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Thrust to weight ratio

Work out the thrust to weight ratio of a multirotor from its all up weight and the bench thrust of one motor. The ratio says whether it flies; the hover throttle says how it will feel.

Everything that leaves the ground: battery, props, camera, payload, cable ties.

From the motor's thrust table, at your prop and pack voltage.

What the ratio means

  • Below 1.5Will struggle to lift, unsafe to fly

  • 1.5 to 2Flies, heavy and sluggish

  • 2 to 3General purpose, good control margin

  • 3 to 5Sport and freestyle

  • Above 5Racing, twitchy, needs rate tuning

    Far more thrust than the aircraft needs to stay up. Hover is a hair off idle and the throttle curve is unforgiving. Needs rate and expo tuning to be flyable.

Thrust to weight

5.54:1

Racing, twitchy, needs rate tuning. Far more thrust than the aircraft needs to stay up. Hover is a hair off idle and the throttle curve is unforgiving. Needs rate and expo tuning to be flyable.

Hover throttle

18%

Total thrust

3600g

Spare lift

2950g

Hovering near 50 % throttle is the efficiency sweet spot for most builds. It leaves authority in both directions and keeps the stick in a range you can control finely.

  • Caution

    All up weight means everything that leaves the ground

    Battery, props, camera, VTX, antenna, GPS, mounting hardware, cable ties, and whatever payload you are carrying. A ratio calculated from a dry frame weight is meaningless, and the battery is usually the single heaviest item. Put the finished aircraft on a scale with the pack in.

  • Caution

    Bench thrust figures are a best case

    Manufacturer thrust is measured on a static test stand, at full charge, with one specific prop, at a specific voltage, in still air. Your pack sags under load, so thrust falls through the flight. By the end of a pack you may have two thirds of the figure you designed around. If the calculated ratio is only just acceptable, it is not acceptable.

  • Note

    Hover throttle is very low

    Hovering at 18 % means the whole useful range of the stick sits in its bottom quarter. Fine for racing, hard to fly smoothly. Expo and rate tuning will help, and so will a smaller prop.

Why this matters

A multirotor does not hold itself level by being stable, it holds itself level by constantly changing the thrust of individual motors. That only works while there is thrust left to give. Below about 1.5 the aircraft asks a motor for more during a correction, finds nothing there, and falls out of the correction rather than recovering from it. The number that makes this go wrong is the weight: an all up weight that leaves out the battery is not a mistake of a few percent, it is usually a third of the aircraft.

Assumptions and sources

  • Manufacturer thrust figures come from a static bench test at full charge with one specific prop and voltage, in still air. Real thrust falls through a flight as the pack sags.
  • All motors produce the same thrust, and the airframe does not disturb the airflow into any of them. Stacked or overlapping props lose thrust to each other.
  • Hover throttle is approximated as the inverse of the ratio. Real throttle to thrust curves are not linear, so treat it as an indication of roughly where hover sits on the stick.
  • The ratio bands are the accepted rules of thumb in the hobby, not measured thresholds. A heavy cinematic build and a light racer can both be right at different ends of the scale.
  • Around 50 % hover throttle is the efficiency sweet spot for most builds. It is a guideline, not a target to optimise against.

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