Using arrays of tiny actuators to modify airflow over aircraft and through engines is showing promise


Small changes can have large effects. That is the principle behind micro adaptive flow control (MAFC), an emerging technology which promises to significantly affect the design of aircraft and engines.

Supported by US Defense Advanced Research Projects Agency (DARPA) funding, researchers are using the technology to actively modify airflows through engine inlets, compressors and nozzles, and flow over aircraft and rotorcraft surfaces, to improve performance.

MAFC involves the use of tiny actuators to exert a small influence on local flow that will have a much larger effect on the total system. Synthetic jets, for example, can be used to re-energise airflow and keep it attached to aerodynamic surfaces.

The first MAFC application to reach large-scale testing is the Active Control of Exhaust (ACE) programme. In ACE, flow control is used to improve mixing of core and bypass air in the exhaust of the Boeing C-17's Pratt & Whitney F117 (PW2000) turbofan. The aim is to eliminate the heavy and complex core-flow thrust reverser used on the C-17 and thereby keep hot engine exhaust away from personnel loading and unloading the large military transport aircraft.

Using adaptive flow control also allows the nozzle to be redesigned to incorporate a large plug, which blocks visibility into the engine and reduces radar signature, says DARPA MAFC programme manager Dr Richard Wlezien.

Rig tests

Earlier this year, P&W rig-tested the new nozzle incorporating adaptive flow control to evaluate the effectiveness of the ACE system, establish the performance of the modified nozzle and demonstrate that it had no detrimental effect on engine performance. The test was a "qualified success", says Wlezien, and cleared the way for follow-on testing on a C-17 next year.

"The only reason for the core thrust-reverser is to keep the temperature down where the loadmasters work," says Wlezien. "Increased mixing of the exhaust gases reduces their temperature and allows removal of the thrust reverser."

In the ACE system, passive fluidic actuators inject unsteady pulses of compressor bleed air into the nozzle to destabilise the exhaust plume and promote mixing of the hot core and cooler bypass flows. The system uses 1.5% of available core airflow.

The test showed that the new nozzle had no adverse effect on engine performance, despite the added surface area of the large plug. "Nozzle performance was excellent, indistinguishable from the current nozzle, which is a testament to the CFD [computational fluid dynamics] work," says Wlezien.

However, tests of the ACE system were "promising, but not conclusive", he says. "It was a challenge to get the actuators to perform effectively. We know how to create unsteady pulses, the problem was getting them out into the exhaust." More work is being done to optimise actuator performance. "We anticipate we are going to get full mixing."

The ACE system emits 30-60Hz pulses of air through slots at the exhaust plane. Pulses are generated by Honeywell-developed fluidic oscillators. Flow is switched between two channels and the frequency of pulses is determined by the length of the tubes. "In the final configuration there will be no moving parts," Wlezien says.

"The actuators work great," he says, but in the test nozzle the two oscillators had to be mounted well forward of the exhaust plane in fairings for the thrust reverser. "The challenge is to get the pulses out into the exhaust plane in a way that is compatible with the outer mould line of the nozzle. We do not want to have any bumps or fairings."

Design work has begun for the C-17 tests, to be funded jointly by DARPA and the US Air Force Research Laboratory. The ground tests will involve measuring temperatures at the loadmaster's position with the ACE nozzle in place. If successful, the system could be installed on C-17s to reduce weight and signature and improve reliability and maintainability.

Other projects under DARPA's MAFC programme include the Massachusetts Institute of Technology's work on an aspirated turbine engine compressor. This involves pumping air through the blades to prevent boundary layer separation. The technique can double the work performed by a compressor rotor and so halve the number of stages required, thereby substantially reducing engine weight.

The Adaptive Virtual Aerosurface (AVIA) project is looking at using synthetic jets to modify the pressure distribution on a wing and so control the aircraft by varying the aerodynamic forces generated on the left and right sides of the wing. This would allow development of a tailless aircraft with no moving aerodynamic control surfaces.

Another MAFC concept uses synthetic jets to delay the stall of the retreating blade of a helicopter main rotor. Retreating-blade stall limits the forward speed of a rotorcraft, and the concept uses on-blade flow control to delay airflow separation. Directed synthetic jets generated by an activated cavity in the leading edge help re-energise flow over the blade.

A different actuator type is being used in a project to improve supersonic inlet aerodynamics, mainly to remove the flow turbulence caused when a shockwave hits the inlet wall. Instead of bleed ducts which remove the turbulent boundary layer air before it can enter the engine, this concept uses an array of tiny flaps, or mesoflaps, which open and close to allow the air to recirculate through a porous surface.

Turbulent air is removed from behind the shockwave and recirculates through the mesoflaps to be injected back into the boundary layer ahead of the shockwave. The Mesoflaps for Aeroelastic Transpiration (MAT) concept reduces drag and bleed losses, and the system can adjust to variable shockwave impingement locations.

Reducing wing download

Another MAFC project moving into large-scale testing involves active alleviation of proprotor download on the wing of a Bell Boeing V-22 Osprey tilt-rotor transport. Testing of a 10%-scale wing at Arizona State University has shown that active alleviation in the hover can reduce download on the wing by 35% - and increase the V-22's payload by 1,000lb (454kg). Tests with a 2-D wing model worked well, says Wlezien, "and with a 3-D model worked better".

The system involves a small "flipperon" control surface running along the trailing edge of the wingbox and a synthetic-jet actuator running along the leading edge of the flap. Together these delay separation of airflow over the wing and fully deployed flap when in the downwash flowfield of the proprotors. The low-frequency jet actuator uses a multi-layer piezo-electric polymer, PVDF, to generate the flow pulses.

The next step, says Wlezien, is to test a quarter-scale model in the large windtunnel at NASA's Ames Research Center, which was previously used for V-22 testing. Potentially the system could be used to reduce drag in forward flight with the proprotors down as well as download in the hover with the proprotors up, he says.

Micro adaptive flow control is still in its infancy, cautions Wlezien, but large-scale demonstrations like the C-17 ACE nozzle show the technology has great potential. It is likely that aircraft and engines will soon incorporate arrays of small actuators that provide large-scale flow control benefits.

Source: Flight International