July 14, 2017, ©. Leeham Co: In the last Corner, we developed a hybrid aircraft propulsion system and looked at system efficiencies. Today, we look a bit deeper at how hybrid propulsion can be implemented on an aircraft.
What are the advantages on an aircraft level, that such a chain can have? Can aircraft level efficiencies compensate for more parts and lower propulsion efficiency of a hybrid systems?
In last week’s Corner, we developed the hybrid propulsion system in Figure 2.
The chain contains more components than for a classical turbofan. We examined the hybrid chain by looking at the efficiency differences such a chain has compared to a Turbofan.
On a chain of the mid-next decade (with superconducting generators and motors) we would lose around 6% in overall efficiency compared with a turbofan with the same core and fan components.
“The same core and fan components” is a limitation that an electric aircraft shall not be constrained by. By clever partitioning and sizing of the components, the hybrid can gain back efficiency. Here is why.
The typical twin-engine airliner has its engines sized by the extremes of its flight envelop. The continued take-off with one engine inoperative is such an extreme, as is the one engine inoperative go-around after a missed landing.
The last part of the climb to cruise altitude (top of climb) is also stressing for the engines. Minimum altitude limits (over mountain ridges etc.) when an engine is lost can also be a sizing case for the engines.
Today’s turbofan propulsion on the twin-engined airliner lose half of its power when an engine breaks. This forces the engines to be larger than needed for normal operation. With a battery as energy store and a more flexible configuration of the propulsion units (motor+fan units), one can avoid having half the propulsive power disappear from a fault in a propulsor.
With three or four propulsors, a third or a quarter of the propulsive power disappear if one of these have a fault, instead of half.
Another area that can be sized more efficiently would be the gas turbine core. The smaller the core, the more difficult it is to get the core efficient. With two turbofans in the 15,000lbf thrust class, the size for the typical regional airliner, we are close to the size of the core where the last axial compressor stages have to be replaced with a radial stage.
The axial compressor blade dimensions get too small and we get high tip clearance losses for the last stages. A radial stage must replace the axial ones. It’s less sensitive to small dimensions.
But a radial compressor at the end of the chain lowers the compressor’s overall efficiency and pressure ratio. With a hybrid setup with a battery to offer energy backup, one could replace the two smaller cores of the turbofan airliner with a single larger core, driving the generator. This is done by Zunum aero in Figure 1.
The efficiency of the core is improved as its larger size makes it suitable for an all axial compressor design.
With a single larger core, an alternative propulsive redundancy scheme must be found. If the core stops, the airliner must continue its flight. The airliner’s batteries can work as a redundant energy source to the core and generator. Batteries and inverters would be duplicated to provide redundancy.
It remains to size the aircraft’s aerodynamics (induced drag) so batteries as an emergency energy source have sufficient power level and endurance to cover the take-off and go-around case.The minimum altitude and emergency landing from cruise cases must also be covered. These will be tougher nuts to crack, as the energy endurance requirement will be higher.
The turbofans of a regional airliner are typically placed on the wings. With their high weight (mass really) the propulsion units (nacelle, turbofan and pylon) are placed where lift is generated and close to the main landing gear forces.
But the wing placement of turbofans is not ideal from an aerodynamic perspective. The engines disturb the flow around the wing and the high-speed fan flow does nothing to improve the aerodynamics of the aircraft. If anything it creates interference drag.
The dominant drag component of an aircraft is the air friction against the airliner’s surface. Laminar flow air generates half the friction of turbulent flow. On a normal airliner, the transition from laminar flow to turbulent boundary layer flow is happening after less than 10% of the surface of the wing/empennage.
With a suitable designed wing profile and propulsors sucking a tired boundary layer off the aft wing/empennage, an electric propulsion system can be designed to keep laminar flow longer and thereby lower the aircraft’s drag. A similar effect is achieved with sucking a tired boundary layer off the last part of the fuselage.
These effects will need new research into the best placements of propulsors. Research of more laminar flow wings and other aircraft parts is ongoing, as is research into boundary layer ingestion propulsion systems. Figure 3.
Sizing of an electric regional airliner
Our next step is to look at size and weight for the components in a typical regional airliner application. How much heavier is a hybrid setup? Can it be motivated with aerodynamic gains? This will be the subject of the next Corners.