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 (R_req)
Endurance (t_req)
Cruise speed (V_c)
Ceiling / Operating altitude
Payload mass (W_pl) in (kg)
Launch & recovery constraints (Runway, VTOL, catapult)

Weight Estimation

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

Initial Maximum Takeoff Weight (MTOW):
W_TO = g × (m_pl + m_batt/fuel + m_struct + m_avionics)

W_TO (or, MTOW) = (W_pl + W_batt/fuel + W_struct + W_avionics)

Aerodynamic Requirements

_stall

_{L_max}

Wing loading: W/S = W_TO / S
Cruise lift coefficient: C_L,c = (2W_TO) / (ρ V_c² S)
Stall speed: V_s = √[2W_TO / (ρ S C_L,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:
M_root ≈ q × (b/2)² / 2

Propulsion Requirements

Required thrust: T_req ≈ 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, T_req=(T/W)×W_TO
Drag coefficient: C_D = C_D0 + (C_L² / (π AR e))
Power Required from T/W,Using actuator disk theory. Power required: P = Drag Force × V_c

or, P = T^3/2 / √(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: P_elec = P / η_p

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 (E_req): The base energy required for the mission (E_req) is calculated from the average power draw and flight time:

E_req = P_avg × t

Battery Energy Capacity (E_bat): A safety reserve of 20–30% is added to determine the actual battery capacity needed: E_bat = 1.25 × E_req
(Using a 25% reserve factor for robustness)
Battery Capacity C_Ah):Capacity in Ampere-hours is calculated considering the battery voltage (V_bat) and system efficiency (η_sys): C_Ah = E_bat / (V_bat × η_sys)
Note: System efficiency η_sys is typically assumed to be around 0.85 (85%).
Battery energy must satisfy:
E_batt ≥ P_avg × t_req
If using fuel (Breguet):
R = (η_p / c_T) × (L/D) × ln(W_i / W_f)
Battery Mass Estimation (W_bat): 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

W_bat = E_bat / Energy Density

Battery Type	Typical 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 = (x_ac − x_cg) / c̄

where x_ac = aerodynamic center location, x_cg = 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

No	Component	Configuration Alternatives
1	Fuselage	– Geometry: lofting, cross section – Internal arrangement – Accommodation: payload, fuel, engine, landing gear
2	Wing	– 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
3	Horizontal 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
4	Vertical Tail	– Configuration: single, twin VT, V-tail – Type: rectangular, swept, tapered – Airfoil
5	Engine	– Type: turbofan, turbojet, turboprop, piston-prop, electric – Location: under fuselage, under wing, beside fuselage, fuselage nose, rear fuselage – Number of engines
6	Landing Gear	– Type: fixed, retractable, partially retractable – Configuration: tricycle, bicycle, tail gear, multi-bogey, no landing gear
7	Control Surfaces	Separate vs all-moving tail, reversible vs irreversible, Conventional vs non-conventional (e.g., elevon, ruddervator)
8	Autopilot	– 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
9	Launch & Recovery	HTOL, ground launcher, net recovery, belly landing

Rule-of-Thumb Table

UAV Weight	T/W	Typical Propeller Diameter
1 kg	2.0	9–11 in
2 kg	2.0	11–13 in
5 kg	2.0	15–17 in
10 kg	2.0	18–22 in
5 kg (Fixed-Wing)	0.4	10–12 in

Power System

Motor–ESC–Battery Matching Table

UAV Scale Motor (KV) ESC Rating Battery (S)
Micro / Racing 2300 – 2700 KV 20A – 40A 3S – 4S
Small / Photo 900 – 1200 KV 30A – 50A 3S – 4S
Medium / Delivery 500 – 800 KV 40A – 60A 4S – 6S
Heavy Industrial 100 – 400 KV 80A – 120A+ 6S – 12S

UAV Scale	Motor (KV)	ESC Rating	Battery (S)
Micro / Racing	2300 – 2700 KV	20A – 40A	3S – 4S
Small / Photo	900 – 1200 KV	30A – 50A	3S – 4S
Medium / Delivery	500 – 800 KV	40A – 60A	4S – 6S
Heavy Industrial	100 – 400 KV	80A – 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

MTOW - Max Take-Off Weight (kg)

Required Thrust (N) - Total for UAV

Propeller Diameter (inches)

Air Density (ρ) (kg/m³)

Disk Area (A):0.05 m²

Ideal Power (P):0.00 W

Thrust-to-Weight (T/W):0.00

Equations: A = πD²/4 | P = T^1.5 / √(2ρA) | T/W = T / (MTOW × 9.81)

UAV Design Phases by Dr Aishwarya Dhara

Main Course

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UAV Design Procedure

Module 4

Initial Design Process

Motor–ESC–Battery Matching Table

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

UAV Propulsion Calculator

UAV Propulsion Calculator

UAV Scale Motor (KV) ESC Rating Battery (S)
Micro / Racing 2300 – 2700 KV 20A – 40A 3S – 4S
Small / Photo 900 – 1200 KV 30A – 50A 3S – 4S
Medium / Delivery 500 – 800 KV 40A – 60A 4S – 6S
Heavy Industrial 100 – 400 KV 80A – 120A+ 6S – 12S