H ANDS ON FIREWALL FORWARD
Always consider the effect of high heat or fire on engine cooling system components and use materials approved for aircraft use.
Aircraft Engine Cooling A baffling situation BY MIKE BERRY
AN INTERNAL COMBUSTION ENGINE is a heat machine, converting chemical energy into mechanical energy at the crankshaft. Since air is used as the cooling medium for 99 percent of reciprocating aircraft engines, this transfer of heat must be carefully controlled to permit reliable, long-term engine operation. Air cooling (as opposed to liquid cooling in automotive applications) is the preferred heat transfer method in aircraft because it saves weight. It is important to ensure that your cooling system is up to the task to ensure reliability and efficiency of your powerplant. Aircraft engine cooling systems consist of the propeller, spinner, engine cowling, engine baffles, cylinders and engine case, exhaust systems, oil coolers, and possibly augmenter tubes and cowl flaps. All of these components are designed as a complete system to remove excessive heat from the engine. Excessive heat is undesirable for three principal reasons: it affects the behavior of the combustion of the fuel/air charge; it weakens and shortens the life of the engine, components, and accessories; and it impairs lubrication. If the temperature inside the engine cylinder is too great, the fuel/air mixture will be preheated and combustion can occur before the desired time. Since premature combustion causes detonation, knocking, and other undesirable conditions, there must be a way to eliminate heat before it causes damage. In a typical aircraft powerplant installation more than half of the heat goes out the exhaust while the remainder is absorbed by the engine. The engine oil picks up part of the heat absorbed by the engine and transfers it through the oil cooler to the outside air, with the engine cooling system taking care of the rest, dissipating heat through the cylinder
96 Sport Aviation May 2011
fins and engine case. An important component of the cooling of an aircraft engine is that of airflow. If the aircraft is stationary or taxiing, there is little or no airflow to carry away the heat so the cooling system will be ineffective. SYSTEM COMPONENTS
A thorough understanding of the system components and the relationships that each share is necessary to properly construct, inspect, and maintain the proper cooling and performance of your engine. As a builder, consider the design of your aircraft and the relationship among cooling system components before making changes or modifications. When maintaining a certified aircraft, it must be maintained in the original and unmodified condition according to acceptable and approved aviation practices or in accordance with a supplemental type certificate or field approval. Any modifications or deficiencies in the system most likely will impact the operation and performance of your aircraft, and errors or oversights can significantly impact your bank account.
PHOTOGRAPHY BY MIKE BERRY
H ANDS ON FIREWALL FORWARD
PROPELLERS AND SPINNERS
These items are not often thought of as components of the cooling system, but they most certainly are. Propellers and spinners must allow smooth airflow to the engine cowling inlet for proper cooling. Substitutions and modifications that disrupt airflow to the cowling inlets can cause insufficient airflow over and through the engine and reduce overall aircraft performance. Operation without a propeller spinner can also have a pronounced effect on aircraft performance and engine cooling.
Modern aircraft cowling design provides for reduced drag and efficient engine cooling.
