An open rotor engine is essentially a turboprop with two rows of blades, or propellers, which can operate efficiently at higher speeds than a conventional turboprop. The blades of a turboprop tend to spin air out, rather than pushing it back. In an open rotor engine, the forward propeller pushes the air backwards, while the rear one sucks it.

"You actually entrain the air in between the two sets of blades, and you can think of it as a virtual fan case," says Nuttall. "It's actually stopping the air going out, and the second one straightens up the air from the first one. Most of the air is doing just what it does in a turbofan, hence you can go a lot faster. You can't go as fast as a turbofan. We think the most efficient speed is about 0.76 Mach." This compares to around M0.845 for typical jet-powered airliners.

A turboprop engine would also need blades twice the diameter of its open rotor equivalent to produce the same power.

"There are only two efficiencies in an engine," says Nuttall. "There's thermal efficiency, which is how efficiently you get the energy out of the fuel and turn it into 'turning'. And there's propulsive efficiency, which is how efficiently you take that energy and turn it into 'push'."

In a conventional turbofan, propulsive efficiency increases as cruise speed goes up. A single propeller can provide 90-92% propulsive efficiency, but this drops off with speed.

"The trick is to try to get an efficiency of that nature, but at [a turbofan] sort of speed, and that's why you put one [propeller] behind the other. That entrains the air and lets you actually drive the aircraft forward at a mach number of 0.75-0.8," says Nuttall.

In terms of lifecycle costs, an open rotor has no nacelle, thrust reverser, or fan case.

"It's an interesting debate at the moment," says Nuttall. "When we're asked this by customers our basic rule of thumb is that it should be about the same as today. You've deleted a lot, but you have added complexity at the back. The main complexity you've added is the gearbox, but the gearbox is half the power of the [P&W] geared turbofan. We accept that the gearbox is a significant thing that we've got to develop, and we've got plans for that."

An open rotor would operate at constant torque, with the pitch of its blades and speed of its rotors controlled by a full-authority digital engine control to provide the thrust demanded by the pilot.

Source: Flight International