What Is An Aerobatic Engine?

A Flyer reader wrote to express interest in a Lycoming IO-360 engine. He went on to say that the engine would be used in an aircraft capable or unlimited aerobatics. A statement like this indicates a need for explanation of the differences between the standard Lycoming engine and the aerobatic Lycoming engine. Aerobatic flight with a non-aerobatic engine could result in engine stoppage from either fuel or oil starvation.

It should first be explained that unlimited aerobatic flight implies that the aircraft may be flown in any attitude with no limitations. Although an aircraft may have excellent aerobatic capability, every aircraft and engine does have limitations which must not be exceeded.

Any engine which employs a float type carburetor for fuel metering is immediately eliminated from use in a fully aerobatic aircraft. Inverted flight for more than a few seconds would cause the carburetor to stop metering fuel and the engine to stop running. While carbureted engines are used in some aircraft with limited aerobatic capability, only positive G maneuvers and very brief periods of inverted flight are possible.

To operate correctly, an engine must have fuel which is properly metered in proportion to the air entering the engine induction system. The fuel injector measures air flow and meters fuel to the inlet ports of each cylinder. Unlike the carburetor, a fuel injector is not affected by unusual aircraft attitudes. Therefore, all Lycoming engines that are designed for aerobatic flight are equipped with a fuel injector.

Delivery of metered fuel to the combustion chamber is not the only challenge addressed in designing an aerobatic aircraft engine. It is also necessary to provide lubricating oil to many points in an operating engine regardless of the aircraft attitude. Two different methods have been used to provide oil for aerobatic engines manufactured by Textron Lycoming.

The flat, opposed cylinder aerobatic engines first offered by Lycoming were designated AIO-320 or AIO-360. These engines were the dry sump type with appropriate oil inlet and outlet connections as well as two crankcase breather connections. Necessary lines and an external oil tank with a revolving pickup capable of reaching oil in almost any aircraft attitude were then supplied by the aircraft manufacturer. This type of installation provided aerobatic capability, but it was complicated enough to be very expensive. A simpler, more universally usable system was needed.

Most Lycoming engines are termed "wet sump" engines because oil is stored internally in a sump at the bottom of the crankcase. When the engine is inverted, the oil will be in the top of the crankcase rather than in the oil sump. To maintain a continuous flow of oil during inverted flight, an oil pick-up line must be provided near the top of the engine as well as in the oil sump. Lycoming aerobatic engines carrying an AEIO designation use inverted oil system hardware to adapt oil pickup lines at the top and bottom of the wet sump engine.

This inverted oil system comprises two major components: the oil valve and the oil separator. Several other items of hardware adapt the system to the Lycoming engine so that oil is available to the oil pump in either the upright or inverted position. These hardware items include a standpipe in the sump which acts as the engine breather during inverted flight, a special adapter or plug at the oil sump suction screen, and other hoses and fittings.

In addition to the inverted Oil system, Lycoming makes other engine modifications to adapt standard engine models to aerobatic use. Some models of the AEIO-540 engine have a baffle added in the oil sump to eliminate oil loss through the oil separator. Also the flow of oil to the oil pickup in the accessory case is limited in the inverted position. To improve this oil flow, holes are machined in the upper rear wall of the crankcase.

With these changes completed, the engine is capable of inverted flight in addition to normal upright flight. Because the oil pick up points are at the top and bottom of the engine, knife-edge flight or flight at very high up or down pitch angles have some limitations; these limitations do not prevent engines from being used in aircraft which perform all the maneuvers required for international aerobatic competition. Engines built with the inverted oil system and incorporating the other modifications discussed earlier are certified by the FAA as aerobatic engines.

Aerobatic engines subjected to the exceedingly stressful maneuvers developed in recent years are also limited by possible damage to the crankshaft flange. Textron Lycoming Service Bulletin No. 465 requires periodic inspections of all crankshafts installed in aircraft that are used for aerobatics

The meanings of the letters and numbers in the Lycoming engine designation are fully explained elsewhere in this publication, but the AE part of the AEIO indicates "aerobatic engine." Lycoming is currently producing AEIO-320, AEIO-360 and AEIO-540 aerobatic engines which range from 150 to 300 horsepower. One of these models should be installed in a general aviation aircraft which is designed for aerobatic flight.