from engine cooling air. While this does improve aircraft performance, it also has an effect of reducing available engine cooling air. With proper operation on the ground and good individual cylinder instrumentation, engine damage can be avoided. ENGINE BAFFLES
Engine cooling baffles serve as “air dams” to direct air over and around the cylinders for proper cooling. These baffles are frequently not given the necessary attention during construction, maintenance, and inspections because minor deficiencies seem to be insignificant, yet are so common. Not only do engine baffles suffer because of engine vibration, but also they are subjected to the demand of heavy air loads and turbulence associated with the airflow into and around the engine cowling, causing constant flexing of the metal and eventual fatigue. Metal engine baffles are constructed of moderately thin aluminum that cracks and deteriorates over time. Flexible baffling material has to withstand the air loads, vibration, and heat associated with the engine environment as well as frequent removal and replacement of engine cowlings. Of particular importance, anywhere in the engine compartment the materials used must be able to not only withstand heat, but also resist burning as required by FAA
ENGINE COWLINGS
Cowlings are another area that not only affect cooling of the engine but also the performance of the aircraft. Missing screws and fasteners, cracks, mismatched panels, missing chafe strips or seals, and loose or cracked access doors or panels all should be repaired. Consider the fact that if a cowling is not sufficiently rigid, is cracked or broken, or is not properly attached during the operation of the aircraft, the cowling can “grow” or move, which allows cooling air to escape without passing over the cylinders and also disturbs the airflow around the fuselage. Next time the cowling is off your aircraft, take a look at it from the inside for cracks, broken parts, and evidence of chafing or movement. Money is well spent on repairs before the situation becomes serious enough that cylinders crack or oil temperatures rise above redline. There are several cowling fasteners available, and not all are of the same strength and ease of use. Each type of fastener must be used in conjunction with receptacles that may be specific to material thickness. Engine cowling design has seen some improvements in recent High performance cowling must be matched with appropriate propeller and years to increase aircraft performance by reducing drag spinner design for sufficient engine cooling.
98 Sport Aviation May 2011
PHOTOGRAPHY BY MIKE BERRY
With their high operating temperatures, turbochargers often require direct cooling via flexible tubing.
regulations. Do not accept or install any material in the engine compartment that is not specifically approved for use in the aircraft engine environment. Black rubber material such as pieces of tire tube improvised as seals, for example, is not only unapproved but also dangerous in case of a fire. The engine baffling is the heart of the cooling system and must be maintained to get the proper cooling for your engine. It has been stated by one engine manufacturer that a void the size of a quarter in an engine baffle can increase the oil temperature during normal aircraft operation as much as 20 degrees Fahrenheit. That is a large temperature increase for such a small hole; consider what may happen when there are multiple air leaks. When you have some spare time, get a helper to hold a strong light inside the lower cowling while you look into the forward cowling air inlet during a dark day or inside a dimly lit hangar to see if you can observe light leaks around where the flexible seals meet with the cowling. When installing the cowling make sure the flexible seals are positioned correctly to allow the air stream to pressure the seals tight against the inner cowling. Some aircraft have a separate air inlet for the oil cooler. This inlet, opening in the cowling, or opening in the engine baffling must also be properly sealed so that all the air intended to
go through the oil cooler does its job. This must not be blocked in any way, and the air must be free to exit without obstructions. Ducting, flexible tubing, seals, and the metal, composite, or fiberglass structure supporting the oil cooler must all be intact and not cracked or damaged. Inner cylinder baffles are another item that is often overlooked in routine inspections and maintenance and deserve more attention, as these do wear out. These baffles, associated ducting, and hardware must be complete and securely attached with the correct hardware and not cracked or broken. The correct
hardware for engine baffles anywhere does not include safety wire; use the proper springs, threaded rods, nuts, and clips as specified in the aircraft or engine parts manual. When assembling or repairing think about the consequences of a lost screw, nut, or washer. Where did it go—into the engine air intake or lodged between cylinder fins? Will a broken threaded rod impair the use of the engine throttle control? This is why special attention must be paid to not only using the correct approved aircraft hardware but also ensuring that it is installed properly. Various engine accessories such as fuel pumps and magnetos require cooling, which is typically provided from ducting that is part of the engine baffle system. The size, shape, and material that the ducting is made of are important to consider as slight deviations from a design may have a significant impact on overall engine cooling. While engine cooling can be baffling at times, it is important to understand the complete system and how the components work together to provide sufficient cooling while not compromising aircraft performance. Making modifications to the original design and not making regular inspections and repairs as necessary can have an adverse effect on overall powerplant longevity and aircraft performance. Mike Berry, EAA 261353, is a retired airline captain with 16,600 hours of flight time and 177 aircraft types flown. He is also an A&P-IA and owned an aircraft repair shop for several years.
A hole the size of a quarter in engine baffling can raise operating temperature by as much as 20 degrees Fahrenheit.
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