Vintage Model Airplane Tips 'n Tricks - #3 - Tube-Within-A-Tube Freewheeling Propeller Assembly
During an SFA forum thread on tube-within-a-tube bearings for freewheel propellers, New Yorker, George Small came up with this elegant description:
"A metal-on-metal journal bearing with the brass ID tube a little longer than the brass OD tube to let the prop free-wheel freely using a latch of choice (Garami, pawl, etc). The OD tube is glued to the prop, the ID tube may or may not be soldered to the propshaft."
Despite being previously unaware of the system, his engineering analysis captured the principle perfectly. Click on the image to see an enlarged version and check out the following explanation of the features on my personal version of the system for further enlightenment.
The ply facings shown will help to protect any soft balsa surfaces from undue wear and pressure.
The tubes are brass but the noseblock main bearing can be a teflon/nylon tube from an RC snake. Lubricate all the bearings with a touch of sewing machine oil.
The smaller inner tube is jammed up against the bend of the winding loop. To prevent it migrating a dab of solder can be used to fix it to the shaft. The larger outer tube is glued to the propeller.
The thrust race and washer can be replaced by a pair of cup washers made (eg) from the heads of drawing pins suitably drilled and deburred.
The prop hook bent onto the propshaft can be formed with a reverse "S" to prevent the motor hook from running off centre.
The anti-reverse stub will prevent the pawl coming around and re-engaging with the loop during the freewheeling phase and killing the glide.
The teflon washer, has no benefit in the flight phase, but when using really powerful motors will protect the prop from considerable abuse when the motor hook is being located on the prop hook when all the motor tension is temporarily applied to the front face of the prop.
Not for competition use, but for low power trimming and test flights, the winding loop can be hooked directly to a winder without having to remove the prop. Bear in mind, however, that winding without a blast tube always carries some risk. More Tip n' Tricks?