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Flying Calmly and Safely Like Never Before

The Air Force Research Lab (AFRL) and the University of Dayton (UD) are using the MicroLabBox to develop and validate control systems to mitigate wind gusts.

As modern aircraft are being developed and Unmanned Aerial Vehicles (UAVs) potentially becoming mainstream delivery bots, or even the future of the air taxi becomes a closer reality, considering the effect of wind gust dynamics on a wing or rotary propeller is critical to long term success. Michael Mongin, an engineer at the Air Force Research Lab (AFRL), is working with graduate students from the University of Dayton (UD), to conduct experiments using the dSPACE MicroLabBox to uncover the secret of calmer flight.
Research funded by AFRL at the University of Dayton is aiming to understand the physics of the flow field surrounding a wing undergoing gusts. AFRL and UD want to understand the changes in the flow field in a way that can unpack the direct physics of why certain movements or dynamics are present as gusts interact with the wings. Once this is innately understood, that information can be used to design controllers for flying through those gusty encounters.

Understanding the Flow Field

Mongin stated, “We are understanding the dynamics on a deeper level and unraveling how we might use this in the future. We use the information we are collecting now to design controllers for flying through encounters of different sizes. We are getting closer to having a controller able to demonstrate some pretty high vortex gust rejection and high mitigation rates on the gusts. We are approaching an area where we have enough of an understanding to make a big impact.” This started as an idea in 2020 with a lab to do it. Mongin and the UD Lab were able to build a gust generator and induce gusts via experiments using the dSPACE MicroLabBox studying the physical effects of the prescribed motion on the wing using closed loop control. Mongin said, “I needed to look at the flow fields and study the physics, and the MicroLabBox has been a good means to an end for us because it allows us, through a very familiar programming architecture, Simulink®, to write and develop these controllers and control structures for our lab that we can just drop in, plug them in, and run the experiments. It is very simple for us to get up and running with an experiment and to modify an experiment and implement things quickly.”

The Experiment: Controls for Unsteady Aerodynamics

Students design profiles for the gust generator to perform through time so that they can generate a vortex gust. This vortex gust has a high shear gradient which creates a complicated problem for the downstream test article. The job of the test article (the wing), in conjunction with the controller that is running it, is to sense changes in lift. Depending on this, the wing adjusts its orientation to counter those changes and try to maintain whatever lift value was targeted. The students might tell it to maintain zero lift or just to maintain a completely static lift profile.

“As the gust hits the wing, the wing will try to pitch down and maintain these zero forces throughout the encounter. If the students want to maintain lift equivalent to a flight condition, they might target a 0.5 Cl, which in the industry means to just maintain the lift that would be on a wing in flight in a typical cruise condition,” explained Mongin.

So as the gust encounters the wing, the wing experiences an increase in lift. A sensor on the wing senses the change and, instead of staying stationary, the wing will pitch down through the encounter as the gust hits the wing, just as the controller has been designed. It is maintaining the same lift throughout the entire encounter itself, but it is not maintaining its orientation, it is free to change its pitch orientation through closed-loop PID control so that it can mitigate that effect.

The MicroLabBox as the Driver of the Experimental Setup

“Before the MicroLabBox was in the lab, before we were able to use it to design controllers, researchers would have to come up with open loop profiles for the wing pitch. They had to have perfect models and run prescribed profiles then analyze in post to see how well it did at mitigating. Now we say, ‘target this amount of lift’, and the controller can make corrections in the loop to get us to the mitigation we want. This takes place faster than we could ever iterate on it or come up with a controller. This is how it would ideally work on the vehicle as well. The MicroLabBox is literally driving all these different elements of our experimental setup, and it can do this easily. The MicroLabBox has so much scalability for what we are doing and we can do so much more with it. It is very good for modern controller development and for general use in a lab environment.”

Mongin expressed that the MicroLabBox has been a highly effective tool for several reasons. He highlighted the utility of the device in triggering high-speed cameras with precision timing during experiments. He went on to add that the MicroLabBox ensures they can capture specific moments in time without concerns. The reliability of the MicroLabBox’s timing was underscored, along with its impressive input output capability, making it the preferred choice in the aerodynamic facilities.

The researchers use an open or free surface, a water channel, where the water itself is moving through the tank. The current experiment is carried out with a gust generator and a flat plate that lies upstream which performs a predetermined movement. For the experiments in this study the flow velocity inside the tunnel was set to 0.28 m/s and the test article used was a flat plate constructed of borosilicate glass with dimensions of 3-inch (0.076m) chord and 6-inch (0.152m) diameter.
The MicroLabBox controls four elements in the laboratory: the PIV camera trigger, the gust generator, the wing control, and the aerodynamic force feedback.

Research into Future Aircraft Design

The outcome of these experiments can lead to future advances in aircraft design. Crucially, a non-commercial platform such as a UAV can only be used in limited flight conditions, because if it is gusty or there are strong winds, the UAV’s ability to fly through these conditions is limited. If these limits can be extended, this would additionally be of great benefit for passenger air traffic. Mongin stated: “In the future, this could be implemented on a commercial platform. If we can figure out how to mitigate gusts, passenger comfort levels could increase. Or maybe there can be extended capability of the aircraft to take off or land in gusty or stormy conditions.”

Today, AFRL and the University of Dayton are looking at more experiments. For instance, if the wing is generating lift, the force can be measured directly using a load cell. That obviously does not exist on an airplane. Airplanes do not have force sensors mounted to the root of the wing to give the exact lift on the wing. What the researchers are trying to do is implement pressure sensors and other types of sensors into the wing that can give enough information about what is going on, to then write these control loops for something that is directly applicable on the wing. This would be a controller that would be able to reduce the movement after a wind gust for small or large airplanes in the future.

Courtesy of University of Dayton