To make a Technic-style propeller spin freely, mount it on an axle that is held still, slide the blades on through their round beam holes, and stop them sliding off with a bush, leaving a hair of a gap. The axle never turns; the blades turn around it. A helicopter rotor works the same way, only with the axle standing upright. Get those three things right and a gentle breath sets the propeller whirring.
Every propeller plane and helicopter in the aircraft series uses this design, from the four-blade Spitfire to the Mi-24 helicopter. This guide shows how it is put together, why it keeps turning, and how to test and fix your own.
The propeller, piece by piece
On the propeller planes, the propeller sits on the very front of the fuselage. It takes two short steps in the booklet:
- Push two black friction pins into the upright leg at the nose and press a long pin-axle block onto them. Two pins, not one, so the block cannot twist.
- Push an axle into the block's cross hole, slide on the propeller beams, and lock them on with a bush.
The blades are ordinary beams. A two-blade propeller is one beam; a four-blade propeller is two beams crossed on the same axle. The Bf 109 has two blades made from a 7-hole beam, the Spitfire four blades from two 5-hole beams, and the P-51 Mustang four long blades from two 7-hole beams.
Here is the key idea: the holes in a beam are round. A cross-shaped axle locks into a cross-shaped hole, like the one in the block, but it turns freely inside a round one. So the axle stays still, gripped by the block, while the beam spins around it.
Why it never falls off
A part that spins must still be trapped. Our build checks look at every propeller and rotor from both ends of its axle:
- Behind the blades, the pin-axle block stops them sliding back into the plane.
- In front, the bush stops them sliding off the end. On a four-blade propeller the two crossed beams sit side by side between the block and the bush.
- All round, nothing else on the model sits inside the circle the blades sweep. If a wing beam or the nose reached into that circle, a blade would hit it on every turn.
Remove the bush and the propeller is only held by friction. Swoop the plane round the room and the blades will eventually fly off forwards.
The helicopter rotor
The helicopters, the Mi-24, Dhruv and AW101, put the same idea on its side. A long pin-axle block is pinned on top of the fuselage. An axle stands upright in its cross hole: that is the mast. Two long beams drop over the mast crossways to make four rotor blades, and a bush on top caps them. The Mi-24 and AW101 rotors are made from 15-hole beams; the Dhruv uses 13-hole beams.
Because the blades are so long, the clearance check matters even more here. Turn the rotor slowly by hand once and watch the tips: they should pass over the tail without touching anything.
How long does yours coast?
Give the propeller one flick and count the turns before it stops. A free propeller coasts for several turns; a sticky one stops almost at once. The slowdown comes from friction between the blades, the axle and the bush, plus the air the blades push.
Try the coasting lab below first. Choose a starting speed and a slowdown, predict how many turns it makes, then scrub the speed graph and see.
Interactive: Coasting prediction lab. Predict stopping revolutions under a chosen constant slowdown, scrub the speed-time graph, compare initial speed and drag, then reset. Modeled behavior, not measured wheel performance.
Now try it on your model. Make three flicks with the bush pushed hard against the blades, then three with a tiny gap, and write down the turns each time.
| Bush setting | Flick 1 | Flick 2 | Flick 3 |
|---|---|---|---|
| Pressed hard against the blades | |||
| Tiny gap |
If the "tiny gap" row coasts longer, you have measured friction yourself. Keep every other thing the same: the same finger, the same flick and the plane held still.
Fixing a propeller that sticks
| What you see | Likely cause | Fix |
|---|---|---|
| Stops after half a turn | Bush squeezed against the blades | Pull the bush out a hair |
| Clicks once every turn | A blade hits the nose or a wing | Check the blade length and that the block is on the right holes |
| Whole block wobbles | Only one pin under the block | Push in both black pins |
| Blades slide forwards when you swoop | No bush, or the bush is on the wrong axle end | Fit the bush on the front of the axle |
Two propellers at once
The twin-engine planes, the C-212 Aviocar and the Fokker F27 Friendship, carry one propeller on each wing. Each has its own block, axle and bush, so the two spin independently: flick one and the other stays still. That is a quick way to check you have built both the same.
Once you are happy with propellers, try the biplane and triplane, the Sopwith Camel and the Fokker Dr.I, whose propellers spin in front of stacked wings.
Interactive: Check yourself. A few quick questions on the ideas in this guide, each with an explanation.
- Why does a propeller beam turn on its axle instead of turning the axle?
- You take the bush off the front of the propeller axle. What will happen when you swoop the plane around?
- Your propeller stops after half a turn. The bush is pressed hard against the blades. What should you try first?
- The helicopter rotor in these models turns about which direction of axle?
