Coax vs flat — hexa and octo tradeoffs

“Flat is always 20–30 % more efficient than coax” only holds when you test the same motor-prop pair once alone and once stacked — the flat layout then has twice the disc area. On a fixed frame size, coax often fits larger props and more total disc area.

What eCalc says

Help on hover efficiency — flat vs coax:

• On a given frame, coax (Y6/X8) can hover more efficiently because max prop diameter is larger.
• Max-prop rules of thumb: flat octo ~0.383×frame, flat hexa ~0.5×; coax X8 ~0.707×, coax Y6 ~0.866× — Prop & frame.
• Coax can keep disc area after one motor failure if the remaining motor takes the load.
• Flat is often cheaper, simpler to wire and easier to maintain.

xcopterCalc’s engine-failure note is a prediction, not a substitute for a safe single-motor test.

Mechanism

Hover efficiency tracks total disc area. Coax stacks two props per arm: more area on a small frame, with upper/lower interference. Help suggests tuning (slightly more pitch/diameter on the lower prop, ~50/50 power split) for up to ~5 % gain.

Cost: more motors/ESCs on X8 vs a flat quad; weight and complexity rise.

Rules of thumb

1. Fix the frame → compare both layouts with each layout’s largest sensible prop.
2. Same target AUW and mission (Sizing).
3. Compare flight time and specific thrust (LiPo).
4. Weigh redundancy needs vs simplicity.
5. Keep pitch/D under ~2/3 when practical.

Worked example: 600 mm frame — flat hexa vs coax Y6

Assumptions:

• Frame 600 mm, target AUW 3.2 kg with camera
• Flat hexa: props near ~280–300 mm (under 0.5×frame, check clearance)
• Coax Y6: larger props possible (theory toward 0.8×frame — less with hub)
• Pack e.g. 6S 8000–10000 mAh, max discharge 80 %
• Similar motor quality class; Kv via Wizard each time

Run both in xcopterCalc: disc area, hover current, hover time, specific thrust, failure note. Per help logic, coax can win in hover on the same frame — if props and cooling are right.

Steps in xcopterCalc

1. Open xcopterCalc.
2. Variant A: 6 rotors, flat, 600 mm, max prop, Wizard, Calculate.
3. Variant B: 6 rotors, coax, same frame, larger prop, Wizard, Calculate.
4. Tabulate results (members: CSV export).
5. Optionally compare X8 coax vs flat octo.
6. Decide on efficiency/redundancy vs cost/complexity.

More practice: cost and complexity

When comparing on equal frame size, also tally motors, ESCs, wiring and spares. An X8 may win hover efficiency and redundancy on paper while doubling powertrain cost versus a flat quad that simply uses a larger frame. Use xcopterCalc for the physics, then decide with budget and maintenance in mind — both are legitimate constraints for hobby and semi-pro fleets.

FAQ

Is coax always better?
No. If you can grow the frame and flat disc area, flat may win. On a given frame, coax often wins per eCalc’s explanation.

Same props top and bottom?
Works; help tuning can improve efficiency slightly.

What about quads?
Four flat rotors: a single failure typically flips the craft. More rotors are not magic — remaining throttle margin matters.

One-sentence decision aid

Need max efficiency/redundancy on a small frame → compare coax seriously in xcopterCalc. Need simplicity and cheap spares → flat, maybe with a slightly larger frame. The calc shows physics; you choose the priority.