“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)

Sunday, July 14, 2019

Eviation Alice

Image Credit: Eviation
As I've mentioned in quite a few previous posts, I'm a pilot by avocation and a dedicated follower of all things aviation. My YouTube feed recently decided that I'd be interested in the Eviation Alice, an all-electric airplane currently being developed by Eviation Aircraft, an Israeli company.

On my first time through the video, I was quite skeptical. The performance claims seemed to be outside of the range of current or near future technology (see below). 

Image Credit: Eviation
As is well-known, the specific energy (energy/mass) of chemical batteries is far below that of aviation fuel and so my first thought was that a 900 kWh battery pack would make the airplane far too heavy to carry any significant payload. However, Eviation claims to have achieved a specific energy "surpassing the 400 Wh/kg mark." This is quite an achievement if true. Such a battery pack would weigh (using 400 Wh/kg and ignoring "surpassed") 2,250 kg., or 4,960 pounds.

On the other hand, Eviation also states that the battery is 65% of the airplanes weight. Let's work back. They state it's a 9+2 airplane, i.e., 9 passengers, 2 pilots. There is no fuel. A standard FAA adult weighs 170 pounds, we'll add a few for (ahem) girth growth and baggage, call it 185 pounds. 11*185 = 2,035 pounds or 923 kg. The maximum gross weight is shown as 6,350 kg. We subtract the payload and get 5,427 kg in airframe and power plant weight. Eviation states that the batteries comprise 65% of the aircraft's weight, yielding (an approximation, of course) an implied battery weight of 3,528 kg.
Image Credit: Eviation

In any case, current lithium ion technology achieves specific energies on the order of 250 wH/kg, but aluminum-air batteries can achieve much higher specific energies. Eviation states on their site (from which the graphic at right is copied) that they have a proprietary aluminum-air system in addition to (?) their lithium ion batteries. However, naive as I am, I don't see how this is feasible, given the fact that in an aluminum-air battery, the aluminum anode is consumed in the oxidation half-reaction. The electrode can be reprocessed, but this is hardly the same as plugging into a charging system! Until I know more about the proprietary system, my skepticism is intact.

But lets suppose that Eviation has conquered this issue and can achieve 400 wH/kg in a practical system. As mentioned, they state that the airplane is a 9+2 configuration (9 passengers and two pilots). The usual tradeoff of fuel for payload with which I deal (and which is a consideration for all fossil fuel powered aircraft) is not a factor here. But we have (at least, depending on which of Eviation's numbers we use) 2,250 kg of batteries and 923 kg of passengers and miscellaneous for a total of at least 3,173 kg and probably more. From the maximum gross weight, this leaves 6,350-3,173 = 3,177 kg for the airframe and power plant.

The YouTube video states that the current prototype uses three Siemens 260 kW electric motors. The best information I can find gives a weight of 50 kg for these motors, so the total is 150 kg. We're down to 3,027 kg for the rest of the aircraft - avionics, fuselage, wings, empannage, propellers, interior furnishings, and miscellaneous. And recall that this is the absolute maximum possible weight in that it assumes the absolute minimum battery weight. With more conservative (not to say plausible!) assumptions for battery specific energy something like 1,830 kg would be the maximum. It's stated that the aircraft is all composite, I'll say it had better be!

All in all, given the contradictory and confusing information on the web site and the weight considerations outlined above, I find it very hard to be anything but skeptical, though I wouldn't go so far (at this point) as to call it a fraud. According to the YouTube video, Cape Air made a double digit "launch order" (airline industry terminology for the first purchaser of a new model). With  such an order and with Honeywell (fly by wire controlls), Bendix (avionics), Siemens (motors), Hartzell (propellers) and others signed on to supply components, there seems to be at least some level of confidence. And Eviation is expecting type certification in 24 to 30 months for the unpressurized version. But I wouldn't book a seat just yet.

What about aerodynamic calculations? None of the key parameters are given to calculate from first principles, so I'll use comparisons to known aircraft. Proceeding in this manner, 260 kW is 349 horsepower (call it 350) so the total power available is 1,050 horsepower and the cruise airspeed is listed as 240 knots. The Pilatus PC-12 uses a Pratt and Whitney PT6A-67P gas turbine engine flat rated to 1,200 horsepower and cruises at 280 knots. The maximum takeoff weight of the PC-12 is 4,740 kg. So, on its face, it would appear that the Alice has sufficient power to produce the listed speed.

How about range? Here we have the statement that the range of the Alice is 650 miles (statute I assume). Again, using the Pilatus PC-12 as a measuring stick, that airplane has a normal range of 1,646 statute miles. It has a fuel capacity of 403 gallons and, if we assume 40% efficiency of the gas turbine engine and use 131 megajoules/gallon, the engine delivers 2.111*10^10 joules or 5,866 kilowatt hours to the propeller to go 1,646 miles. The Alice has a battery capacity of 920 "usable kilowatt hours" (yes, different than 900 used above, but the statements from Eviation are widely variable depending on which interview or site I look at). Then, if we take (920 kWH/5,866 kWH)*1,646 mi., we can estimate that the Alice should have a range of 258 miles. It's implausible that the Alice has an aerodynamic efficiency of over twice that of the PC-12 so, again, I'm very skeptical.

Eviation states that the Alice on display at the recent Paris Air Show is a flying prototype and they are only awaiting FAA approval to begin flight tests. They state that they expect to fly later this year. Given the lack of consistency of their claims and the rough estimates above, I'll await the results. But, despite the apparent confidence of the very reputable OEM suppliers listed above, I'm putting this in my "I'll believe it when I see it" file.

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