UAV Design Procedure

Module 4

Design Guidelines
Design Process
Conceptual selection
Power System
Design Guidelines
UAV Design Guidelines
FAA currently regulates for UAVs

Under Part 107, for design and operation, the requirements (or constraints) are more about safe flight operation rather than structural certification. Key points:

The drone must be registered; operators must place the issued registration number visibly on the aircraft.

Operational limitations: typically within visual line-of-sight (VLOS), daylight (or twilight with proper lighting), max altitude ~400 ft above ground (or above structure under certain conditions), max speed ~100 mph (87 knots).

Pre-flight safety checks: the operator is responsible for ensuring the drone is safe to fly (communications link, control surfaces, etc.).

  • DGCA / FAA category compliance
  • BVLOS / VLOS operational constraints
  • Weight class limits
Conceptual Design Phase

Key questions to consider include:

a) Does the product meet the customer’s actual needs?

b) What is the estimated market size—how many units can be sold?

c) Will the production cost plus mark-up be perceived by customers as good value for money?

d) Are the projected operating costs and system reliability acceptable to customers?

e) Can the programme’s non-recurring costs be recovered within a reasonable timeframe through sales revenue?

f) Are there any political, regulatory, or external factors that may restrict the sale of the aircraft or system?

Preliminary Design Phase

Stepwise procedure includes:

1. Expand the initial UAV system outline into detailed configurations, performing optimisation trade-offs to ensure maximum performance across expected missions and environmental conditions.

2. Identify which UAV components will be built in-house and which will be externally sourced, along with approximate procurement costs and supplier options.

3. Finalise a complete system design definition, including subsystem interfaces and a full UAV system specification for further development.

4. Reassess programme costs for upcoming development phases and operational expenses to confirm whether continuation of the UAV project is justified.

5. Avoid rushing this phase, as careful early consideration of manufacturability, reliability, maintenance, and operational aspects prevents costly corrections in later UAV development stages.

Detailed Design Phase

Stepwise procedure includes:

1. Detailed studies of UAV aerodynamics, dynamics, structures, and onboard systems are carried out, along with layouts for control stations and support subsystems.

2. Designers create production-ready drawings for all UAV components, including ground support and test equipment, while applying value analysis to optimize cost and performance.

3. Specifications for externally purchased components are finalized, and tenders are invited from suitable suppliers for procurement.

4. Required manufacturing jigs and tools are identified and designed unless sourced externally to ensure accurate UAV production.

5. Test schedules for upcoming evaluation phases are drafted, and initial content for UAV operating and maintenance manuals is prepared.

UAV Design Requiremnets

1. Mission-Oriented Design

2. Keep Weight Low

3. Optimize Aerodynamics

4. Ensure Stability & Control

5. Use Efficient Propulsion

6. Ensure Structural Integrity

7. Choose Reliable Avionics

8. Add failsafe settings

Simulation and Iteration

CFD, XFOIL, AVL, MATLAB performance iteration.

Design Process

Initial Design Process

Data Collection: This is the foundational step. It involves defining the mission requirements (e.g., payload, flight duration, range, operating environment, size constraints) and gathering component data (e.g., motor specifications, battery energy density, propeller performance curves) that will drive all subsequent decisions.

Data collection Sample Sheet

Trade-Off Studies: This step evaluates different combinations of design parameters to find the optimal configuration. It often involves analyzing the impact of factors like increasing battery weight (more energy, less payload capacity) versus reducing flight duration (less battery weight, more payload capacity) to balance competing objectives.

Sizing: Based on the trade-off studies, this step determines the preliminary size and weight of the overall aircraft and its key subsystems. It sets the required thrust-to-weight ratio and estimates the total takeoff weight needed to meet the defined mission.

Initial Sizing
Mission Requirements
  • Type of mission: Surveillance, mapping, delivery, agriculture, etc.
  • Operational range (Rreq)
  • Endurance (treq)
  • Cruise speed (Vc)
  • Ceiling / Operating altitude
  • Payload mass (Wpl) in (kg)
  • Launch & recovery constraints (Runway, VTOL, catapult)
Weight Estimation

Payload Requirements based on Payload mass (mpl),Payload type (Camera, sensor, sprayer, communication module), and Power requirement for payload (Ppl).

