Balance and CG
Balance and CG at https://rotorlab.app/cg answers two questions the Builder's performance numbers cannot: where does the centre of gravity of this build actually sit, and is that a problem? You position each part of the aircraft (battery, payload, individual components), and the CG marker moves to match the resulting mass distribution. On a multirotor the tool compares the CG against the point the lift rotors balance about; on a winged aircraft it reports the CG as a percentage of the mean chord, the standard aviation measure.
Placement affects balance only. It never changes the performance figures the rest of the app reports: the tool distributes the build's mass, it does not add or remove any. The sum of every item on this page is exactly the all-up weight the analyzer reports. Dry mass is split into the motors (at their real positions, using the Motor mass, each (g) field) plus a "Frame and electronics" remainder, so distributing mass can never invent or lose grams.
The coordinate frame#
Positions are millimetres from the frame datum: +x right, +y aft, +z up. The datum depends on the airframe:
| Airframe | Datum |
|---|---|
| Multirotor | The centre of the rotor ring, because hover wants the mass directly under the thrust |
| Winged aircraft and VTOLs | The wing leading edge at the root, so "CG at 30% of chord" falls straight out of the arithmetic |
An item you have not placed yet sits at the datum, and the tool counts its mass there. The verdict line reports how many grams are still unplaced, and a finding flags the balance as provisional when unplaced mass is 5% or more of the aircraft.
Choosing a build#
The Build selector at the top offers:
- Current build (from app): the working build shared across the app (see The working build). Positions you set here are saved back to it, so they follow the build into other tools.
- Example profile: the app's default example.
- Library: (name): any build saved in your library.
Next to it, Start from a frame loads a dimensioned frame preset into the current build. Your batteries, payload, and components are kept; only the airframe underneath them changes. A short description of the selected frame appears beside the selector.
Frame dimensions#
A row of dimension fields sits under the selectors, and the fields shown depend on the airframe:
- Multirotor: Arm length, hub to motor (mm). The rotor positions are derived from it.
- Winged aircraft: Wingspan (mm), Body length, nose to tail (mm), and Wing chord (mm).
On a winged build that has not placed its motors, the tool derives a plausible motor layout from the wingspan and body length (a tractor prop ahead of the wing, a pusher behind it, lift rotors on booms), so the drawing shows a real aircraft instead of every motor stacked on the datum. Any motor position the build has actually set wins over the derived layout.
Placing parts#
There are two ways to position a part, and they stay in sync:
- Drag it in a view. The Top view (looking down, nose up the page) moves a part in x and y. The Side view (nose to the left) moves it in y and z. The isometric view is a viewer only; you cannot drag in it. While you drag, the CG marker follows live in every view; when you release, the server recomputes the authoritative balance and refreshes the findings.
- Type coordinates in the table. The table lists every part with its mass and its x, y, z fields. Batteries, components, the payload, and the frame remainder are editable. Motor rows are locked and marked "(from the frame)" because their positions come from the frame geometry.
In the views, each part is a coloured dot whose area is proportional to its mass. The legend maps colours to kinds: battery, component, payload, motor, frame.
Reading the result#
The headline verdict reads Balances at y (mm), adding the x offset when it is 0.5 mm or more, and on a winged build the CG position as a percentage along the chord. Below it, the sub line reports the distance from the point the rotors balance about, the mean chord, which datum the numbers are measured from, and any unplaced grams.
In the views:
- A red cross marks the centre of gravity.
- A dashed circle marks where the lift rotors balance.
- On a winged build, a shaded band marks 25% to 33% of the chord, the range a conventional wing is built around.
What the balance means#
The What this means for the way it flies panel lists findings from the build checks, each rated ok, info, warn, or fail. The worst finding colours the verdict. The checks cover:
- Multirotor offset: how far the CG sits from the rotor centre, relative to how far the rotors are from the centre (10 mm is nothing on a large hex and a lot on a 5 inch quad). Ratings run from "Balanced over the rotors" through "Slightly off center", "Off the rotor center", "Badly out of balance", and a failure when the loaded motors would reach maximum thrust just holding level.
- Winged CG range: where the CG falls against the 25% to 33% chord band, warning when it is far forward (stable but heavy in pitch) and failing when it is dangerously aft (roughly past 35% to 40%, where the aircraft pitches away from level instead of settling back).
- VTOL conflict: a quadplane or tiltrotor has to satisfy hover and cruise at once. If the lift rotors balance outside roughly 20% to 38% of the chord, no single battery position satisfies both, and the finding says to move the lift rotors rather than the battery.
- Unplaced mass: a reminder that mass without a position is being counted at the datum.
Tip
On a symmetric airframe the motors and frame contribute almost nothing to a CG offset. A build's real balance error is nearly always the battery and the payload, so place those two first.