The cooling problem in an aircraft is getting enough air past the engine, or enough coolant through a radiator, when the engine is working hardest and the aircraft is slowest, which is the climb after take-off. Air-cooled engines need baffles that force the air through the fins and an exit for it; liquid-cooled ones need a radiator in clean airflow and a header that can purge air. Fit a temperature gauge on the hottest cylinder and watch it, because an overheated engine seizes without warning.
Part of Aerodynamics and Flight
Installing an engine’s cooling in an airframe: ducting, cowling, radiators and temperature monitoring.
The Engine’s Own Cooling Is Someone Else’s Article
How cooling fins, shrouds, radiators, pumps and thermostats work in an engine is part of the engines section; see Internal Combustion for the carrier of that topic, which lists air cooling and water cooling among its subtopics. This article is about the installation: what changes when the engine is inside a cowl on an aircraft rather than in the open on a bench or a vehicle.
Why Climb Is The Hard Case
An engine producing high power makes the most heat, and the cooling air flows because the aircraft is moving. In a climb the power is high and the airspeed is low, so cooling air is at its least. On the ground at idle or high-power run-up there is even less. A bench run in the open air says little about a cowled engine in a slow climb.
Air-Cooled Installations
- Baffles: sheet partitions that force air to flow through the fin gaps rather than around the cylinders.
- Inlet and outlet: the exit area for the hot air matters as much as the entry; a cowl that traps air cools badly.
- Pressure difference: air flows from high pressure at the front to lower pressure behind the engine. Gaps that let air leak around the baffles reduce cooling.
- Cooling drag: all the air moved through the engine costs drag, so more opening cools better and slows the aircraft; the balance is found in test.
Liquid-Cooled Installations
- Radiator position: in clean airflow, not behind the propeller hub’s turbulence, and strong enough not to be punctured by stones.
- Header and bleed: air trapped in the system forms a pocket that stops circulation.
- Hoses and clamps: rubber hoses perish with age and heat (Hoses and Tubing); carry spares.
- Weight: coolant adds weight high in the aircraft.
- Freezing and boiling: coolant must suit the weather.
Monitoring Is Part Of The Installation
Fit a temperature sensor on the hottest cylinder head (air-cooled) or coolant outlet (liquid), where the pilot can see it. Record the temperature in each ground and flight test and set a limit from the engine maker’s data or from the temperature at which this engine first misbehaved, with a margin you decide in advance. Do not guess a limit; find it on the ground with the aircraft tethered.
Common Mistakes
- Testing cooling in a cold, windy field and flying on a hot day.
- Leaving gaps in the baffles.
- A cowl with no exit.
- Ignoring oil temperature — oil carries much of the heat.
Safety
Overheating seizes the engine and starts fires
Hot coolant under pressure scalds; never open a hot radiator cap. A seized engine is a sudden stop, which at climb height is a forced landing. A hot engine beside a fuel leak is a fire.
What’s Next
- Learn next: Flight Instruments, including the engine gauges
- Combine with: Engine Mounting, with which the cooling ducts must agree