2.0 Aircraft Flutter
2.1 Phenomenon of Flutter
Aircraft flutter is an aeroelastic phenomenon, which simply means it combines aerodynamic effects with elastic effects. Because all bodies are elastic, to some extent, they will deform some amount as external forces are applied. Airplanes are designed to be as light as possible to maximize their performance characteristics, which implies they are just as elastic, if not more so, than most structures. Due to this attribute of elasticity, most of an aircraft’s lifting surfaces are extremely susceptible to deformation (Stearman).
To illustrate this, imagine a wing undergoing an aerodynamic force. The wing would deform a certain amount, producing a change in the aerodynamic forces applied to it. These “new” forces would then in turn lead to a different deformation. This feedback process may result in a self-excited oscillation as energy is absorbed from the free flowing airstream (Stearman). This phenomenon is defined as flutter.
If the flutter is defined as divergent, these forces would lead to greater and greater deformations until the wing or other lifting surface fails (Stearman). Flutter is a potentially dangerous condition and can easily damage or destroy a poorly designed aircraft.
2.1 History of Flutter
Aeroelastic effects surfaced as early as nine days prior to the famous Wright Brothers first flight at Kitty Hawk, North Carolina. After Professor Samuel P. Langley crashed for the second time while trying to fly for the first time in the history of the world, he was trying to grasp why it had happened. He concluded that the forces applied to the rear wing and tail caused it to twist in a torsional divergence that collapsed the structure, initiating his descent into the Potomac River. Since then, there have been many incidences involving flutter from World War I, to the Concorde rudder failure in 1992, to the present. Extensive research in many countries has been conducted over this span of time to understand and resolve this aeroelastic dilemma (Stearman).
2.2 Flutter Speed and Frequency
The subject of aircraft flutter involves two very important aspects: the speed and frequency at which flutter occurs. These are defined to be the flutter speed and the flutter frequency. Flutter speed is the minimum velocity at which flutter occurs, and flutter frequency is the frequency at which flutter occurs. These characteristics change as do the characteristics of the aircraft such as the center of gravity, on which aeroelastic effects are extremely dependent.
2.3 Symmetry of Flutter
Flutter can occur in one of two main mode shapes with respect to the axis parallel to the fuselage. The two types are symmetric flutter and anti-symmetric flutter. Symmetric flutter is defined as the deformation and oscillation of components of an aircraft, which bend in the same direction. This can be easily visualized by imagining a bird flapping its wings. When one wing flaps upwards, the other one does as well. Anti-symmetric flutter, on the other hand, is characterized by opposite directions of deformation. When one wing deforms upwards, the other deforms downwards. These oscillate at relatively high frequencies and can be hard to detect when the amplitude of the oscillations is small.
2.4 Stability
Wing oscillation can exemplify one of three system stabilities: stable, marginally stable, or unstable. For the aircraft to be stable, the aircraft must have the ability to dissipate the energy absorbed from the free stream, thus diminishing the amplitude of the oscillation or damping the vibration (Gross et. al., 8). If the aircraft exhibits marginal stability regarding oscillations, the amplitude of the oscillations will remain constant. If the system is unstable, the oscillations will increase without bound resulting in eventual failure of the wing or airframe. Flutter will occur.
Figure 2.1: Marginal Stability (a), Stability (b), and Instability (c)
The horizontal axis in the figures represents the time scale and the vertical axis represents the amplitude of the oscillations the lifting surface undergoes. Figure 2.1a is a representation of marginal stability. Note the constant amplitude of the oscillations. The stable flutter is represented in Figure 2.1b as the oscillations are damped out or decrease with time. In the unstable case (Figure 2.1c), the oscillations grow without bound. This event would lead to the eventual failure and destruction of the aircraft. Our primary objectives are to see at which speed, given our new changes to the wing tanks, will cause flutter to occur, implying an unstable process.