Module 2/5 · Weeks 4–6 · 27 h

Power and propulsion

DRT 341 Advanced Unmanned Aircraft Systems Technology and System Architecture

About 85 minDraft, awaiting reviewLast updated 26 September 2026

By the end of this module you will be able to

Compare multirotor, fixed-wing, hybrid VTOL and helicopter aircraft in energy use and mission fit
Calculate fixed-wing power required from the lift-to-drag ratio
Estimate endurance from specific energy and battery mass fraction
Explain battery-electric, hybrid and fuel-cell propulsion architectures
Choose a power system to meet mission requirements

Prerequisites: DRT 341 Module 1

Energy decides what a drone can do

The first question users always ask is “how long can it fly?” The answer depends on two things: how much power the aircraft needs to fly and how much energy it can carry. This module compares aircraft types and energy sources so you can choose a system for a mission on sound grounds.

Aircraft types and power use

Type	Strengths	Limitations	Suited missions
Multirotor	Vertical take-off, hover, easy control	Very high power, short endurance	Close inspection, single-point imaging
Fixed-wing	Low power, long endurance and range	Needs a runway or launcher; cannot hover	Wide-area survey, long linear patrols
Hybrid VTOL	Vertical take-off plus efficient wing-borne cruise	Complex, heavier from two systems, risky transition	Wide-area survey without a runway
Single-rotor helicopter	Better hover efficiency than multirotor, heavy lift	Complex mechanics, heavy maintenance	Agricultural spraying, heavy lift

The key difference is how lift is produced. A multirotor must push air down continuously to support its weight (see momentum theory in DRT 112). A fixed-wing aircraft gets lift from forward motion, so its propeller only has to overcome drag.

Fixed-wing power required

In level flight, lift equals weight and thrust equals drag. The lift-to-drag ratio ( $L / D$ ) measures wing efficiency, so

D = \frac{W}{L / D} P = D V = \frac{W}{L / D} V

Example 1. Comparing power use

A 3 kg fixed-wing aircraft with $L / D = 10$ flies at 15 m/s.

Drag $D = 29.43/10 \approx 2.94$ N
Mechanical power $P = 2.94 \times 15 \approx 44$ W
With combined propeller and motor efficiency 0.6, electrical power $\approx 74$ W

Compare the 2 kg multirotor using about 237 W in hover (DRT 112 Module 4): the heavier fixed-wing uses roughly a third of the power. That is why wide-area survey favours fixed-wing or VTOL aircraft.

Energy sources

Specific energy ( $e$ ) is energy per unit mass, in Wh/kg. The higher it is, the more energy you carry for the same weight.

Energy source	Approximate specific energy	Note
LiPo battery (whole pack)	about 150–200 Wh/kg	Delivers high current well; common in multirotors
Li-ion battery (whole pack)	about 200–250 Wh/kg	More energy but lower current capability; suits fixed-wing
Gasoline (fuel only)	about 12 000 Wh/kg	Small engines convert only a small share to work
Hydrogen (fuel only)	about 33 300 Wh/kg	High-pressure tanks are heavy and bulky

The hydrogen and gasoline figures are lower heating values from U.S. Department of Energy documents. They look far higher than batteries, but the comparison needs care: they are fuel only, excluding engines, tanks and conversion efficiency. At system level the gap narrows a lot; hydrogen’s advantage is clearest on missions lasting several hours.

Estimating endurance from battery mass

E = m_{b} e_{b} t = \frac{η _{u} E}{P _{elec}}

where $m_{b}$ is battery mass, $e_{b}$ specific energy and $η_{u}$ the usable fraction (often set to 0.8).

Example 2. Fixed-wing endurance

The aircraft in Example 1 carries a 1.0 kg Li-ion pack at 200 Wh/kg.

Energy $E = 1.0 \times 200 = 200$ Wh
Endurance $t = 0.8 \times 200/74 \approx 2.2$ hours

Raise the battery to 1.5 kg and total mass rises, so power required rises too; endurance does not grow by 50%. Designers must find a balanced battery mass fraction.

Propulsion architectures

Three rows: battery-electric is battery, ESC, motor, propeller; hybrid is engine, generator plus battery, motor, propeller; fuel cell is hydrogen tank, fuel cell plus battery, motor, propeller — Figure 1. Three propulsion architectures

Battery-electric is simple, quiet and low-maintenance, the most common type, but limited by battery specific energy.
Hybrid uses an engine to drive a generator; the battery supplies peak power for take-off. Endurance is far longer, at the cost of noise, vibration and engine maintenance.
Hydrogen fuel cell converts hydrogen straight to electricity, quietly and for long durations, but requires handling high-pressure tanks and a hydrogen supply, with its own safety requirements.

