January 10, 2020, ©. Leeham News: We continue our series why e in ePlane shall stand for environment and not electric.
Before we continue the discussion about low CO2 footprint propulsion opportunities we look into some of the distributed propulsion concepts proposed for electric/hybrid airliners.
In this and the next Corner, we examine the substance in claimed aerodynamic gains and increased efficiency from such concepts.
The proponents for electric/hybrid propulsion solutions claim one of the advantages is it enables distributed propulsion like in Figure 1. This is an ONERA concept called Ampere, and it will represent the ideas of spreading the propulsion over the wing, by it increasing the aerodynamic and propulsive efficiency of the aircraft.
The other idea which is often presented as enabled with electric/hybrid is the aft Boundary Layer Ingestion (BLI) fan concept shown in Figure 2.
The graph is from a recent JADC (Japan Development Corporation) study which promised a 3% gain in efficiency with this technology.
We will now examine the realism in these ideas. We can do this in two ways:
We will use the second method as this makes for an easier and clearer discussion. We will start this week with the BLI concept and look into wing distributed propulsion next week.
Figure 2 shows the concept. The normal wing-mounted turbofans of an airliner are fitted with generators that can deliver an additional 1MW of power per side, which drives a 1MW electric motor/fan combination on each side in the tail of the aircraft.
The suction of the thick boundary layer at the tail of the fuselage into the fans increases the aerodynamic efficiency of the aircraft by reducing the thickness of the turbulent (and thus energy consuming) boundary layer at the aft end of the fuselage.
Now to the acid test of the idea: Could this have been done with today’s technology and if so, why wasn’t it done?
The sobering fact is the answer is yes. And more sobering: it’s more efficiently and conveniently done with today’s technology. Even worse, every airliner for 50 years has a powerful gas turbine where these motors/fans are placed, sitting idle during cruise, the APU, Figure 3.
This APU is perfectly capable to power any BLI fans sitting where the exhaust of the APU is depicted (the exhaust can be placed in the middle of the fans or any other convenient place). These APUs are generic power generators, generating pneumatic/electric or hydraulic power to the aircraft. They are used on the ground to provide the aircraft with these powers but they also work as standby power if needed all the way up to maximum flight level for the aircraft.
During cruise climb, cruise and descent they are not used, so they would be free to drive BLI fans via a gearbox/clutch and short driveshaft during these flight phases. With the present use spectrum, they are not designed to be highly efficient but this is just a requirement/design choice as they use standard gas turbine technology. The weight/volume and cost consequences of making them efficiently drive BLI fans during cruise would be marginal. No new technology or knowledge is needed.
Despite being almost for free and sitting there for 50 years on every airliner in the world, the aft BLI fan idea has not been realized. It hasn’t even been tested to my knowledge. If the gains are so obvious as the electric/hybrid studies show, why is this the case? The volume, weight and system complexity of the fan power source is already there on our airliners?
And it’s not because the benefits of boundary layer suction/ingestion are recent knowledge. This is known since the 1950s and its use was more popular then than now. It was implemented on both military and civil aircraft. But the gains are not that large. It’s been better to spend the money, weight and system complexity elsewhere on the aircraft.
And if we adopt the JADC and other BLI electric/hybrid aft fan ideas we have to find a (less ideal) place for the APU. With two 1MW electric motors, it gets crowded in the tail.
So instead of adding a gearbox/clutch and short shaft to the APU we now shall add 1 MW to each engine generator. Then a 3000V electrical distribution system to the rear, pumping 330 Amps per side to motors driving the fans. The propulsion chain loss of this is between seven and 13% dependent if we use cryogenic generators/motors or not and there would be additional weight, system complexity, and cooling requirements. But hey, it’s a good idea, it’s electric!
This shall be compared with less than 0.5% gearbox losses for the drive from the APU and marginal weight/cost consequences.
The conclusion from this lacmus test is clear. This concept, if it had the large gains that are now put forward would have found its way onto our airliners 30 to 40 years ago. It hasn’t.
And it’s not because we didn’t have a suitable distributed propulsion system available. The perfect power source was there since the 1970s. Time to think instead of just buying into what is presented as enabled with electric technology for airliners.
Next week we will look at wing distributed propulsion.