Thrust-to-weight ratio
Summary¶
Thrust-to-weight ratio (T:W) is the ratio of the maximum thrust the propulsion system can produce to the all-up weight of the aircraft. A T:W of 1.0 means the motors can exactly balance gravity at full throttle; the aircraft can barely hover and has no reserve for manoeuvre or wind. A T:W of 2.0 means the motors produce twice the aircraft weight at full throttle; hover requires roughly 50% throttle, leaving the other 50% for authority. For libdrone variants, T:W at full throttle ranges from approximately 2.3:1 (Ghost, survey- optimised) to 4:1+ (Pro and Bandit in sport trim). T:W determines hover throttle position, climb rate, wind resistance ceiling, and the margin available for attitude recovery from disturbances.
Concept¶
The hover throttle relationship¶
In steady hover, thrust equals weight. If maximum thrust is T_max and aircraft weight is W, hover throttle position is approximately W / T_max. At T:W = 2, hover requires 50% throttle. At T:W = 4, hover requires 25% throttle.
The hover throttle position matters for two reasons:
First, control authority: the flight controller modulates throttle ±N% around the hover point to maintain altitude and respond to attitude changes. If hover is at 90% throttle (T:W ≈ 1.1), only 10% throttle is available for upward authority — the aircraft cannot accelerate upward quickly and will lose altitude in any manoeuvre that demands more than the available 10%. If hover is at 50% (T:W = 2), 50% remains for authority — full attitude recovery is possible.
Second, motor temperature: motors near maximum throttle for sustained hover run hot. Motors at 50% throttle for sustained hover run cool. Motor temperature under hover load is a direct function of hover throttle position.
T:W and wind resistance¶
In forward flight tilted into wind, the motor thrust vector has both a vertical component (opposing gravity) and a horizontal component (opposing drag and wind force). The maximum wind speed the aircraft can resist is the speed at which the horizontal thrust component equals the aerodynamic drag — which depends on the reserve thrust available above hover. Higher T:W means more reserve thrust, which means higher maximum sustainable wind speed.
For a survey platform like Ghost with T:W ≈ 2.3, maximum sustainable wind is approximately 8 m/s. For a sport platform like Pro with T:W ≈ 4, wind resistance exceeds 15 m/s. This is why Ghost's mission weather limit is 8 m/s and Pro's is much higher.
T:W in motor and battery selection¶
T:W cascades through the entire design. Selecting heavier motors lowers T:W (more weight, similar thrust) unless matched with larger props. Adding battery mass lowers T:W. Adding payload mass lowers T:W. The design cycle for any new variant starts with the target T:W, derives maximum AUW from the motor thrust curves, then allocates that mass budget across components.
For Ghost: 4× MN4108 at 4S produce ~880 g thrust each = 3520 g total thrust. AUW 1360 g. T:W = 3520/1360 = 2.6. Hover at ~38% throttle. This is appropriate for a survey platform with calm-weather mission constraints.
Reference¶
| Platform | Max thrust (4 motors) | AUW | T:W | Hover throttle |
|---|---|---|---|---|
| Pro (6S) | ~3 200 g | ~600 g | ~5.3 | ~19% |
| Bandit (4S) | ~2 436 g | ~500 g | ~4.9 | ~21% |
| Core (4S) | ~2 436 g | ~250 g | ~9.7 | ~10% |
| Ghost (4S) | ~3 520 g | ~1 360 g | ~2.6 | ~39% |
Minimum T:W for stable flight: 1.5 (provides control authority and 6 m/s wind resistance). Below 1.5, altitude hold becomes unreliable in any wind.
T:W for ArduPilot Autotune: Autotune requires T:W ≥ 2.0. Below this, the aircraft cannot perform the rapid attitude changes the procedure demands.
Procedure¶
Calculate T:W for a new build¶
- Find the motor thrust curve from the manufacturer's datasheet (thrust at 100% throttle on the intended battery cell count and prop size).
- Multiply single-motor max thrust by motor count.
- Estimate AUW: sum BOM component masses + printed parts + battery.
- T:W = total max thrust / AUW (both in the same unit, grams or Newtons).
- If T:W < 2.0, increase motor size, reduce AUW, or accept limited wind resistance and reduced Autotune compatibility.
Rationale¶
T:W is the first design parameter checked when evaluating any build or modification — before motor temperature, before flight time, before regulatory weight. A build with T:W < 1.5 is unsafe regardless of how good its other parameters are. Establishing T:W analysis as a first-step check prevents the common error of selecting motors that are correct for the frame size but insufficient for the actual loaded AUW after all components are added.
Connections¶
yaml requires: - [lift-and-thrust](<./lift-and-thrust.md>) - [brushless-motors](<./brushless-motors.md>) - [propellers](<./propellers.md>) related: - [hover-and-forward-flight](<./hover-and-forward-flight.md>) - [lipo-batteries](<./lipo-batteries.md>) - [li-ion-batteries](<./li-ion-batteries.md>) - [ardupilot-autotune](<./ardupilot-autotune.md>) - [motor-mixing](<./motor-mixing.md>) leads_to: - [hover-and-forward-flight](<./hover-and-forward-flight.md>) - [ardupilot-autotune](<./ardupilot-autotune.md>)