Initial Maximum Takeoff Weight (MTOW):
WTO = g × (mpl + mbatt/fuel + mstruct + mavionics)

WTO (or, MTOW) = (Wpl + Wbatt/fuel + Wstruct + Wavionics)

Aerodynamic Requirements
    For the initial design, we select a maximum stall speed (Vstall) constraint of 12 to 15 m/s, which is typical for a small UAV, and we target a maximum lift coefficient (CLmax) in the range of 1.2 to 1.6 to ensure adequate low-speed performance and lift capability.
  • Wing loading: W/S = WTO / S
  • Cruise lift coefficient: CL,c = (2WTO) / (ρ Vc2 S)
  • Stall speed: Vs = √[2WTO / (ρ S CL,max)]

    • If wing-based uav, determine wing area (S).

    The selection of AR is a primary trade-off study because it directly impacts performance, where Aspect ratio, AR = b2/S

  • For small-to-medium fixed-wing UAVs, the typical operational ranges are:Low-to-Moderate AR: Commonly in the range of 2 ≤ AR ≤ 7 for man-portable or hand-launched small tactical UAVs.
  • Medium-Velocity AR: Often around 8 ≤ AR ≤ 15 for conventional, medium-velocity UAVs.
  • Glider/High-Endurance AR: Typically AR > 15.
Structural Requirements
  • Material selection (Carbon fiber, composites, aluminum)
  • Safety factor: 1.5 – 2.0
  • Wing root bending moment estimates:
    Mroot ≈ q × (b/2)2 / 2
Propulsion Requirements
  • Required thrust: Treq ≈ D

    • The Thrust-to-Weight ratio (T/W) is a critical metric used to ensure the UAV can perform its required maneuvers.Any VTOL (Vertical Take-Off and Landing) UAV must have a T/W of at least 1.0 to simply hover and overcome gravity.To ensure safe and rapid ascent, VTOL designs often select a much higher T/W margin, typically between 1.8 and 2.5, for takeoff.

  • Mathematically, Treq=(T/W)×WTO
  • Drag coefficient: CD = CD0 + (CL2 / (π AR e))
  • Power Required from T/W,Using actuator disk theory. Power required: P = Drag Force × Vc

    • or, P = T3/2 / √(2ρA).Where Disk area per prop,A = πD²/4, . Note that increasing the T/W ratio leads to an exponential increase in power consumption, directly impacting battery sizing.

  • Hover power Estimation: number of motor * Per motor power.
  • Electric power input: Pelec = P / ηp
Energy & Endurance Requirements

These calculations determine the required energy capacity and resulting mass of the battery, which is crucial for closing the design loop and verifying the initial weight estimates.

  • Total Energy Required (Ereq): The base energy required for the mission (Ereq) is calculated from the average power draw and flight time:
    • Ereq = Pavg × t
  • Battery Energy Capacity (Ebat): A safety reserve of 20–30% is added to determine the actual battery capacity needed: Ebat = 1.25 × Ereq

    (Using a 25% reserve factor for robustness)

  • Battery Capacity CAh):Capacity in Ampere-hours is calculated considering the battery voltage (Vbat) and system efficiency (ηsys): CAh = Ebat / (Vbat × ηsys)

    Note: System efficiency ηsys is typically assumed to be around 0.85 (85%).

    Battery energy must satisfy:
    Ebatt ≥ Pavg × treq

    If using fuel (Breguet):
    R = (ηp / cT) × (L/D) × ln(Wi / Wf)

  • Battery Mass Estimation (Wbat): The final, critical step is to estimate the mass of the battery pack, which heavily influences the overall UAV sizing and T/W ratio. This is determined by dividing the required energy by the selected battery's Energy Density
    • Wbat = Ebat / Energy Density
    Battery TypeTypical Energy Density
    Li-ion (Lithium-ion)~240 Wh/kg (Better for long endurance/range)
    Li-Po (Lithium-Polymer)~200 Wh/kg (Better for high power/burst current)
Stability & Control Requirements
  • Static margin: SM = (xac − xcg) / c̄

    • where xac = aerodynamic center location, xcg = center of gravity, c̄ = mean aerodynamic chord. Typical static margin: 5–15% of c̄ (fixed-wing, stable).

