The cowling of an engine is used to streamline the airflow outside the aircraft but, more importantly, also on the inside improving cooling. As most aircraft engines are air cooled they need proper airflow to prevent them from running too hot. In the good old days the engine sometimes just sat in the airflow with minor baffling, which is not enough for todays high powered engines.
As you may expect, high performance engines need something better than that. The cowling completes the top section of the baffles which guide the air around the cylinders thus keeping them cool. Even Rotax engines uses some form of cooling airflow to keep the cylinders cool so this story applies to those engines too.
A proper cowling will usually have two cold air inlets and a larger warm air outlet underneath. Water cooled engines have a separate inlet for the coolant radiator.
Modern cowlings use a form of ram air pressure so that small amount of air is doing all the work by virtue of pressure and airspeed. This way the cowling can be made very streamlined and sleek so that drag is minimized and the aircraft does not suffer in performance.
Inside the cowling the cooling can be either up-flow or down-flow. Most engines use the down-flow method where air enters the cowling above the engine and flows downwards to exit the cowling underneath the aircraft. The up-flow type is sometimes used with pusher type engines.
There are several parts or areas we can identify with engine cooling and cowlings, they are listed below:
The exact area needed so that the correct amount of cool air enters the engine depends on several factors: aircraft speed, heat developed by the engine (function of engine power) and this varies with different conditions of flight. If the inlet area is too large it will result in a lot of drag and this will reduce the speed of the aircraft and the engine runs much too cool.
Keep in mind also that during a high angle of attack climb air flow will also be disrupted and the propeller will chop that incoming air into a turbulent mass before it enters the cowling.
Most kit manufacturers, which supply the engine kit, will also have all baffles and installation materials in a complete set. This saves the builder an enormous amount of time installing these. Baffles guide the air around the engine and its ancillaries without leaks. This air flow must be smooth and efficient for proper cooling.
If the top cowling is part of the engine pressure chamber the baffle seal will need to be replaced more often compared to a separate top cover for the pressure chamber. This top cover also needs to be removable for easy daily engine inspection and maintenance.
Located underneath the bottom cowling where the hot air exits. This area is quite a bit larger than the inlet area as the air has expanded due to the heat it picked up from the engine and exhaust. During flight a pressure differential will exist between the inlet and outlet area, this helps the air flow and reduces cooling drag.
Slower and lower powered aircraft usually have a fixed size of air outlet. Although not ideal, this works good enough. Higher performance aircraft, on the contrary, develop more power and more heat comes from the engine and exhaust. Thus we need to regulate the outlet area in some way for the different conditions of flight.
These are pilot operated flaps and are used to increase the outlet area thereby improving air flow during taxi, climb and slow flight where high power is developed by the engine.
These flaps are rather easy to install with a piano hinge and a separate control cable running to the cockpit for control by the pilot.