A novel approach to determining aircraft stability derivatives in extreme flight conditions

The concept of Aircraft Stability first originated in 1911 with the creation of the aircraft equations of motion. These expressions detailed the aircraft’s dynamics in flight and were formulated using parameters termed as aircraft stability derivatives. The aircraft stability derivative is a direct measure of an aircraft’s response to a disturbance. It is presented in the form of Cmα , for example, which denotes the pitching moment response to change in angle of attack. This formulation of aircraft stability was created as a linearized approximation and proved successful for aircraft of that generation. However, with the advances toward flight in more extreme regimes, it suffered as a consequence of non-linear phenomena within that envelope.

The aim of this research project is to merge the fields of aircraft stability, computational fluid dynamics (CFD) and high angle of attack aerodynamics in the investigation of aircraft stability derivatives. Employing a combination of low-speed wind tunnel experiments and sub-scale and free flight CFD simulations, the project focuses on the application of forced oscillations to a generic aircraft geometry. This aircraft is known as the Standard Dynamics Model (SDM) and was established in 1978 for the purpose of comparative dynamic wind tunnel testing.

Within this thesis, the specification of a frequency and amplitude-based methodology is explored under the premise of enhancing the stability derivative determination capability in extreme orientations. To date, this appears to have only been explored in the linear regime, based on information available in the public domain. In examining the forced oscillation methodology and the frequency effects upon the aircraft stability derivative resolution, it was found that forcing frequency played a pivotal role at high angles of attack. In the case of the Standard Dynamics Model, an oscillation of 5 Hz was required when an angle of attack of 15° was exceeded in order to accurately capture the high frequency content associated with high angle of attack phenomena. The recommendation is to perform frequency assessments prior to employment of this methodology should it be employed at high angles of attack. With this knowledge an accurate forcing frequency may be specified in order to isolate the dynamics at play and obtain the most accurate representation of the aircraft’s stability. In doing so, extend the traditional forced oscillation methodology successfully into the high angle of attack regime.