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Aviation History
1918
1918 - 0453.PDF
APRIL 25, 1918. infinitely near to zero the centre of pressure has moved But at very small values for angle of attack, the lift-draglnnniteiy lar behind the aerofoil. On an infinitely small ratio is generally better for the multiplane form than for the iurtner decrease of 1, L becomes negative, but of value monoplane form, and the critical angle slightly increases asinfinitely near zero, and the centre of pressure has moved the number of sup.rposed aerofoils is increased. So for an infinitely far ahead of the aerofoil. aeroplane requiring a great range of speed, a biplane or 1 apologise lor talking in such alarming terms as infinitives, triplane (quads are still rather dark horses) is a better formbut 1 wish to emphasise this enormously great movement ' of the centre of pressure of an aerofoil very near its attitudefor zero lift. Of course, though the leverage is very great, the force is very small, so the moment is not very large. Rounding off the ends of the aerofoil, in plan view, somewhat than a monoplane ; whilst for maximum efficiency at any onespeed the monoplane form is the best. Generally, the greater the number of superposed aerofoils,the smaller will be the chord length and the higher the aspect ratio of each of such aerofoils. Small chord length meansincreases the efficiency, since a turbulent or eddying air flow small shift of centre of pressure, as this is proportional to is set up at a sharp corner and power is wasted in so doing, chord length, and therefore allows of small tail plane forAltering the aspect ratio has a variety of effects upon the stability, as we shall see later. A multiplane form usually properties of an aerofoil. Speaking broadly, the higher the •aspect latio the greater will be the maximum lift value per square foot, and the greater maximum lift-drag value, butfor very small values of i the lift-drag ratio seems to be quite as good for an aspect ratio of about 5/1 as for any other. FORMS OF MULTIPLANES. L POSITIVE STA6GER NE6ATIVE 5TASQER L*jJ2 BIPLANE FORMS IV TRIPLANE FORM FlS.6. POSITIVE &TA6S&R QUAORuPLANE FORM. Nearly all modern aeroplanes are multiplanes, that is,they have two, three, or even four aerofoil surfaces more or less superposed (Fig. 6). The vertical distance between anytwo superposed aerofoils is called the " gap," whilst the dis- tance that the leading edge of one of these perofoils is aheadhorizontally of the other is termed the " stagger"— " positive stagger " if the upper aerofoil is ahead of the lower," negative stagger " if the lower be ahead of the upper. In a biplane form, a pair of superposed aerofoils, the lowerone is less efficient than the upper ; in a triplane form, three superposed aerofoils, the lowest aerofoil is of about the sameefficiency as that of a biplane form, and the centre aerofoil is slightly less efficient still; in a quadruplane form, four super-posed aerofoils, the lowest is of about the same efficiency as that of a biplane form, whilst the two centre aerofoils areboth about as efficient as the centre aerofoil of a triplane. Thus broadly, the greater the number of superposed aerofoils,the smaller will be the maximum lift value per square foot and the lower the maximum lift-drag value. To Assist Disabled Soldiers and Sailors. RECOGNISING that the average disabled man has not asufficient pension to keep him when discharged from the Army, the employees of Messrs. Wm. Cole and Sons, of 235,Hammersmith Road, W.6., have started a scheme which it is hoped will become a national affair. The idea is that allwho can afford ic should contribute 3d. per week until the end of the war, and that the money subscribed should be distributedthrough local committees in each borough or district. Any official appointed must do the work voluntarily in orderthat every penny subscribed shall go to the sailors and soldiers and their dependants. Messrs. Wm. Cole and Sonshave opened the fund with 100 guineas, and every one of their workers has agreed to subscribe the necessary 3d.It is hoped that the example will be largely followed, for, after all, a little practical help is worth a great deal ofsympathy, We refer to the matter in our leaders on page 443. •.:..-;:...• . .r ••.-. . 45* possesses a great total depth of girder compared to its span,and this means a lighter aerofoil structure for the same- strength to resist the stresses due to lift. It must be noted,however, that for the stresses induced by drag, the greater, the number of superposed aerofoils the weaker do they tendto become, because the depth of girder to resist this direction of applied force (that is, the distancebetween the front and rear main spars of such! aerofoils) usually becomes smaller compared to thespan. . ' As regards stagger. Positive stagger tends toincrease the maximum lift slightly, and also the maximum lift-drag ratio. Moreover, in a posi-tively staggered biplane form the lower aerofoil is working in a slight down draught from the upperaerofoil, which means that its angle of attack is vir- tually slightly less than that of the upper. Now in asystem of two aerofoils, one of which is ahead of the other—if the virtual angle of attack of the rearaerofoil be less than that of the front, the tendency is to decrease the shift of centre of pressure of thetotal reaction on the system. The same applies of course to triplane and quadruplane forms. Positivestagger, therefore, tends to allow of a smaller tail plane. Negative stagger tends to produce, generallyspeaking, the opposite results to positive. In neither case, however, is the difference producedby stagger of really serious amount, unless we go to values of stagger which are hardly practicable forstructural reasons. Positive stagger tends to in- crease the stresses in the aerofoil structure, slightnegative stagger tends to decrease them. In other words, for the same strength, positive staggertends to increase the weight of the aerofoil structure, slight negative stagger to decrease it. A biplane or multiplane structure is held together by com-pression struts, usually of solid spruce, and by tension wires. The tension members in modern aerofoil structures aregenerally in the form of what are known as R.A.F. wires. These are formed from sclid steel rod of circular section, ofabout 60 tons per square inch, ultimate tensile strength ; they are swaged to a flat elliptical section for their entirelength except a few inches at each end, these ends being threaded for attachment. This form of tension membergives an extremely low drag value compared to its strength. As regards gap value, naturally, the greater the gap value,the less the aerodynamic effects, already noted, due to a superposed form. Structuially, large gap means deep girder,and this, for the same strength, means lighter wing spars, lighter tension but heavier struts, and higher strut drag.Modern practice is to keep gap about equal to chord length ; taking all considerations into account there is usually littleto be gained by departing far from this value. (To be continued.) , ^ • D3 H The Friedrichshafen Disaster. '"'- ] ACCORDING to lat^r information received iri Paris the fire at the Zeppelin airship works at Friedrichshafen now used for building the F.F. bembing machines would appear to have been started by the explosion of a benzine reservoir, which may have been the result of carelessness. The enemy certainly appears to have sustained a considerable loss, and the reconstruction of works, similar to those he has lost, will take several months. The story that the fire was the result of an air raid is not confirmed ; in fact it is stated that the bad weather would have been an obstacle to any such enterprise. THE following is the official German version of the catas trophe :—" On the evening of April 13th a fire broke out at the Friedrichshafen yards, involving also a former Zeppelin hangar. The fire was quickly subdued, and the damage is slight. Work at the yard continues without interruption"
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