Trade-off Study 01: Propulsion, Aerodynamics, and Endurance Optimization¶

Status: Finalized
Author: Sarthak Rathi
Methodology: Theoretical modeling, thrust-table extrapolation, and material science analysis.

1. Introduction & Objectives¶

The primary objective of this project was to design a multirotor platform capable of ~30 minutes of autonomous flight while carrying a heavy avionics payload (Raspberry Pi 4 + Wi-Fi adapters) and adhering to a strict 6-inch maximum propeller constraint.

Standard 5-inch and 6-inch FPV drones are traditionally built for freestyle or racing, prioritizing peak thrust and instantaneous acceleration. These builds typically achieve 3 to 7 minutes of flight time. To achieve a 30-minute endurance target, the design philosophy had to shift from "High Thrust / High Current" to "Low Drag / Maximum Efficiency." This trade-off study documents the component selection process across four domains: Propeller Aerodynamics, Motor Stator Sizing, Battery Chemistry, and Airframe Material.

2. Propeller Aerodynamics (5" vs. 6" | Tri-blade vs. Bi-blade)¶

The constraint of a 6-inch maximum propeller size imposes inherently high disk loading. To maximize flight time, we must reduce the RPM required to generate hover thrust (~300g – 375g per motor depending on the battery weight).

2.1 Diameter and Pitch Evaluation¶

I initially considered 5-inch propellers (e.g., Gemfan 5129 Tri-blade) to keep the frame as compact as possible. However, theoretical thrust models indicated that to hover a 1.2kg to 1.5kg drone, 5-inch props would require very high RPMs, drawing inefficient levels of current. Expanding to 6-inch propellers drastically increases the swept area, lowering the required RPM and moving the motor into a more efficient power band.

2.2 Blade Count (Tri-Blade vs. Bi-Blade)¶

Freestyle drones predominantly use tri-blade propellers for cornering grip and "bite." For endurance, the third blade introduces parasitic aerodynamic drag with diminishing returns on thrust.

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xychart-beta
    title "Estimated Flight Time by Propeller Geometry (1.5kg AUW Model)"
    x-axis ["5129 Tri-blade", "6030 Tri-blade", "6030 Bi-blade", "6026-2 Bi-blade"]
    y-axis "Hover Time (Minutes)" 10 --> 40
    bar [16, 26, 31, 34]

Conclusion: The Gemfan LR 6026-2's low pitch (2.6) and bi-blade design minimizes aerodynamic drag, offering up to a 30% theoretical efficiency gain over standard 5-inch tri-blades during steady-state hover.

3. Motor Selection (Stator Volume and KV)¶

Thrust table analysis revealed a critical flaw in using standard freestyle motors for 6-inch endurance props.

3.1 Evaluating the 2306 Class¶

Standard FPV builds typically use 2207 or 2306 motors with a high KV (1750KV–2450KV) and my initial BoM also included 2306 1750KV motors. But, thrust table analysis revealed a critical flaw: a 2306 stator lacks the low-end torque required to swing a 6-inch bi-blade efficiently. To overcome the rotational inertia of the larger prop, the 2306 motor draws excessive current, generating heat rather than thrust. Note: I modeled a software workaround using ArduPilot's MOT_THST_MAX = 0.65 parameter to artificially limit a high KV motor to an effective ~1100KV. While mathematically viable, it does not fix the physical lack of torque.

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flowchart TD
    subgraph Freestyle Motor
    A[2306 1750KV] -->|Low Stator Volume| B[Low Torque at Low RPM]
    B -->|Swinging 6-inch Prop| C[High Current Draw to overcome inertia]
    C -->|Heat Loss| D((Poor Endurance))
    end

    D -.-> E

    subgraph Endurance Motor
    E[2807 1500KV] -->|High Stator Volume| F[High Torque at Low RPM]
    F -->|Swinging 6-inch Prop| G[Low Current Draw]
    G -->|Peak Efficiency Band| H(((High Endurance)))
    end

    style D fill:#ff4444,stroke:#ff0000,stroke-width:3px,color:#ffffff
    style H fill:#44ff44,stroke:#00aa00,stroke-width:3px,color:#000000

3.2 Scaling Up: 2506 vs 2807¶

To achieve optimal efficiency, the motor must hover the drone (producing ~350g of thrust) exactly at its peak efficiency curve. * 2506 1500KV: A strong middle ground, providing more torque than the 2306 while remaining lightweight. * 2807 1500KV: Provides maximum torque, the more stator volume means the motor can easily spin a 6-inch prop at low RPMs.

Conclusion: The platform is optimized around the 2807 1500KV class. This provides immense control authority (a thrust-to-weight ratio of ~6:1) while operating in its peak efficiency band during hover.

4. Battery Chemistry and Configuration¶

The energy system dictates the absolute ceiling of endurance. The choice was between standard Lithium Polymer (LiPo) and Lithium-Ion (Li-ion) packs using 18650/21700 cells.

