Sopwith Camel

Time between repairs was ca. 30 hrs of running despite an almost perfect lubrication by modified castor oil. This oil should be mentioned in greater detail. Obtained from oil tree seeds, it provides very good lubrication in a range of working temperatures in petrol engines. Its disadvantage was that in time it turned into fatty acids, which “stuck” engine installations. This degradation depended on UV rays, contained e.g. in the sun’s rays. Castor oil was a strategic product in WW1. That was not an advantage to the Axis, as the tree usually grew in Africa or the Far East, and economical ties with those countries were limited, even more so for the war. This effected attempts to used mineral oils as substitutes, especially in rotary engines, which had an “open-circulation” lubricating system.
Construction-wise, reliability of the rotary was a difficult task to accomplish. The search for an optimized solution resulted in various modifications usually related with timing gear. Originally it was a “monosoupape” system with one tappet-controlled exhaust valve. A return suction valve was in the piston, feeding fuel on its downward movement. There was also a similar solution, but without the valve in the piston, fuel being sucked the same way as in a two-stroke engine. A third solution had two tappet-controlled valves, being similar to the static four-stroke construction. All those solutions were very vulnerable to centrifugal forces, which influenced not only the timing gear but also lubrication and load distribution. That caused great trouble with using these engines, especially affecting attempts to build higher-rated units of the type.
Flying-wise, the engine’s moment of reaction was very bad, being transferred onto the airplane. It caused the aircraft to tend to rotate in the direction opposite to the engine’s rotation. It was difficult to control this phenomenon, especially with lighter aircraft, the Camel being one of them. This was addressed by the use of bi-rotational two-bank units (the banks rotating in opposite directions). But this again caused a weight increase, while further complicating the construction; also, it caused a much worse cooling as a result of two times fewer rotations of the banks of cylinders.
Despite such troubles many firms focused on rotary engine production, their units usually not exceeding 150 hp. The Entente forces bought them mainly from Clerget, Le Rhône and Bentley, various technical solutions being employed in their constructions. These companies also delivered engines for various versions of the Sopwith Camel. It appears, though, that time filtered all those varieties and types of rotaries, for on analysis by technicians and pilots it was found that the best construction of the type was the original Gnome Rhône of 80 hp. It should be assumed, then, that this rating and related constructional features aided by experience were optimal.

Sopwith F1 Camel, US Navy No. A5721,  March 1920, Guantanamo Base, Cuba. [Painting by Arkadiusz Wróbel]

Famous combat and test pilot Cecil Lewis confirmed this by expressing his opinion saying about the Morane Parasol that the 80-hp Le Rhone tractor engine was in its front part, this engine being just perfect – the best rotary ever built. It was unusually flexible and could be made to work extremely evenly at low throttle – it was tuned perfectly across the entire range. When at full throttle, it gave a high-pitched, full sound, a kind of ringing. If it was heard from a distance, it seemed like a wasp returning to her nest. On top of that, this engine was extremely reliable.
The many attempts to modify the construction of the rotary badly influenced production issues. Attempts to perfect this type of aircraft engine ended when well-performing radial engines came along, particularly the high-performance liquid cooled V-type units. But that was after WW1 and those engines were the last piston-type high-rated aircraft power plants. Still, rotary engines continued to be used in many countries until mid-30s.

The first pre-prototype Sopwith F.1 was finished on Dec 22, 1916 and first flown at Christmas by Harry Hawker. Armed with two synchronized 7,7 mm Vickers machine guns on the fuselage in front of the pilot, it was propelled by a Clerget 9Z rotary of 110 hp. The guns’ breeches were enclosed in a hump which, slightly raised, was a substitute for a windscreen. The single-piece upper wing had no angle of setting in order to simplify production, while the lower one had and angle of 5 deg. This layout was the common feature of Camels throughout and was an effect of trying to achieve the best aerodynamics available. The wings were rectangular with rounded corners.
Initial tests showed the machine was unusually maneuverable, while its weight distribution and wing layout made it react very quickly to even the slightest movement of control surfaces and ailerons. Pre-prototype tests continued till Feb 1917, mainly at Brooklands field. At the same time next three machines were being built, they being prototypes proper.