The Builder
The Builder at https://rotorlab.app/app is where you describe an aircraft and read its predicted performance. The left side is a live console of input groups (airframe, propulsion, batteries, payload, power rails). The right side is the readout: stat cards, charts, a checks block, and a performance detail table. Every edit recomputes the whole readout after a short pause, so you see the effect of a prop size or a heavier pack immediately.
Reach for the Builder when you are sizing a new aircraft, judging a payload or battery change, or checking whether a build will fly at a hot, high site. If you are starting from nothing, the Setup Wizard walks the same inputs in order and hands the finished build back to this page.
The console#
The console is a stack of collapsible input groups. Click a group header to collapse or expand it; the collapse states persist across reloads. The groups, in order:
- Airframe: Airframe type, Dry mass, Arm length (center to motor), Motors, Coaxial (X8 / Y6 stacked). See Airframe types.
- Batteries and buses: the motor bus picker and one card per battery pack. See Batteries, buses, and power rails.
- Propeller: Diameter, Pitch, Blades.
- Motor: KV, Stator (label), Max thrust per motor, Measured loaded RPM (WOT).
- ESC & wiring: Current per motor, continuous; Main connector; Main wire (AWG).
- Environment: Site altitude, Site temperature.
- Forward flight: Flat-plate drag area f.
- VTOL transition (wing), Fixed wing (wing), Lift rotors (dead in cruise), and Lift rotor motors: these appear only when the selected airframe type uses them.
- Payload: the Other payload (mounts, wiring) mass plus itemized components from the payload catalog.
- Power rails: BEC and UBEC rails with their loads.
- Advanced model factors: figure of merit, coaxial power penalty, usable energy fraction, and the other tuning factors. This group starts collapsed.
Every input carries a small ? badge. Hover or focus it for a short note on what to enter, its units, and what a value of 0 does.
The most important input
Max thrust per motor in the Motor group. Enter a measured or datasheet value and thrust-to-weight, hover throttle, and the wide-open-throttle current checks become trustworthy. Leave it at 0 and the tool estimates thrust from a coarse prop coefficient and marks every dependent figure as an estimate.
The in-progress build is kept in your browser, so navigating to another RotorLab page and back does not lose unsaved work. Your first visit opens a full example build so you can see a complete analysis; after that, an untouched session opens blank, and Load example in the Build menu brings the example back.
The readout#
The readout recomputes about a fifth of a second after you stop typing. A small live indicator in the page bar pulses while it works, next to a chip showing the loaded build name and a one-line geometry summary.
Hero stat cards#
The four large cards at the top depend on the airframe type:
| Airframe type | Cards |
|---|---|
| Multirotor types and VTOL transition | All-up weight (g), Hover thrust-to-weight (:1), Hover endurance (min), Power loading (g/W) |
| Quadplane and Tiltrotor types | All-up weight (g), Hover thrust-to-weight (:1), Hover endurance (min), Cruise endurance (min) |
| Fixed wing types | All-up weight (g), Stall speed (km/h), Cruise endurance (min), Range (km) |
The Hover thrust-to-weight card carries a quality tag: under 2 is underpowered, 2 to 3 is adequate, 3 to 5 is good, and above 5 is very agile. A row of secondary figures sits under the cards; for a multirotor it includes flight bus current, WOT current, max speed, disc loading, total pack energy, hover thrust fraction, and air density.
Charts#
- Power distribution: the animated diagram of batteries, buses, motors, rails, and loads. Wire thickness scales with current.
- Prop power required vs airspeed: shown when forward flight is active (flat-plate drag area above 0, or a winged type), with the cruise speed marked.
- Flight endurance vs flight-pack capacity: hover and cruise endurance swept from 0.4x to 2x your pack capacity, with your pack marked.
- Mass breakdown: frame, each battery, each component, and other payload.
- Hover power split: prop hover power versus auxiliary pack-input power.
- Thrust-to-weight vs flight-pack capacity.
- Flight bus current vs limits: hover and WOT totals against the flight pack's continuous and burst ratings.
Checks#
The Checks block opens with a verdict line: All checks passed, Flies, with cautions, or Will not fly or risks hardware, followed by a count of failures, warnings, and notes. The individual findings flag, among other things:
- ESC over limit: WOT current per motor against the ESC's continuous rating, with the headroom margin when it passes.
- Flight pack limits: hover current against the pack's continuous C rating, and WOT current against its burst rating.
- Regulator headroom: any power rail whose output current exceeds its regulator rating, and any rail assigned to a bus with no battery.
- Accessory pack runout: a warning when a non-motor bus runs out of energy before the flight pack does.
- Main connector and main wire ampacity against hover and full-throttle current.
- Prop clearance: a propeller larger than the arms allow.
- Over-massed hover: hover using more than half the available thrust.
- Transition feasibility on winged VTOL types.
Performance detail#
Expand Performance detail under the checks for three tables: Power buses (each bus with its energy, hover minutes, and continuous utilization), Performance (max thrust, RPM, pitch speed, throttle band, prop clearance, energy, range, and the forward-flight speeds when active), and Auxiliary power rails (each rail's output current against its rating and its load and pack-input watts).
Reading the numbers#
- Hover thrust fraction is exact: the share of available thrust used to hover.
- The hover throttle band shows two bounds: a linear one equal to the thrust fraction, and an aerodynamic one equal to its square root (thrust scales with RPM squared). Real stick position sits between them.
- Anything tagged est or rough is a model estimate, not a measured value. Entering a measured max thrust per motor or a measured loaded RPM replaces the corresponding estimate.
Toolbar#
The page bar holds the Wizard button, which opens the guided Setup Wizard, and the Build menu:
- New build: clear to a blank build.
- Load example: load the full example profile.
- Import profile: read a build from a profile JSON file.
- Export profile: write the full build, including all batteries, buses, rails, and loads, to a JSON file.
- Build library: open your saved builds. See Build library.
- Download report (HTML): a self-contained report file with the stat cards, power diagram, and charts. See Reports.
- Download report (PDF): the same build as a document for a proposal or design review.
- Help: an in-app panel covering the hover model, airframe types, forward flight, the VTOL transition, the power system, reading the output, and calibration.