Flight, July 1924

THE DE HAVILLAND TYPE D.H.51
A Two-Three-Seater Biplane of Low Cost and Economical in Running

   Low power, reasonable cost, economy in operation, and good performance; these were the features aimed at in the design of the latest machine to be produced by the De Havilland Aircraft Co., Ltd., of Stag Lane, Edgware, Middlesex. The D.H. 51 is not, and this is significant, a light 'plane in the sense in which the term is now used in this country. It is a machine of low power, certainly, compared with service machines and other high-power craft of modern times, but its engine power is such as to give an excellent performance, while the wing area is sufficient to reduce the landing speed approximately to that of many light 'planes. The machine is naturally more expensive to buy than a light 'plane of 30 h.p. or so, but on the other hand it is not as much more costly as one might imagine, and it will carry three without being in any way overloaded. Add to this the fact that at cruising speed the engine is only running at about 60 per cent, of its full power, and it will be seen that the reliability should be all that can reasonably be expected. As an owner-pilot's machine the D.H. 51 should appeal by the fact that it will carry one passenger and a considerable amount of luggage, or two passengers without a great deal of luggage. The ease with which the machine can be flown, and its stability and controllability, should enable a pilot of only average skill to use the D.H. 51 for cross-country work with perfect safety. The landing speed is so low as to enable the machine to be brought down almost anywhere, while the power reserve is such as to give an ample margin for climb out of a small field. In this latter respect the D.H. 51 certainly appears to score over the light 'plane, which, with its ultra low-power engine, will scarcely have a climb of more than about 300 ft./min., while the ground rate of climb of the D.H. 51 is about 580 ft./min.
   The power plant provided is either a 90 h.p. R.A.F.1A or an 80 h.p. Renault, both 8-cylinder air-cooled engines. The engine fitted in the first type, illustrated herewith, is the R.A.F.1A. In this connection a somewhat curious state of affairs seems to exist. The R.A.F.1A engine is not, it appears rated by the Air Ministry as an airworthy engine, having plain single ignition. In order to improve the reliability and to make the engine conform to modern requirements the De Havilland Aircraft Co. have fitted the engine with Remy coil ignition in addition to the standard magneto ignition system. In the view of the Air Ministry, however, this constitutes a change in the engine, and a 10-hour run is insisted upon. The Air Ministry at first wanted the engine to be run on the ground for 10 hours, but it was pointed out to them that if this were attempted the engine would probably melt. As a special concession the Air Ministry has now agreed to accept a 10-hours' flying test around the Stag-Lane aerodrome as proof of the engine's "worthiness." Now it is quite conceivable that in such a test something may go wrong with the engine, something which has nothing whatever to do with the addition of the second ignition system, but the De Havilland Aircraft Co. must bear the blame if anything goes wrong, and must try again until the 10-hour run has been successfully completed. At the best it will probably cost the firm L100 or so to get the engine passed, which, in view of the fact that the De Havilland Co. has no proprietary interest in the engine, seems rather unfair, to say the least. Incidentally, the official distrust of an engine that was produced at the Royal Aircraft Establishment is not without its humorous side. In the meantime the D.H. 51 is not allowed to go more than 3 miles away from its aerodrome. And, of course, if and when the engine does obtain its certificate, no private owner-pilot will be able to fly his machine unless he has a ground engineer qualified for this particular engine. As there are large numbers of these engines to be obtained at low cost, it might have been thought that here was a good opportunity for producing low-priced machines of really good performance, but if all these restrictions are to be applied the cost will soon go up to such an extent that no machine can be produced at a price that is likely to appeal to more than a very few wealthy sportsmen. Thus once more the Air Ministry has provided an example of the "encouragement" which it is anxious to extend to people wishing to popularise flying. Truly we are a long-suffering race.
