42 n TRANSPORTATION July/August 2023 www.drivesncontrols.com Active control could help eVTOL aircraft to fly faster The advantage of fixed-wing vertical take-off and landing (VTOL) aircraft – commonly referred to as tilt-rotors – is that they can be launched from areas as small as a building roof, yet use conventional aeroplane wings to reach high speeds. While aircraft such as the V-22 Osprey and the AW609 are well-established, smaller electrically-powered VTOL (or eVTOL) aircraft could take to our skies within a few years as taxis and delivery vehicles. A challenge to all tilt-rotor aircraft is the phenomenon known as whirl flutter – an instability that occurs with elasticallymounted propeller rotors that causes the entire wing-rotor system to vibrate. This happens when a critical speed, specific to each type of aircraft, is reached. Above this speed, whirl flutter can create vibrations powerful enough to shake the wing apart. Current tilt-rotors typically use thick wing sections to increase their rigidity and push whirl flutter to higher speeds. This, however, comes at the cost of a heavier structural weight, and higher aerodynamic drag, requiring more power to lift the aircraft in the helicopter mode, and to overcome aerodynamic drag in the aircraft mode. Active control A potentially attractive answer is to use active aerodynamic control devices that can alter the wing loads in real-time to counteract vibrations. Such a system would reduce the need for thick wings and allow the aircraft to push past the critical speed. With the aim of developing a new aerodynamic control technique to tackle whirl flutter, researchers at the Institute for Propulsion and Mobility (IPM) at the University of Bath have designed and built a novel wind tunnel experiment that simulates whirl flutter vibrations, while deploying an aerodynamic control strategy. “Testing aerodynamic control strategies for whirl flutter is not a simple process,” explains the Institute’s Dr Sam Bull. “Not only do we have to assess their performance across a wide range of deployment speeds, we also have to observe how that performance changes on a vibrating wing – similar to what would occur on the real aircraft undergoing whirl flutter.” The IPM team is investigating an aerodynamic control device known as a minitab – a miniature spoiler that deploys above and below the surface of the wing to alter the aerodynamic load. The theory is that when an aircraft approaches its critical speed, the minitabs deploy rapidly, protruding above and below the wing by up to 2% of the wing width – about 10mm in the experiments – to calm the vibrations. “On an aircraft, conventional control surfaces, known as flaperons or ailerons, deploy to change the loads around the wing,” Bull continues. “However, the size and weight of these control surfaces mean you can’t move them very fast. Mini-tabs, on the other hand, have a relatively low mass, meaning you can deploy them much faster, giving the potential to counteract the effect of whirl flutter.” To achieve this, the IPM team has developed a wing model with integrated UK researchers are investigating how spoiler-like devices could help future electric tilt-rotor aircraft to fly faster by counteracting potentially damaging vibrations. To test the proposal they have used a novel wind tunnel test rig which relies on precision motion control. Existing tilt-rotor aircraft, such as this V-22 Osprey, avoid the effects of whirl flutter by using thick wing sections. Future, lighter eVTOL craft may need more sophisticated approaches such as mini-tabs.
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