February 4, 2021, ©. Leeham News: We did a simple reality check on two high-profile ideas for Sustainable Air Transport last week, the Eviation Alice project and Heart Aerospace’s ES-19.
We now look at energy usage when performing Sustainable Air Transport flights, but it can be timely to recap some fundamentals of such flights before we discuss this.
I’ve covered this previously, so here’s a recap. In cruise, an aircraft develops lift to counter weight and thrust to counter drag. In a steady-state, these are equal opposing forces (the model in Figure 1 is simplified but serves the purpose).
So to counter drag, we need thrust. Drag is composed of two components (if we ignore minor drags, typically less than 5% for airliners), parasitic drag (drag due to size) and induced drag (drag due to weight).
Induced drag is at highest directly after rotation and declines with speed (Figure 2). To alleviate it, we need wingspan.
Parasitic drag increases with speed, is low at rotation but high at cruise and descent. The dominant source of parasitic drag is air friction against the aircraft’s wetted area (thus it scales with size).
All air vehicles live with these forces. In an aircraft design process, a good trade between these is sought.
A troubling fact for the new concepts is the development of mass with flight time, Figure 3.
The figure is to scale and depicts a typical Commuter flight with nine passengers for a 200nm hop (A to B or a-b) with the typical reserves of 100nmm alternate (B to C or b-c) and 30 minutes circling at alternate (C to D, c-d with EASA turbine rules, the US FAA says 45min circling). The principle of the figure is valid for larger aircraft; it’s just the proportion of fuel as a % of the total mass that increases, and thus the amount of change of mass with time.
Mass vs. time for a battery aircraft is at max all the time, including when diverting to an alternate and circling there. The fueled aircraft takes off with less than half the mass and then burns off fuel mass during the whole flight. It lowers the drag and thus energy consumption for the later parts of the flight.
The hybrid is somewhere between these depending on the relation between battery stored energy and gas-turbine generated energy.
The aircraft designer iterates to an optimal trade for the drags by varying wingspan and area so the fueled aircraft has induced and parasitic drag in balance at the midpoint of the typical flight for the aircraft type.
As he designs a battery aircraft, he must upsize the wingarea to reach the more than double lift force at rotation and span to keep induced drag down after rotation. It results in a larger wing, resulting in a larger empennage (to control the increased moments). It increases the parasitic drag (drag due to size) and masses, thus induced drag, which requires more span, which.. . This is called the weight spiral and it’s an art to get it to stop at an optimal point.
So the conclusion is:
A major problem for battery and hybrid aircraft is the required reserves for safe air transport. All pilots, my included, have had the bad stomach feeling when your planned flight energy runs tight because of a non-predicted weather change.
Therefore, the regulatory reserves are an ABSOLUTE minimum; all pilots I know add on top. The problem for heavy energy concepts is that as the weight creep hits the projects (as it always does), the only variable is cutting flight range. Reserves are untouchable and stay the same whether the aircraft has an 800, 400, or 200 nm range.
If a project has a 200nm range on the Powerpoint, it will surely have less when flying in the big windtunnel (the reality). The problem with the energy budget is that reserves grow to the flight budget’s size pretty quickly.
It’s refreshing to see the honest performance summary in Figure 4 of the Tecnam company that makes the P2012 Traveller nine-seater Commuter tailored for Cape Air in Boston (left column). Together with Rolls-Royce, Tecnam projected the P-VOLT battery version of the aircraft (right column, the black % annotations are mine)). To my knowledge, it’s the first passenger battery Commuter that has reached a final specification stage from an experienced company.
I contacted the company about the reserve assumed and got a straight answer; it’s a 30 minutes VFR reserve.
The tables show the reality of where we are today and tomorrow. My conclusion is; if an IFR flight would be required, the P-VOLT is a non-starter as there is no energy for any alternate (it has to come out of the route energy, and this is tight to the point of non-usable).
The above mass realities and drag trades explain why upgrading an existing fueled aircraft with battery or hybrid powertrains results in large payload and range losses. The span and structural strength are not there to compensate for any increases in mass, so the weight increase of an alternative powertrain with energy store will hit the payload.
At the end of the day, an increase in mass hits energy consumption, and it makes a hybrid consume more fuel than the design it replaces. This is only evident when every aspect is covered, and the final mass and drag bill is run in a competent flight model (or a functional prototype).
Typically after about a year or two, the projects go quiet about their initial lofty claims and then follow a total silence, or it seeps through hydrogen is studied.
We will model the energy consumption of some typical aircraft in the following Parts. I wanted to do this recap, as we can then avoid the base discussion of why the alternative aircraft consumes more energy than the ones they replace.
For those that think new, ingenious aerodynamic concepts (like multiple propellers) will fix the above, type in ePlane in the search box top right and read from Part 4. None of these concepts helps us; most tax the budget further.
Part 3 of the ePlane series describes the perhaps only propulsive change we can do which will have a positive effect; the Open Rotor. I will cover the CFM RISE project later in the series. Part 3 describes the runup work to RISE.