home | capabilities | question index | engine services | airframe services | logbook research | inspections | articles | aircraft tool rental | experimental-projects | clearance parts stock | about us

engine cooling

    most aircraft engines are cooled by air, oil, and fuel; some other aircraft engines are water cooled by use of air-cooled radiators.  this article will cover the air-cooled engines.  

basic principles of air cooling

the majority of aircraft piston engine cooling is done by air.  the aircraft engine cowling  is equipped with air inlets that allow the air to be directed into the engine compartment.  on a standard design aircraft (with engine in front) the propeller and the relative wind drive air into the inlets.  in cruise, the relative wind does most of the work of pushing air through the inlet.  when the aircraft is flying the air entering the inlets is denser due to the ram action of flight.  this ambient air creates a high pressure area inside the engine cowling than the static air around the aircraft.  there are two types of air cooling systems on most air-cooled piston engine general aviation aircraft; downdraft and up draft. the "downdraft" cooled engine is cooled when air from the high pressure area is forced down between the cylinders to a low pressure area.  the air inlets are typically behind and above the propeller center on the downdraft cooled engine and the oulet(s) are generally located on the bottom of the engine cowling.   when the high pressure air below the engine is forced up between the cylinders to the low pressure area the engine is an "updraft" type; the air inlet(s) are typically behind and below the propeller center on the updraft engine.  the majority of the heated exit air is then turned and directed out the bottom of the cowling, additionally louvers in the upper side of cowling allow thermo siphoning of cooling air after shut down of engine.  both of these systems function the same and they both require baffling. baffling constrains the high pressure air directing it through cylinder and head cooling fins.   the aluminum baffles typically box in the high pressure air.  the flexible baffle seals (usually a type of neoprene rubber or silicone rubber) are normally incorporated to seal off  prevent excess air  leakage and still allow for considerable engine movement.  these baffle seals may be mounted on either the perimeter baffles or cowling.  

     most cooling problems are due to a lack of high pressure from old worn baffle seals, but not always. some earlier popular aircraft had design flaws in baffling that left gaping holes causing lower cooling pressure differential. some of these installation  require considerable ingenuity and time to correct. however, there is still more to engine cooling. engine cooling is also dependent on the low pressure area.

the low pressure area is due to outlet gaps (intentional) at the bottom of cowling. the faster moving air passing by the bottom of the cowling drafts the slower moving air from inside the cowling creating the low pressure area. some aircraft are equipped with cowl flaps that regulate how much air is drafted from inside the cowling.  the cowl flaps varying in position from open to closed depending on the angle of attack and engine power settings.  the cowl flaps almost always protrude into the relative wind.  there are a couple of considerations concerning  the low pressure area. in most installations, the total cooling air exit(s) at the bottom of the cowl needs to be significantly larger in surface area than the air inlet(s) due to the fact that heated air expands.  to generate enough low air pressure at slow airspeeds, cowling exit lips or cowl flaps are usually added to the outlet.   if there is not enough draft, the aircraft will experience insufficient cooling and elevated engine temperatures.  some times the draft is sufficient but engine installation obstructions or a sharp lower firewall edge will cause turbulence slowing the movement of the air out of the low pressure area.   the combination of high air pressure and low air pressure creates air flow;  the airflow is increased when the relative difference between the high and low air pressures is increased. any increase in airflow results in an increase in cooling.  

    to assist with the cooling, engine manufacturers cast or machine large cooling  fins on the cylinders heads and to a lesser amount on the cylinder barrels. the larger more dense fins are located at the head of the cylinder, since cylinder heads concentrate the most of the engine heat generated.  a large part of the engine combustion cooling is accomplished with head cooling fins; since the head and cooling fin cast is usually made of an aluminum alloy that readily transfers heat.   to direct airflow around the cylinders fins, wrap around baffles are incorporated.  the outlet area between the baffles is critical in dimension to obtain optimum heat transfer to cooling air.  the inter-cylinder baffles are typically supplied with lycoming engines, but generally are supplied by the airframe manufacture for continental engines.  additional wrap around baffles direct the air around the forward and aft cylinders to balance the airflow with the inter-cylinder baffles.  

see aircraft air-cooling differential pressure check

oil cooling

     the airframe manufacturer typically selects the oil cooler, determines the location and designs the mounting  for lycoming engine installations.  these installations usually include oil hoses that direct engine oil from the rear of the engine to the oil cooler and back.  continental engine oil coolers are typically mounted to the engine and installed by the engine manufacturer without use of external plumbing.  some smaller continental engines have no oil coolers, but use an air inlet below the propeller to direct cooling air along the engine oil sump by use of a baffle trough.  in all cases the oil serves as both lubricant and liquid coolant.  to control the oil temperature from getting too cold or too hot, a veratherm or thermostatic valve is incorporated in the engine or oil cooler mounting base.   these engine oil thermostats typically close off  an internal engine oil passage and direct the oil through the oil cooler when reaching desired oil temperature.  some engines have a bypass plunger instead of a thermostat; on these engines the oil cooler is bypassed when the oil is too cold and too slow to move.   the location of the oil cooler varies from the front to the back of the engines, some are mounting on the cowling or the firewall.  .  

cooling by fuel

    generally when engine horse power approaches 200 or more additional engine cooling is accomplished for take off and climb power by supplying additional fuel to the engine.  the excess fuel, more than is needed for proper combustion, vaporizes (evaporates) causing a cooling effect. this added fuel enriches the fuel/air mixture typically lowers the exhaust gas temperature ( egt) from about 150  to 200 degrees (f) rich of peak.   the result is less engine heat generated and therefore less heat to air and oil cool.   cooling the engine by fuel comes at a cost, since the additional fuel is not free.  and cooling by fuel typically only occurs at full throttle positions for takeoff and climb operations; as a result most aircraft require full throttle to take off.   the additional fuel is typically activated by a throttle - mixture link on fuel injected systems and an additional fuel valve in the carburetors.  most smaller engines do not use fuel enrichment for take off and climb. . 

