February 11, 2022, ©. Leeham News: In a sister article, Part 6P. Energy consumption, the deeper discussion we use Leeham’s Aircraft Performance Model from our consulting practice to generate the aeronautical energy consumption for aircraft like Eviation’s Alice and Heart Aerospace’s ES-19.
This is the energy needed to combat the drag of the airframe during flight (Figure 1). We then add the losses in the chosen propulsion system to arrive at the energy drawn from the energy source.
The energy needed for the aircraft is found through the force required to combat the drag on the airframe as it flies through the air, Figure 1. When we multiply the force with the traveled distance, we get the energy consumed.
When we climb, the force combines drag and the force needed to get the aircraft to a higher altitude. The potential energy we gain can be used to neutralize the drag on the way down.
A passenger aircraft must descend at an acceptable rate for the passenger’s ears, therefore, we cannot descend at a rate where we gain energy back in the descent, but the energy consumption is low in this phase.
The claimed range for the Alice is 440nm with reserves, for the ES-19 216nm including reserves. The results from the model doesn’t agree. Neither aircraft can fly the distances promised, not even with a 30 minutes VRF (fair weather) reserve.
The Alice, with its large 3720kg 820kWh battery, can fly a still air 200nm route in Europe and land with a 30 minutes regulatory VFR reserve. For the 45 minutes US reserve, there is not enough energy.
If an IFR alternate of 100nm is required with 30 minutes circling (EU rules), the range falls below 100nm.
The ES-19 can’t fly 200nm and have any reserves; in fact, the battery is empty before we land. It can’t even fly a 100nm flight and land with European VFR reserves. This assumes the 3,000kg (four times 750kg in the nacelles) batteries give us 660kWh of energy (the same energy density as the Alice batteries).
The problem for these aircraft is not that they consume a lot of energy, it’s the amount of energy they carry. The ES-19 takes off with 1/12 the energy of the 19 seat Beech 1900. As a result, everything is done to mimize energy consumed with a low cruise speed and wide wings. But it’s not enough when the energy is 8% of what a normal Commuter has.
Eight percent is not enough to fly to the destination and have anything left for reserves, not for bad weather conditions nor for good weather conditions if we have a headwind.
Didn’t the organizations behind the aircraft realize this?
The described companies are upstarts, started by enthusiastic entrepreneurs wishing to change how air transport is done. Unfortunately, architecture choices and presentations on what’s possible were made before aeronautically competent people arrived. When experienced engineers work through the problem, it’s too late. Media have been briefed on the “fantastic news,” and investors promised groundbreaking new aircraft.
The reality was all the time what we see in the Tecnam slide below. Realistic data from an experienced manufacturer. Solid, though not very exciting information.
If we go by the more than handful of battery aircraft projects that have preceded these two, the next step is to look at “range-extending” by adding a gas turbine driving a generator to get more energy. As you calculate this through, the battery gradually shrinks to gain weight (the gas turbine with generator and fuel systems is not for free), and the fuel burned from the gas turbine increases.
In the end, you have a more complex and expensive aircraft than the one you replace without gaining anything in fuel burn. Then you start looking at hydrogen as an alternative.