Title: Aeroelasticity: The "Dancing Mystery" of Military Aircraft Design
Recently, there has been a trend of "softness and rigidity" on the stage of military aircraft design, and behind this trend, there is a puzzle of aeroelasticity that is a headache for designers. In this article, we will dig deep into the profound impact of aeroelasticity on aircraft design, reveal three typical problems, and explore the high standards that modern aircraft design needs to address.
The structure of the aircraft is far from being a rigid body, and a certain elastic deformation is bound to be caused by the action of aerodynamic force, which is the aeroelastic phenomenon of the interaction between structural deformation and aerodynamic force. This design challenge was enormous, as we needed to manage elasticity to accommodate complex aerodynamic fields while maintaining structural toughness.
One of the main troubles of aeroelasticity is the divergence of structural deformation, which simply means that at a certain height and speed, the elastic deformation of the structure and the equilibrium state of aerodynamic force are out of control, resulting in monotonic divergence. The peak of deformation divergence is called the deformation divergence velocity, and once triggered, it can cause the structure to collapse and cause a flight accident. This phenomenon is reminiscent of some of the resulting disasters in history, such as the flutter crash of the British aircraft "Moth".
Under the interference of aeroelasticity, the efficiency of the control surface will be greatly reduced, and even the opposite effect will occur. For example, the deflection of the aileron may not produce the expected rolling moment, but instead cause the aileron to fail, leaving the aircraft out of the pilot's control. The risk of this kind of maneuvering backlash is a matter of great concern to designers, because the maneuverability of an aircraft in extreme situations is a matter of life and death.
Flutter is an important problem in aeroelasticity, especially when the wing vibrates, and the interaction of aerodynamic forces with elastic deformation causes the vibration to expand continuously. This self-excited vibration is often accompanied by the bending and torsional deformation of the wing, resulting in wing flexional and torsional flutter. The design must ensure that the flutter critical speed is higher than the maximum flight speed to avoid the continuous expansion of vibration leading to wing destruction.
Modern aircraft design requires that aeroelasticity and aeroservoelasticity be considered throughout all phases of aircraft design. This requires designers to not only consider the light structure, low damping, and large flexibility of the structure, but also solve the problems of aeroelasticity and aeroservo elasticity when the frequency band of the flight control system becomes wider and the authority increases.
The flight control system becomes the key to solving the problem of aeroelasticity. Modern large aircraft use aerodynamic shapes with large aspect ratios, full-time fly-by-wire flight control systems, and an increasing number of composite materials. This makes the aircraft more lightweight, but it also creates new challenges for aeroelasticity. The flight control system strives to balance the lift distribution and reduce the weight of the structure by adopting active control technologies such as load reduction, gust easing, and structural modal suppression, so as to improve the stability of the aeroelastic bomb.
In the face of aeroelastic problems, it is difficult to achieve the optimal level by simple structural optimization or flight control improvement. Designers must broaden the field of design to include multidisciplinary constraints and collaborative design. This multidisciplinary collaborative design is a major challenge for modern aircraft design and a catalyst for technological advancement.
The requirements for aeroelastic design are not only based on practical experience, but are also specified in the form of regulations. In the design of civil aircraft, the relevant aeroelastic stability requirements must be met to avoid possible risks. Especially for pneumatic servo elasticity, it is necessary to ensure the stability and amplitude margin and phase margin requirements in different flight control configurations. All of this needs to be verified by means of theoretical analysis, wind tunnel tests, ground resonance tests and flight tests.
Aeroelasticity is a daunting puzzle in the design of military aircraft, but it is also in the challenge that the power of technology is revealed. Through in-depth research and perseverance in the problem of aeroelasticity, we may be able to break through the technical bottlenecks and open up new possibilities for future aircraft design. In this arena of challenges and innovations, the future of military aircraft is quietly revealed.