Meeting new flutter challenges head on

Preparing aircraft to carry new stores in the face of ever evolving new threats and countermeasures

Mitigating the risk of flutter – a self-sustaining structural vibration that can destroy an aircraft in a second – is imperative for the South African Air Force (SAAF) as they prepare aircraft to carry new weapons and sensors to mitigate new threats and countermeasures that evolve constantly.

Introducing new external stores on an aircraft always creates the risk of flutter. Without exception, aircraft with new stores undergo a flutter clearance process. The flutter clearance process requires the determination of aircraft structural dynamic properties and to identify flutter risk in the intended operating range of the aircraft with the stores. A flutter-flight test should follow to confirm the analysis.

During flutter flight tests the aircraft is ‘shaken’ while flying at a constant speed. Accelerometers distributed over the airframe measure the structural response, which is analysed to determine the flutter speed.

The CSIR’s involvement in flutter-flight testing for more than thirty years has recently led to the development of a simple and effective flutter excitation system (FES) that uses a rotating annular wing. An aerodynamic exciter is more effective at low frequencies and control surface excitation becomes less effective at high frequencies due to the inertia of the control surface.

Since the CSIR’s rotating annual wing rotates rather than oscillates, it remains effective at high frequencies. The aircraft requires no structural modification for the testing, as the annular wing exciter is built into the flutter-flight test dummy of the store that needs to be tested. Some wiring is required, but once installed to a particular store station it can be used for any store to be carried at that station. The current FES has been successfully utilised in a number of flight test campaigns within South Africa and abroad.