FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1917
1917 - 0297.PDF
MARCH 29, 1917. pressure moves forward of or past the centre of the strut.The struts are thus subject not only to compression, but also to bending. The bending moments, however, in the averagesize machine are comparatively small, and can easily be taken care of by sockets as well as by the struts. In thelatter case it is the maximum moment of inertia of the strut- section that comes into play, and the fibres of material affectedare almost idle when buckling occurs. Geometrically and aerodynamically the sockets can be treated as a well filletedinter-section of a strut of streamline section with half of a streamline body, thus offering little resistance to motion. Advantages of Single Lift-Truss. The disadvantage of the single lift-truss system againstthe double lift-truss is that it cannot be adopted for staggered biplanes, especially those with a pronounced stagger. Theadvantages, however, are great, and can be demonstrated as follows :— (1) The strength of the struts varies as the moment ofinertia of the strut section. At a constant ratio of diameters of the strut section, theleast moment of inertia varies as the fourth power of the thickness' (small diameter) of the strut. Therefore the thickness of strut varies as the fourth rootof the load that the strut can stand. For the double load of a single lift-truss strut against thetyro struts of a double lift-truss, the increase of the thickness aad therefore of the air resistance will be only about 100 (s/^ — 1) = 20 per cent, or the air resistance will be reduced by about 100 ( ~ ) = 4° per cent. The weights being proportional to the squares of the thick- ness, the gain in weight of struts of a single lift-truss against a double lift-truss will be about 100 •• 30 per cent. (2) The diameters of wires or cables vary as the square rootof the load that the wire or cable can stand. For the double load of one single lift-truss cable againsttwo cables of the double lift-truss, the increase of diameter and therefore of the air-resistance will be about 190 ( ^2 — 1) = 40 per cent, ar the air resistance will be reduced by about 100 (2~ v2\ - 30 per cent. There is no gain in weight, however, in this case. 3) The larger size wires and cables of a single lift trusssystem allow a further gain in reduction of air resistance by using streamline wires or cables. The latter are made ofstreamline form by means of sharp specially attached trailing edges, which might be impracticable on the small cables ofthe double lift-truss system. Forces Acting on each Truss. 4) In the double lift-truss system the forces acting on eachtruss depend upon the position of the centre of pressure. In fast flight and a small angle of incidence, the rear truss carriesa greater part of the total load—roughly speaking, between, two-thirds and three-fourths of the total (see Fig. 37). At*slow flight and large angle of incidence the reverse is the case, Fig. 36. Thus considering the two extreme attitudes,each of the trusses is either partly idle or has an extra strength and therefore weight. The Aeronautical Society's Educational Lectures. THE fourth of this series of lectures was delivered beforea well-filled hall at Cricklewood last Thursday evening, when Capt. F. S. Barnwell, R.F.C., read his paper on " TheModern Aeroplane." Bearing in •mind that the raison d'etre of these lectures is to educate, Capt. Barnwell is to be con-gratulated on the manner in which he treated his subject. In the space of an hour and a half he took his audience fromthe first elementary principles of the aeroplane right into the technical complexities of aeroplane design so lucidly, that bothtyro and expert were instructed and interested. He also touched upon the various types of machines in use and theirperformances, and then indulged in a cursory glance into the future, giving examples of two " useless " machineshaving remarkable performances. Two Aeroplanes from Batavia. THROUGH Mr. Beckett, the British Consul-General in Batavia, the British residents in the Dutch East Indies have forwarded to the Overseas Club ^3,815 for the purchase of tw« aeroplanes to form part of the Imperial Aircraft Flotilla. In the single lift-truss the forces acting on the truss arealmost independent of the centre of pressure, as the reaction of the load at the root of the cantilever is equal to the loadand independent of the bending arm. Thus, the forces acting on a single lift-truss will be about25 t° 35 per cent, smaller than the forces of which the front and rear trusses of the double system must take care in theworst cases. This will result in another considerable reduction of weight and air resistance. Special Types of Side-Bracing.Fig. 38 shows a K-type side-bracing proposed by Capt. Martin (Scientific American, 1911). It is another interestingtype of the single lift-truss that has all the advantages mentioned. Besides, as can easily be seen from the action offorces in Figs. 40 and 41, the extra bending moments in the I-type strut due to the cantilevers (sockets) are eliminatedentirely. This advantage, however, is strongly jeopardised by the extra weight and resistance of the braces. The Curtiss single lift-truss strut (Fig. 39), which is usedin the wireless-truss biplane and in the new triplane (as far as it can be understood from the published details), is built upof two steel tubes, one straight, the other bent in two places ; both tubes being inclosed by a covering. The character ofcover determines whether this strut should be considered either as of the Martin's K-type, without the advantage ofeliminating bending, or as a simple and cheaper type of the Dorner cantilever or I-strut type. Finally the X-type side-bracing (Figs. 42 and 43) offersalso certain advantages, especially as compared with the standard type (Figs. 28 and 29) and even with the N-type(Figs. 30 and 31), a,s it belongs to the double lift-truss class. The advantage over the N-type is that there is one strut less.The advantage over any other double lift-truss system is that the amount of load carried by each truss is practically inde-pendent of the variation of the centre of pressure, although there is a certain increase of the forces due to the angularityof the struts. (See action of forces in the two extreme cases in Figs. 44 and 45). The trusses therefore do not have asmuch extra strength as in all the cases of two parallel lift- trusses. The gain in resistance and weight is evidently somewhatsmaller than in the case of I-strut, but in return the X-type side-bracing is adaptable to staggered biplanes and to largebig-chord wings. The I-strut, and in general the single-truss, would offer too many constructive difficulties in these cases. Numerous interesting and entirely new possibilities inbiplane construction can be obtained by combining the types of trusses and side-braces previously described. Examplesare:— 1. Wireless truss (Figs. 16, 17 or 18) combined with N-typebracing (Figs. 30 or 31) can be trated as (a) double lift-truss system or (b) single lift-truss system. In case (b) remarkableresults can be obtained by placing the front and rear bars of the N in vertical planes parallel to the plane of symmetryof the airplane. 2. Wireless truss combined with V-type bracing can alsobe treated as (a) on a double lift-truss system or (b) on a single lift-truss system. 3. Wireless truss combined with the I-struts—an almostideal structure for small fast machines. 4. Wireless truss combined with the X-type bracing, givinga construction with the least number of members for big-span and big-chord machines. 5. Strutless truss (Fig. 20 or 21) combined with the X-typebracing, and so ou . . . as I do not attempt to exhaust, but merely to indicate the possibilities. Prominent German Airman Missing.IT was officially announced from Berlin on March 22nd, that " the aeroplane piloted by Prince Frederick Karl ofPrussia, which went for a flight over the enemy lines between Arras and Peronne, has not returned." Berlin, also onMarch 22nd, announced that the crack military pilot Fritz Mannschott had been killed in an aerial fight on the Westernfront. According to the German story, Prince Frederick, on March 21st, took part with an air squadron in a flight towardsthe enemy. North of Bapaume the squadron sighted some enemy aeroplanes belonging to a squadron of one-seaters. Inthe ensuing battle, concentrated fighting by the squadron was impossible owing to the numerous clouds. The Prince,who was flying in the rear, became engaged in an air battle unnoticed by the others, and his aeroplane was only seenwhen descending in steep spirals. It was then 600 ft. from, the ground, and any assistance was impossible. The aeroplanelanded close to some enemy troops near Lagnicourt-Vaulx, and the Prince, apparently wounded, was removed from hisaeroplane and carried away by some soldiers who had hurried to the scene. 297
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
