The motor is the hole down the middle

Nine printed pieces push together into a hand-launch biplane. One of them is hollow, and the twist you wind into it is the whole engine.

Nine pieces

Two wings, a fuselage, a tail, a propeller and four posts. Every joint between them is printed: a peg in a hole, or a tab in a slot.

Plugged together

They go together one way. The four posts stand in the lower wing, the fuselage drops onto two pegs between them, and the upper wing lands on what is left sticking up.

The cage

That is the whole trick of a biplane: two flat plates held 29.6 mm apart by four posts, and nothing else holding the top one up.

The hollow

Inside the fuselage: a shell 2 mm thick around 125.5 mm of nothing, with a Ø1.4 mm bore at the nose and a Ø3 mm post standing across the far end.

Wound

Turn the propeller and the band knots up between those two features. Nothing else on the aeroplane moves while you wind it.

Launch

Let go. The band hands every turn back through the same bore, the propeller runs free, and the plane leaves your hand.

The fuselage drawn back to a transparent shell, showing a twisted band running from the nose down the length of the hollow bay.
The fuselage drawn back to a shell. The band runs from the nose bore to the post at the back of the bay, wound. Rendered from the shipped assembly; the band is drawn in, because the CAD has no mesh for one.

The inside is the engine

Open the fuselage along its axis and there is almost nothing in it, which is the point.

A solid nose block 11.5 mm long carries a Ø1.4 mm bore on the centreline. That bore is the propeller's bearing, and it is the only hole through the front of the aeroplane. Behind it the walls run 2 mm thick — exactly 2 mm at every station, top, bottom and both sides, tapering with the outside all the way aft — around 125.5 mm of open bay.

118 mm back from the nose face, a Ø3 mm post stands across that bay. Those two features are the entire motor: one end for the propeller to turn on, one end for the band to hook over, and the length of the fuselage in between for the twist to live in.

And the channel is open at both ends. Put a straight line down the axis and it goes in at the nose bore, past the post, and out of a 2 × 4 mm mouth in the back of the tail without touching anything else — which is exactly what a rubber-band motor needs and nothing else on the aeroplane provides.

Four posts do all the work

Two wings cannot hold themselves apart. Four printed posts hold them, and that is the difference between a biplane and a plank.

Each post is 43.2 mm long and changes diameter twice along its length: Ø3 mm where it passes through a wing, Ø3.5 mm in the open span between them. The wing holes are Ø3.2 mm, so all eight of those joints are a peg 0.2 mm under its hole — stiff enough to stand on its own, loose enough to push together by hand. The fuselage sits on the same idea, on two Ø2 mm pegs in the lower wing and two Ø2.2 mm sockets in its underside.

Both wings are flat plates 3 mm thick on the same chord line, with no stagger between them. The posts finish flush with the top of the upper wing; the little coloured discs you can see up there are their ends.

Rear elevation of the biplane: the yellow upper wing and violet lower wing held apart by four teal posts, with the orange fuselage and the pink tail between them.
Rear elevation, drawn straight off the assembly at true scale and dimensioned in millimetres. The blade standing behind the fuselage is the propeller — it reaches further than the fuselage is deep, so it shows above and below.

Nine pieces, printed flat

The whole aeroplane on one plate, at the size it prints, in the colours the print project loads.

Fuselage

The hollow spine. Walls 2 mm thick, a Ø1.4 mm bore at the nose and the Ø3 mm anchor post inside.

Upper wing

The long plate, 3 mm thick. Four Ø3.2 mm holes, and the posts come through them flush with the top face.

Lower wing

The short plate. The same four holes, plus a pad on the centreline with two Ø2 mm pegs that locate the fuselage.

Propeller

One flat blade on a Ø8 mm hub, 1.5 mm thick, with the same Ø1.4 mm bore. It runs 0.5 mm clear of the nose face.

Tail

Fin and stabiliser in one piece, both 1.5 mm thick. A tab under the fin drops into a slot in the top of the fuselage's back.

Post ×4

Ø3.5 mm between the wings, turning down to Ø3 mm where it goes through them. 43.2 mm end to end.

All nine printed parts laid out flat and seen from above: the yellow upper wing, violet lower wing, orange fuselage, teal propeller, pink tail and four teal posts.
Every piece at true relative scale — nothing here is normalised, so the upper wing really is four times the length of a post. Plates face down, posts on their sides, the tail standing on its tab.

It looks like what it is

Extruded plastic, printed one layer at a time, with the layers still showing on every face.

The assembled biplane standing on a pale studio floor, seen from the front quarter, with the propeller blade turned nearly flat.
It does not stand on its wing. The four posts run right through the lower wing and finish 0.3 mm proud of the underside, so the plane is standing on their feet.
Close view of the nose: the teal propeller blade with its small central bore, sitting just in front of the orange fuselage.
The nose, close. The dot in the middle of the propeller hub is the Ø1.4 mm bore, lined up with the matching one in the nose block behind it.

The numbers

Printed parts 9
Overall 6.1 × 7.1 × 2.4in
Weight 1.6 oz
Print time 5.3hours
Material PETG
Moving parts One

One printed aeroplane: 1.6 oz of it, 6.1 × 7.1 × 2.4 in overall. Wind the propeller, hold it by both wings, and let go.