As a natural side-effect of combustion, piston engines generate large amounts of heat that needs to be managed. An engine’s internal components are cooled by the oil system, but the external components, such as the cylinder heads, need to be cooled as well. While some engines feature water cooling systems and radiators, most general aviation aircraft use air cooling introduced by the cowling at the front of the front of the engine compartment, and a system of baffles within the compartment to distribute air.
Cowlings typically feature two cold air inlets at the front of the engine compartment, and larger warm air outlet underneath to vent hot air away. Modern cowlings use ram air pressure so that a small amount of air does the majority of the work via pressure and airspeed. This allows cowling to be sleek and cause minimal amounts of drag. Cowling can be either up-flow or down-flow, which refers to the direction that air ultimately flows through the engine compartment. Most designs feature down-flow configurations, although up-flow is used on some pusher-type engine housings.
A cowling’s inlet is designed to accommodate for factors like air-speed and the engine’s heat, both of which depend on various flight conditions. If the inlet area is too large, it will create too much drag, causing the aircraft to fly too slow, and make the engine run too cool. Once air is past the inlet, a set of baffles inside the engine compartment control and guide air over the engine and its various components. Some aircraft will also include cowl flaps in their design, which allows the pilot to control how much air flows through the cowling.
While efficient, air cooling does have its drawbacks. During a climb, less air than normal enters through the cowling, causing heat to rise. It is also much less effective during high-power, low-speed operations such as takeoff, landing, and taxiing. On the other hand, high-speed descents can cause too much air to flow into the engine compartment, causing the engine to drop in temperature and the cylinder heads to crack. Therefore, pilots are trained to carefully manage their cooling needs, and never run their engines too close to the red line.
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