Canadian researchers are claiming a breakthrough in rotary-wing fly-by-wire with the development of a control technique that gives pilots “stability when they need it and agility when they request it,” without having to make a deliberate switch between modes when flying conditions change.Photos: NRC Aerospace
The technique has been developed by the Ottawa-based National Research Council Canada Institute for Aerospace Research (NRC Aerospace
) and demonstrated using its Bell 412 research aircraft equipped with an experimental fly-by-wire (FBW) system. NRC Aerospace has led research into rotorcraft handling qualities for decades, using a series of variable-stability FBW helicopters as in-flight simulators.
The new control technique could prove a key to wider adoption of fly-by-wire technology in helicopters, as it combines the stability required in poor visibility when the pilot needs to make slow control inputs, with the agility demanded in good visibility when the pilot wants to make rapid inputs. The system blends the control response types based on how aggressively the pilot moves the stick.
The new control structure “is a way of splitting the pilot’s input into its frequency content,” says Bill Gubbels, a researcher at NRC Aerospace’s Flight Research Laboratory. In good visibility, when the pilot has ample visual cues to the helicopter’s attitude and velocity, moving the stick aggressively channels the control input to a rate command system that provides agility.
When visibility reduces and the cues disappear, the pilot makes slower, more deliberate inputs that are channeled to a translational rate control (TRC) system. When external references are lost suddenly—in a brownout caused by blowing sand or a whiteout from swirling snow—the pilot can release the stick and the aircraft will slow under TRC to a stable hover, by itself.
Bill Gubbels Right) in the NRC 412 with test pilot Stephan Carignan
Rate command, in which moving the stick commands yaw, pitch or roll rate, allows the helicopter to be maneuvered aggressively, but does not provide the stability or fine control needed when flying in the absence of external cues. TRC commands ground speed, which depends on how far the stick is moved; and returning the stick to the center brings the helicopter back to a hover.
The breakthrough claimed by NRC Aerospace is in enabling the stability of a TRC system to be coupled with the performance of a rate command system without requiring discrete mode switching by the pilot.
“Helicopter pilots adapt how they fly to the cues they have. If they don’t have external cues, they slow down and look inside the cockpit,” says Gubbels. “It’s like driving in fog without a speedometer and trying to accelerate to 50mph. If the fog lifts and you can see more of the world outside, you start to accelerate. When you lose the cues, you slam on the brakes. It’s on-demand aggressiveness.”
With the new control structure developed by NRC Aerospace, “if the helicopter hits fog, the pilot can freeze the stick and search for cues,” he says. Crucially, the pilot does not have to make the decision to select a different control mode; it is done automatically based on stick inputs. “When you have bad cues, workload goes up, and that’s not when you want to ask the pilot to hit a button.”
NRC Aerospace is looking to license its new FBW control architecture to manufacturers. “Manufacturers are still using classic control laws. This is the next generation,” says Gubbels.