the wake, andthus the thrust required from the jet. The downward momentum is decided by the weight, but the kinetic energy involved isminimized if as large a volume of air as possible is deflected. That is why large spans and high speeds reduce this "induceddrag" as it is called. That is also why a helicopter is operating inefficiently while hovering and can reduce power once it movesforwards and influences more air. The strength of the vortices, and the energy wasted in them, also diminishes if the aeroplane'sspeed and span are increased. The downwash eventually reaches the ground, where its down-ward momentum is destroyed. The force exerted on the ground by the impinging downwash is equal to the rate at which momen-tum is destroyed. We have already said that the rate at which it is created is equal to the aircraft weight, so that we find a force onthe ground equal to the weight of the aircraft. We are not usually conscious of an increase in air pressure as the aeroplane flies over,because the increase is small. If the aircraft is flying supersonically, however, the sudden jump in air pressure as it passes may breakwindows and be heard as a "sonic bang." This consideration of the motion of the air is a valuable illus-tration of the concepts of momentum and energy. Cage Birds. We can apply these principles to the solution ofsome well-known puzzles. Imagine a bird at rest on a perch in a very large bird-cage made throughout of very-fine-gauge wire.The whole is suspended from a spring balance and weighs, say, 10 lb. The bird then takes off and flies round the cage. Doesthe reading of the spring balance alter? Would the answer be the same if the cage had a solid floor? The weight of the bird is no longer being supported by thereaction of the perch, which was included in the ten pounds recorded by the spring balance. The reading will thus fall by theweight of the bird. The bird must now impel air downwards and generate momentum by flapping at a rate equivalent to its weight.If the cage has a wire base, this air moves down out of the cage without hindrance. If, however, the base is solid, then the momen-tum is destroyed and a downward force equal to the weight of the bird is added to the weight on the spring balance: clearly it nowreads 10 lb again. The answer to the problem is, therefore, that the reading of thespring balance changes only if the floor of the cage allows air to pass through it—or round its edges if it is small. (FISH-TAILPIECE: A goldfish cannot avoid transferring hisweight to the bowl in which he swims. If his density is not the same as that of the water, the load on the bowl is the same whetherhe swims or lies on the bottom.) FORTHCOMING EVENTS June 5-7. June 8-10. June 9-10. June 9-10. June 9-10. June 9-16. June 10. June 10. June 13. June 15. June 15-16. Institute of Navigation: "Avoidance of Collision by Ship- June 15-17. borne and Airborne Means," a Three-day Conference in June 17-20. conjunction with the French and German Institutes. June 17-29. Frejus-St. Raphael Aero Club: 4th International Rally, Frejus. June 19. Austrian Aero Club: International Rally, Vienna/Aspern. June 21-24. British National Model Aircraft Championships, R.A.F. Waterbeoch. June 24. Leinster Aero Club: Annual Display, Weston Airfield, June 26. Dublin. Western France Aero Club: Angers Gliding Week, Angers. June 29-30. Royal Aero Club: Aerobatic Competition, White Waltfiam. July 5. Royal Dutch Aero Club: 6th International Air Display, July 12-13. Ypenburg. Aerodrome Owners Association: Forum on Helicopter July 14. Landing Sites. July 14. Northampton Gliding Club: Open Day, Sywell. July 27- Lower Normandy Aero Club: 30th International Rally, Aug. 5. Flers. Sept. 2-8. Le Zoute Aviation Club: Invitation Rally. I.A.S.: National Summer Meeting, Los Angeles. Institute of Welding: British Commonwealth Welding Con- ference, London and Saltburn-by-the-Sea. Kronfeld Club: Film Show. Palermo Aero Club: 9th International Aerial Tour cf Sicily, Palermo. F.A.I. Annual Conference, Palermo. Kronfeld Club: "The R.A.F. Formation Squadron," by S/L. Topp. Invitation Rally, La Baule. Tiger Club: Aerobatic Contest, Sywell. Royal Aero Club: National Air Races, King's Cup, and Lockheed Aerobatic Trophy, Baginton. Yorkshire Aeroplane Club: At Home, Sherburn-in-Elmet. WoWerhampton Aero Club: Invitation Rally. National Gliding Championships, Lasham. S.B.A.C. Show, Farnborough.
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