Propellers

Summary¶

A propeller converts rotational energy into thrust by accelerating air downward. Its two primary specifications are diameter (tip-to-tip) and pitch (theoretical travel per revolution through solid medium). Larger diameter moves more air per revolution and is more efficient for hover. Higher pitch moves the drone faster through the air per revolution but draws more current. Blade count affects the thrust-to-noise trade-off. libdrone uses HQ 6-inch tri-blade polycarbonate props in two pitch variants: 6×3×3 for standard conditions and 6×2.5×3 for calm conditions and maximum flight time.

Concept¶

Diameter and efficiency¶

The relationship between diameter and hover efficiency comes directly from momentum theory. The power required for a given thrust scales inversely with the square root of disk area:

P ∝ T / sqrt(A)

Doubling the rotor area (1.41× diameter) reduces power for the same thrust by approximately 30%. A large, slow propeller is always more efficient for hover than a small, fast propeller producing the same thrust.

The practical constraint on diameter is tip clearance — adjacent prop tips must not approach each other. At 330 mm wheelbase, 6-inch (152 mm diameter) props give a minimum 15 mm tip clearance. 7-inch props would violate this clearance.

Pitch¶

Pitch is the theoretical distance the propeller would advance in one full rotation through a solid, non-slipping medium. A 6×3 propeller would advance 3 inches per revolution if there were no slip. In real air, slip means the actual advance per revolution is less than the pitch.

Higher pitch: more air displaced per revolution, more thrust at high RPM, more current draw, better wind penetration, higher motor temperature. Better for windy conditions and fast forward flight.

Lower pitch: less air per revolution, less thrust at high RPM, lower current draw, cooler motors, better efficiency in calm hover. Better for mapping missions in calm conditions and for maximum flight time.

The pitch-to-diameter ratio beyond approximately 1:2 produces diminishing returns from aerodynamic losses. Both libdrone prop options (6×3 and 6×2.5) sit below this limit.

Blade count¶

More blades for a given diameter increase thrust at a given RPM but also increase aerodynamic drag and noise. Two-blade propellers are efficient but produce more vibration per revolution. Three-blade propellers balance thrust, noise, and efficiency. Four-blade propellers are used where noise reduction and compact diameter are important (urban environments).

libdrone uses three-blade props. The tri-blade design provides more thrust per diameter than two-blade at the same RPM, allowing a lower RPM for the same thrust — which reduces motor heat and aerodynamic noise.

Rotation direction and installation¶

Adjacent props must counter-rotate to cancel net angular momentum. Props are manufactured for a specific direction and are not interchangeable without reversing the motor. Standard (CCW) props used on clockwise motors push the drone into the ground — a common first-build error with an unmistakable result: the drone pushes down on arm.

Polycarbonate (PC) vs nylon vs carbon fibre: - PC (libdrone standard): durable, consistent weight, balanced from factory. Bends under impact rather than shattering. Safe for community builds. - Nylon: cheaper, more flexible, lower performance. Acceptable for training. - Carbon fibre: stiffest, lightest, most efficient. Shatters on impact into sharp fragments. Not recommended for community builds or proximity flying.

Prop balancing¶

An unbalanced propeller vibrates at exactly its rotation frequency. At 30,000 RPM, an imbalance of 0.1 g at the blade tip generates approximately 75 N of rotating centrifugal force. This force propagates through the motor into the floating mount (which attenuates but does not eliminate it) and reaches the gyroscope.

Three minutes on a magnetic balancer per propeller, adding tape to the lighter blade until the prop rests level in any orientation, measurably reduces the Blackbox noise floor and extends motor bearing life.

Reference¶

libdrone V2.4.6 propeller specification¶

Betaflight has two profiles configured, one per prop set. Switch profiles when changing props — the PID values differ slightly.

Inspection and replacement criteria¶

Replace immediately if: - Any visible crack, chip, or nick in a blade - Hub shows any cracking or white stress marks around the shaft bore - Drone has experienced a ground impact at any speed (props absorb significant impact energy and may have internal damage invisible externally) - Vibration level in Blackbox increased compared to previous flights

Polycarbonate props do not have a flight-hour lifespan limit if undamaged. Replace on damage, not on schedule.

Procedure¶

Pre-flight prop check¶

1. Visually inspect each blade from tip to hub on both faces.
2. Flex each blade gently — cracks produce an audible click.
3. Check hub for looseness on shaft: prop should not rock or wobble.
4. Verify correct rotation direction: top face should face upward on CCW motors, downward on CW motors (or check the small "R" or "CW" marking on reversed props).

Prop installation torque¶

Finger-tight plus 1/8 turn. Over-torquing a polycarbonate prop cracks the hub. Under-torquing allows the prop to shift position during flight. M5 prop nut: approximately 0.5–0.8 N·m. Never use standard M5 bolts — always use the prop nut with the nylon lock built in or a dedicated prop bolt.

Rationale¶

Why two prop sets are specified¶

The 6×3×3 and 6×2.5×3 props have meaningfully different performance profiles at libdrone's operating conditions. The lower pitch (2.5) draws noticeably less current at hover — approximately 3–5% less per motor — directly translating to longer flight time. In calm conditions where wind authority is not needed, this is a worthwhile gain. In wind, the higher pitch (3.0) provides better authority and speed. Maintaining both sets and knowing which to use based on conditions is part of operational competency.

libdrone Pro propeller selection¶

Two HQProp sets are specified for Pro:

Set A (higher pitch) produces more thrust at a given RPM and handles heavier payloads better. Set B (lower pitch) draws less current at the same RPM, extending flight time by 10–15% at the cost of reduced top-end thrust. For typical sensor payload missions (860g AUW), Set A is the standard choice. For long loiter missions with lightweight payloads, Set B extends endurance.

Connections¶

requires: - lift-and-thrust - brushless-motors related: - electronic-speed-controllers - floating-motor-mounts - vibration-isolation-theory - propeller-balance - acoustic-signature-design leads_to: - electronic-speed-controllers - propeller-balance