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Aviation History
1944
1944 - 0379.PDF
FEBRUARY 24™, 1944 4 FLIGHT Advertisements. 13 EVEN I CAN UNDERSTAND—17. Q. How is lift obtained from a moving aerofoil ? A The pressure in a fluid becomes smaller when it is in motion relative to its surroundings. Familiar illus trations are the suction exerted by a scent spray or the lowered pressure in the choke of a carburetter. It was Bernoulli in 1738 who reduced this phenomenon to law and order in his well-known Theorem. This may be rather crudely stated by saying that when part of a mass of fluid is flowing steadily and therefore has energy of movement (proportional to mass per second x velocity2) the pressure in it will go down to such an extent that the energy of movement or kinetic energy added to the pressure is the same in any part of the fluid, still or moving. (It may seem odd to add a pressure to a quantity of energy. It must be remembered that pressure is potential energy and the two forms can be converted into one another.) If there is no movement the pressure will'be greatest (generally known as " total head ")• If movement starts the kinetic energy must be deducted from the total head to get the pressure in the moving stream. The immediate means by which lift is imparted to a wing, and which cause the larger movements —already described (No. 8, Induced Drag)—-depend on this principle. In Figs. 1 and 2 the well-known venturi meter is shown as it gives a very clear picture of Bernoulli's Theorem at work. If the end of the pipe is closed with no flow, both pressure gauges will show the same reading—the " total head " which depends on the height of the reservoir supplying it. If the water is allowed to flow both gauges Fig. I.—Flow stopped. Both gauges show the total head less the kinetic energy of stream (which in this case is nothing) HIGH PRESSURE LOW PRESSURE h VALVE OPEN Fig. 2,—Fluid Flowing will show less pressure but that at B less than A owing to the higher speed. The same amount of water passes all sections and as the area of the throat is less than that of the pipe the speed will be higher and the pressure corre spondingly lower. It is clear that the greater the difference of pressure between A and B, the greater the flow. The difference of pressure is therefore a measure of the flow of water. The fact that pressure decreases as speed increases can be very simply shown as in the experiments illustrated in Figs. 3 and 4. In connection with aerofoils, such expressions as " partial vacuum " are often heard. It is important to realise that up to near the speed of sound—say, 700 m.p.h., Fig. 3 If two pieces of card are curved and supported on pencils as shown, they will be drawn together when air is blown between them, because the pressure between the cards be comes less than that outside. Fig. 4 One card held on a pencil resembles an aerofoil section. Air blown on to leading edge will cause it to 'lift' as shown by the dotted line. air in steady flow does not alter in density at all—behaves as an incompressible fluid like water and is subject to the same laws. In Figs. 5 and 6 an aerofoil section is shown with the air streaming past. That part of the air which takes the longer path over the top surface and joins the part going under the wing at the trailing edge must necessarily go faster. Kinetic energy increases and pressure drops. Under neath the wing there is a smaller increase of speed and so a smaller decrease of pressure, the net result being a lower pressure on the top than the bottom of the section and therefore a lifting force. If the incidence is increased the speed and pressure drop over the top surface increases, while the speed over the bottom surface decreases and may become less than the surrounding air, thus giving rise to an increase of pressure. These conditions cause an increase of lifting force. Fig. 5 Small Incidence £1 gher speed and lower pressure T Little or no speed increase. Small change of pressure >I8' .. 1 Air further accelerated. Pressure Greater incidence J stiM ,ower Air slowed up. Pressure increase This is one of a series of articles on technicalities sponsored by The de Havilland Aircraft Co., Ltd., in the belief that they will prove of interest and use to students and others in the Services and the aircraft industry.
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