The Mystery Behind Airplane Engine Placement: Why Aren't They in the Middle?

When pondering the design of modern aircraft, one question often arises: why are airplane engines not placed in the middle? The configuration of airplane engines is a complex intersection of aerodynamics, weight distribution, and practical considerations. This article delves into the reasons behind this design choice, incorporating examples and explanations to provide a comprehensive understanding.

The Current State of Airplane Engines

The only flat aircraft engines still in widespread use are Continental, Franklin, Lycoming, and Rotax, installed in small general aviation (GA) airplanes. These engines range from 40 to 400 horsepower, with a typical 2-seat trainer airplane using a Continental O-200 or Rotax 912 engine, producing around 100 horsepower. The real limitation is air-cooling, making it difficult to route sufficient cooling air to the rear cylinders on engines like the 8-cylinder Lycoming O-720, which produce about 400 horsepower. Beyond this limit, turbo-props are a reliable alternative, but come with a higher cost.

Other Engine Configurations and Considerations

A few small helicopters, such as the Robinson, still use air-cooled, horizontally-opposed engines, but they also incorporate fans to move cooling air during hovering. The Robinson brand also offers a turbine-engine version of their R-44 4-seater helicopter, primarily catering to regions where 100LL gasoline is difficult to obtain, such as Romania or Africa. For military operators, engines that burn jet fuel are preferred since they only need to stock one type of fuel across all their aircraft. Notably, diesel-powered trucks and ships can also use jet fuel, known as kerosene.

Jet Engines and Ground Clearance

All jet engines are designed to be round and enclosed within shrouds or nacelles. In the newer B-737-300 and subsequent models, the nacelles are flattened on the bottom to allow for ground clearance.

Aerodynamics and Engine Placement

The fast-moving air over the wing of a plane causes a difference in pressure on the airfoil due to Bernoulli's principle. This principle explains why the top of the wing is under less pressure than the bottom, creating lift. Given this information, one might wonder why engines aren't placed in front of the wings, as the exhaust from the engine could help during takeoff. However, placing the engine in front would disrupt the airflow over the wings, significantly compromising lift generation, and could lead to performance issues.

In the context of single-engine GA planes, many do have engines in the middle. For example, the Cessna 152 and Piper aircraft feature their engines centrally located. However, for multi-engine planes, such as commercial airliners, the middle section is reserved for passengers and cargo, ensuring optimal weight distribution and space utilization.

Conclusion

The design of airplane engines is a fine balance between various factors, including aerodynamics, weight distribution, and practicality. While the current design of placing engines in certain positions may seem counterintuitive at first, it ensures the most efficient and safe operation of aircraft.