Airframe

Airframe is the skeleton of the drone: the frame you choose or draw, its arms, the hardware that holds it together, the straps and pad under the pack, and whether everything you bolt to it fits.

Where it is

Click Airframe in the section row. The section list shows five rows: Frame, Arms, Screws & standoffs, Layout & fit and Straps & pads.

Airframe selected in the section row, the drone in the viewport and the Frame, Arms, Screws & standoffs, Layout & fit and Straps & pads rows in the list

Frame

Line 2: the frame’s size class and wheelbase (motor to motor across the diagonal), for example 5" · 220 mm.
Line 3: the frame mass that flies. For an unedited catalog frame this is the vendor’s weighed figure. Once you edit the frame in the designer, it becomes an estimate and carries a ~.
Choosing a frame: click line 2 to open the part finder on the frame catalog.
The page is the frame designer, described below.

A frame from the catalog also has its published figures (built around, material, frame mass, arm length, largest prop, motor mount) on its sheet, so the computed figures have something to be compared with.

The part finder open on frames, with 5” Freestyle highlighted

Arms

Line 2: the arm material.
Line 3: the arms’ first bending mode with the motors fitted, for example ~180 Hz 1st mode. It carries a ~ because the model behind it is scaled from a guessed reference (see Guessed, sourced and measured).
The page draws the arms in plan beside the Arms sheet: arm length, section at the root, taper, the stock it is cut from, mass per arm and for all arms, tip stiffness, bending stiffness, bare first mode, root stress per newton, twist per newton-metre, and a strength ranking against the catalog’s plate frames. The two page numbers are the loaded first mode and the arm stock thickness.

There is nothing to pick here. Arms are cut from the same sheet as the frame, so you change them in the frame designer.

Screws & standoffs

Line 2: the hardware the frame needs, for example M3 · 4 standoffs.
Line 3: the hardware mass, for example ~20 g hardware. It is computed from the geometry, so it is an estimate of a typical hardware bag, not of the one you own.
The page shows the Fasteners sheet: standoffs, screws, hardware mass and its share of the frame mass, stack height, top plate height, motor screw engagement, and the assembly checks.

Layout & fit

Line 2: the stack mount pattern, for example stack 30.5x30.5.
Line 3: the part closest to a propeller disc, for example pack 9 mm from a prop.
The page draws the layout in plan beside two sheets:
- Fit: mounts, shims and standoffs. These change the fit, not the flight numbers. There is a Pack mount choice (which plate the pack straps to) and sliders for prop spacer, motor soft mount, stack standoffs, pack fore/aft, prop imbalance, GPS mast height and camera uptilt. Each slider shows its value in millimetres (or grams, or degrees) and its range comes from the parts you have fitted. Reset to as-built puts them back. The pack’s overhang fore/aft and each side, and its clearance to the props, are measured here.
  
  Shims and standoffs change only the geometry: what clears what. Where the pack is mounted and how far fore or aft it sits also move the centre of mass and the inertia. These settings are stored once for your Lothal install (assembly_tweaks.json), not per drone.
- Layout: where the motors are and what that geometry alone decides: whether the layout can be controlled, roll, pitch and yaw authority, and how it compares with a standard quad mixer.

The Layout & fit page with the pack moved aft into a prop disc, drawn in red, and the Layout & fit row marked red

Straps & pads

Line 2: the pad and the strap that hold the pack, for example TPU pad 2 mm · 20 mm strap (guess), or no pad · 20 mm strap (guess) on a build with no printed pad fitted.
Line 3: where the pad starts isolating, as ~N Hz pad isolation corner. With no pad fitted, this reads the slot count instead, for example 2 strap slots under the pack.
The page draws the seat plate’s slots against the pack’s footprint beside the Straps sheet, which has these rows:
- Battery pad: the pad’s material, thickness and plan size when a printed pad is fitted; not fitted otherwise, with a pointer to the Fitted tick on Printed → Battery pad.
- Loaded area: the pack’s footprint minus the two slot openings, with the pack’s mass beside it.
- Pad stiffness: the pad’s spring, in meganewtons per metre. A ranking figure, not a measurement.
- Resonance with the pack: the natural frequency of the pack sitting on the pad, in hertz.
- Isolates above: the frequency above which the pad lets through less than it would without any pad, in hertz. Below this a pad amplifies rather than isolates.
- At the frame’s 1st mode: how much of the frame’s ring the pad passes to the pack at the frame’s loaded first mode, with the verdict isolates or passes it through.
- Damping ratio: labelled (guess).
- Lifts the pack: how far the pad raises the pack above the plate, and the pad’s own mass.
- Strap: the strap’s width and thickness, labelled (guess) when they are the defaults rather than something you set on Printed → Battery pad.
- Strap slots: how many slots are cut in the seat plate, each one’s size, and how many sit under the pack.
- Strap & pad checks: all pass, or the number flagged and the most serious one.
The two page numbers are Pad isolates above (the pad’s corner frequency and the frame’s own ring, side by side) and Strap slots.

