Propellers for frame size — largest prop for efficiency

Many builds start with “motor first, prop later”. On a multicopter the better order is the opposite: frame size caps propeller diameter. Fit the largest sensible prop with tip clearance for hover efficiency, then find matching Kv.

Frame size and tip clearance

In eCalc, frame size is the distance between opposite motors (X), or twice the centre-to-motor arm length. For fully symmetric frames, xcopterCalc checks prop tip clearance between props (not automatically to the centre hub — that depends on the frame).

From the eCalc help, approximate max prop at zero tip clearance:

In practice stay slightly under the theoretical max (hub, guards, tolerances). Coax vs flat: Coax vs flat.

Why “larger” usually hovers better

Hover means lift = weight at minimum electrical power. Larger disc area moves more air at lower induced velocity — typically higher specific thrust (g/W). eCalc treats ≥ 6 g/W as a good band.

Trade-off: very large, slow props feel less snappy. Race setups sacrifice efficiency for response — different hover targets (Size a drive).

Pitch/diameter: above ~0.667, stall and vortex-ring risk rise. Aim for pitch under 2/3 of diameter outside deliberate high-pitch race props.

Prop-KV-Wizard

Once frame and prop (diameter/pitch) are set, the Prop-KV-Wizard in xcopterCalc suggests a Kv starting range for your estimated AUW. It does not replace picking a motor from the database, but it beats folklore like “always 2400 Kv on 6S”. See Kv vs prop diameter.

Worked example: 7” long-range quad

Assumptions:

• Flat quad, frame 300 mm motor-to-motor
• Theoretical ~150 mm prop; in practice often 7” (177.8 mm) with clear hub clearance
• AUW 950 g including camera/VTX
• 6S 4000 mAh, max discharge 80 %
• Prop e.g. 7×4 or 7×3.5 (check pitch/D)
• Kv class much lower than typical 5”/4S — let the Wizard guide you

Comparison idea: Same motors with 6” vs 7” at the same AUW in xcopterCalc — compare hover current, specific thrust and hover throttle. The larger prop usually wins in hover if clearance and pitch are sane.

Steps in xcopterCalc

1. Open xcopterCalc.
2. Enter rotor count, flat/coax, frame in mm.
3. Set AUW or weight without drive.
4. Size prop diameter to frame and clearance (not “what was in the box”).
5. Keep pitch/D < ~0.667 unless you accept a race trade-off.
6. Run Prop-KV-Wizard with estimated AUW → read Kv band.
7. Pick motor, Calculate, read clearance notes and efficiency metrics.
8. For coax, see help on upper/lower prop tuning.

No invented prefill URLs — enter values by hand. Members can use Share.

FAQ

Can I always run the theoretical max prop?
Only if the centre hub and wiring allow it. eCalc checks prop-to-prop, not automatically prop-to-hub.

Folding props?
Enter effective tip-to-tip diameter including the yoke; standard yoke widths are listed in the xcopterCalc help.

Generic prop?
If your prop is missing: generic thin/normal/wide approximates PConst. Prefer manufacturer data when you have it.