Bjorn’s Corner: Electric aircraft, Part 6

By Bjorn Fehrm

August 03, 2017, ©. Leeham Co: In our search for an electric regional aircraft configuration, we found that a jet aircraft requires too high power levels. The higher speeds of a jet aircraft take the power levels beyond what we can handle with an electric hybrid propulsion system.

Our ambition is to transport 50 passengers on a regional network. For networks which have sectors around 200-300nm, the turboprop is the preferred regional aircraft. We will now re-direct our hybrid regional aircraft project to this market segment.

Figure 1. An ATR 42-600 48 seat regional turboprop. Source: Wikipedia.

Characteristics of a turboprop

The thrust to drive an aircraft forward is developed in the same way between a turboprop, propfan and turbofan. The air approaching the aircraft is caught by the propeller/fan and is kicked backwards at higher speed. The generated thrust is:

                  Thrust  = air massflow * air overspeed

The higher the air mass flow and the lower the air overspeed compared to the surrounding air, the less work is needed to generate the thrust. The propulsive efficiency is increased. This is where the turboprop excels. It accelerates a large mass of air to a low overspeed. So, to produce the thrust needed for take-off, we can spend less energy.

The drawback of the turboprop comes from the same physics. When the aircraft is moving forward, the aircraft speed will quickly reach a speed which is close to the speed of the airstream from the propellers. The overspeed reduces and the thrust falls. Figure 2 shows in the left graph the suitable speed range for different propulsion systems.

Figure 2. Speed range for propulsive concepts and drag vs. thrust for jet and turboprop aircraft. Source: MIT and US Navy. Click to see better.

The turboprop is a very high bypass ratio propulsion. It’s efficient at low speed, but loses efficiency over 300kts. The turbofan does not lose thrust as speed increases. But the propulsive efficiency of the turbofan is lower around 100-150kts, where the critical V2 speed is. This is why we don’t need 7000kW to propel an engine out turboprop through V2.

Our ATR 42 sized turboprop

The reference turboprop we will use to design our hybrid regional airliner is the ATR 42-600, Figure 3.

Figure 3. ATR 42-600 regional turboprop 3D drawing. Source: ATR.

The ATR 42 adds means to bring down the power requirement further. As we see in the right-hand part of Figure 2, the drag of an aircraft shoots up at low speed, where the V2 speed is. It’s the induced drag that increases as speed decays (induced drag increases with lower speed, parasitic drag, where air friction drag is dominant, decays).

A turboprop uses a high aspect ratio wing to take the induced drag down. The aspect ratio for the ATR 42 wing is 11.0. The high aspect ratio and the low V2 speed, 112kts (Figure 4), gives a low drag level during take-off.

Figure 4. Key data for the ATR 42-600. Source: ATR

The power requirement for a One Engine Inoperative (OEI) take-off for the ATR 42-600 is therefore 2400hp or 1800kW.

In the next Corner, we will use the lower power requirement of the turboprop to design our hybrid power chain for our 50-seat regional airliner.

10 Comments on “Bjorn’s Corner: Electric aircraft, Part 6

  1. Using the Airbus Perlan II as a basis with its Aspect Ratio of 27 and scale/redesign it to a 18 seat Jetstream 31 size plane will give a smaller Power requirement that might be realistic in the pretty near future.

    • The market for a ATR42 sized plane would be greater- remember the goal is 50 pass.
      You would think the D0-228 would make a better starting point than the very old design Jetstream if you wanted to go for a 19 seater

      • The suggestion was starting with the Perlan II and scale it bigger and modify the fuselage to fit 18 pax. Keeping the wing aspect ratio of 27. The idea is to get a low V2 speed and low power requirement thanks also to the high CL value (Lift/Drag). I think Björns further analysis will show that the mass and cost of the hybrid powerplants and li-ion batteries for a 50 seater with modest aspect ratio wings will be too high.

        • A traditional structure; a rigid tube and hollow wing for liquid fuel power, probably won’t show practical benefits when loaded with batteries, motors and generators, so I tend to agree that if Bjorn uses this type of design for the model, the conclusion might be its impractical.

          The move to electric power, software control, new opportunities present themselves that may permit fundamentally new design principles to be used that are impractical with gas turbine cores as the propulsive power.

          New design thinking is required – current tube and wing design is driven by the need to store large amounts of liquid fuel, smallest number of engines because they are heavy and expensive. Current, non-super conducting motors can make 5 to 5.40 kW/kg (Yasa / Siemens). Such a vehicle with the electrical storage providing the strength, plus directional control by power control or gimbals could change the picture.

          I will agree this is further away than 5-10 years!!

        • No ones going to be buying a 19 seater these days, its not because the fuel cost is too high , which is a pity.
          Where I live they used The beech 1900 with its raised roof for many years, now they are gone. Most services were replaced either by older 30 seaters or newer 50 seaters. Cabin size expectations are greater is one of the reasons.

  2. I wonder if it really make sense to adapt the existing plane concept to the electric plane, as assumend in these articles. In my opinion, this doesn’t use the benefit of electrical engines.

    I like the Lilium concept (https://lilium.com/technology/) and wonder if this would scale (not with a battery but as hybrid with jet engine cores as energy supply).

  3. My guess is the best way to introduce electric propulsion is in a substantially rethought transportation system. Say as a very frequent walk up air taxi service running on a very limited number of suitable city pairs (eg US North East Corridor) using vehicles certainly below 20 seats. Electric ensures the vehicles are quiet enough to go close to downtown, use autonomous electric tractors on ground to accelerate the vehicles (ie reduce engine out power required) if a benefit is seen, add in autonomous flight to cut out much of the staffing expense, something similar to the Cirrus parachute system to add safety reassurance, free routing and run a flight every 15 minutes or less. Get these technologies in use and learn about and improve them before trying anything ‘airliner’ sized.

    Of course, each of these elements also needs to be suitably mature in itself before combining into such a system. Free routing is coming, Cirrus parachute would need to be trialed on larger, somewhat faster aircraft, autonomous flight of a Jetstream (so the same sort of size) is certainly already accomplished and under active trial plus is as much about legal issues as the technology, while small, simple electric air vehicles should be relatively inexpensive to design, certify and manufacture.

  4. I think the amounts of energy required for serious passengers service are not best stored in batteries. Bjorn showed. On shorter routes high speed trains/ cars are superior anyway. On top of that if the electricity still comes from coal/ natural gas like today, who are we fooling?

    Maybe hybrid propulsion, energy regeneration, taxi etc can produce profitable cases, without green naivity and subsidies.

    • There are a number of short routes over water where you commute by boats taking 0.75-3 hours that are targets for electrical Aircraft commute. Another is massive morning traffic jams making a short & cheap electrical flight sensible especially to the Airport dedicated quadcopter areas. Like from upstate NY to JFK, SFO to Palo Alto/Mountainview where the Apple bus would pick you up att Moffet Field. The key besides batteries, noise, cost is automated flight Control and 4D ADB-S in/out FAA/Eurocontrol route allocation that can handle massive amounts of electrical Aircrafts/quadcopters. Before such system is in place and can handle route changes due to wind shear, storms, icing conditions, medical emergencies, technical faults etc. it will just be trails and low volume flights.

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