Electric Aircraft

Question 1

What is the main problem with electric aircraft?

Answer 1

Flight time is limited by available energy supplies. (Batteries)

Question 2

Give some advantages of electric aircraft.

Answer 2

• Zero emissions
• 90% energy efficiency and lighter
• Better reliability and as much as 10 times the time between overhaul.
• Immune to carburettor ice, or to water and ice in the fuel tanks.
• Easy to start
• Can be deliberately overpowered for a short time, generating as much as three times their normal power.
• No loss of power with altitude.
• Fairly flat torque-vs-rpm curve.
• Motors are small: can be placed far away from the power supply, reduce cowling and/or transmission shaft size.
• Multiple small motors can be used as a means of flight control.

Question 3

What are some disadvantages of electric aircraft?

Answer 3

• Total weight including the power supply is almost certainly heavier.
• Longer to refuel/recharge
• Upfront cost
• Need gear reduction
• No range benefit from the ‘fuel burn’
• Temperature effects on batteries
• Risk of battery explosion
• Batteries tend to lose power as they reach the end of their charge
• Batteries don’t last forever
• Environmentally hazardous materials (lithium, cobalt)

Question 4

What rechargeable batteries are used for aircraft?

Answer 4

Li-ion batteries. Energy density is still very low compared to traditional fuels.

Question 5

What do fuel cells use for electric aircraft?

Answer 5

Hydrogen fuel. Higher energy density than batteries.
- Can be quickly ‘recharged’ by replacing fuel.
- Bulky, explosive and needs pumping hydrogen infrastructure on ground

Question 6

Describe a hybrid electric engine.

Answer 6

Using conventional hydrocarbon engine to charge electric batteries.
Or uses electric engine as a ‘booster’
10-15% efficiency lost during energy conversion.

Question 7

Discuss solar cells used for electric aircraft.

Answer 7

• 5m^2 required for 1kW
• Solar cells don’t age well
• Higher attitudes are generally good for efficiency; there’s less air to filter out sunlight.
• Roughly 30% more sunlight available outside the atmosphere vs. at sea level.
• Typically weigh about 0.3kg/m^2 + installation weight
• Not formed to the desired aerofoil counter.

Question 8

What is BMF for electric aircraft and why is it important?

Answer 8

Battery mass fraction used for sizing for flight segments.