Flight, April 1921

THE AVIA B.H. (EXP.) 35 HP. MONOPLANE

   As we have previously remarked in FLIGHT, Czecho-Slovakia is determined not to lag behind other European Nations in aeronautical matters, and is already making rapid progress in the design and construction of new machines. Owing to their awkward political position during the War they had little opportunity of gaining actual experience in the design and construction of aircraft, as other nations did. Thus it is, that now they are starting to build up their own aircraft industry their knowledge of modern practice is somewhat limited, and they are faced with the necessity of having to worry out certain problems for themselves. In a way, we are inclined to think that this is a slight advantage, for, although the road may be uphill, are they not free from the temptation of blindly following the ruts of others ?
   At the Czecho-Slovak Aeronautical Exhibition held at Prague last year, a very interesting little machine was exhibited by the Avia Go., of Prague, which excited considerable attention in both home and foreign circles. We are indebted to our Bohemian contemporary Letectvi for the accompanying particulars of this machine, which is known as the "Avia" B.H.Exp. It was built from the designs of Messrs. P. Benes and M. Hajn as an experiment, with the object of producing a low-powered machine for economic flying built on somewhat unusual lines. It is a two-seater monoplane having what at first glance appear to be cantilever wings, mounted on the bottom longerons of the fuselage; the wings, however, are not true cantilevers, but are braced by a pair of struts running from the main spars up to the top longerons, on each side of the fuselage. This position of the wings was chosen for the purpose of obtaining the greatest possible visibility.
   The wings, which have a comparatively high aspect ratio, are of rather unusual form, for in addition to tapering from root to tip, their maximum thickness occurs at the point of attachment of the bracing struts, about one third of the span of the wing from the root. They are set at a slight dihedral angle.
   It will be noticed that the fuselage is comparatively deep from nose to stern, tapering to a vertical knife-edge at the latter, and has a clean, streamline form.
   Throughout the design the questions of simplicity and ease of construction have been a main consideration, with the result that the machine is built up of an exceptionally small number of component parts, and is easily assembled and dismantled. For the greater part of the construction of the Avia B.H. wood and three-ply is employed, the fuselage being entirely covered with the latter.
   The long overhang of the wings demands rather thick spars, and these are of box construction, sufficiently strong to withstand any tendency on the part of the overhung portion of the wing to twist. Where the wings are attached to the fuselage the latter is provided with strong cross members. The wing attachment, and also the attachments of the bracing struts, are of the knuckle-joint type, whilst the control cable passing from the fuselage through the wings to the ailerons has a coupling - accessible through a small window - near the wing root. There are only 14 nuts and bolts to be manipulated for dismantling or assembling. The bracing struts have adjustable ends, so that it is possible to alter the angle of the wings. With a wing loading (including weight of bracing struts) of 1.23 lbs./sq. ft., the factor of safety for the wings is 12. Previous to the first flight being made the wings were subjected to a test up to five times normal load without fracture. Tests were also made on the tail planes, which withstood successfully a load of 132 lbs. applied at each end. It should be mentioned here that the tail plane is of the cantilever type, with a non-lifting streamline section, and is divided into two units mounted near the top longerons of the fuselage, a little way above the line of thrust. The construction of the tail plane is similar to that of the wings; large divided elevators are fitted, and a balanced rudder is hinged to the stern post of the fuselage - no vertical fin is employed. The wings are covered with fabric, but the ailerons - which are comparatively narrow - are constructed of three-ply.
   The engine is a 4-cylinder 35-40 h.p. Austro-Daimler, mounted in the nose of the fuselage, driving a 6 ft. 6 ins. walnut tractor air-screw. The radiator is mounted in the nose of the fuselage, below the engine, and is fitted with adjustable shutters. Behind the engine is a petrol tank of 13.2 gals, capacity; behind the tank is the passengers' cockpit, followed by the pilot's, in line with the trailing edge. The usual "joy-stick" control is fitted.
   The landing chassis is of the Morane-Saulnier "M" type, with divided axle, the outer strut members - forming the usual V, viewed from the side - being completely covered in with three-ply. During flight the axle lies enclosed in a three-ply fairing, which really forms a small, narrow lifting plane. Provision is made for the free upward movement of the axle when landing. Rubber shock-absorbers are fitted.
   During its first trials - carried out by Josef Novak - an engine of only 26 h.p. was fitted, and with the loadings at 31 lbs./h.p. and 7 lbs./sq. ft. a speed of 68 m.p.h. was obtained , and the climb was 3,300 ft. in 10 mins. Unfortunately, we have no other particulars of this machine's performance with full power. The stability is said to be remarkably good, in spite of high C.G., and it manoeuvres with great facility, and climbs quickly. The gliding angle is small, and the get-off and run along the ground on landing are short.
   The following are the principal characteristics of the Avia B.H. :-
   Span 35 ft. 5 ins.
   Chord (max.) 4 ft. 2 1/4 ins.
   Overall length 19 ft.
   Overall height 6 ft. 8 ins.
   Area of main planes 116.2 sq. ft.
   Weight empty 582 lbs.
   Weight full load (pilot and passenger) 1,012 lbs.
   Weight full load (pilot only) 845 lbs.
   Weight per h.p. 23.6-25.3 lbs.
   Weight per sq. ft. 7.3-8.7 lbs.