“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Saturday, January 18, 2020

More on Eviation Alice

Image credit: Jasper Juinen/Bloomberg
I published a post on the "Alice," a fully electrically powered airplane being designed and built by the Israeli Company "Eviation." The Alice is being promoted as a "9+2" airplane, that is, two pilots flying nine passengers. It is claimed that the airplane will have a range of 650 miles at a speed of 260 knots. Cape Air had made a "double digit" (actual quantity not stated that I can find) order (the "launch order") for the Alice. I expressed considerable skepticism, particularly with respect to the battery pack and to the claimed range.
Image credit: Eviation

The latest news is that two more airlines have placed orders for the Alice, bringing the total ordered to over 150. So three airlines have made substantial orders and several well-known OEM vendors (Honeywell, Bendix, Siemens, Hartzell) are providing equipment for the airplane. Is my skepticism unwarranted?

In my previous post, because the parameters needed for a direct calculation were not given anywhere that I could find, I got my estimate for the range by comparing the energy stated for the battery pack in the Alice to the energy in the Jet A fuel in a Pilatus PC-12. The number I came up with was 258 miles, far short of the claimed 650 and likely a deal breaker for the orders. Can I derail a multi-million dollar endeavor by back of the envelope calculations on an obscure blog?

There are two factors contributing to my vagueness on the range calculations: actual energy available in the battery pack; and the drag force on the airplane in flight. A rudimentary dimensional analysis show that the range is directly proportional to energy available and inversely proportional to drag, that is, ~R\propto\frac{E}{F_{d}}~, where R is range, E is total energy available, and Fd is the drag force. This, of course, makes intuitive sense but, at the moment, I don't know the proportionality constant.

Eviation claims a capacity of 900 kWh in the battery pack, though it's not at all clear how this can be accomplished. Eviation states that they use Li-Ion chemistry and also make a claim for a proprietary aluminum-air chemistry. I don't see how the aluminum-air chemistry can be feasible in an airplane, but who knows? Per the Wikipedia page for the Alice, the aluminum-air battery will be used on a later evolution of the Alice.

But, for Li-Ion chemistry, the current state of the art is about 260 watt hours/kilogram. At this energy density, 900 kWh would require 3,460 kg, or a bit under 7,630 pounds. At a maximum takeoff weight of 6,350 kg, this leaves 2,890 kg or 6,371 pounds for airframe, power plants, passengers, pilots, and baggage. Again, I don't have any data on the weights of the airframe and power plants. And, in my effort to be generous, the 260 watt hours/kilogram doesn't include the pack.

As to drag, I found a site that stated that the "L/D" (lift to drag) ratio of the Alice to be 24. Now, in cruise flight, lift is equal to weight. We'll assume a full load, giving a weight of 6,350 kg or 62,230 Nt. With a L/D ratio of 24, drag would be 2,293 Nt. Now, drag=thrust in straight and level flight, so we're looking at a thrust delivered by the propeller of 2,593 Nt. And P=F*V where P is power, F is force (thrust) and V is speed. So we have P=2,593 Nt * 134 m/s (260 knots converted to meters/second) = 346,817 watts or 347 kilowatts required in cruise.

Now, a constant speed propeller may be about 90% efficient, so the electric motors must deliver 347/.9 = 385 kilowatts. We have 900 kilowatt hours available so that's 900 kWh/385 kW = 2.34 hours. IFR (instrument flight rules) flight requires a minimum 45 minute (0.75 hour) reserve (we'll hold it to the minimum though I doubt a procedures manual for an air carrier operator would do so, and my policy is to never fly into my last hour of fuel) so we now have 1.59 hours or an hour and 35 minutes of battery capacity for cruise. Note: the specifications page for the Alice has been updated since my earlier post and gives some numbers that aren't too far off of mine, but I'm sticking with mine because they're derived from Eviation's performance claims.

I'm ignoring climb and this is generous because more power is used in climb (though that may not be the case for an electric airplane) and is at a slower speed (in all airplanes). So we can cruise at 260 knots for an hour and 35 minutes for a range estimate of 413 nautical miles or 475 statute miles. And, given the minimal reserve and ignoring climbing at low speed, this is generous.

I will agree that my rough calculations result in a range estimate higher than that I got using the Pilatus comparison, but it's significantly less than the 650 miles claimed by Eviation (I can't determine whether this is nautical or statute miles).

And this might be practical for a flight from, say, John Wayne Airport in Orange County to Las Vegas McCarran International, a distance of 226 (statute) miles, or Kennedy to Dulles, a distance of 228 (statute) miles. You wouldn't want to fly it to Reagan Airport because flight to
or from Reagan requires an air marshall and now you've lost 11% of your paying passenger capacity. There are many such city pairs. At right are 300 statute mile radius circles centered on New York City, Chicago, Houston, and Los Angeles. Such city pairs  as NYC - Philadelphia, Chicago - Detroit, Houston - Dallas, and Los Angeles - Las Vegas seem to be feasible.

And the economics seem favorable. 900 kWh of electricity probably would cost something on the order of $100, and a crew of two might be $100/hour. Maintenance on electric motors is much less demanding than on turbine or piston internal combustion engines.

So, taking everything into consideration, and if the data that's been provided so far is accurate, I think there may be a role for such an airplane.