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Aviation History
1923
1923 - 0689.PDF
NOVEMBER 8, 1923 Teading at 30,000 it. Major Wimperis then referred to thefigure adopted by the French Section Technique of a tempera- ture decrease of 6 -5' C. per kilometre of ascent, taken inconjunction with a constant surface temperature of 15° C. Using the S.T.Ae. rule as a basis the probable error would bereduced to less than 500 ft. A large proportion of Major Wimperis' paper was devotedto the subject of the magnetic compass. The very nature of the subject makes it almost impossible to condense it and stillretain its value, and those interested should obtain a copy of the paper so as to study it in detail. The lecturer referred tothe difficulties of damping the oscillations set up even in moderately " bumpy " weather, and to the peculiarity of themagnetic compass of giving a misleading indication during a turn off north—known as the " Northerly turning error."Weakening the magnetic moment or increasing the inertia of the moving system did not solve the problem, as both tendedto make the needle stick. A solution was found by Campbell and Bennett at the Admiralty Compass Observatory, who in1918 showed a way in which the desired slowness of response could be obtained without lowering the magnetic momentvery much on the one hand or increasing the moment of inertia on the other. This method consisted in attachingradial " feelers " of thin wire to the magnetic system. These feelers " on moving through the liquid would set up smalleddies and thus dissipate energy'. In this way it was found that the compass could even be made " aperiodic," butexperience had shown that the best results were obtained when the amount of damping was kept within 70 per cent, ofthat which would just cause the motion to be aperiodic. Brief reference was also made to the gyro turn indicator. In conclusion the lecturer referred to the development ofaero engine indicators, to the use of the Challenor electric air-flow meter and to the R.A.K. indicator, in which theinstantaneous pressure in an engine cylinder opposes on one side of a small piston a steady and known air pressure on theother, so that at the moment when the two are equal an electric current is broken which sparks through a drum of paper movingat a speed proportional to that of the engine. Two appendices to Major Wimperis' paper gave respectivelyan explanation of the theory of the Bygrave " position line " slide rule, and an exposition of the variation of atmosphericpressure and density for a constant lapse rate. Both are extremely interesting and should be carefully studied. 13 H CORRESPONDENCE The Editor does not hold himself responsible for opinions expressed by correspondents. The names and addresses of the writers, not necessarily for publication, must in all cases accompany letters intended for insertion in these columns. LIGHT 'PLANES WITH "SIDECAR"[2077] In FLIGHT of October 25 last reference was made tothe possibilities of interchangeable fuselage-wing units for one-two-seater light 'planes. It would seem that to changethe engine, the seating part of the fuselage, and the wings would leave only the undercarriage, the rear part of thefuselage, and the tail unit applicable-to both types. Seeing that the latter parts are by no means the most expensive partsof the 'plane, and that they form only a small proportion of the whole, there will be little saving on the expense of main-taining two separate machines. The following construction which I have devised makes the proportion of temporary partsmuch smaller, and no part of the single seater is changed, the two-seater being formed from it by added parts only. In the attached drawings the figures to the left of the lineXY are the single-seater, those to the right being the two- seater. The full lines represent the permanent parts of thestructure, and the broken, lines the additions. The bracing is of Warren-truss form, and the additions consist of one widetop centre section, two short extensions to the lower centre section (which is permanent), and the necessary bracing.Fig. 1 shows how the single-seater top 'plane is removed in its two halves, with the outer bracing struts and the end fairingof the lower 'plane ; these parts being re-connected to the added parts. Fig. 2 shows how the ends of the added topcentre section are " raked," giving the planes of the two-seater form a " sweepback." Fig. 3 shows how the lower planeextension is " raked " with the same object. By this means the " sweepback " (necessary to balance the altered centre ofgravity due to the passenger) is achieved without alteration to the permanent planes and bracing. As regards the accommodation for the passenger, the cockpitprovided for him can be covered over when the plane is in its single-seater form.From the point of view of stability, the increased span should equalise the aileron effect for the increased area, and the" sweepback " of the planes should virtually increase the tail plane area. As the side area is unchanged, the existing ruddershould suffice. Altogether this scheme offers two machines, with the extraexpense of only three short pieces of plane and a few extra struts. With the truss bracing the necessary spar joints are asimple matter. Concerning the change of engines : this point arises in anyscheme for one-two-seater machines. Provided the power of the engine is sufficient to fly the two-seater, retaining the sameengine will result in a fast single-seater; but its increased speed will probably be at the expense of economy. So it might bedesirable for single-seater " runabout " use to substitute a lower-powered engine, which can still bear the same relationto the centre of gravity by being set farther forward than the heavier engine, preferably being designed as a unit with itsbearers and tanks. In any case the propeller of the two-seater will have to be changed for the single-seater form. It .even seems that with three propellers, two engine units and the spare parts mentioned above, one could have the useof three machines. I must mention that I have no financial interest in thedevelopment of the light 'plane ; but the above ideas occurred to me, and I think they may be of interest to you and yourreaders. ARTHUR PRIESTLEYHounslow, November 3, 1923. Dominion Premiers at CroydonTHE visit to Croydon by Prime Ministers and other representatives of the Dominions and India which has beenarranged for Saturday next (November 10) will, we learn from the Air Ministry, be a private one, and the public'will not beadmitted to the aerodrome. A similar visit was made by the Dominion Prime Ministers when they attended the Imperial Conference in 1921. An opportunity will be afforded ofinspecting the present traffic arrangements at the aerodrome, and various types of military and civil aircraft will bedemonstrated. The visitors will arrive about 2.30, and it is expectedthat the programme planned will be finished shortly after 5 p.m. 689
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