Battery Powered Aircraft — Chapter 13

Electric Motor: Core principles

Electric motor converts electrical energy into rotational mechanical energy to turn the aircraft propeller, similar to how a piston engine drives a propeller via a crankshaft.
The basic principle mirrors a generator/magneto: electricity drives rotation rather than rotation generating electricity.
Most electric motors use magnets to create the turning force.
A magnet produces a magnetic field; magnets attract or repel based on pole orientation: opposite poles attract, like poles repel. This magnetism creates rotational motion to turn the propeller.
The system relies on two main magnetic components: permanent magnets and temporary electromagnets (electromagnets).

Magnets and electromagnetism

Permanent magnet: a material that naturally exhibits magnetism (has fixed North and South poles).
Temporary magnet: magnet-like behavior created only when an electric current flows through a conductor (e.g., a wire). When current flows, a magnetic field forms around the conductor, creating poles; when current stops, magnetism disappears.
A conductor with current flow acts as a temporary magnet with a North and South pole determined by current direction.
An electric motor may use both permanent and temporary magnets inside a rotor or a stator. The fixed part (stator) is outside and does not turn; the rotating part (rotor) is in the middle and connects to the aircraft’s propeller via a shaft.

Rotor and stator: basic arrangement

The rotor turns around inside the fixed stator.
In the simple rotor-stator setup, when current flows through the rotor winding, the rotor becomes a temporary magnet whose poles interact with the permanent magnet in the stator, causing rotation due to attraction/repulsion.
If the poles align perfectly, rotation would stop, so the motor must continuously flip the rotor’s magnetic field to keep turning.

How rotation is sustained

In a DC motor, flipping the rotor’s magnetic field is accomplished by reversing the current flow through the rotor winding. A commutator switches the direction of current to ensure the magnetic field flips at the right times, keeping the rotor turning.
In an AC motor, the motor uses a rotating magnetic field produced by energizing multiple stator coils in a specific sequence; the rotor (often with permanent magnets) follows the rotating field, resulting in continuous rotation.
A coil (many turns of wire) is used to intensify the magnetic field in the conductor, enhancing motor performance.

DC vs AC motors in aircraft applications

DC motor (commutator-based): rotor is a temporary magnet, stator typically uses a permanent magnet. Simpler construction but has a mechanical commutator.
AC motor: more common in electric aircraft/cars due to higher efficiency; rotor usually contains the permanent magnet(s); stator contains windings (coils) energized in sequence to create a rotating magnetic field that the rotor follows.
Despite differences, both rely on magnetism to spin the rotor and drive the propeller.

Weight considerations

Electric motors generally weigh less than an equivalent-sized piston engine, which is advantageous for aircraft.
However, battery weight largely offsets these savings, influencing overall aircraft performance and range.

The motor’s internal complexity

Opening a basic electric motor reveals many closely wound wires (the coil) to strengthen the magnetic field.
The choice of permanent vs temporary magnets and their placement (rotor vs stator) often depends on whether the motor is powered by AC or DC electricity.

Battery: central energy storage

A battery is a self-contained pack of electricity stored in chemical form.
Most batteries contain three basic components:
- Two electrodes (plates) made of different materials, labeled positive (+) and negative (-).
- An electrolyte (chemical solution) between the electrodes.
A cell is the basic unit; multiple cells are connected to form a battery capable of delivering higher electricity.
When electrodes are connected in a circuit, chemical energy converts to electrical energy, allowing current to flow from the negative electrode to the positive electrode.
Figure references (13.6) illustrate a basic one-cell battery.

Battery types and storage

Primary battery: non-rechargeable; once all chemical energy is used, it cannot be recharged.
Secondary battery: rechargeable; can be replenished with another electricity source, reversing the chemical reaction.
Lead-acid and NiCad (Nickel Cadmium) rechargeable batteries are common in piston and gas turbine aircraft but are not ideal for the large energy storage needs of battery-powered aircraft.
Lithium-ion batteries are heavily used due to high energy density, high voltage capability, fast recharge, and low self-discharge. They can store about ten times more electricity than a lead-acid battery of the same size.
Lithium-ion advantages: high storage density, high voltage operation, rapid recharge, low self-discharge, widely used in small electronic devices.
Battery capacity is usually described in kilowatts (kW). Higher capacity enables longer flight duration and greater range.
Despite high energy density, Li-ion batteries are still roughly ten times larger than the equivalent amount of fuel needed for the same range, illustrating the ongoing density challenge.
Battery technology is a key area of development; very high-density batteries are seen as essential to extending the range of battery-powered aircraft.

Supporting systems for battery-powered aircraft

Inverter: converts DC from the battery into AC required by many electric motors; placed between the battery and the motor.
Cooling: crucial for motor, battery, and inverter; batteries are especially sensitive to heat from fast charging or heavy use, which degrades capacity and can pose safety hazards in flight.
Most small electric aircraft use liquid cooling, even on the ground, to manage temperatures. A typical cooling system includes a radiator to release heat and a pump to circulate coolant through components.

Voltage: a safety and design consideration

Voltage describes how quickly electricity flows through a circuit.
Examples from the text: a standard AA battery is about 1 ext{ V}, small piston aircraft batteries around 12 ext{ V}.
In battery-powered aircraft, the main electrical system is typically around 400 ext{ V}.
Some electric cars use voltages exceeding ext{≥ } 800 ext{ V}.
High voltage is necessary to deliver sufficient electrical energy to the motor and other components, but it presents significant hazards.
Safety cautions: high voltage is kept away from main airframe areas; pilots should treat wires as live even when the aircraft is shut down and avoid contact during pre-flight checks.

Practical implications and future directions

The general principle of a battery-powered aircraft is straightforward, but current battery technology is a major hurdle that constrains range.
Battery-powered engines are currently suitable for small to medium-sized aircraft with short flights.
Hydrogen is discussed as a potential method to extend range while maintaining environmental benefits, a topic to be explored next.

Real-world relevance and connections

Weight savings from electric motors are offset by battery weight, impacting aircraft performance.
Battery density and thermal management are critical design considerations for reliability and range.
Inverter and cooling systems are essential for safe and efficient operation of electric propulsion systems.
The transition from piston/gas turbines to electric propulsion hinges on energy density improvements and charging infrastructure.

Conceptual and historical notes

The motor operates by the interaction of fixed (stator) and rotating (rotor) magnetic fields, guided by permanent magnets and electromagnets.
The rotating magnetic field in AC motors is created by energizing stator coils in sequence, moving the rotor magnet in step with the field.
The DC motor uses a commutator to flip current, keeping the rotor magnet aligned with the opposite pole on the stator to sustain rotation.

Key figures referenced (conceptual descriptions)

Figure 13.1: Basic components of a battery-powered aircraft.
Figure 13.2: Magnetic attraction and repulsion between magnets.
Figure 13.3: Magnetic field around a conductor when current flows.
Figure 13.4: Rotor turning due to temporary magnet interacting with a permanent magnet.
Figure 13.5: Poles flip to maintain rotor motion.
Figure 13.6: Basic components of a battery cell (two electrodes, electrolyte).

Quick summary

Electric motors turn electricity into motion using magnets; both DC and AC variants exist, with different rotator/stator arrangements and switching mechanisms.
Batteries store chemical energy and convert it to electrical energy; lithium-ion is preferred for its high energy density but still presents size/weight challenges.
Supporting systems like inverters and cooling are essential for safe operation and efficiency.
High-voltage systems pose safety risks that must be managed through design and procedures.
Range limitations currently constrain battery-powered aircraft, but ongoing advances and alternative approaches (e.g., hydrogen) may extend future capabilities.