Aeroelastic Stability of Turboprop Aircraft: Whirl Flutter

Abstract

This chapter is focused on a specific type of dynamic aeroelastic stability phenomenonwhirl flutter. Whirl flutter is caused by the effect of rotating parts of a turboprop power plant (propeller and, gas turbine engine rotor). The chapter presents fundamental facts regarding the whirl flutter phenomenon, including a historical overview and information regarding the occurrence of whirl flutter in aerospace practice. After that, the physical principles of whirl flutter are explained using a simple mechanical system with two degrees of freedom. Next, an analytical solution to determine the aerodynamic forces caused by the gyroscopic motion on each of the propeller blades is provided and the influences of the main structural parameters on the whirl flutter stability are discussed. The second, practical part is focused on the experimental research of the whirl flutter phenomenon and on the certification-related issues. The methodology of certification according to the FAR/CS 23 regulation standard is demonstrated on the example of a twin wing mounted tractor engine commuter aircraft.

Nonlinear Analysis and Bifurcation Characteristics of Whirl Flutter in Unmanned Aerial Systems

“…As illustrated in Figure 4c, the first four cubic-and fifth-order cases resemble mainly hardening behaviors. However, the last case, 0.5ψ − 20ψ 3 + 500ψ 5 , has initial softening between 0 • and 5 • , after which it begins to exhibit hardening. This case within the cubic-and fifth-order investigation of this system introduces a new parameter to the study by making the softening cubic term dominate the given angles.…”

“…A study based on the linear results is considered for the nonlinear first case of the linear-cubic-fifth-order configuration, shown in green in Figure 7, 0.5ϕ − 0.1ϕ 3 + 2500ϕ 5 . It should be noted that the parameters investigated for this particular case were run with both increasing and decreasing velocities to discover the effects of each parameter on the subcritical or supercritical instability of the system under investigation.…”

“…Further, the subcritical nonlinearity is more prominent when the number of blades is decreased, and the flutter speed is higher. Next, the impacts of the system's properties and number of blades on the system's response and instability are investigated when considering the fifth case of the linear-cubic-fifth representation, shown in pink in Figure 5, 0.5𝜑 − 20𝜑 3 + 2500𝜑 5 . It should be mentioned that this scenario showed subcritical Hopf bifurcation in Figure 5.…”

“…Whirl flutter can cause propellers and the surrounding structures to fail and therefore must be accurately predicted. Whirl flutter is the result of a coupling of the structure of the wing, the aerodynamic forces and moments on the wing and the rotor, and the gyroscopic effect of the rotor [5,6]. The rigid nature of the rotors in these VTOL systems introduces additional couplings not seen in traditional helicopters [7].…”

Bifurcation Analysis and Sticking Phenomenon for Unmanned Rotor-Nacelle Systems with the Presence of Multi-Segmented Structural Nonlinearity

“…The onset of whirl flutter has been shown to damage surrounding structures of the rotor-nacelle, especially the mount location and wing depending on the mounting position. The onset of whirl flutter results from a coupling between the structure wing/mounting material, the aerodynamic forces and moments on the rotor, and the gyroscopic effect of the rotor [8][9][10]. Depending on the aircraft type and the rotornacelle's location, the system may respond differently when aeroelastic instability occurs.…”