Expert: Paul Soderman - 4/18/2009

Question
Hi Paul

 There are a few small electric - powered aircraft in existence today. What would it take to build an all electric version of a King Air, a Dash-8 or the equivalent of a 747? Here is an outline of my calculations so far-

The electric power system will consist basically of a battery, motor and controller. In doing preliminary calculations, the weight of the electric motor vs the turbine engine must be calculated. The fuel weight must be subtracted from the required battery weight.

For calculating the battery weight, the KwH or the motors must be multiplied by the number of hours of flight. Then batteries including the required number of KwH rating and containing the total energy must be included.

Finally, for power requirements, could we take full power for 30 minutes and then 50% (cruise power ) for the rest of the flight?

Please tell me if I am on the right track.

Some information:
The ElectraFlyer-C airplane has a an 18 hp electric motor weighing 29 lbs and battery pack weighs 79 lbs for 90 minutes of flying.

If we take a Cessna 310 for example and replace the engines with two 300 hp electric motors and battery packs for the same endurance how much would it weight?

A simple calculation would be 300/18 x 29 x2  = 962 lbs for the motor and 300/18 x 79 = 1311 lbs for the battery pack.

Is this calculation reasonable? What is the similar weight of Avgas ? at 6 lbs per gallon 100 gallons would be 600 lbs.

Answer
I can't see anything wrong with your calculation except that I don't know enough about batteries to know if you can scale on power required so easily.  In other words, the high energy density batteries in the ElectraFlyer may not function as well at large size and power required.  You are scaling from 5.6 kWH ElectraFlyer battery to a 358 kW Cesna power plant (358 kWH for one hour flight).  My solar batteries (Surrette S530) are designed for deep, multiple cycle use and are very rugged and heavy (127 lb), but they can only put out 144 amp hours for one hour or 0.86 kW for one hour at 6 volts.  To get the 358 kW required for a Cesna 310 would require a lot of solar batteries.  You need to talk to a battery expert.

Gasoline has an energy density of about 45 kJ/kg or 8.76 kwH per liter.  You can do the math for the Cesna.
Paul