Glossary

Every term ThrustLab uses, defined once in plain language. The rest of the User Guide and the ⓘ popovers throughout the app link back to these definitions, so if a word ever looks unfamiliar, this is the place to look it up. Each term opens with a sentence anyone can read; the engineering detail follows.

Kv

A motor's speed constant: the RPM it spins per volt applied, with no load. A higher-Kv motor spins faster on the same battery but makes less torque, so it pairs with smaller propellers; a lower-Kv motor turns slower with more torque and swings a bigger prop.

In ThrustLab, Kv is the headline number on the motor picker and the main custom-motor input. It is tied to the motor's torque constant (one is essentially the inverse of the other), so changing Kv shifts both the speed and the torque the motor produces.

Advance ratio (J)

How far the propeller moves forward in one revolution relative to its diameter — a dimensionless way of saying how "loaded" the prop is by forward speed. J = 0 at static (hover); it climbs as airspeed rises.

J is the horizontal axis of a propeller's performance curves. Both the thrust coefficient (Ct) and power coefficient (Cp) fall as J rises, which is why a prop that makes lots of static thrust can make far less at speed.

Related: Thrust coefficient (Ct) · Power coefficient (Cp)

Thrust coefficient (Ct)

A dimensionless number that captures how much thrust a propeller makes, with air density, RPM, and diameter divided out. It lets you compare props of different sizes on equal footing — the same Ct means the same "thrust personality" regardless of scale.

Together with the power coefficient, Ct describes a propeller's entire character at a given advance ratio. The diagram below shows the typical shape.

Related: Power coefficient (Cp) · Advance ratio (J)

Power coefficient (Cp)

The power-side twin of the thrust coefficient: a dimensionless measure of how much shaft power a propeller absorbs, with density, RPM, and diameter divided out. Together, Ct and Cp tell you everything about how a prop behaves at a given advance ratio.

A high-pitch prop has a higher Cp — it works the air harder and demands more power for the same RPM. Comparing Ct to Cp is essentially comparing the thrust you get against the power it costs.

Related: Thrust coefficient (Ct) · Advance ratio (J)

Disk loading

Thrust divided by the total swept rotor area — it tells you how hard the rotors are working the air. Lower disk loading is more efficient in hover but needs bigger props; higher disk loading is more compact and agile but burns more power to hover.

It is one of the clearest single numbers for explaining why a big, slow-turning prop hovers more efficiently than a small, fast one.

Six-step vs FOC advanced

The two ways an ESC drives a motor. Six-step (trapezoidal) commutation switches the windings in six coarse stages — it is simple and common in drone ESCs. Field-oriented control (FOC) shapes the current smoothly to follow the rotor angle, which runs quieter and a little more efficiently but needs more processing power.

ThrustLab models both. The choice mainly affects ESC losses and how the motor current waveform is treated — for most steady-state estimates the difference is small, but it grows at high current.

State of charge (SOC)

How full a battery is, as a percent of its usable capacity — 100% is fully charged, 0% is empty. It is the battery's fuel gauge. Most flights stop at a safe reserve well above 0% to protect the cells.

In a dynamic run, SOC falls over time as the battery drains; you can set a cutoff so the simulation ends when SOC reaches a chosen floor.

Related: Specific energy · C-rating

C-rating

How fast a battery can safely deliver current, expressed as a multiple of its capacity — a 1300 mAh pack rated 100C can supply about 130 A. A higher C-rating means a stiffer pack that sags less voltage under load.

ThrustLab can convert a C-rating into an internal resistance for the pack, which is what actually causes voltage sag in the simulation.

Related: Specific energy

Tip Mach number advanced

How fast the propeller's blade tips move compared to the speed of sound. The tip travels far faster than the hub, so it meets compressibility effects first — as it nears Mach 1, drag and noise climb sharply, and ThrustLab warns you.

It is the reason very large props are spun slowly and small props can only spin so fast before they become inefficient and loud.

Related: Drag-divergence Mach (M_dd)

Drag-divergence Mach (M_dd) advanced

The local Mach number at which a blade section's drag starts to shoot up because the airflow over the section goes locally supersonic and forms a shock. It is a property of the airfoil section, not of the whole blade. Running blade sections past M_dd wastes power and adds noise, so ThrustLab reports how much of the blade is above it.

The Diagnostics tab flags frac_above_Mdd — the fraction of the blade running past drag divergence. A non-zero value means the prop is being pushed into a noisy, inefficient regime.

Related: Tip Mach number

Specific energy

How much energy a battery stores per unit of weight — the number that ultimately sets how long you can fly. A pack with higher specific energy carries more flight time for the same mass, which is why it matters more than raw capacity alone.

When you are weight-limited (almost always, on an aircraft), specific energy is the figure of merit for choosing a chemistry.

Related: C-rating · State of charge (SOC)