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Aviation History
1918
1918 - 0452.PDF
APRIL 25, 1918. THE MODERN AEROPLANE. By F. S. BARNWELL, Captain, R.F.C. (Continued from page 421.) NEXT to consider the properties of the wings or aerofoilswhose function is to sustain the weight of the whole machine with the least possible expenditure of energy. The shape of the section is the most important feature of anaerofoil, the next most important is its aspect ratio, and the next the form of its ends. Consider an aerofoil (Fig. 4)moving at some constant speed and attitude relative to the air ; it does not matter, for the affect on the aerofoil, whether theair be still and the aerofoil in motion, or whether the aerofoil be stationary and the air flowing past it, the only importantpoint is the relative motion between air and aerofoil. For convenience we call a line tangent to the under surface of the SPANr CHORD LEN6TH l LEAPING EDGE SPAN-i-CHORD LENGTH-ASPECT RATIO PLAN FORM WITH SQUARE ENOS - -.'.TRAILING EDGE SECTION section of the aerofoil the " Reference Chord," and the anglebetween this " Reference Chord " and the relative direction of the air the " angle of attack " or " i." For any onevalue of " i " there is a corresponding total reaction exerted upon the aerofoil, because it is altering the direction of theair flowing past it; we shall denote this by R. For convenience we usually consider separately the component of R verticalto the air motion, and the component of R parallel to the air motion. The vertical component we call the " Lift,"and denote by L, the parallel component we call the " Drag," and denote by d. The point in which the line of action of Rcuts the " reference chord," we call the " centre of pressure," and we usually define the position of this " centre ofpressure " as its distance from the " leading edge " of the chord expressed as a fraction of the chord. This fraction we termthe " centre of pressure coefficient." Now the vertical component, L, is the force whichthe aerofoil supplies to support the whole weight of the aeroplane. The horizontalcomponent, d, is the force which must be supplied to the aerofoil; so the ratioL-7-d, called the "Lift-drag ratio," is a measure of the efficiency of the aerofoil. The reaction of an aerofoil, varies ap-proximately as the square of the speed. The aerodynamic laboratory supplies uswith figures for lift, drag and position of centre of pressure, from experiments onsmall model aerofoils suspended in a cur- rent of air in a wind tunnel. Forconvenience in applying the figures to full size aerofoils, they are given in the form ofcoefficients (Fig. 5) :— (1) Absolute lift coefficient, Lc. (2) Absolute drag coefficient, dc. (3) Centre of pressure coefficient. It is necessary to bring in the value of the density of the air,since it decreases at any point, as the point is raised higher above the earth's surface ; the density of the air at 10,000ft. is only about .7 of what it is on the earth's surface. These curves show how the absolute lift and drag coefficient,the lift-drag ratio, and the position of centre of pressure, vary, as i is varied. I wish to call attention to one or two pointsonly which are important in considering an aerofoil as a working member of an aeroplane. Firstly, the lift vanisheswhen i is about minus 2J°. Secondly, the lift is at a maximum at about 15°, and decreases beyond this. So between — z\°and + 15° is the total range of useful attitude, and the value of the lift coefficient atabout 15 ° determines the slowest possible speed atwhich the aeroplane can fly. For modern wingforms it is then about .55, which means that at50 m.p.h. the wings will support about 7 lbs. persquare foot ; so if the " Loading "of an aero-plane, which means the total weight divided bythe number of square feet of total aerofoil area, be7 lbs. per square foot, the slowest possible flyingspeed will be about 50 m.p.h. near the ground,and must be greater higher up. The value of i formaximum lift value is known as the " criticalangle." The lift-drag ratio is amaximum when i is about 3£°; for a good modern wing form it is then about 17, which means that for every pound ofthrust which is supplied to the aerofoil it can lift 17 lbs. The lift coefficient is about .26 at 3^°, so the speed must beabout 73 m.p.h. to support 7 lbs. per square foot at this attitude. This brings out the point that all modern aero-planes fly, at their highest speeds, at values for i less than that for maximum lift-drag, since to retain safe landing speed wecannot go to very high loading. Nowadays machines of 7 lbs. per square foot loading are generally driven at about120 m.p.h., which means that they are flying at about 0° value for i, that is, with the aerofoil chord horizontal. Lastly, we note that practically throughout the range ofuseful attitude, an aerofoil by itself is instable. As * increases the centre of pressure moves forwards, as i decreases it moves Fie.4. p x A x V* " p~x K'xV*where L = lift in lbs. per square foot. d — drag in lbs. per square foot.p — density of air in lbs. mass. A — area of aerofoil in square feet.V =• speed in feet per second. BSQLUTE LIFT COEFFICIENT. L.,- ._ PRA& A BAS6 OF vm.ug OF ANGLE OF ATTACK. backwards. At o° the centre of pressure is back at about.55 of the chord length from the leading edge, at 140 it is right forward at about .28 of the chord length from the leadingedge. When * is zero, L has still a considerable value, and d is at its maximum value. When L is still positive but is 450
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