Why Arent All Pieces of a Jet Fighter Conformal?

Introduction

Overview of Conformal Designs and Splitter Plates

In the world of jet fighters, the term 'conformal' has gained significant traction, especially in discussions about design efficiency and aerodynamics. Conformal designs are those where external components, such as fuel tanks or weapon stores, are integrated into the wing or fuselage, reducing the vehicle's overall drag and improving its aerodynamic efficiency. However, this isn't a one-size-fits-all solution. One of the first instances of conformal use was in the F-15E Strike Eagle, where it was employed to extend range, reduce air drag, and increase payload options. Over time, conformal designs have become integral to various fighter jets, including the F-16, Rafale, and Eurofighter, among others. These designs present an aerodynamic challenge, requiring extensive time and money to study any configuration, particularly at AoA 0.

The Role of Boundary Layer Splitter Plates

A particular component that significantly impacts the performance of jet fighter engines is the boundary layer splitter plate. As detailed by Martin Smith, its primary function is to maximize engine performance by excluding the intake from the boundary layer, ensuring a steady and high-quality airflow. The boundary layer is the layer of air closest to the surface of the wing or fuselage, characterized by slower and turbulent air. Intake design on jet fighters often includes a splitter plate to route this slower air away from the intake, thereby preventing the formation of a shock wave within the boundary layer.

The splitter plate not only helps in boundary layer rejection but also in the formation of a shock wave that is less affected by the body of the aircraft. This is particularly crucial for supersonic flight. This design ensures that the air entering the intake is at a high enough speed and quality, which is essential for optimal engine performance.

Blending External Components with Conformal Designs

However, despite the advantages of conformal designs, not all elements of a jet fighter can or should be conformal. One must consider the inherent constraints of fighter jets, such as the thin wings needed for high-speed performance. These thin wings typically have limited internal volume for fuel and weapons. As a result, many legacy fighter jets rely on external stores, like fuel tanks and weapons, to maintain operational flexibility. These external components, while aerodynamically inefficient, serve critical missions and are a necessity in certain scenarios. For example, the Strike Eagle has external fuel tanks and weapons stores that, though aerodynamically inefficient, are essential for missions requiring a larger payload.

Modern advances in design and manufacturing methods, coupled with the growing demand for stealth, have led to the reduction of reliance on external stores. Integrating these components into the aircraft through conformal designs has become a key focus. This trend is evident in projects like the So You Are Not Naive, which aims to minimize external storage for improved stealth and aerodynamic performance.

The F-35, developed with conformal designs, represents a step in this direction. Its diverterless inlet, also known as a Diverterless Supersonic Inlet (DSI), demonstrates the feasibility and benefits of fully conformal designs. The DSI does away with traditional splitter plates, further optimizing the intake design for both performance and stealth.

Conclusion

While conformal designs offer significant advantages in terms of aerodynamic efficiency and stealth, they aren't a universal solution. The inclusion of external components like fuel tanks and weapon stores remains necessary in many cases due to design constraints. However, these designs continue to evolve, and projects like the F-35 illustrate the progress made in integrating these elements conformally, paving the way for a more efficient and effective future in fighter jet design.

References and Further Reading

FAST HISTORY: LOCKHEEDS DIVERTERLESS SUPERSONIC INLET TESTBED F-16
A CFD Investigation of a Generic Bump and its Application to a Diverterless Supersonic Inlet