Energy-Saving Helicopter Blades

Helicopters can perform some incredible aerobatic feats, but they’re also noisy, shaky, and expensive to run. NASA researchers are developing helicopter blades featuring a shape-shifting smart material that could lead to a smoother, quieter, more fuel-efficient ride.

The blades use piezoelectric actuators–mechanical devices incorporating a material that changes shape when subjected to an electrical field. This shape change deforms the rotor blade as it spins, improving a helicopter’s aerodynamic performance.

Last year, NASA, in collaboration with aerospace company Boeing, the Defense Advanced Research Projects Agency (DARPA), and the U.S. Army, tested the first full-scale rotor blade to use the technology in a wind tunnel that simulates flight conditions. The system significantly reduced vibrations, saved energy, and allowed rotor movement to be more precisely controlled. In the future, the system could also reduce noise. It is now ready to be flight-tested, although a date for the first flight has not yet been set.

“Right now, we are trying to understand and appreciate everything that we have accomplished in the full-scale wind tunnel,” says William Warmbrodt, the project leader from the Flight Vehicle Research and Technology Division at NASA’s Ames Research Center, in California.

As a helicopter blade passes through the air, it leaves behind a wake, and as the blade behind it passes through that wake, it experiences a periodic vibration. “Having blade actuation allows you to put a periodic motion into the blade flaps with the right amplitude, phase, and frequency to cancel out that vibration,” says Steven Hall, a professor of aeronautics and astronautics at MIT and a consultant on the NASA project.

“People have been talking about using smart materials in aircraft for a long time, but what [has] really been lacking is the right kind of actuator to make it practical,” says Hall. Previous efforts, involving hydraulic actuators, proved too heavy and slow to be practical. “It is hard to do hydraulics in a rotating frame: you need enough force to deflect the flap because the air loads are very high, and you have to do it at the frequency required,” Hall says.

The new actuator sits inside the steel frame of a rotor blade near both the tip of the blade, where aerodynamic forces are greatest, and a flap on the rear portion that moves up and down as it turns. Power amplifiers transmit an electric field to the piezoelectric material inside the actuators, and that material responds by changing length, expanding a very small amount (roughly 10 to 20 thousandths of an inch). This moves a rod perpendicular to the blade flap, which pushes the flap. “You are taking a small motion, amplifying it enough to move the flap a few degrees,” says Hall.

But movement of the flap creates a dramatic aerodynamic change to the blade. The flap can help generate lift or air speed, and, whereas an airplane can only use flaps for takeoff and landing, such flaps can be used anytime during a helicopter flight.

What is really important is that the piezoelectric materials are stiff and can change shape rapidly. “That is what makes it an acceptable actuator,” says Hall. The smart materials also make the actuator system lightweight and compact. Furthermore, the NASA researchers have designed the actuator system to fit into the blade structure of existing helicopters without significantly modifying the rotor blades’ design.

“Smart materials hold a tremendous promise for revolutionizing how we design, build, and operate our helicopter aircraft,” says Warmbrodt.

The project could have several spin-offs: the U.S. Army is developing a second rotor using electric motors, and DARPA just announced a Mission Adaptive Rotor (MAR) program, which is going to look at a number of technologies, including smart materials, to improve the rotor blades used in military helicopters.

Warmbrodt adds, “The DARPA MAR program is the next step in looking at how we are going to radically change the design of helicopter blades to achieve a new level of performance.”