Designing UAV Propulsion

Given a mission profile (endurance, range, payload, speed envelope), select propulsion architecture using this decision order:

1. Endurance < 30 min, small payload → Electric (LiPo battery + BLDC motor + FOC ESC)
2. Endurance 1-5 hrs, MALE-class → Electric (Li-ion or Fuel Cell) or 4-stroke ICE
3. Endurance 5+ hrs, HALE-class → Fuel Cell (PEM) or ICE with EFI
4. High speed (>150 kt) / jet-like mission → Micro-turbojet/turboprop, Brayton cycle
5. Match propeller diameter/pitch to selected powerplant via BEMT + motor-propeller matching

Example quick calc for electric multirotor:

Required hover thrust T = MTOW × g / n_rotors
Motor Kv selection: pick Kv so that at battery voltage V, no-load RPM ≈ 1.3-1.5x required RPM
Propeller pitch: static thrust favors low pitch; cruise efficiency favors high pitch
Battery sizing: capacity(Ah) = (P_avg × t_flight) / (V_nominal × η_system)

Progress:

• Step 1: Define mission envelope (endurance, range, speed, altitude, payload, VTOL/fixed-wing)
• Step 2: Select energy source category (battery chemistry / fuel cell / ICE fuel / turbine fuel)
• Step 3: Select motor/engine type and size for required power-to-weight
• Step 4: Select or design controller (ESC/FOC, EFI, turbine controller)
• Step 5: Optimize propeller/rotor via BEMT for the specific flight regime
• Step 6: Validate system-level efficiency across full flight envelope (takeoff, climb, cruise, descent)
• Step 7: Check thermal, vibration, and structural integration constraints

Step 1 — Mission Envelope

Extract: required endurance (min/hr), range (km), max speed, cruise speed, altitude ceiling, payload mass, VTOL vs fixed-wing, environment (urban/high-altitude/marine).

Step 2 — Energy Source Selection

Step 3 — Motor/Engine Sizing

Electric (BLDC/PMSM):

• Choose Outrunner for high torque/large props (multirotor, direct-drive fixed-wing); Inrunner for high-RPM ducted/geared applications.
• Solve for Kv, internal resistance Rm, and no-load current I0 to maximize efficiency at the cruise operating point (not just peak power point).
• Verify continuous current rating vs. thermal limits at sustained cruise power.

ICE (piston):

• 2-stroke for power-to-weight; 4-stroke for fuel efficiency and endurance.
• Specify EFI over carburetor for altitude-compensated fuel-air mixture; avoids flame-out at altitude.
• Account for vibration loads on airframe/avionics mounts.

Micro-turbine/turbojet:

• Model Brayton cycle: compressor efficiency, combustor temperature rise, Turbine Inlet Temperature (TIT) limit.
• Address bearing lubrication and thermal management at >100,000 RPM.

Step 4 — Controller Selection

• Electric: prefer FOC-based ESC over trapezoidal PWM for efficiency, low acoustic signature, and fast torque response (critical for VTOL stability).
• Use CAN-based ESC for multi-motor coordination in larger platforms.

Step 5 — Propeller/Rotor Optimization (BEMT)

1. Discretize blade into radial elements.
2. Compute local lift/drag from local angle of attack and relative airflow velocity.
3. Integrate for total thrust and torque across the disc.
4. Iterate diameter and pitch:
   • Larger diameter, lower pitch → better static thrust (VTOL, takeoff)
   • Smaller diameter, higher pitch → better cruise efficiency (fixed-wing high speed)
5. Consider variable-pitch mechanism if mission spans distinct regimes (hover + high-speed cruise) requiring efficiency in both.
6. Evaluate ducted fan configuration if operating in urban/confined environments — reduces tip vortex losses, increases static thrust, adds mechanical guard.
7. Choose pusher vs. tractor layout based on airframe flow quality requirements (pusher preserves clean flow over fuselage/sensors).

Step 6 — System Validation

Build power-required vs. power-available curves across the full flight envelope (takeoff, climb, cruise, loiter, descent). Confirm battery/fuel sizing meets endurance target with margin (typically 20% reserve).

Step 7 — Integration Checks

• Thermal: motor/ESC/fuel-cell/engine heat dissipation paths.
• Vibration isolation for ICE/turbine on sensitive payloads (EO/IR, comms).
• Structural mounting loads from thrust and torque reactions.

Example 1: Input: Quadrotor, MTOW 5 kg, required hover endurance 40 min, no forward flight requirement. Output: LiPo battery (high C-rating, ~250 Wh/kg), Outrunner BLDC motors sized for ~2:1 thrust-to-weight per motor, FOC ESC for efficiency and stability, propeller optimized for static thrust (large diameter, low-moderate pitch), no ducting needed unless operating near people.

Example 2: Input: Fixed-wing VTOL, MALE-class, 8-hour endurance, 300 km range, cruise speed 90 km/h. Output: PEM fuel cell system (or Li-ion battery-electric hybrid if fuel cell infeasible) for cruise motor; separate electric lift motors for VTOL phase only (used briefly, sized for takeoff/landing, not endurance-critical); tractor-mounted cruise propeller optimized via BEMT for high pitch/cruise efficiency; consider variable-pitch prop if VTOL-to-cruise transition efficiency is critical.

Example 3: Input: High-speed UAV target, 250 kt dash speed, 20 min mission. Output: Micro-turbojet propulsion, Brayton cycle sizing for required thrust at dash speed and altitude, JP-8 or equivalent fuel, no propeller (direct jet thrust), thermal/bearing design for sustained >100,000 RPM operation.