Why Most Turbofans Are Tractors While Most Propfans Are Pushers

Introduction to Jet Engine Types

Modern commercial and military aircraft rely primarily on jet turbine engines, specifically axial flow turbofans. These engines consist of several key components, including the compressor, burner, and turbine sections, which must be arranged in a specific order to ensure efficient operation. In an axial flow turbofan, air enters the compressor at the front, passes through the burner section, and then is expanded through the turbine before exiting the engine.

The turbine drives the compressor via a central shaft, and in some designs, this shaft can be split into two concentric spools, with the low-pressure turbine connected to the low-pressure compressor via the inner shaft, and the high-pressure sections using the outer shaft. This design ensures that the components receive the appropriate thermal and mechanical loading, but it also poses challenges for the integration of propellers.

Engine Architecture and Propulsion Design

The arrangement of the propeller in relation to the engine is a critical consideration in aerospace design. One of the main challenges in placing a propeller at the rear of an aircraft is the management of exhaust gas temperatures. Propellers must be kept away from the high-temperature exhaust gases, which can lead to rapid wear and failure.

Conversely, placing the propeller at the front (or tractor configuration) allows for a more compact, efficient propulsion system. This placement also provides direct thrust at the aircraft's center of gravity, improving overall flight dynamics. However, it requires the use of a gearbox to match the propeller's rotational speed to the engine's output, which adds complexity and weight to the system.

Pusher Propfans and Their Rarity

The rarity of pusher propfan configurations in modern aviation is largely due to the engineering complexities involved. One notable example is the Piagio Avanti, which employed a pusher propeller configuration. However, it was not widely adopted, and the Beechcraft Model 99, which also featured a pusher propeller, faced similar challenges and was eventually retired. Most other turboprops made today are tractor configurations.

The reasons for this can be traced back to the aviation industry's preference for simpler, more reliable systems. The pusher configuration necessitates a complex gearbox or use of an electric motor, which adds both weight and cost. Additionally, the gearbox, especially in turboprop applications, can be subject to failure, leading to maintenance and safety concerns.

Conclusion

While there are exceptions, such as the Piagio Avanti, the vast majority of modern aircraft use tractor configurations for their turbofans and propfans due to their proven reliability and efficiency. The pusher configuration, while theoretically viable, faces significant engineering challenges and is more complex to manufacture and maintain.

Understanding these principles is crucial for students and professionals in the field of aerospace engineering, as well as for those interested in the history and development of aviation technology.