September 1, 2017, ©. Leeham Co: in the nine previous Corners, we looked at 50-seat regional jets and turboprops with hybrid electric propulsion systems.
We have seen that at the state of technology until the mid-next decade, such aircraft have dubious efficiency. The hybrid propulsion chain weighs too much, and can at best match the propulsion efficiency of gas turbine based aircraft when one includes any airframe gains that can be made.
We will now finish the series by looking at a pure electric concept, designed for extremely short-haul routes. The Zunum Aero 10-seat commuter in Figure 1 will be our reference for such a design.
Our commuter aircraft, similar to the concept in Figure 1, will serve routes below 500nm. It’s a pressurized 10 seat commuter, used for short-haul routes of 100-450nm in its initial version.
The electric propulsion is now battery-based, Figure 2. This is simpler than a hybrid system. By using motors with 95% efficiency we can stay with air cooled motors and can therefore avoid a cryogenic cooling system in the aircraft.
Power switches as discussed in last Corner will still be needed, as the battery will be split in two systems for redundancy, each capable of feeding each motor with power.
The commuter will use electric fan based propulsors, each of 500kW maximum power. The reason for using fan-based propulsors for an aircraft with a maximum cruise speed of ~300kts is the lower noise levels from shrouded fan blades.
A nacelle shrouds the fan blade tip noise in the radial direction and it contains sound damping sections in front of the fan and aft. A low noise level is important for a commuter, which shall use close-by urban airfields.
The 500kW of the propulsor motor is only used for five minutes during One Engine Inoperative take-off scenarios. Normal take-off power is 450kW per propulsor with cruise consuming 350kW each.
Normal empty weight for such 10 seat commuters lies around three tonnes (6,600lbm) with Max Take-Off Weights of five tonnes (11,000lbm). Fuel load is one tonne with full passenger load, which gives a range of 900nm.
If we assume a commute sector of 150nm, our fully electric aircraft would consume 45kWh during take-off, 160kWh during climb, 250kWh during cruise and 20kWh during descent and landing.
This totals 430kWh, which with 0.3kWh/kg specific power for our battery, gives a battery weight of 1435kg. This is the battery weight for our mission consumption. To this we shall add the safety reserves and a margin as not all routes are 150nm.
If we assume the routes shall stay below 450nm, our battery will weigh four tonnes. To this we shall add the propulsors of 150kg each. Our total propulsion system weighs 4,300kg.
This shall be compared to a turbofan system of 900kg and one tonne of fuel, giving us a range double that of our electric aircraft.
Our electric commuter would have an empty weight of around six tonnes with a Max Take-Off Weight of seven tonnes.
This shall be compared with the turbofan variants three tonnes empty weigh plus the one tonne of fuel = four tonnes weight before passengers are loaded.
Our electric aircraft weighs 50% more and has half the range.
The question is now: can an aircraft which is tanked from the Electric Grid compensate the higher weight with the cheap energy from our power stations? We will explore the operational economics of our 10-seat electric commuter in the next Corner and compare it with a turbofan equivalent.