Flight, September 1924

THE BRISTOL "BROWNIE" MONOPLANES (NOS. 1 AND 2)
Bristol "Cherub" Engines

   OF exceptional interest are the two monoplanes entered for the Lympne competitions by the Bristol Aeroplane Co., Ltd., of Filton, Bristol. Not only do they represent the first designs by Captain F. S. Barnwell since his return from Australia, but they, or rather one of them, represent the only example in the competitions of all-metal construction. The Short monoplane has an all-Duralumin fuselage, but the wing is of the usual composite wood and metal construction. The same applies to the other Bristol monoplane, which has a steel tube fuselage but a wooden wing. In the following notes any reference to the fuselage applies equally to both Bristol “Brownies," but the sketches showing wing details, and the notes dealing therewith, refer specifically to the all-metal wing. That no detailed reference is made to the wooden wing is not due to any lack of interest, for, as a matter of fact, even the wood wing incorporates many features that might have been illustrated and described, but in an issue like the present we are naturally somewhat limited as regards space, and we have, therefore, thought it well to devote such space as is available to a fairly detailed reference to the all-steel wing as being the more interesting of the two, especially as this is the first time one has had an opportunity of illustrating in detail the Bristol methods of steel construction, upon which, although it has not been generally known, the Bristol Company has been doing a great deal of experimental and research work.
   As regards aerodynamic design, the Bristol "Brownies" are low-wing, cantilever monoplanes with bi-convex aerofoils tapering in chord and depth from root to tip. The section, although of great maximum depth, is not a high-lift section, but is rather on the lines of some American aerofoils which were described in FLIGHT last year. As a matter of fact, the particular section employed is not identical with any of the American sections, but is one developed by Captain Barnwell. Wind tunnel tests have been carried out in the Bristol wind tunnel, so that very complete data should be known to the designer, the more so as the first machine was finished and flying some six weeks ago, so that there has been ample opportunity to discover whether or not the actual light 'plane tallies with the wind tunnel model.
   The fuselage is of fairly large maximum cross-sectional area, and is of the flat-sided, flat-topped variety. The machine is of fairly high aspect ratio, and the body is of rather more than usual length, so that one would expect the machine to be pleasant to fly. The flying tests have, we believe, proved this to be the case, and the "Brownies" are said to handle particularly well. Large tapering ailerons are fitted, which should give ample lateral control. With reference to the latter a somewhat novel arrangement has been adopted. Instead of the usual "positive" cables, the ailerons on the "Brownies" are pulled down by rubber cords suitably tensioned, and the "return" cable is connected to the controls and positively operated. The object of this arrangement is to impart to the wing to some extent the principle of automatically variable camber, since, as the speed increases and the pressure on the ailerons with it, the flaps will automatically rise.
   With reference to structural design, steel is, as already mentioned, the material used, exclusively in one machine and in the fuselage of the other. The fuselage is a girder of steel tube longerons and struts, braced by piano wire and turnbuckles. Fig. 8 in our set of sketches shows a typical fuselage joint, from which it will be seen that a short length of sleeving is slipped over the longeron at the strut attachments, the bolts for the latter being slightly staggered so as to clear each other. Exceptions from this form of construction occur where local considerations demand. Thus, at the point, of attachment of the wing halves wood bulkheads are incorporated, as shown in Fig. 7. These are frames covered both sides with three-ply so as to form boxes. The undercarriage strut attachments, also, are located on these wooden bulkheads, as indicated in the sketch.
   The cockpits are arranged tandem fashion, and with the low position of the wing the view is exceptionally good, except straight down from the rear cockpit, where the large chord at the root of the wing obscures it to some extent, although this is probably of small importance as the pilot can look forward over the leading edge for landing. The controls are of usual type except for the aileron controls already referred to.
   The undercarriage is of somewhat unorthodox design, the "legs" consisting of two vertical streamline tubes secured to the front spar boxes and braced fore and aft by wires. The high-tensile steel axle extends a considerable distance beyond the strut supports and absorbs some of the shock by its deflection. No rubber shock absorbers are fitted. The wheel track is just over 4 ft. Lateral bracing between struts is in duplicate.
   Perhaps the most interesting feature in the design, from a structural point of view, is the all-metal wing, and this is consequently shown by several sketches. The steel spars are of very original design, especially as regards the webs, which are formed of zig-zag steel tubing. This tubing, normally of circular section and some If ins. in diameter, runs through from root to a few feet from the wing tips. This tube is flattened out and bent over as it meets upper and lower flange respectively, to which it is attached by small, flat plates and rivets. The tube does not extend right out to the wing tip, as the spar here becomes so shallow that the zig-zag tube arrangement would scarcely be suitable. The outer portion is built up as shown in Figs. 4 and 5. The spar flanges are double, with a flat inner flange, to which the corrugated outer flange is riveted. This point also is illustrated in our sketches. The wing ribs are N-trusses of channel section. Fabric covering is employed for wings as well as fuselage.
   The tail plane, elevator, rudder and fin are also of metal construction, but of more orthodox form. The elevator is operated via a layshaft, the cranks of which can be seen above the fuselage deck. The tubular fin post is braced by struts below and by two wires above. The fin post, it will be observed, slopes forward. Presumably this has been done in order to bring the pull on the rudder cables at right angles to the post, thus avoiding any side load on the rudder cranks.
   The Bristol "Cherub" engine is mounted in a somewhat unusual manner. The fuselage terminates in a ply-wood bulkhead bolted to the tubular longerons. To this bulkhead are secured, by ordinary pipe unions, four pillars which carry the engine. These pillars are cantilever beams, and there is no diagonal bracing of any sort, the pillars taking the direct load as well as the torque. Details of this mounting are shown in Fig. 6, but it might be pointed out that the pillars are normally secured to, and form part of, the engine crank-case, the joint being made by the pipe unions on the engine bulkhead. The engine is thus very accessible, and is easily removed by undoing the four unions and the petrol and oil leads. The petrol and oil tanks are housed above and behind the engine bulkhead, and gravity feed is employed.

Flight, September 1926

British Light ‘Plane Development & Lympne Meeting

THE 1926 MACHINES

No. 3. The Bristol "Brownie." (Bristol "Cherub" Engine.)

   This machine is, to all intents and purposes, identical with that which took part in the 1924 Lympne competition, in which the "Brownie" obtained second place in the general classification and first place in the take-off tests. The machine has a steel tube fuselage, and the wing framework is also of steel, with corrugated steel strip spars, so that except for the fabric covering it is of all-metal construction. Although a light 'plane, the "Brownie" is not a particularly small machine, having large wing area in order to obtain light wing loading. Among the modifications carried out for the present competition may be mentioned the substitution of a sprung undercarriage and the lowering of the line of thrust due to the fitting of a Series III Bristol "Cherub" engine, and the addition of a deck fairing.