The Straps & pads page with the seat plate’s slots drawn under the pack, and the Straps sheet listing pad, strap and slot figures

The frame designer

Clicking the Frame row opens the frame designer on the stage. It has four areas:

Left, the shelf. Nine layouts to start from, drawn as diagrams with each motor’s rotation: Quad X, Quad I, Hex I, Hex V, Hex Y, Hex IY, Oct X, Oct I and Oct V. Below them, the frames you have saved.
Centre, the canvas. 2D is the plan you draw in. 3D shows the same frame extruded: click a plate to select it, drag it to move it, and drag empty space to orbit.
Right, the controls. Frame name; arm length, width at root and tip, arm stock and plate stock; centre plate; stack height; motor and stack bolt patterns; Alternate motor directions. With an arm selected: its angle, length, root and tip width, and Clockwise. Every dimension is a slider you can drag and a box you can type in.
Below, the numbers. Drawn mass, CG offset, roll and yaw inertia, and the checks. Details ▾ opens the full structure, arms, fasteners and layout figures.

The frame designer with an imported X-frame outline on the canvas, the start-from shelf on the left and the plate controls on the right

Button	Does
New, Open, Save, Duplicate	Start, open, save and copy frame documents. Saved frames appear on the shelf.
Export	Writes one of five files. See Exports.
Import	Brings in an outline from an SVG or DXF file as plates you can edit. See Importing frame outlines.
Print	Opens the 1:1 print sheet in your viewer, ready to print at 100%. Print it and lay your parts on it to check the scale.
+ Plate, + Arm	Add a plate or an arm.
+ Hardware	Adds an M3 standoff and its two screws on the selected plate.
+ Motor mount	Cuts a motor’s four bolt holes into the selected plate.
+ Stack mount (and its pattern picker)	Cuts the flight stack’s four bolt holes into the selected plate, on the pattern chosen beside the button. The picker lists `20×20 M2`, `25.5×25.5 M2`, `25.5×25.5 M3` and `30.5×30.5 M3`; the thread is spelt out because a bare `25.5x25.5` is an M2 stack. The pattern is centred on the selected plate and the add is refused if it does not fit, with the plate unchanged and no undo step.
+ Camera cage (and its camera picker)	Adds two upright side plates on the camera bay, spaced for the camera class chosen beside the button (`Nano 14 mm`, `Micro 19 mm` or `Full 26 mm`, the widths of the catalog’s three analog cameras). Each plate has an M2 pivot hole and a 0–50° tilt slot. No selection needed: the cage seats itself on the bottom plate’s front edge.
Copy	Copies the selected plate.
Mirror	Repeats the selected arm around the centre, once per arm the layout has.
Delete	Deletes the selected plate, and the motor standing on it if it is an arm.
Undo	Steps back one edit.
−, +, Fit	Zoom out, zoom in, and frame the whole drawing.
2D, 3D	Switch the canvas view. In 3D, upright side plates stand on their base and flat plates lie, so a cage reads as a cage.
snap	Snap step: 0.5 mm, 1 mm, 5 mm or off. Hold `Ctrl` while dragging to ignore it.
Symmetry	When on, dragging a corner of one arm moves the matching corner on every arm.

Keys on the canvas: F fits the drawing to the view, S turns symmetry on and off, and Ctrl+Z (Cmd+Z on macOS) undoes.

What reaches the drone

The designer edits the frame that is fitted. An unedited catalog frame flies with its published mass and arm length. When you edit it, the frame that flies changes by what your edit changed, in mass and in arm length, and those figures then carry a ~. Plate thickness and standoff heights drawn in the designer do not move where parts sit on the drone in this version.