4.1 LiPo vs. Li-ion¶

LiPo (e.g., 1300mAh 6S): Offers very-high discharge rates (100C+). Ideal for racing, but highly detrimental to endurance due to terrible energy density (Wh/kg).
Li-ion (e.g., 21700 cells): Much higher energy density compared to LiPo, but severely limited discharge rates (usually 3C to 10C max).

Because the theoretical hover current of this drone is extremely low (14A – 18A total), a high discharge rate is irrelevant. The design benefits hugely from Li-ion chemistry for its superior energy density.

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flowchart LR
    B[Battery Selection] --> LiPo[LiPo Chemistry]
    B --> LiIon[Li-ion 21700 Chemistry]

    LiPo --> |100C+ Discharge Rate| HighP[High Peak Power]
    LiPo --> |Low Wh/kg| LowE[Poor Endurance < 10m]

    LiIon --> |Max 45A-70A Discharge| LowP[Sufficient for Hover & Cruise]
    LiIon --> |High Wh/kg| HighE[High Endurance > 25m]

    style LiPo fill:#cc3333,stroke:#ff6666,stroke-width:2px,color:#ffffff
    style LiIon fill:#33cc33,stroke:#00aa00,stroke-width:2px,color:#000006

4.2 Configuration: 4S1P vs. 4S2P¶

I mathematically modeled two battery architectures based on modern 21700 cells (e.g., Molicel P45B or Samsung 50S):

Metric	4S1P Li-ion (P45B)	4S2P Li-ion (50S)
Capacity	4,500 mAh	10,000 mAh
Weight	~300 g	~600 g
Max Safe Discharge	45 A	70+ A
Projected AUW	1.2 kg	1.5 kg
Estimated Flight Time	13 – 16 min	28 – 35 min

Conclusion: The physical drone is currently built to accommodate the 4S1P pack to keep initial weight down and ensure structural survivability during early testing. However, the theoretical model proves that migrating to a 4S2P pack yields the optimal 30-minute endurance threshold, as the capacity more than doubles while the weight penalty is easily absorbed by the 2807 motors.

5. Airframe Material Efficiency Impact¶

A unique constraint of this project was utilizing a 3D-printed structural frame. The choice of filament is not just a mechanical decision; it is a critical aerodynamic and electrical one.

The "Hidden" Power Drain of Flexible Frames¶

If a frame is printed in a flexible material like standard PETG or PLA, the arms will flex under thrust. This induces high-frequency micro-oscillations. The flight controller's gyroscopes detect these vibrations, and the PID loop attempts to correct them by rapidly varying motor RPMs. * Result: The motors constantly accelerate and decelerate hundreds of times a second, drawing massive current spikes and dissipating energy as heat. Studies estimate a flexible frame can cause a 10% to 20% loss in total flight efficiency.

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flowchart TD
    Flex[Flexible Frame e.g., PETG/PLA] -->|Motor Thrust Causes Flex| Vib[High-Frequency Micro-Oscillations]
    Vib --> Gyro[Gyroscope Detects Mechanical Noise]
    Gyro --> PID[Flight Controller PID Overcompensates]
    PID --> RPM[Rapid Motor RPM Fluctuations]
    RPM --> Heat[Energy Wasted as Heat in ESC/Motors]
    Heat --> Loss((10 - 20 percent Flight Time Loss))

    Stiff[Stiff Frame e.g., ASA/PA6-CF] --> Damp[Vibrations Resisted]
    Damp --> Smooth[Smooth Gyro Traces]
    Smooth --> Eff((Efficient Motor Output))

    style Loss fill:#ff4444,stroke:#ff0000,stroke-width:3px,color:#ffffff
    style Eff fill:#44ff44,stroke:#00aa00,stroke-width:3px,color:#000000

Material Selection¶

To prevent this, the frame material must possess high stiffness and rigidity. 1. PA6-CF (Nylon Carbon Fiber): The absolute best choice. Extremely stiff, absorbs vibrations, and reduces PID workload. (Rejected only due to printing complexity/cost). 2. ASA / ABS-CF: Excellent stiffness, high glass-transition temperature (won't warp under hot motors), and UV stable for outdoor flights.

Conclusion: The frame must be printed in ASA or Carbon-reinforced ABS/Nylon. PETG and PLA are structurally and electrically non-viable for a highly tuned, long-endurance autonomous platform.

6. Final Propulsion Architecture Summary¶

By abandoning FPV racing dogmas and optimizing for steady-state cruise, the finalized theoretical architecture achieves the 30-minute goal:

Motors: 2807 1500KV (High torque, low-RPM efficiency).
Propellers: Gemfan LR 6026-2 (Bi-blade, minimal aerodynamic drag).
Battery: 4S Li-ion 21700 (High energy density, low C-rating requirement).
Frame Material: ASA/PA6-CF (High stiffness to minimize electrical PID-correction losses).