The Relationship Between Aircraft Forward Speed and Reverse Thrust: Propeller vs. Jet Engines

Introduction

Understanding the dynamics of aircraft reverse thrust is crucial for pilots, engineers, and aviation enthusiasts. Reverse thrust is a mechanism employed to assist in deceleration and maneuvering, particularly during takeoffs and landings. This article explores how aircraft forward speed influences reverse thrust, focusing on both propeller and jet engines. We will delve into why reverse thrust is more effective at high speeds and the unique characteristics of propeller and jet reverse thrust mechanisms.

The Influence of Forward Speed on Reverse Thrust

Contrary to the common misconceptions, reverse thrust is not linear in relation to forward speed. As the aircraft gains higher forward velocity, the reverse thrust increases. This phenomenon can be explained by the increased interaction between the onrushing airstream and the engines, akin to pushing against water versus pushing against solid ground. The faster the aircraft moves, the more powerful the reverse thrust becomes.

Effectiveness of Reverse Thrust at Different Speeds

One of the most compelling examples illustrating this relationship is the ATR 72 aircraft. The table below, extracted from ATR 72 documentation, showcases the differences in reverse thrust effectiveness at various speeds. It is evident that the reverse thrust is significantly higher at high speeds compared to low speeds.

Table: Propeller Thrust at Different Speeds

This table indicates that at 0 kt, the reverse thrust is -610 daN or 620 kgf per engine, while at 100 kt, it increases to 2900 daN or 2950 kgf per engine. This significant increase in reverse thrust at higher speeds provides a clear demonstration of the relationship between forward speed and reverse thrust.

Comparative Analysis: Propeller vs. Jet Reverse Thrust

Propeller reverse thrust, also known as Beta thrust, is a unique mechanism where the propeller blades simply change pitch to push air forward instead of backward. This change is instantaneous, making it independent of the aircraft’s forward speed. Unlike jet engines, which use shutters to redirect thrust, propeller reverse thrust is more effective and straightforward.

Jet engines, on the other hand, employ a more complex mechanism for reverse thrust. Shutter devices redirect the engine’s thrust to a forward angle, typically around 30 degrees. While this approach is effective, the redirection results in a significant loss of power compared to the straight-line reverse thrust of propellers. Additionally, not all jet engines are equipped with reverse thrust capabilities, further limiting their effectiveness.

Factors Influencing Reverse Thrust Generation

The speed of the airplane affects reverse thrust generation in different ways for propellers and jets. For propeller aircraft, the thrust generated is primarily influenced by the change in pitch of the blades. These changes occur instantly, regardless of the aircraft's forward speed. In contrast, jet engines’ reverse thrust is more dependent on the activation of specific mechanisms, which are less responsive to changes in speed.

Conclusion

In summary, the correlation between aircraft forward speed and reverse thrust is complex yet fascinating. Higher forward speeds magnify the effectiveness of reverse thrust, primarily due to increased interaction between the onrushing airstream and the engines. Propeller reverse thrust, driven by blade pitch changes, is notably more effective than jet reverse thrust. Understanding these principles is essential for optimizing aircraft performance and safety, especially during critical phases of flight.