Mode 2 Drone + FPV reference

Tools

Prop clearance

Whether a prop size fits a wheelbase on a symmetric X, and by how much. Exact geometry, narrow scope.

Frames are advertised by wheelbase, props by diameter in inches, and the two are never in the same units or the same geometry. This works out whether the discs actually miss each other, and what gap is left.

Frame and prop

The figure frames are advertised with: the distance between diagonally opposite motor shafts.

The diameter, so the 5 in a 5x4.3x3.

Result

Enter a wheelbase and a prop size.

The geometry

Prop clearance on a symmetric X, 220 mm wheelbase with 5 inch props Four motors at the corners of a square. The advertised wheelbase of 220 mm is the diagonal between opposite motor shafts. Adjacent motors sit 155.6 mm apart, the prop discs are 127 mm across, and 28.6 mm is left between blade tips. wheelbase (diagonal) 220 mm gap 28.6 mm adjacent motors 155.6 mm prop 5″
Drawn to scale. The wheelbase is the diagonal, not the side, which is exactly why 220 mm is not the distance the props have to clear.

On a true or symmetric X the four motors sit at the corners of a square. The wheelbase quoted by the frame maker is that square's diagonal, measured shaft to shaft between diagonally opposite motors. Adjacent motors are therefore the square's side apart, which is the diagonal divided by the square root of two.

Two adjacent props miss each other when that spacing is greater than one prop diameter, and the gap between their blade tips is the difference. That is the whole calculation, and within the geometry it models it is exact rather than approximate.

  • Adjacent spacingwheelbase / sqrt(2)
  • Prop diameterinches x 25.4 mm
  • Tip gapspacing minus diameter

Checked against frames we have documented

The archive on this site records the wheelbases of several frames the domain's previous occupant sold, along with the prop sizes they were built for. Running them through the same arithmetic gives results consistent with what those frames were specified to take, which is a reasonable sanity check on the model even though it is not a substitute for the manufacturer's own figure.

Documented frames, run through the calculation above
Frame Wheelbase Prop Tip gap
Kilowatt 145 mm 3" 26 mm
Shredder, 5 inch 220 mm 5" 29 mm
Shredder, 7 inch 280 mm 7" 20 mm
Wheelbases and prop sizes from the archived product pages, documented on Kilowatt and Shredder. Tip gaps calculated here, not published by the manufacturer.

The pattern in that table is worth noticing: the gap gets tighter as the class gets larger. A 7 inch frame is a much more constrained design than a 3 inch one, which is part of why long range builds tolerate compromises that a small freestyle build does not.

What runs out of room first

In practice the neighbouring prop is rarely the binding constraint. The arm itself, the camera, the stack, an action camera mount or a chunky antenna will usually intrude into the disc before another prop does. This calculator cannot see any of that. The prop size a frame maker specifies accounts for all of it, and it is what binds.

Where the model does not hold

A stretched X, which the frames page describes, moves the motors so front and rear spacing differ. The wheelbase is still quoted as the diagonal, but adjacent spacing is no longer the diagonal over the square root of two, and this calculation will be wrong. The same applies to hexacopters and octocopters, where motors sit at different angular spacing entirely.

Prop pitch and blade count do not enter the calculation at all, because they do not change the diameter. A 5x4.3x3 and a 5x3x2 occupy exactly the same disc.