FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1918
1918 - 0482.PDF
\ blade of the air screw is passing the line of fire, the lag of thegun mechanism and of the buUet th&n allows for the bullet's passing between this blade and the next one coming roundlhe gear is usually made up of a cam on the air screw hub or driven elsewhere by the engine, working a tappst which'by means of sundry bell cranks and pull or push rods operates the trigger of the machine gun. Some form of toggle or tripdevice, is incorporated, and is worked by a Bowden lever attached to the pilot's control lever, so that the gear is nor-mally thrown out of action until the pilot presses this lever, ibe pusher machine is as bad for firing astern as is the tractorfor firing ahead. But to revert to the consideration of the body. I shallonly deal with that of a tractor, as much the same considera- tions are involved in that of a pusher. The body must bedeep enough to afford ample protection to the pilot and gunner, 36 in. is about the minimum. It must be wideenough to allow ample room to work the guns, 30 in. is about the minimum. So the body is an item of large cross sectionalarea, and is the part of an aeroplane which individually offers the greatest " residual " drag. It is the part of an aeroplanewhich calls for the greatest numbsr of conflicting requirements. It must be as small as possible and have as few openings aspossible to keep its drag as small as possible. It must be large enough to contain, with workable room, the pilot, gunnertanks, ammunition, instruments, &c, yet the wider it is the worse the view. It must have openings of size sufficient toget in and out of easily, and generally the opening must be large enough to allow of a machine gun fitted upon a rotatingorm of mounting. & - The form of body which offers the lowest drag per squaretoot of cross sectional area, is a torpedo shape of length equal MAY 2, 1918. lh a rotary engined tractor aeroplane, of which the noseis necessarily a circular -section covering, or cowl, round the engine, it does pay to continue this circular section well alongthe body, otherwise there will be a greatly increased drag due to the sudden "falling in " from the circular sectioncowl to the inscribed rectangular section body. I shall consider the landing gear very briefly as at presentit is probably the most standard member of an aeroplane (Fig- 9)- The landing gear may be considered in two parts, the " main under carriage " and the " tail skid."The " main under carriage " is almost invariably two- wheeled. These wheels must be far enough apart to givesufficient stability for " taxying " across a strong wind, they must be far enough ahead of the centre of gravity of the wholeaeroplane to prevent of its pitching on to its nose too readily but they must not be too far ahead or we get an aeroplanedifficult to land without bouncing, and very apt to " swing " or " chase its tail " when taxying. The distance apart, ortrack "of the wheels is seldom less than 4 or more than 6 ft., and the best distance for them ahead of the centre ofgravity (for a normal machine) lies between 8 in. and 14 in. The wheels are of a specially light wire-spoked type, withpneumatic tyres, very long hubs to give transverse strength, and fabric discs attached over the spokes to reduce drag.The rim is generally offset as far as possible towards the inner end of the hub to reduce the bending moment on the axle.Indiarubber is almost invariably used for shock absorbing at present. It is capable of absorbing more work per lb.weight than any other substance, and in the form gener- ally employed for aeroplane shock absorbers, that is, braidedelastic cord of about f in. diam., it is extremely simple to fit to vary and to renew. AXLE FAIBINS SHOWN CKT AWftY AT CENTRE TO SHOW AXLE 8MB MIW3ES CLOSING PLATS ATTACHED TO AXLE CONTINUOUS AXLE TYPE NO AXLE FAtRING OIVIPED AXLE TYPE MAIN LAND!N<S SEAR 'VEE' TYPES. FIG.9. tc about five times its greatest diameter. A body of thisform may have as low a drag as 1 lb. per square foot of cross sectional area, at a speed of 100 ft. per second. The corres-ponding resistance of the bodies of modern fighting aeroplanes is from about 3 to 7 lbs. per square foot, because they arenecessarily compromises. A final point to note about the body is that it is usuallyinstable, in the " weather cock " sense. If a torpedo form have its axis inclined at a small angle to its direction ofmotion, there will be of course a certain total reaction force on it, and the line of action of this force will be very near itsnose (perhaps in front of it) ; that is, will be ahead of the centre of gravity of the torpedo and will tend to swing itsaxis to a still greater angle to the direction of motion. Now the body of an aeroplane is generally sufficiently nearly oftorpedo form to render it instable in this way, and this must be taken into account in dealing with the question of stability. The bodies of most modern machines are of a rectangularcross section, as this is the lightest shape for strength and is the most simple to make. It is also the most economicalshape for stowing things into, and is the most convenient form to which to attach the other members of the aeroplane.It probably does not pay, unless a rotary engine be employed', to transform a rectangular or square section body by theaddition of a light fairing structure, in to a round'section • for it adds weight, and adds flimsy parts where such are verylikely to be roughly handled, whilst the drag of the larger section round body thus made up is about the same as thatof the smaller inscribed rectangular or square section. The tendency at present is to keep the under carriage assimple as possible, in order to reduce weight and drag. Hence the almost universal adoption of the " Vee type." The axleis sometimes a plain continuous tube, sometimes it is of the " divided type," that is, it is in two parts, one for each wheel ;the inner ends of these two parts being mounted on horizontal' hinge pins connected to the cross members near their centre.The latter method allows of a lighter axle, as the bending moment falls from a maximum at the shock absorber to zeroat the hinged inner end, so that the two axle tubes may be tapered in strength and therefore in weight. The plain continuous tube axle on the other hand is sub-jected to the same maximum bending moment for its whole length between the shock absorbers and so must be of uniformstrength and weight throughout its length. It is, however, a simpler structure and works out little heavier than thedivided type. It should be noted that in the " divided type " an additional tension wire must be led from the body to thecentre of the cross members, otherwise the push of the inner , ends of the axle will tend to bow these cross members down-wards. To be continued.) ... . .-— -, m m m * --••• - -" ••; Aerial Mails from Denmark. A DANISH aeronautic company has been formed with ashare capital of £111,000, with the object of establishing an air route after the war between Copenhagen, Esbjergand London, the route to be connected with other Scandina- vian air routes. 48O
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
