Radial Engine - Radial Versus Inline Debate

Radial Versus Inline Debate

Liquid-cooled engines often weigh more and their cooling systems are both more complex and are generally more vulnerable to battle damage. Minor shrapnel damage easily results in a loss of coolant and consequent engine seizure, while an air-cooled radial would be unaffected. Additionally, radials have shorter and stiffer crankshafts, a single bank radial needing only two crankshaft bearings as opposed to the seven required for a six-cylinder inline engine of similar stiffness. The shorter crankshaft also produces less vibration and hence higher reliability through reduced wear.

While a single bank radial permits all cylinders to be cooled equally, the same is not true for multi-row engines where the rear cylinders are affected by the heat coming off the front row, and air flow being masked. Additionally, having the cylinders in the airflow increases drag considerably, adding turbulence that destroys the laminar airflow over the fuselage and adjacent wings. The answer to both these problems was the addition of specially designs cowlings with baffles to force the air over the cylinders. The first effective drag reducing cowling that didn't impair engine cooling was the British Townend ring or "drag ring" which formed a narrow band around the engine covering the cylinder heads, not only reducing drag, but adding a small amount of thrust. National Advisory Committee for Aeronautics then studied the problem, developing the NACA cowling which further reduced drag, increased thrust and improved cooling. Nearly all aircraft radial engine installations since have used NACA type cowlings. The thrust generated by both the Townend ring and the NACA cowling was due to the Meredith Effect (discovered by British researchers) which was also put to use in the radiators of several aircraft that used liquid-cooled engines such as the Spitfire and Mustang.

Because each cylinder on a radial engine has its own head, it is impractical to use a multivalve valvetrain on a radial engine. Therefore, almost all radial engines use a two valve pushrod-type valvetrain and as a result produce less power for a given displacement than multi-valve inline or vee engines. The radial engine typically has a larger frontal area, and is less amenable to streamlining and drag reduction than an inline engine. Pilot visibility is often poorer due to the width of the engine, and the designer is limited in engine placement as ensuring adequate cooling air, either in a buried engine installation or in a pusher configuration is more difficult.

While inline liquid-cooled engines continued to be common in new designs until late in World War II, radial engines dominated afterwards until overtaken by jet engines, with the late-war Hawker Sea Fury and Grumman Bearcat, two of the fastest production piston-engined aircraft ever built, used radial engines. Until the development of the jet engine, large aircraft commonly used radial engines. Factors influencing the choice of radial over inline were reliability and simplicity in maintenance. The large frontal area of these aircraft meant the radial engine's own frontal profile was a less significant factor when it came to drag.