By Bjorn Fehrm
July 8, 2021, © Leeham News: Last week, we looked at the cost of running an electric 19 seat airliner based on energy stored in batteries. We found the energy costs were lower than for the equivalent turboprop aircraft, but when we add the maintenance costs for the batteries, the operating costs were higher than today’s 19 seat commuter.
This was under the assumption that the battery aircraft had the same energy consumption as today’s aircraft. We now run this check. The result is eye-opening.
Last week’s article assumed our battery-based 19 seater had the same energy consumption as a 19 seat commuter like the Beech 1900 (Figure 2) or Jetstream 31 (Figure 3). The block fuel for the 200nm trip was 320kg with 308kg reserve fuel (IFR reserves of 5% route contingency, 100nm alternate, and 30 min circling) for the turboprop.
It equates to 1,037kWh block energy and 998kWh reserves when we convert with 12kWh energy per kilo for fuel and take 27% of that as the shaft energy transferred to the propellers (the gas turbines for the turboprop have 27% fuel to shaft energy efficiency).
Transposed to battery energy, this equates to a 4,508kg block energy battery with 4,339kg reserves. To reach these values, we used a 92% efficiency for the electric propulsion chain, battery-to-propeller-shaft, with 0.250 kWh energy per kg of battery (the energy density on a system level). Densities are today at typically 0.160 kWh per kilo but we upped this with 56% to cater for development in batteries during this decade.
So instead of 628kg jet fuel, where 320kg gets consumed during the flight, we have a constant 8,847kg battery system. The electric energy is 14.09 times heavier for the maximum range flight for the electric airliner.
It’s a devastating difference for an aircraft, making it practically impossible to design an aircraft with any reasonable performance around this excess weight.
But the bad news doesn’t stop there. The heavier turboprop, the Beech 1900, has an empty weight of 4,900kg. Laden with 19 passengers at 100kg we are at 6,800kg. The rest up to takeoff weight is fuel, in our example, 628kg. So we takeoff with 7,428kg and land with 7,108kg.
If we want to fly longer, we can fill an additional 338kg of fuel, doubling our range, before we hit Maximum TakeOff Weight (MTOW). If we want to fly longer still, we load only 16 passengers, and we can fill an additional 300kg of fuel. And so on, until the tank volume is the limit.
The battery aircraft is different. If we assume we can store the batteries in the wings (distributed along the wing to put the weight where the lift is), we could best case get away with an additional tonne of empty weight according to our performance model’s weight estimation part (this includes lighter electric motors instead of turboprop engines).
To this, we shall add the batteries. We land on an operational empty weight of 14,757kg. The only variable component is the payload, 1,900kg of passengers with bags. The takeoff weight for the 200nm trip is then 16,657kg. It stays constant during the whole flight and is also our landing weight.
Our wings must lift 16.6t of aircraft whether we fly 50nm or 200nm, and we can’t extend our range by offloading passengers as our energy doesn’t increase (the range increases with a few nm as the whole aircraft is now 300kg lighter, but this is only 1.8% of the total weight).
How do we design a 19 seater that weighs 214% more than a typical 19 seater? With difficulty! In fact, it’s virtually impossible to make something sensible around this weight.
Heart Aerospace says it’s possible. It’s even a good idea, as it’s a greener aircraft than the Beech 1900 or Jetstream 31. Let’s examine their proposed aircraft and apply the learning we have so far.
Figure 4 shows the ES-19 airliner that transports 19 persons 200nm according to the company.
The aircraft looks pretty normal compared with the Beech 1900 except for two aspects: the very large wing and wingspan and four engines instead of two. The four engines have little to do with any efficiency gains by having more propellers. Together with the large wing, with a wide span, it’s there to try and handle the hurdles this aircraft faces in performing a flight.
The hurdles start at takeoff field length and how the aircraft handles an engine out situation at takeoff. The field length for the ES-19 should be shorter than normal, according to Heart Aerospace. I can’t understand how this is possible with an aircraft that takes off with 2.4 times the weight of the Beech 1900.
Using our airliner performance model, the ES-19 has twice the runway requirements of the Beech 1900 at 7,600ft when we dimension the takeoff thrust of the engines to keep the V2 climb at over the required 3.0%.
The 3.0% are connected with the V2 safety speed directly after rotation, where we shall climb with a minimum angle of 3.0% with One Engine Inoperative (OEI). Four engines make us lose only 25% of the thrust instead of 50% for a two-engined aircraft. This certification requirement explains why we have four engines instead of two.
The next problem for the aircraft is that the climb drag is 55% higher than the Beech 1900, despite optimizing the climb speed to a lower value to decrease the drag.
It continues in the cruise where the drag is 91% higher, once again after cruising at an optimum 180kts instead of 230kts to keep the drag down. The 91% additional drag comes from the larger wing (parasitic drag) and the extra weight (induced drag).
This problem of higher drag continues through the trip’s descent and landing.
The increased weight of the aircraft makes it accelerate slower to takeoff speed. It doubles the runway distance needed for takeoff. Once in the air, the increased drag from the weight and the larger wing increases the aircraft’s drag. To compensate, we need to run the engines harder to generate more thrust. It increases energy consumption.
Despite optimizing the aircraft speed in all flight regimes to lower drag and thus thrust, the aircraft consumes more energy than the Beech 1900. As it flies slower than the Beech 1900 to minimize drag, it keeps this higher energy consumption for longer.
The result is the aircraft consumes three times more energy for the 200nm trip, at over 3,000kWh. As we have 1,000kWh available for the trip (the rest has to remain reserves), our range of the aircraft is less than 100nm.
There is no point in increasing the battery size to increase the possible range, as when we increase the battery size, and by it the aircraft weight, the range decreases. The aircraft drag rises faster than our available energy increases.
The problem with available energy means we can’t size the battery for optimal time on aircraft before changes. We need to load the battery to 100% for each flight to get somewhere (not the 200nm we should though). The operating costs skyrocket as we need to change batteries after 1,000 flights, which with six flights per day is every six months.
The finding is the battery energy density has to climb above 1kWh per kg to change this, and we are today some 600% from this point.
Electric aircraft came in vogue when electric cars worked, with Tesla as a good example. The point all missed was that our petrol cars are miserable energy hogs. They use about 5%-7% of the energy in the gasoline as they coast from stoplight to stoplight.
Battery or hybrid cars recover energy as you stop for the light. But there are no stoplights in the sky and our airliners use between 25% to 50% of the energy in the jet fuel. It means electric aircraft, whether with batteries or hybrid, doesn’t improve state of the art; they degrade it.
It’s typical that non-aeronautical people run companies like Heart Aerospace.
Aeronautical companies, like Airbus, Embraer, Britten-Norman, and Pipistrel, started by propagating for battery aircraft. Then they swung to hybrids and are now at hydrogen. With SAF (Sustainable Aviation Fuel), it’s the only technology that can bring us greener air transport.