March 11, 2022, ©. Leeham News: After our articles about Serial Hybrids and Parallel Hybrids showed they were unsuitable for airliners, where do these make sense?
The obvious answer is for our stop-and-go cars (as we then can recover the brake waste energy). Still, there are aeronautical special cases where hybrids can bring advantages. Let’s look into these.
If a vehicle can gain major architectural benefits from the power output being electric rather than shaft power, a hybrid powertrain can be motivated. An example is to replace the complex and expensive mechanical rotor gearbox and rotor head of a helicopter and go for an electric-driven multirotor concept instead.
As explained in Figure 1, a helicopter needs to vary the rotor blades’ angle continuously during flight. It comes from the helicopter’s forward speed adding to the forward-going blade’s speed and decreasing the speed of the reward-going blade. To have an even lift, the angle of attack of the blades are continuously varied during the rotation.
A multirotor design like a drone or VTOL simplifies this by having at least four rotors, one in each corner, half rotating clockwise and half anti-clockwise. Now you can control the lift and pitch/roll/yaw by varying the RPM of the electric motors driving the rotors, Figure 2.
So a complex gearbox plus rotor head has morphed into several electric motors with fixed rotors. Control is now by shaping the alternating current from the motor control electronics to the motors, a much simpler design.
Here a Serial Hybrid makes sense, as it produces electric power that can be converted to the alternating current needed to control the motors.
The Parallel Hybrid can solve some other problems. One is as a supplementary safety system to a gas turbine propulsion system. Airbus is exploring a parallel hybrid configuration for its helicopters, where an electric motor working in parallel with the gas turbine can improve the helicopter’s autorotation emergency mode.
As it’s a short-term power injection to the rotor gearbox, the energy store weight problem we’ve seen, doesn’t become acute (we talk power for minutes only). Still, the electric motor and its energy store, the battery, is dead weight for all non-emergency situations. We will, therefore, see configurations where the motor is used as the main electric generator for normal flight and the battery used as the standard vehicle battery. In an emergency these switch to a short-term power boost for the helicopter rotor.
Finally, we have the enlarged starter-generator that helps a gas turbine be optimized closer to its limits, assisting with speed up and down transients. But this has always been called a “more electric “ technology, and we will continue to call it such.
A hybrid works in some instances, our everyday car being the prime example. For the car, it’s because of the stoplights; for air vehicles where we have no stoplights, the exceptions are different, as described above.
For the general aircraft case, where a hybrid delivers shaft power to a propeller or fan, the hybrid’s added complexity only increases manufacturing and operating costs, with no tangible reduction of greenhouse emissions.