   The D.H.51 is an orthodox tractor biplane of typical de Havilland appearance, and standard de Havilland practice has been followed in its construction. The fuselage is the usual plywood covered structure with flat sides and bottom and curved deck fairing. A very neat feature of the cockpit arrangements are the sliding fairings carrying the wind screens. One of our sketches shows the front cockpit, with the fairing pulled back and the "door" swung open so as to facilitate access to the cockpit. In the case of the front cockpit the object of the sliding fairing is, of course, to allow two passengers to be carried, while the pilot's fairing has a much smaller movement and is intended to be adjustable to the most comfortable position. The D.H. sliding cockpit fairing is just one of those obvious things which nobody thinks of, and which are yet extremely useful and practical.
   The R.A.F.1A engine is mounted on transverse tubular bearers, and rubber packing pieces are inserted between the tubular bearers and their lugs on the fuselage. It has been found that this mounting reduces to a minimum the vibration transmitted to the fuselage. The usual scoop is fitted above the engine, between the cylinder banks, but somehow the de Havilland designers have managed to design a nose that looks considerably neater than do the majority of machines fitted with this type of engine. The exhaust pipe from the port cylinders is taken right across the fuselage, behind the engine, to a very long pipe on the starboard side, which carries the gases from all cylinders well aft, and contributes in no small measure to the silence of the engine. A large-diameter four-bladed airscrew is fitted, the engine being, of course, of the geared type. The petrol system is of the direct gravity feed type, the petrol tank, of aerofoil section, being mounted in the top centre-section. The fuel is conducted to the engine by a "Petroflex" tube. The tank is divided into two compartments, the second of which contains but a small quantity of fuel. Thus when the main compartment begins to run dry, and the engine shows signs of faltering, the pilot knows that he has a certain quantity left in the second compartment, and can begin to look for a suitable landing ground. The quantity of petrol carried is 30 gallons, which should be sufficient for about 4 1/2 hours' flying at cruising speed. It should be pointed out that a fireproof bulkhead is interposed between the engine and the fuselage, and as the petrol tank is well away from the engine the danger from fire should be very remote.
   The wings of the D.H. 51 are of standard construction, and the section employed is a slightly modified R.A.F.15. The structure is a two-bay biplane with streamline wire bracing, and the only unusual feature is the omission of the incidence bracing on the port side of the struts carrying the top centre-section. This bracing has been omitted so as to give easy access to the front cockpit, and the wires on the starboard side, plus those in the centre-section itself, take over the duties of the missing wires.
   The undercarriage is of usual de Havilland type, with combined rubber and oleo shock-absorbing gear. The first shock is taken by the oil, and then the rubber blocks, working in compression, begin to take up the load, the oil being forced past its plunger and damping the recoil. The travel provided is not very long, but has been found in practice to be sufficient.
   The control surfaces and controls are all of standard de Havilland type, the differential aileron control patented by this firm being incorporated. There is no tail trimming gear, but the elevator is spring-loaded so that by setting it any slight changes in trim due to varying loads can easily be taken care of.
   In conclusion it may be pointed out that by careful structural design the tare weight of the D.H.51 has been reduced to the very low figure of 1,312 lbs. As the machine carries three, and is not a particularly small machine, the wing area being 325 sq. ft., tins figure is extremely good, yet adequate factors of safety are provided everywhere, so as to make the machine suitable for the rough handling usually associated with school work. The flying qualities are excellent, not only in the matter of performance but also as regards handling, and the D.H. 51 is exceptionally pleasant to fly.
   The main dimensions, areas, etc., are given on the general arrangement drawings published on page 438. The figures of weight are as follows: Tare weight, 1,312 lbs.; fuel (30 gallons) and oil (3 gallons), 250 lbs.; useful load (including pilot), 678 lbs.; total loaded weight, 2,240 lbs.; wing loading 6-9 lbs./sq. ft.; power loading (at 90 b.h.p.), 24-9 lbs./h.p. Fitted with the R.A.F.1A engine, and carrying pilot, one passenger, 50 lbs. of load, and with full tanks, the performance is approximately as follows: Maximum speed near ground, 94 m.p.h.; cruising speed at low altitude, about 80 m.p.h.; stalling speed, 36 m.p.h.; maximum speed at 7,150 ft., 89 m.p.h.; ground rate of climb, 580 ft./min.; rate of climb at 6,500 ft., 300 ft./min.; service ceiling, 11,000 ft.; absolute ceiling, 13,000 ft.