Fig. 82.—Water Cooling of Salmson Seven-Cylinder Radial Airplane Engine.
A typical water-cooling system in which a pump is depended upon to promote circulation of the cooling liquid is shown at [Figs. 82] and [83]. The radiator is carried at the front end of the fuselage in most cases, and serves as a combined water tank and cooler, but in some cases it is carried at the side of the engine, as in [Fig. 84], or attached to the central portion of the aerofoil or wing structure. It is composed of an upper and lower portion joined together by a series of pipes which may be round and provided with a series of fins to radiate the heat, or which may be flat in order to have the water pass through in thin sheets and cool it more easily. Cellular or honeycomb coolers are composed of a large number of bent tubes which will expose a large area of surface to the cooling influence of the air draught forced through the radiator either by the forward movement of the vehicle or by some type of fan. The cellular and flat tube types have almost entirely displaced the flange tube radiators which were formerly popular because they cool the water more effectively, and may be made lighter than the tubular radiator could be for engines of the same capacity.
Fig. 83.—How Water Cooling System of Thomas Airplane Engine is Installed in Fuselage.
The water is drawn from the lower header of the radiator by the pump and is forced through a manifold to the lower portion of the water jackets of the cylinder. It becomes heated as it passes around the cylinder walls and combustion chambers and the hot water passes out of the top of the water jacket to the upper portion of the radiator. Here it is divided in thin streams and directed against comparatively cool metal which abstracts the heat from the water. As it becomes cooler it falls to the bottom of the radiator because its weight increases as the temperature becomes lower. By the time it reaches the lower tank of the radiator it has been cooled sufficiently so that it may be again passed around the cylinders of the motor. The popular form of circulating pump is known as the “centrifugal type” because a rotary impeller of paddle-wheel form throws water which it receives at a central point toward the outside and thus causes it to maintain a definite rate of circulation. The pump is always a separate appliance attached to the engine and driven by positive gearing or direct-shaft connection. The centrifugal pump is not as positive as the gear form, and some manufacturers prefer the latter because of the positive pumping features. They are very simple in form, consisting of a suitable cast body in which a pair of spur pinions having large teeth are carried. One of these gears is driven by suitable means, and as it turns the other member they maintain a flow of water around the pump body. The pump should always be installed in series with the water pipe which conveys the cool liquid from the lower compartment of the radiator to the coolest portion of the water jacket.
Fig. 84.—Finned Tube Radiators at the Side of Hall-Scott Airplane Power Plant Installed in Standard Fuselage.