AOPA Pilot Magazine - January 2021 - 83

P&E 

TECHNOLOGY

hp versus kW
Why eVTOLs and batteries don't hold up to horsepower
BY PETER REZ

THESE DAYS WE HEAR A LOT about electrically powered airplanes. Harbor Air
introduced an electrically powered version of the de Havilland DHC-2 Beaver;
Bye Aerospace is bringing to market an
electrically powered trainer called the
eFlyer 2; Pipistrel introduced the Velis
Electro; and the Eviation Alice is undergoing flight tests. We all see electrically
powered drones buzzing around, so why
can't the same technology be used to make
eVTOL (electrically powered vertical takeoff and landing aircraft) air taxis that whisk
their passengers through the skies above
crowded city streets?

Exhibit
Now!

to about 90 kW. At that setting fuel flow is
about 10 gph, which translates into about
360 kW. That means only 25 percent of
the energy from burning the fuel is used in
moving the airplane through the sky. This
is because of the poor efficiency of the piston engine. Fundamental thermodynamics
would say that the piston engine should
have an efficiency of almost 60 percent, but
various losses conspire to lower the efficiency-such as the engine's bearing and
piston frictions, poor valve seating, and the
need to drive accessories. All contribute to
power loss. This is where the electric motor
shines, with efficiencies as high as 95 percent. So, what's the problem?

POWER

When we fly our piston or turbine airplanes,
we're always concerned with power, which
is the rate energy is used-in other words,
energy divided by time. Our pilot's operating handbooks publish fractions of the
maximum rated horsepower for different settings at different altitudes-say, 65
percent or 75 percent-and give the corresponding fuel flows in gallons per hour. And
because fuel is a store of energy, fuel flow is
also equivalent to a quantity of power.
With electrical power the same principles apply. But now it's not avgas or jet
fuel that's the energy store. Instead, it's
the charge in the battery. So, it's the size
and charge of the battery that determines
the speed and range of the aircraft. Since
power is energy divided by time, we can
also say that energy is the product of power
and time. This is how electrical energy consumption is measured, as the product of
power in kilowatts (kW) and time in hours-
in other words, the kilowatt-hour (kWh).
Meanwhile, the unit of power for internal combustion engines is horsepower.
You can convert horsepower into its electrical equivalent, so the 200-horsepower
Lycoming IO-360 in my Mooney produces
a rated power of 150 kW. I usually fly at a
60-percent power setting, which translates

ENERGY DENSITY

The performance of batteries is specified in
terms of the energy they store divided by
their weight, expressed in kWh/kg. This is
the battery's energy density. The theoretical
maximum for a lithium-ion battery with a
lithium metal oxide cathode and graphite
anode is 0.27 kWh/kg. In practice, battery
packs have not yet approached this value.
For example, the high-performance Tesla S
uses a 100-kWh battery that weighs about
1,375 pounds, which equates to an energy
density of 0.16 kWh/kg. Bye Aerospace optimistically quotes a value of 0.25 kWh/kg for
its eFlyer 2's battery.
But those figures are still very low compared to avgas, which has an energy content
of 12.3 kWh/kg-about 50 times greater.
Putting it in perspective, the energy content in the Tesla S battery is the same as 2.1
gallons of avgas.
The low energy content for a given
battery weight does not offset the electric
motor's increased efficiency. An electric airplane with a battery that weighs the same as
a piston airplane's full fuel load simply won't
travel very far.
If the battery weighed the same as a
full fuel load one might think the range of
the electric airplane was one-fiftieth of the
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AOPA Pilot Magazine - January 2021

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