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The Defense Advanced Research Projects Agency’s CRANE (Control of Revolutionary Aircraft with Novel Effectors) program is progressing toward the development of a new X plane maneuvered by active flow control (AFC) rather than mechanical flight-control surfaces.

For its part of the CRANE program, BAE Systems will evaluate the benefits of using AFC integrated into different air-vehicle concepts leading to a conceptual design review. BAE Systems will mature design, integration, and de-risking activities, including wind tunnel testing at its facilities in 2022.

To discuss the program, the details behind AFC, and BAE Systems’ experience in developing AFC technologies, we talked with Professor Clyde Warsop, a BAE Systems Engineering Fellow who acts as the chief technologist on the company’s portion of the CRANE project.

Breaking Defense: When I first heard of active flow control I thought it might be about using jets of air in place of actuator-operated flight controls, or that it acted like water jets on a speedboat to provide propulsion. But they’re not correct, are they?

Professor Clyde Warsop, BAE Systems Engineering Fellow

Warsop: Air jets are a way of introducing energy into a flow, but unlike your boat analogy where you’re using water jets as a means of propulsion (just like a jet engine provides propulsion to steer an aircraft), what we’re doing with active flow control is very different. With AFC, we’re using air jets to manipulate the aerodynamic flow and change the way the air flows around the aircraft to induce a reduction in drag or an increase in lift or a change in aerodynamic performance. It’s about adding small amounts of energy to get big effects out of that energy addition.

Breaking Defense: So active flow control is about aerodynamic control?

Warsop: One of the application areas is to provide control forces and moments (the turning effect of a force) so that you can steer and maneuver the aircraft. But there are other ways that you can use active flow control to reduce the aerodynamic drag. You can also delay flow separation at high angles of attack so that you can actually fly at higher lift coefficients and maneuver more aggressively or fly at slower speeds without imposing or incurring huge amounts of drag. It’s not just maneuvering the aircraft, it’s also increasing maximum lift so that you can takeoff and land more slowly or from a shorter airfield. You can actually apply some active control systems in cruise flight to lessen drag and reduce fuel burn so you can increase the range of a mission or the duration of the flight.

It has multiple applications. The key to doing it successfully is to actually minimize the amount of energy you introduce into the flow with the active flow control system, while at the same time trying to maximize the benefit. You want to put as little energy in as you can because you have to pay for that in some way (e.g. through burning more fuel to generate that energy), and obviously you want to maximize the outcome of that energy introduction.

Breaking Defense. What does active flow control look like? I’m envisioning a thrust vectoring exhaust nozzle?

Warsop: Thrust vectoring is one way of applying active flow control. You’d actually have a fixed-geometry nozzle and inject jets of air in a controlled way to change the way the main jet flow comes out. By tailoring and controlling the direction and quantity of injected flow you can cause the main jet to deflect upwards, downwards, or sideways allowing it to provide a ‘steering’ force similar to the way that deflecting a water jet propelling a boat can be used to steer it. That’s called fluidic thrust vectoring.

Other application areas where we’re more interested in at the moment in the CRANE project is where we have what we call ‘blowing slots’ along the trailing edges of the wing through which we blow very high-speed supersonic air jets over the surface of a trailing edge. We call this ‘circulation control’. What that does is interact with the flow around the entire wing, causing an increase or decrease in lift on that part of the wing where we’re actually blowing the air jets. When we do that we can change the pitching, rolling, or yawing moment generated by the wing, which can be used to maneuver the aircraft. The idea is that we can use this technology to replace the more conventional moving control surfaces such as flaps and ailerons that are currently used for this purpose with a system that is lighter and has fewer moving parts.

Other ways we can apply it is through a technology called ‘forebody blowing’. There we have, again, narrow slots located around the edge of the aircraft nose which we use to blow air jet ‘sheets’. If we blow through these slots, what we end up doing is changing the way the vortices around the nose of the aircraft form. That’s done through a mixing and an entrainment process that changes the way the aerodynamics around the aircraft behave. If only one side of the aircraft forebody blowing is activated it creates asymmetry in the forebody vortices, and that asymmetry then generates a side force and yawing moment to act on the front of the aircraft. With appropriate control over the blowing these side forces and yawing moments can be used to maneuver the aircraft.

Breaking Defense: Active flow control has been studied for a century. What advancements have been made recently to now make it a viable capability?

