DARPA Funds Research to Achieve Super-Lift Emulating Birds
Have you ever seen an owl catch its prey? It swoops down silently and swiftly without warning onto its prey and takes off with equal stealth with its prize. The secret of the owl’s noiseless swoop and liftoff lies in its wings, which allow it to fly almost vertically without being stalled and generate little turbulent wake. Throughout history, men have sought to mimic birds with flight devices. In fact, the key observation that helped the Wright brothers fly was the wing warping of birds. Yet, over a hundred years after the Wright brothers first took off, humans have not succeeded to fly as efficiently as birds.
In 2003, Professor in the University of Miami College of Engineering’s (UMCoE) Department of Mechanical and Aerospace Engineering Ge-Cheng Zha, then a new faculty member, was looking for a summer job in his field. He landed a summer research assignment in Dayton, Ohio with Wright Patterson Air Force Base (WPAFB). There he was assigned the task of improving the efficiency of aircraft jet engines. By summer’s end, he worked out an off-beat solution: blow air near the leading edge of the jet engine’s compressor blade and suck the same amount of air near the trailing edge. In principle, the solution would be able to reduce the number of compressors from six stages to two stages, a substantial weight reduction. Zha named the technology Co-Flow Jet (CFJ) active flow control.
Although the invention helped improve the efficiency of jet engines, Zha was still stumped on how to apply it to thin compressor blades. Upon returning to UMCoE, he discovered a high impact application area: the aircraft wing. Over the next 15 years, with support from various government agencies including the Defense Advanced Research Projects Agency (DARPA), NASA, Air Force Office of Scientific Research (AFOSR), Army Research Office (ARO) and CIRA, Zha created a revolutionary CFJ wing technology.
The Aerodynamics of Flight
“While the aerodynamics of flight is complex, the wing is the primary device that gives an airplane its lift,” explains Zha. “The wings of a plane are angled upward and as the plane moves through the air, the difference in air pressure between its lower and upper surfaces creates lift.”
This process is how all planes fly – from the single seater propeller planes to the ocean crossing behemoths called the jumbo jets. While the tilt of the wing gives lift, it also adds to the resistance of air on the entire plane – called drag. The drag is overcome by the engines which provide the forward thrust. To increase lift, aeronautical engineers and aircraft designers increase the tilt of the wing. Unfortunately, this serves to increase the drag, also increasing the risk for the aircraft to stall. This constant tussle between lift and drag is what makes conventional flight inefficient. The invention of the CFJ airfoil is revolutionary in its ability to substantially suppress this conflict.
Coflow Jet Airfoil Technology
A CFJ airfoil is sketched in Fig. 2, which withdraws a small amount of mass flow into the airfoil near the trailing edge, pressurizes it using a micro-compressor actuator embedded inside the airfoil, and then injects the mass flow near the leading edge tangentially in the main flow’s direction.
The CFJ airfoil is revolutionary because of its unique feature to enhance airfoil performance with a low energy expenditure mechanism; the air jet gets injected at the leading edge peak suction location, where the main flow pressure is the lowest and gets sucked out at the trailing edge, where the main flow pressure is the highest. The low energy expenditure is the key factor enabling the CFJ airfoil to achieve ultra-high cruise efficiency.
In addition to high cruise efficiency, CFJ airfoil achieves a super-lift coefficient that breaks the theoretical limit, which was thought to be unsurpassable. In one of Zha’s recent projects, funded by DARPA and published in AIAA Aviation’s 2018 issue, a lift coefficient of 8.6 was achieved, substantially higher than the theoretical limit of 7.6. The maximum lift coefficient that current high lift flapped wings can achieve is usually less than 3. The super-lift coefficient will enable aircraft to achieve ESTOL (extremely short takeoff/landing) performance without using engine jet blowing and the heavy and expensive high lift flap system. It also fosters new VTOL (vertical takeoff/landing) concepts that are much more efficient and quieter than conventional technology relying only on propeller vertical lift.
The CFJ airfoil mimics the flight of birds by creating a super-suction effect that reduces pressure drag. Birds create super-suction by flapping their wings at powered down strokes. However, the CFJ airfoil’s super-suction is so strong that an aircraft doesn’t need to flap its wings. Such a radically different aerodynamic performance largely impacts conventional aircraft design.
Fig. 4 is a CFJ airfoil at AoA of 70deg with no stall simulated by computational fluid dynamics software. It achieves a lift coefficient of 10.6 without any flaps. Furthermore, the wake is filled with reversed velocity deficit, which is the owl effect to have quiet flight with little turbulent wake noise. Fig. 5 is the recently tested CFJ airfoil under the DARPA project, in which the Super-Lift coefficient is experimentally proved. Fig. 5 also shows the five micro-compressors embedded inside the CFJ airfoil.
Big Dreams for the Aviation Industry
Recently, with funding from the Emil Buehler Perpetual Trust, the Center for Green Aviation has been formed at UM with Zha as the co-director. They have been able to apply the CFJ technology to ultra-quiet and efficient VTOL aircraft, drones, ESTOL transonic transports, long range electric aircraft, ESTOL amphibious aircraft, and high-altitude long endurance aircrafts.
“This is just the start,” says Zha. “Our aim is to not only transform the entire aviation industry by making flight more efficient, but to do so with little or no direct carbon emission pollution. Building environmentally friendly technologies is very important to us, as we want to set an example in planning for a sustainable tomorrow.” Dr. Zha hopes to ultimately usher the patented CFJ technology to the industry and market place to replace conventional technology.
Figure 6 and 7 depict the CFJ-Seagull, a full electric four-seater general aviation airplane with the CFJ wing as a part of the distributed propulsion system. The aircraft currently uses a battery energy density of 250 watt-hours per kilogram. The design achieves a range of 512 kilometers at a cruise speed of 184 kilometers per hour. The airplane cruises at a lift coefficient of 1.3 and an excellent aerodynamic efficiency. The typical cruise lift coefficient limit for conventional subsonic aircraft is about 0.5. Thanks to the CFJ airfoil technology developed by Zha, the CFJ-Seagull has the compact size of a conventional general aviation airplane, but with a substantially longer range. In addition, the high cruise lift coefficient also provides a very high wing loading of the aircraft, which significantly increases the flight comfort due to the high resistance to atmospheric turbulence disturbance. Comparisons with existing technology show that the advantages of using the Coflow jet technology increases the energy efficiency and the range of the aircraft by 30% or more.