  • Control authority based on mission maneuver requirements
Conceptual selection


Conceptual Design selection
UAV major components with their conceptual design alternatives
NoComponentConfiguration Alternatives
1Fuselage – Geometry: lofting, cross section
– Internal arrangement
– Accommodation: payload, fuel, engine, landing gear
2Wing – Type: rectangular, swept, tapered, dihedral
– Airfoil, twist angle, setting angle
– Location: low-wing, mid-wing, high wing, parasol
– High-lift devices: flap, slot, slat
– Attachment: cantilever, strut-braced
3Horizontal Tail – Configuration: conventional, T-tail, H-tail, V-tail, inverted V
– Type: rectangular, swept, tapered, dihedral
– Airfoil, twist angle, setting angle
– Installation: fixed, moving, adjustable
– Location: aft tail, canard, three-surface
4Vertical Tail – Configuration: single, twin VT, V-tail
– Type: rectangular, swept, tapered
– Airfoil
5Engine – Type: turbofan, turbojet, turboprop, piston-prop, electric
– Location: under fuselage, under wing, beside fuselage, fuselage nose, rear fuselage
– Number of engines
6Landing Gear – Type: fixed, retractable, partially retractable
– Configuration: tricycle, bicycle, tail gear, multi-bogey, no landing gear
7Control Surfaces Separate vs all-moving tail, reversible vs irreversible,
Conventional vs non-conventional (e.g., elevon, ruddervator)
8Autopilot – UAV dynamic model: linear, nonlinear
– Controller: PID, gain-scheduling, optimal, robust, adaptive, intelligent
– Guidance subsystem: proportional navigation, line-of-sight, command guidance, three-point, lead, waypoint
– Navigation subsystem: inertial navigation (strap-down, stable platform), GPS
9Launch & Recovery HTOL, ground launcher, net recovery, belly landing
Rule-of-Thumb Table
UAV WeightT/WTypical Propeller Diameter
1 kg2.09–11 in
2 kg2.011–13 in
5 kg2.015–17 in
10 kg2.018–22 in
5 kg (Fixed-Wing)0.410–12 in
Power System

Motor–ESC–Battery Matching Table

UAV ScaleMotor (KV)ESC RatingBattery (S)
Micro / Racing2300 – 2700 KV20A – 40A3S – 4S
Small / Photo900 – 1200 KV30A – 50A3S – 4S
Medium / Delivery500 – 800 KV40A – 60A4S – 6S
Heavy Industrial100 – 400 KV80A – 120A+6S – 12S

Design Tip: Always verify the motor's manufacturer data sheet to ensure the selected propeller does not draw more current than the ESC or motor can handle at maximum throttle.         


UAV Propulsion Calculator

UAV Propulsion Calculator

Disk Area (A):0.05 m²
Ideal Power (P):0.00 W
Thrust-to-Weight (T/W):0.00
Equations: A = πD²/4 | P = T1.5 / √(2ρA) | T/W = T / (MTOW × 9.81)
UAV firmware: connection · calibration · analysis
⚙️ Firmware design · Quadrotor UAV
1. HARDWARE CONNECTION PROCEDURE

1.1 Pre‑Connection Checklistverify all components

□ Verify all hardware components are present: - ST NUCLEO-L432KC microcontroller board - MPU-9250 IMU sensor (accelerometer, gyroscope, magnetometer) - BMP-180 barometer - FrSky antenna with X8R receiver - DFRobot MicroSD card module - 4x Brushless motors (Tarrot MT1806 kv2300) - 4x Propellers (T-motor 5035) - Battery/power supply - USB cable for PC connection

1.2 Physical Connection Steps

Step 1: Microcontroller board setup
Mount ST NUCLEO-L432KC on drone frame · connect power (USB or VIN/GND) · verify 3.3V/5V output.
Step 2: I2C sensor connections (MPU-9250 & BMP-180)
MPU‑9250 pinNucleo pin
VCC3.3V
GNDGND
SDAI2C SDA pin
SCLI2C SCL pin
BMP‑180 (same I2C bus)
VCC3.3V
GNDGND
SDAI2C SDA pin
SCLI2C SCL pin
Step 3: SPI connection (SD card module)
DFRobot moduleNucleo pin
VCC5V or 3.3V
GNDGND
MISOSPI MISO
MOSISPI MOSI
SCKSPI SCK
CSGPIO (CS)
Step 4: SBUS connection (FrSky X8R)
X8R SBUS OUTUART RX (inv. logic, 100k, 8E2)
GNDGND
VCC5V
Step 5: Motor connections (PWM, X config)
MotorNucleo PWM pin
Motor 1 (front)PWM1
Motor 2 (right)PWM2
Motor 3 (back)PWM3
Motor 4 (left)PWM4

Step 6: USB (CLI, programming, MavLink) – micro‑USB to Nucleo.

