Review of Liquid-Cooled Aircraft Engine Installation Aerodynamics

The aerodynamic behavior of liquid-cooled aircraft engine cooling installations is reviewed. Design considerations for inlets, diffusers, and exists are discussed. It is shown that the design of an efficient liquid-cooled installation is a techically sophisticated problem. This problem should not be underestimated in the development program of liquid-cooled aircraft engines. Questions are raised concerning the availability of suitable radiators for aircraft installations. INTRODUCTION To many, liquid-cooled aircraft engines seem to have an aura, a fascination that has not been given to air-cooled engines. Some of this is certainly due to aesthetics. Liquid-cooled engines are synonymous with streamlining and aerodynamically clean profiles. The connection is readily made to the legendary Spitfire, M 1 ustang, and Messerschmitt. The unappealing radial-air cooled engine does not enjoy this subjective attribute. Today's horizontally-opposed air-cooled engines fare no better. The mention of liquid-cooling immediately stimulates designers to visualizing low-drag shapes and improved performance, and stimulates marketing to thinking in terms of lightning-bolt and shark-mouth paint schemes. However, if more objective criteria are applied, the air-cooled engine compares more favorably with the liquid-cooled engine. It is apparent from literature during the 1930s period that each had strong proponents and that it was relatively easy to start an argument over the relative merits of each. Most of the same technical points apply today. The author, though, will separate these points into propulsion concerns and aerodynamic design concerns. From the standpoint of propulsion, the liquid-cooled engine offers better fuel efficiency and longer engine life. Better control of cooling paths and subsequently of component cooling is realizable. On the other hand, the air-cooled engine offers system simplicity and is less vulnerable to system component failure. The weight of the cooling system is less. From the standpoint of external aerodynamic design, the liquid-cooled engine offers reduced frontal area and the potential for reduced drag, particularly compressibility drag. The internal combustion engine aircraft speed record was held by a liquid-cooled engine; however, it is now held by a radial air-cooled engine. Engine power must be factored into the comparison. Ground cooling is more of a problem for liquid-cooled engines. From the standpoint of internal aerodynamic design, the liquid-cooled engine has a definite advantage. Aerodynamically, each component of the system is in theory well behaved, i.e., no separated flows. The liquid-cooling system is more tractable to analytical aerodynamic modeling and design. The horizontally-opposed air-cooled engine configuration leads to large separated flows, because the relatively large internal volumes and ducting necessary to do otherwise are not practical. This is inherent in the geometry of the engine. The radial air-cooled engine configuration is aerodynamically much easier to deal with. Liquid-cooled aircraft engines are presently available up to approximately 400 hp. There are developmental programs under way that could extend this range. In some cases, these are derivatives of automobile 1 Presented as Paper 86-2587 at the AIAA General Aviation Technology Conference, Anaheim, CA., Sept. 29-Oct. 1, 1986; received Nov. 12, 1986; revision received Aug. 22, 1987. Copyright (c) engines; in other cases, these are unique designs, such as the rotary type. The author's recent experience with one of these programs has brought forward an appreciation for the level of technology required to design an effective and efficient liquid-cooled installation. The technical problem of achieving the required cooling for minimum drag penalty should not be underestimated. It is the purpose of this paper to identify and discuss the various design problems of a liquid-cooled installation, indicating, where possible, what is known and what is unknown. Much of the information in this paper is taken from literature, particularly the works of Kuchemann and Weber