A commonly glossed-over subject on complex-airplane checkouts can cost you several knots in cruise speed, and perhaps several hundred hours of operating life from your cylinders. It's not the landing gear, or a controllable-pitch propeller, or even the mixture control. Cowl flaps can have a profound, long-term effect on the health and longevity of a high-performance airplane engine -- types that already often stand a poor chance of reaching the factory recommended time before overhaul (TBO).
Although the last couple of decades have seen attempts at liquid cooling for general aviation (GA) engines, most light-plane powerplants still depend on external airflow for cooling. Cowl flaps increase cooling air flow through the engine compartment and around cylinder cooling fins. They also add air flow through the oil cooler, which is basically a radiator required to keep the inside of the engine more cool. If you're flying a turbocharged engine with an intercooler, cowl flaps may also help offset the heat of turbo boost, increasing available engine power.
An opening or door at the rear of the engine cowling, cowl flaps deflect the slipstream enough to create a low pressure area in the bottom of the engine compartment. Some designs may have cowl flaps on the sides of the cowling as well. Reduction in air pressure itself helps cool the engine; but more importantly, the low pressure that cowl flaps create pulls air through the engine compartment for increased cylinder and oil cooling. All that air flowing through the engine compartment, though, also creates a tremendous amount of drag, which will rob you of cruise airspeed.
In airplanes built for cross-country speed, this cooling drag is minimized by keeping the rear cowling opening just big enough for adequate airflow in cruise. At lower airspeeds, like takeoff and climb, the pressure differential created by the opening alone may not be enough for sufficient cooling. For cooling during these high-power/low-airspeed phases of flight, cowl flaps (controllable openings that provide a greater airflow deflection and therefore greater pressure differential) are usually standard equipment for high-performance airplanes.
Many airplanes don't have cowl flaps. They depend on fixed slipstream-deflectors on the lower, rear cowling (a "cooling lip") to serve as a permanently open cowl flap. The cooling lip works most of the time with lower-horsepower engines. But even in those airplanes, in hot temperatures, or if you've upgraded to a more powerful engine ("hotter" in more ways than one), you'll have to sacrifice climb rate and climb at a higher indicated airspeed to effectively cool the engine.
How does your cowl-flap operating technique affect performance and cylinder longevity?
In general, cowl flaps should be an all-or-nothing exercise: Keep them fully open, or fully closed, as appropriate. You may be able to slightly cool a slightly hot engine by running the cowl flaps part-way open, but this takes finesse and generally isn't worth the effort. An exception is when cruising a turbocharged engine at high altitude, where cooling airflow is thin but cooling drag is still a speed drain if the cowl flaps are fully open.
Some airplanes have electrically operated cowl flaps with "Cowl Flap Open" annunciator lights. The annunciators often come on when the cowl flap is about 50-percent open, so a technique I've used in Beech Baron 58TCs and 58Ps is to run the cowl-flap motor only until the light first comes on, then turn the motor off. This leaves the cowl flaps "in trail" for high-altitude cooling with minimal cooling drag. But again, except for this limited application, it's usually best to open cowl flaps all the way when needed, and close them fully when they're not.
Cowl-flap use is critical but easy, and following a few simple steps can save you a lot of unnecessary expense. Here's how to remember, using the acronym COWL:
Checklists: Follow the appropriate ones -- before, during and after flight.
Open your cowl flaps for start-up, taxi, run-up, and for taxi-in after your clear the runway.
Watch cylinder and oil temperatures and close the cowl flaps as you level the aircraft into cruise flight. But if the engine is getting abnormally hot, or you're in a configuration where it's likely to get hotter (taxi, takeoff or climb), open the cowl flaps.
Leave the cowl flaps closed for descent and landing, open them again as you tax to parking, and close (and plug) them if possible when you hangar or tie down.
Proper cowl-flap use is an exercise in pilot procedure and temperature control. Follow your checklists, but do not be afraid to deviate from them if your engine temperatures begin to get too hot or cold. It's easy (and common) to brush over cowl flap use when first checking out in a complex airplane, and to dismiss improper cowl flap use as unimportant if you forget after the checkout. But along with good-condition engine baffles and proper fuel flow set-up, correct cowl flap use is vital to getting the best life out of your engine.
Fly safe, and have fun!