1.3 Power‑up sequence – verify connections → connect battery last → observe LED → check serial monitor.

2. CALIBRATION PROCEDURE

FSM SYS_INIT → SYS_STARTUP → SYS_SAFE → [running states]

2.2 Pre‑calibration setup
Connect USB, open serial (baud = firmware), SYS_SAFE, type help.

> thread_info
> display
> top

2.3 Sensor calibration

SensorCLI command / methodverification
Magnetometermag_calib (figure‑8)compass heading matches known direction
Accelerometerautomatic @SYS_INIT (level)roll/pitch ~0°, Z ≈ 9.81 m/s²
Gyroscopeautomatic @SYS_INIT (stationary)rates ≈ 0°/s
Barometer@SYS_STARTUP (ground reference)stable altitude, detects small changes

2.4 ESC & motor calibration – disarm → arm_req → min throttle → check directions (X config, diagonal same).

2.5 Radio calibration (SBUS) – verify channels via display: 1:roll,2:pitch,3:throttle,4:yaw.

2.6 Complete checklist

□ IMU sensors calibrated & verified
□ Magnetometer calibrated
□ Barometer referenced
□ Motors respond & correct rotation
□ Radio channels mapped
□ Failsafe works
□ SD card mounted
□ MavLink heartbeat (if used)
□ Commander flags = working
3. ERROR ANALYSIS PROCEDURE

3.1 Error detection framework – Commander flags:

struct flags {
struct { bool imu_status; bool mag_status; bool baro_status; bool imu_calibrated; bool mag_calibrated; } sensors;
struct { bool comm_joystick; bool mavlink_rx; bool mavlink_tx; } communications;
struct { bool controller_active; bool planner_active; bool ekf_active; } control;
bool pwm_enabled; bool sys_armed_flag;
};

3.2 Pre‑flight error analysis – SYS_INIT, SYS_STARTUP, SYS_SAFE checks. Sensor must respond, ID match, no timeout.

3.3 Run‑time monitoring – automatic/manual mode checks (loop timing, data freshness, radio link).

Failure classexamplesaction
MAJORIMU dead, critical timeout, battery critically lowimmediate disarm → SYS_FAIL_MAJOR → log → prevent arm
MINORintermittent read, single packet loss, temp warninglog, continue, notify, attempt recovery

3.5 Diagnostic CLI commands

> thread_info – thread state & stack
> top – CPU usage
> display – real‑time sensors & flags
> display_repeat – continuous monitor

SD card log format (TIME, ALT, ROLL, PITCH, YAW, ACCEL_XYZ, GYRO_XYZ, MAG_XYZ, MOTOR%, STATUS, ERROR)

3.6 Common errors & troubleshooting

IMU not respondingcheck I2C, pull‑ups, power, speed
Mag calibration failedmove from metal, figure‑8 slowly
SBUS timeoutcheck baud 100k, inverted logic, receiver power
Motor not spinningPWM pin, ESC cal, battery, individual test

3.7 Systematic debug process – isolate, check logs, test components, environment, recent changes.

3.8 Error prevention checklist

Pre‑flight: calibration done, all flags OK, battery full, SD ready, radio link, no errors in log.
During flight: monitor critical params, watch behaviour, manual override ready.
Post‑flight: review logs, inspect components, update calib, document issues.
4. SUMMARY – quick reference
PhaseKey actionsVerification method
ConnectionPhysical wiring, power‑upvisual, LED indicators
CalibrationMag, Accel, Gyro, BaroCLI display command
Pre‑flight checkCommander flags, radio linkCLI status commands
Run‑time monitoringError detection, loggingReal‑time display, SD logs
Post‑flight analysisLog review, error classificationData analysis tools
Critical success factors
• Always verify calibration before flight
• Monitor Commander flags continuously
• Use CLI for diagnostics in SYS_SAFE
• Review SD card logs after each flight
• Document errors for pattern recognition
• Never arm with active error flags
UAV Design Phases by Dr Aishwarya Dhara
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