Sim flies quads only. If the drawn frame does not have four motors, it is not flown, and the Frame row says drawn frame not flown: not a quad.

Exports

Export	Use it for
Print sheet (SVG, 1:1)	Printing on paper at 100% to check sizes against real parts.
Cut sheet (SVG, nested)	The plates laid out for cutting.
Cut file (DXF)	Sending to a CNC or laser-cutting service.
Assembly (STL)	Opening the assembled frame in another 3D program.
Build sheet (HTML: parts, hardware, cost)	A printable page listing every plate (role, thickness, material, size and quantity), every screw by length and every standoff, strap and pad sections when they are there, a camera cage section when there is one, an assembly order and a cutting cost range from placeholder rates.

Below the five exports, the menu has three more entries:

Cutting bit: 1/16” (1.59 mm, the typical shop value), 1/8” (3.18 mm) or 1/32” (0.79 mm). The ready-to-cut check and the export notes assume the bit chosen here.
Round inside corners to the bit: fills every inside corner tighter than the bit’s radius, on every plate, with the fillet the router would leave anyway. It is one undo step.
Countersink selected plate’s screw holes (toggle): marks the selected plate’s round screw holes as countersunk. On the DXF they move from the plate’s <ROLE>_HOLES layer to <ROLE>_COUNTERSINK. The SVG draws a dashed blue ring over each one at 1:1.

Exports are written to the exports folder and Lothal opens it. See Exports.

What to watch for

Warnings on this section’s rows include (short forms as the rows show them):

Frame: frame has no plates, frame has no arms, frame has no motors, motor layout cannot be controlled, drawn frame not flown: not a quad, custom frame — unchecked, frame mass outside catalog range.
Arms: an arm has no centreline, an arm centreline leaves its plate, arm section outside catalog range, and notes when the frame rings near where the drone hovers.
Screws & standoffs: a screw bottoms out, too little thread engaged, hole too close to the edge.
Layout & fit: a part is in a prop disc, pack is in a prop disc, pack is wider than the plate, a part is larger than the plate, two parts overlap, a part overhangs its mount, mount pattern does not match, a part has nothing to mount to.

Not in v0.3.0. These warnings join the list:

Frame: N corner(s) sharper than the bit (an inside corner the current cutting bit cannot follow), a gap narrower than the cutting bit.
Arms: arm weaker than every <class> frame, when the arm breaks under a lower load than every catalog frame of its size class. When the arm’s length does not place it in one size class, it is compared with the whole catalog and reads arm weaker than every catalog frame.
Straps & pads: no strap slots in the plate, slot N mm, strap needs M mm, a strap slot is not under the pack, pad resonance ~N Hz, in the loop (a pad whose natural frequency lands in the flight-control loop’s bandwidth, where it amplifies rather than isolates), pad passes the frame's ring.
Video: camera wider than its side plates (the fitted camera is wider than the drawn cage’s spacing).

Warnings never stop you. See Fit checks and warnings.

Honest limits

The arm’s first mode is scaled from a reference frequency that has not been measured. Treat it as a ranking between frames, not a number to trust on its own.
The drawn frame models plates and arms. A real frame’s side plates, camera plate and small details are not all drawn, so its drawn mass can differ from the weighed one.
Straps and pads are not modelled.
An imported outline has limits; see Importing frame outlines.

In the next release, camera side plates can be drawn upright, and the battery pad and strap slots are modelled. The new figures have limits of their own:

The strength rating is a ranking under one steady load at the motor, not a crash prediction. Impact energy is not modelled, so it gives no fall height.
The strength references are Lothal’s generated presets of the catalog’s plate frames, with arm widths assumed where the vendor publishes none. Moulded frames, such as the 65 mm whoop, have no plate arm to rank.
The pad’s stiffness, natural frequency and transmissibility rank one pad against another. The pad material’s stiffness is only known as a span, so its frequencies are not to be trusted on their own. The damping ratio is a guess.
The pad does not quiet the gyro. A pack pad isolates the pack from the frame, not the flight controller.
The cutting cost is a range from placeholder rates, not a quote.

Frame bench: test how the frame’s inertia changes how fast the drone rolls.
Physics is the source of truth
Parts catalog

Documentation licensed underCC BY 4.0. Lothal itself is GPL-3.0-or-later.Report a problem.Roadmap.