Design of Cowlings for Air-Cooled Aircraft Engines 370193
RECENT work on cowlings for air-cooled engines has been characterized by the correlation of the cooling function of the cowl with the drag-reducing function into a rational design procedure, whereas earlier work was devoted largely to drag reduction and this was a cut-and-try proceeding.
The fundamental relations between the pressures and velocities of the external and internal air flows are discussed here in their relation to the quantity of air available for cooling and the effect on drag.
Experimental results are outlined, and a design procedure is indicated. It is pointed out that certain factors must be determined by the engine manufacturers in order that a rational design of cowl may be laid out.
The shape of the cowling nose is not critical, and the part of the drag that is subject to control is determined by the air flow out the cowl exit. For an efficient cowling and for control of the air flow, the exit is the important part. A procedure is given for designing an efficient skirt and inner cowl which form the exit.
The propeller has an important effect on the flow at low air speeds, but, in general, is insufficient to provide the necessary flow of air required for cooling on the ground. A new type of cowling providing more flow in this condition is discussed.
Finally, the in-line air-cooled engine is mentioned. The cooling problem here is shown to depend on air-flow conditions differing considerably from those of the radial engine. Economical cooling is dependent on the reduction of internal losses, particularly the large turning loss at the entering side of the engine cylinders.