Warsop: You’re correct and it’s been used for a number of purposes as we’ve discussed. There have been some major programs undertaken by Boeing and Airbus to demonstrate hybrid laminar flow, for example, for drag reduction purposes on large civil transport aircraft, and that’s quite mature as a technology.

In terms of the military application, the challenge is a bit different. We’re not necessarily looking at optimizing cruise performance through drag reduction. What we’re looking for is other ways of improving the performance of the aircraft by replacing conventional control surfaces, which have moving parts and a lot of complexity, with much more simple fixed-geometry components.

Until recently, the main barriers to implementing a real active flow control system on a production aircraft relate mainly to the inability to actually demonstrate adequate cost benefit for a fully integrated system. That’s because you need an excessive energy requirement to achieve the benefit, or because such a system adds significant amounts of weight.

But things have changed over the last decade or so. Now we’ve got new technologies and digital engineering toolsets that allow us to develop and manufacture more intricate, lighter weight, and more efficient structures through technologies like 3D printing, for example. We’ve also gained a lot of knowledge about how to make flow control efficient. That’s been driven by big advances in computing power and improvements in our computational fluid dynamics simulation capability, and also in the ability we have now to apply very advanced test diagnostics in wind tunnels and systems.

So we actually understand a lot more about the way that active flow control works. And we know now through the research done over the last 10 or 15 years, how to do it more efficiently. And so now we can actually consider it as a concept that actually buys its way onto the airplane.

BAE Systems has been actively involved in flow-control research both with its in-house programs, and also in partnership with academia.

That led us to develop the DEMON demonstrator, a flapless unmanned air vehicle with active flow-control capabilities, which was flown in 2010. More recently in 2019, we flew the MAGMA proof-of-concept subscale demonstrator UAV that—for the first time in aviation history—successfully demonstrated fully controlled flight using active flow control technologies, employed by supersonically blown air.

Those were quite advanced proof-of-concept demonstrators of the technology. Neither of those would have come about unless we had developed an improved understanding of how to implement the technology to make it efficient and effective.

Breaking Defense: Describe the tactical benefits of active flow control.

Warsop: When you have a technology like active flow control, you can do things like replace conventional, moving control surfaces with fixed geometry devices. That offers the potential to significantly reduce the number of moving parts on the aircraft, with some obvious implications for maintenance and supportability.

There’s a potential to have flight control systems that are actually lighter in weight, which could result in a smaller, more affordable aircraft to achieve a certain set of mission requirements. Or you can trade that mass reduction for the ability to carry extra fuel for greater range and endurance of your aircraft, or also trade it for additional payload. That gives you more flexibility as a warfighter to fly a mission and to achieve an end objective.

Technologies like forebody blowing actually change the vortex structures that develop around the aircraft, so they work very well at high angles of attack when other control methods tend to lose their effectiveness. You can, therefore, potentially use that technology to enhance the maneuverability of the aircraft to avoid a threat or to provide flight control during landing approach at high angles of attack.

Active flow control also allows you to fly at slower speeds, which lets you takeoff and land in shorter distances and from a greater variety of air bases. That gives the warfighter a greater amount of flexibility on how to deploy assets to achieve an end objective.

So there are quite a number of different tactical and military benefits. And that’s the main focus of the CRANE program, which is to actually show how these benefits can lead to military advantage.

DARPA’s CRANE program will develop a new X plane maneuvered by active flow control rather than mechanical flight-control surfaces. Image courtesy of BAE Systems.

Breaking Defense: Under the DARPA contract, BAE Systems is using its digital engineering and development skills for not only the AFC system but also ultimately for the building of a full-scale X plane. Tell me about that.

Warsop: The primary focus for CRANE is understanding and designing an X-plane concept that makes full use of active flow control, to demonstrate the operational military benefits of doing so. It’s to understand the practical issues of implementing it at full scale, and knowing what the potential benefits of applying the active flow control could be.

Our role so far is to mature and test the AFC technology, and to develop conceptual X-plane designs. A lot of that design work is focused around modeling and simulation. We’re also conducting some large wind-tunnel testing activities to validate the modeling and simulation activities.

In addition, we are producing a comprehensive set of computational design and analysis tools for the conceptual design of practical aircraft that uses active flow control. These toolsets and design capabilities will build heavily on BAE Systems world-class AFC experience, knowledge, and computational engineering capabilities that have been built up over the past 20 years or so. The result will be a revolutionary AFC design capability that will allow designers of the future to implement active-flow control in the aircraft design process efficiently and with confidence.