Avia Av.51

Flight, January 1934

THE AVIA 51
A Czech High-performance Passenger Plane

   MR. ROBERT J. NEBESAR will be remembered by readers of FLIGHT as the author of an article entitled "Supercharging the Aeroplane Engine and increasing Speed with Altitude," published in THE AIRCRAFT ENGINEER (Monthly Technical Supplement to FLIGHT) on December 25, 1931. At that time Mr. Nebesar was Project Engineer to the Detroit Aircraft Corporation. He has since returned to his native Czechoslovakia, and during last year has been engaged on the design of a small three-engined commercial monoplane for the Avia Company of Prague. This machine, the Avia 51, is shown in the accompanying illustrations. It made its test flights some time ago, and came up to its designer's estimates in every respect. Mr. Nebesar carried out extensive model tests both at Prague and at the French laboratory at Saint Cyr. It is, Mr. Nebesar informs us, largely a result of these careful model tests that he has been able to obtain such high aerodynamic efficiency. The value of the Everling "High-Speed Figure" is 20.9, which must be regarded as extremely good for a three-engined machine, and shows that the minimum drag coefficient is low. Another very important factor in a commercial machine is the maximum ratio of lift to drag. The maximum L/D of the Avia 51 is 10.2 or, expressed otherwise, the best gliding angle is 5 degrees 35 minutes. The wing loading is high (19.3 lb./sq. ft.), and as the maximum lift coefficient is about normal (0.75 in British "absolute" units), the landing speed is also high. The figure 62 m.p.h. is quoted by the firm, but for the wing loading and kL max. mentioned above one would expect the minimum speed to be about 70-71 m.p.h. As no air brakes are fitted, and the gliding angle is very flat, this landing speed appears rather high in comparison with current British practice. Since, however, the Avia 51 is capable of maintaining flight with one engine stopped, forced landings should rarely occur, and the high landing speed may be tolerated.
   In external appearance the Avia 51 is of fairly orthodox design, with the wing engines faired into the leading edge and the central engine mounted in the nose of a streamline fuselage. The somewhat "stilty" appearance caused by the long undercarriage legs is doubtless a result of the designer's desire to provide a good ground angle coupled with a long travel of the wheels.
   In the construction of the Avia 51 nothing but metal and fabric has been used (with the exception of certain cabin decorations). Duralumin and high tensile steel are the materials employed. Painting, lacquering and cadmium plating are the precautions taken against corrosion.
   The cantilever monoplane wing is of orthodox two-spar construction. The spars are built up of duralumin, with booms of "D" section and ties of channel section, forming an N-girder. The "D" section spar booms are, of course, built up of a "U" section strip with a corrugated covering strip closing the open side of the U section. It is noted that the rounded side of the "D" is facing inwards. One would have thought that the more logical way was to turn the rounded side outwards so as to get the riveting a little away from the area of maximum stress. The reverse arrangement possibly makes the attachment of the channel section ties easier, and Mr. Nebesar informs us that he has been able to get 45,000 lb./sq. in. column stress out of his spar booms. The wing ribs, also of duralumin, are built up of "bulb" sections.
   Ailerons of Frise type are used, the aileron spars being duralumin tubes. Fin, rudder, tailplane and elevator are of duralumin construction, and the hinges of rudder and elevator are of the set-back Handley Page type.
   In the construction of the fuselage the so-called "mixed monocoque" system is used. The unsupported panel areas are very small, as there are six main longerons with intermediate stringers, while double-walled bulkheads are spaced fairly closely, and lighter formers are placed between them. The fuselage cross-section is of oval shape, and the covering is duralumin sheet, riveted to longerons, stringers and double-walled bulkheads, but not to the intermediate formers.
   A divided type of undercarriage is used, with long telescopic legs running to the front wing spar. These telescopic legs are of the oleo-pneumatic type, and have a long stroke. The wheel track is wide, and wheel brakes are fitted. If desired, the Avia 51 can be fitted with floats.
   The three Avia R-12 engines which form the power plant of the Avia 51 are seven-cylinder radial air-cooled, of a rated power of 200 b.h.p. each. They are mounted on welded steel tube structures, easily detachable as complete units, and rubber bushes are interposed to absorb vibration. Engine starting is by compressed air, the central engine driving the compressor. Two-bladed adjustable pitch metal propellers are fitted. All three engines are enclosed in complete N.A.C.A. cowlings.
   Petrol is carried in two soldered brass tanks, each of 16 1/2 gallons capacity. The tanks are mounted in the wing, between the wing spars. Fuel is supplied to the engines by engine-driven pumps. The oil tanks have a capacity of 5 gallons each, and adjustable oil coolers are combined with the tanks.
   Seating accommodation is provided in the cabin for five passengers, the seats having deep cushions and head rests. Ventilation is by ducts from cowls in the wing roots, and adjustable ventilators are placed at each seat. Heating is by hot air from a muff around the exhaust pipe of the central engine. The cabin has a length of 10 ft., a height of 5 ft. 1 in. and a width of 4 ft. 11 in. The height is not sufficient to give room to stand upright, but in any case it is doubtful if there is room to walk about in such a small cabin. Behind the cabin is a lavatory, and there are three luggage and mail compartments, one forward, one in the cabin, and one behind the cabin.
   A door in the front wall of the cabin communicates with the pilots' cockpit, which has two seats side by side. The chief pilot occupies the left seat, while the second pilot, who is also the radio operator, occupies the right seat. The windscreen is of non-splintering glass, and the side windows can be opened. View upward is afforded by the cockpit skylight, but to the back the view is cut off, and a mirror is so fitted that the pilot can see in it what is behind the machine.
   The usual instrument equipment is supplied with the standard machine, but if night-flying or blind flying is contemplated, special equipment can be supplied at extra cost. The usual navigation lighting equipment is always provided.

Flight, November 1934

THE FOURTEENTH PARIS AERO SHOW

THE EXHIBITS DESCRIBED

Large Civil Machines

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   Next in size is the three-engined Avia 51 with an all-metal stressed-skin fuselage not unlike the form first brought into prominence by the Lockheed "Vega," and with fabric-covered metal wing and tail units. Six passengers are provided for in comfortable seats, although the large cabin appears to be slightly overcrowded.
   The engines, three Avia R 12S of 200 h.p. each, are fitted with a form of N.A.C.A. cowling, but, despite this, their arrangement makes it appear that their respective slipstreams might interfere considerably with each other. The airscrew paths overlap, so that the resultant airflow, particularly under the wing roots, is probably very disturbed indeed.
   With 600 horse-power, this machine is certainly not under-powered, but it did not impress us as being "just anybody's machine."
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