additional cooling considerations

    flying at slower speeds or at high altitudes for turbocharged engine require more perfect cooling than faster moving aircraft with the same engine power.   as mentioned earlier the air directed into the inlets is dependent upon the speed of the aircraft.  therefore, tow aircraft, floatplanes, fixed gear aircraft and other slower high power aircraft often suffer from more cooling problems.    

proper engine temperatures

typical continental engine temperatures should be: maximum oil temp 240�f, oil temperature 150-200�f, and maximum cylinder head temperature (cht) 460�f.  suggested lycoming engines temperatures are: cht below 435�f in high performance cruise, cht below 400�f in normal cruise, maximum cht 500�f, cht temperatures 150�f-435�f, maximum oil temperature 245�f, oil temperature 165�f-200�f.  check engine and aircraft type certificate data sheets for  exact limits on your particular engine / airframe combination.  hopefully your engine temperature indicators are ranged marked correctly.  

correcting engine cooling problems

    a hot running engine has a shorter life-span due to elevated operating temperatures; this can be a tricky problem to fix on most aircraft. 

common air cooling problems

there are a number of things that can lower cooling air differential pressure and flow from engines:  

1.  winterization components not removed for warm weather operations

2.  leaks between the cowling and air inlet / filter smaller carbureted engines

3. cooling pressure leakage from baffling discrepancies due to wear, poor fit, out of position

4.  inadequate baffle seals and baffles.  gaps in baffles.  worn or short baffle seals.

5.  air leak around alternator, starter and ring gear (usually earlier lycoming installations)   

6.  inter-cylinder baffles that are not installed properly, gaps at bottom cylinder head fins, gaps at engine case,  weak springs or insufficient support allow baffles to sag under air pressure.  

7.  wrap around baffles improperly fitted and or are not secured properly

8.  insufficient pressure differential across cylinder fins

9.  insufficient draft and low pressure inside of cowling

10.  insufficient cooling air inlet or outlet(s) areas

11.  turbulence near or at cooling air inlets / outet(s) 

12.  cabin fresh air systems sourced from engine cooling high pressure area

13.  engine combustion air intakes sourced from engine cooling high pressure area

14.  incorrectly rigged nose landing gear doors affecting cooling pressure areas

15.  missing nose gear to cowl fairings

16.  excess lean mixture, induction leaks

17.  advanced or retarded ignition timing.  retarded timing can be caused by weak magneto impulse spring

common oil cooling problems

1.  malfunctioning veratherm or thermostatic valve, poor seat condition

2.  bypass valve/spring broke

3.  kinked or restricted oil hoses

3.  internal restricted oil cooler, incorrect size

4.  bent / smashed oil cooler fins, insufficient cooling air  

5.  oil cooler heat deflector missing or misplaced

6.  failure to remove winterization baffle for warm weather operation

7.  insufficient air draft through oil cooler

common fuel problems

1.  overly aggressive engine mixture leaning procedures

2.  not using full throttle for take off on normally aspirated engines

3.  not using full throttle for take off on turbocharged engines with automatic waste-gate controls

4.  using improper fuel for engine

5.  clogged injectors causing a lean air/fuel mixture.

6.  restricted or not fully opening distributor valve / flow divider injected engine, vent port clogged 

what to avoid

1.  shock cooling the engine.  lycoming recommends that cylinder heat temperatures should not change in excess of 50�f per minute.  

airframe specific cooling problems

cessna 172n 1977-1979 service kits are still available at ten times the original cost.  the information is included in se79-13.  the cooling improvements are service kit sk172-55 and sk172-56.  sk172-55 is an oil cooler relocation kit and can be purchased for $1200.  sk172-56 is a kit to put an improved lip on the bottom of the cowl to improve airflow through the cowling.  these kits only apply to sn: 172675585 - 17271953 and f17201515 - f17201864

m20e a kit for installing an outlet duct on the bottom of the cowl.  the kit is no longer available, but a majority of the parts for the kit can be ordered.  the kit is m20-182-1.  the kit works on other models, such as the m20c, to to improve cooling.  a little extra work has to be done on fitting it to these other models.  the modifications make a significant improvement.

maule aircraft have some cooling improvement options.  for m-5's, a duct and cowl lip can be purchased from maule to improve cooling.  early m-4's are mentioned in service letter sl12. m-4-220c are mentioned in service letter sl26.  m-5 edo floatplanes are mentioned in sl28. 

links to other cooling pages on this site

aircraft air-cooling differential pressure check

have a small aircraft problem you can't solve?  click below to send the question.

home | capabilities | question index | engine services | airframe services | logbook research | inspections | articles | aircraft tool rental | experimental-projects | clearance parts stock | about us

send mail to tim@littleflyers.com with questions or comments about this web site.
copyright � 2000 little flyers
last modified: february 26, 2007

oil analysiscompression checksengine top overhaulengine overhaulengine coolingengine guage accuracy checksengine adjustmentfuel injector service

jordan 11 legend bluelegend blue 11slegend blue 11slouis vuitton outletblack infrared 6sjordan 11beats by dre outletlegend blue 11slouis vuitton outletblack infrared 6scheap oakley sunglassescoach outlet onlinelegend blue 11sjordan 11 legend bluebeats by dre cheapjordan 11 legend bluejordan 6 black infraredblack infrared 6sjordan 11 legend bluelegend blue 11s