Selecting and installing equipment

Why this matters

A drone built from good parts may still not fly if the parts do not fit together: connectors that look identical but have different pinouts, a propeller that draws more current than the ESC can handle, or an added camera that shifts the centre of gravity so one motor works hard all the time. A builder’s job therefore starts before the first bolt is tightened: calculate and plan so every part works with the others, then record it so others can check.

This module uses one hypothetical case throughout the course: a quadcopter for survey photography belonging to an agency, running a 6-cell battery with a camera mounted at the front.

The connection diagram

A connection diagram maps every wire in the aircraft: which lines carry power, which carry data, and which way the data flows. It is like the wiring plan an electrician needs before running cables in a house.

• Power flows from the battery through the power module to the ESCs and motors. These lines carry tens of amperes and need wires and connectors rated for that current
• Voltage and current data from the power module go to the flight controller, which uses them to estimate remaining energy and trigger failsafes
• Motor commands go from the flight controller to the ESCs. Modern ESCs commonly use the digital DShot protocol; the ArduPilot documentation recommends DShot600 for most boards. ESCs using DShot or the CAN bus (DroneCAN) do not need the throttle-range calibration that PWM ESCs need
• Peripherals such as the GNSS and compass, RC receiver, telemetry radio and camera each use different ports and protocols, such as UART, I2C or CAN

Mass budget and centre of gravity

A mass budget lists the mass of every part, including those that are often forgotten: wires, connectors, mounts and straps. The centre of gravity (CG) is the point where the drone would balance on a fingertip. It is the average of each part’s position weighted by its mass, with every position measured from the same reference point (origin).

$x_{CG}=\frac{{\sum }_i{m}_i{x}_i}{{\sum }_i{m}_i}$

If the CG is not beneath the centre of the four motors, the motors nearer the CG must work harder to keep the aircraft level, leaving less reserve thrust and draining the battery faster. The FAA’s aviation maintenance technician handbook uses the same principle for aircraft weight and balance.

parts = {  # name: (mass kg, position x m; positive = towards the nose)
    "frame": (0.62, 0.0), "motors": (0.34, 0.0), "ESCs": (0.10, 0.0), "FC + GNSS": (0.09, 0.02),
    "battery": (0.72, -0.03), "camera": (0.35, 0.12), "wiring and mounts": (0.12, 0.0),
}
mass = sum(m for m, _ in parts.values())
cg_x = sum(m * x for m, x in parts.values()) / mass
print(f"total mass {mass:.2f} kg, CG {cg_x * 1000:+.1f} mm from the frame centre")

others = sum(m * x for name, (m, x) in parts.items() if name != "battery")
battery_x = -others / parts["battery"][0]
print(f"move the battery to x = {battery_x * 1000:+.1f} mm to centre the CG")

total mass 2.34 kg, CG +9.5 mm from the frame centre
move the battery to x = -60.8 mm to centre the CG

Thrust, current and energy

The thrust-to-weight ratio (TWR) is the total maximum thrust divided by the weight, both as forces in the same unit (newtons). Weight $W=mg$ is not the same as mass $m$. Thrust must come from the manufacturer’s test table for the motor, propeller and voltage actually used. A motor’s KV rating does not tell you its thrust: the same motor with different propellers gives different thrust and draws different current.

g = 9.81
weight = mass * g
max_thrust_per_motor = 14.0   # N, from the test table for this motor, propeller and voltage
twr = 4 * max_thrust_per_motor / weight
hover_share = weight / 4 / max_thrust_per_motor
print(f"weight {weight:.2f} N, TWR {twr:.2f}, hover needs {hover_share:.0%} of max thrust per motor")

esc_rating, max_motor_current = 30, 22   # A
print(f"ESC current margin {(esc_rating - max_motor_current) / esc_rating:.0%}")

weight 22.96 N, TWR 2.44, hover needs 41% of max thrust per motor
ESC current margin 27%

Battery energy is approximately the nominal voltage times the capacity; divide by average power to get time. This is a starting model, because voltage and power change with load, temperature and battery age.

v_nominal, capacity_ah = 22.2, 5.0          # 6S 5000 mAh
usable_wh = v_nominal * capacity_ah * 0.80  # assume 80% of nominal energy is usable
hover_power = 4 * 62                        # W, from the test table at hover thrust
print(f"nominal {v_nominal * capacity_ah:.0f} Wh, usable {usable_wh:.1f} Wh, "
      f"hover about {usable_wh / hover_power * 60:.1f} min")

nominal 111 Wh, usable 88.8 Wh, hover about 21.5 min

The 80% and 62 W per motor are assumptions of this example; do not apply them to another aircraft without checking. The time must be confirmed by a test flight whose voltage and current log you read, which you will do in module 2.

Installing for compatibility

Compatibility has three dimensions, and all three should be checked before first power-up.

The compass should be kept away from high-current wires and motors, because current creates interfering magnetic fields; this is why GNSS and compass units are often mounted on a mast. The flight controller should sit on a vibration-isolating mount that is not so loose it moves freely.

Module lab

Common mistakes

Summary

• The connection diagram and register show power, data and protocol for every line
• The mass budget must include every part; CG is the sum of $m_i{x}_i$ divided by total mass, from one origin
• TWR uses thrust from a test table matching the real set-up, and the ESC current margin must be checked
• Flight time from energy divided by power is a starting model to be confirmed from logs
• Check mechanical, electrical and data compatibility, and always remove propellers for bench tests

Check your understanding

1. A 2 kg part is at $x=0$ and a 0.5 kg part at $x=0.20$ m. Where is the CG?
2. A 2.0 kg drone has 49.05 N of total maximum thrust. What is its TWR (use $g=9.81$)?
3. What is the nominal energy of a 4S (14.8 V) 5200 mAh battery in Wh?
4. With 88.8 Wh usable and 400 W average power, about how many minutes can it fly?
5. Why must propellers be removed before bench-testing motors?