[.041 Safety Wire]

Actually on Radial powered aircraft they are referred to as Cowl Flaps and are normally open during idling and extended periods on the ground. With no forward movement they will allow more air to pass through the engine cowl. Since most aircraft have pressure cowlings as the aircraft moves forward air is forced through the pressure cowling to cool the powerplant. In these situations the cowl flaps are closed. In liquid cooled powerplants the radiator doors control the amount of air passing through the radiator core. On the 51,s the design of the scoop and cowling actually accounts for a slight speed increase. dependent on the setting of the exit flap. Typically aircraft of WWII have a flap or door arrangement on their oil coolers also with them being controlled by a thermoswitch. This allows the aircraft engine to heat up faster. Some even had gasoline dilution systems to thin out the oil for cold starts.