Power-system compatibility

When choosing power-system parts, check several things together:

Voltage: battery, ESC and motor must support the same cell count.
Current: ESCs and wiring must carry peak current with margin, and the battery must deliver it (C-rating).
Thrust: enough thrust-to-weight for control and wind.
Heat: ESCs, motors and battery must shed heat in Thailand’s hot climate.
Weight and centre of gravity of the installed system.

Class activity

Case study

Compare the specifications of three survey drones (multirotor, fixed-wing, VTOL) from maker documents. Calculate area surveyed per flight at equal GSD.
Using eCalc or a spreadsheet, estimate endurance as battery mass varies from 0.5 to 2.0 kg, and find where gains start to shrink.
Discuss whether your organisation should invest in hydrogen drones, considering fuel supply, safety and lifecycle cost.

Common mistakes

Watch out

Comparing fuel specific energy directly with whole battery packs, ignoring system weight and conversion efficiency.
Assuming endurance grows in proportion to battery added.
Choosing aircraft type from familiarity rather than mission requirements.
Forgetting heat in hot weather.

Summary

Multirotors use high power because rotors support the weight continuously; fixed-wing power $P = W V / (L / D)$ is much lower.
VTOL combines both strengths at the cost of complexity and weight.
Battery packs offer about 150–250 Wh/kg; fuels are far higher but must be assessed at system level.
Endurance $t = η_{u} m_{b} e_{b} / P_{elec}$ , and extra battery gives diminishing returns.
Power systems must match in voltage, current, thrust, heat and centre of gravity.

Check your understanding

A 5 kg aircraft with $L / D = 12$ flies at 18 m/s. What mechanical power does it need?
At 0.55 overall efficiency, what electrical power does question 1 need?
A 1.2 kg battery at 180 Wh/kg, 80% usable, powers question 2. How many minutes can it fly?
Why is comparing hydrogen’s 33 300 Wh/kg directly with a 200 Wh/kg battery unfair?
Which aircraft type suits inspecting cracks under a bridge, and why?

Answers

$W = 49.05$ N, $D = 49.05/12 \approx 4.09$ N, $P = 4.09 \times 18 \approx 73.6$ W
$73.6/0.55 \approx 134$ W
$E = 216$ Wh, $t = 0.8 \times 216/134 \approx 1.29$ h, about 77 minutes
The hydrogen figure is fuel only, excluding tank, fuel cell and conversion efficiency
A multirotor, because it must hover close to the structure and move slowly and precisely

Key formulas

Level-flight power (fixed-wing)	$P = D V = \frac{W}{L / D} V$
Electrical power	$P_{elec} = \frac{P}{η}$
Battery energy	$E = m_{b} e_{b}$
Endurance	$t = \frac{η _{u} E}{P _{elec}}$

Key references

Fahlstrom, P. G., Gleason, T. J., & Sadraey, M. H. (2022). Introduction to UAV systems (5th ed.). Wiley.
Anderson, J. D. (2016). Introduction to flight (8th ed.). McGraw-Hill.
Quan, Q. (2017). Introduction to multicopter design and control. Springer.
U.S. Department of Energy, Fuel Cell Technologies Office. (2017). Target explanation document: Onboard hydrogen storage for light-duty fuel cell vehicles. link
Leishman, J. G. (2006). Principles of helicopter aerodynamics (2nd ed.). Cambridge University Press.

In class / field

Lecture, case discussion and in-class problem solving

Learning evidence: Quiz results and submitted exercises

Module quiz

This is a formative self-check, not a graded exam

Knowledge domain: Electrical, electronics and power systems · Aircraft, structures and design

Power and propulsion

By the end of this module you will be able to

Energy decides what a drone can do

Aircraft types and power use

Fixed-wing power required

Energy sources

Estimating endurance from battery mass

Propulsion architectures

Power-system compatibility

Class activity

Common mistakes

Summary

Check your understanding

Key formulas

Key references

Further reading

UAS power distribution systems

BLDC motors, ESCs and propeller matching

In class / field

Module quiz

Power and propulsion

Lesson

By the end of this module you will be able to

Energy decides what a drone can do

Aircraft types and power use

Fixed-wing power required

Energy sources

Estimating endurance from battery mass

Propulsion architectures

Power-system compatibility

Class activity

Common mistakes

Summary

Check your understanding

Key formulas

Key references

Further reading

UAS power distribution systems

BLDC motors, ESCs and propeller matching

In class / field

Module quiz