Re: Prop hub - paper tubes

From: Tapio Linkosalo <tapio.linkosalo_at_helsinki.fi>
Date: Sat, 19 Jan 2008 12:01:38 +0200 (EET)

On Fri, 18 Jan 2008, Fred Tellier wrote:

> They don't need to be that heavy, use less wraps of tissue and thinner
> glue. 4 or 5 wraps are enough and just enough thin amberoid to wet the
> tissue.

Thanks, Fred, I'll try with thinner glue next. Actually those tubes that I
did seem to be closer to 50 than 100mg per pair...

I uploaded a couple of pics of my latest attempt to build a F1M VP hub.
The layout and functioning is rather conventional, except that I used
pultruded carbon tube (0.5mm ID 1.0mm OD) as the main body supporting the
blades. The tube is reasonable in weight, about 800mg per metre, so a 40mm
piece for the hub weights about 35mg. It is super stiff and much stronger
than balsa at the same weight.

I also used the tubing as hinges. I had tried the ukrainian style before,
where a T-shaped wire goes into the tube, attaches to the blade, and then
turns down to meet actuating arm. This seemed quite flimsy to me, wit only
the vertical wires supperting towards the actuating arm to hold blades in
place. So instead I used two pieces of wire, bent 90 degrees at the end.
to go into the hub tube and act as the hinges. The outer one can be easily
seen at the end of the tube, but wire that changes the blade pitch is
another hinge. The VP hub tube has a slot underneath so that the wire hook
can go into the tube through the slot. The wire is stiff enough the handle
such a slot, even though it weakens it somewhat.

The weight are reasonable, the hub with paper tubes and all is 190mg, prop
shaft wih actuating arm 90mg and the spring 30mg. AUW 320mg. I plan to try
the same setup next for F1D, hoping to get to 100mg, by using thinner
carbon tube and wire, and use 00-90 bolts instead of the M2 used in this
hub.

The hub is built up with (ordinary laminating) epoxy. I drilled a hole
through the hub for the shaft, cut a length of the tube as support (inside
the spring, the set these up on a piece of waxed piano wire, wrapped the
joint in carbon and wetted with epoxy.Once cured, I grinded all extra
stuff of. The same thing with spring pretension adjust, I waxed a steel M2
bolt, wrapped it onto the carbon tube with carbon towing, wetted and
grinded. For the pitch adjustment fork I lanimated a piece of U-shaped
carbon, cut to shape, and glued onto place with epoxy with carbon tow in
1mm pieces hacked and mixed in.

I think, for a neater and lighter hubs, simple silicone molds could be
used to increase carbon to epoxy ratio and make the parts neater. One mold
to make the tube centre joint and pre-tension screw housing, another for
the pitch fork. The latter would need carbon that was T-shaped in cross
section, as flat carbon seems too fimsy, that is why I had to glue the
ugly piece of carbon sheet across the U in this prototype...

I think I still have something to learn about sizing the springs. I read
that people use up to 90 degrees of pretension. Seems like with my springs
I can use only around 10 degrees, otherwise they get too strong. Maybe I
should use thinner wire?

Sorry for the quality of the pics. Took them under lamp light, so color
balance was way off, and looks like I over-adjusted them to be too blue...
But then, not much color with black carbon, silver steel and white nylon
:)



-Tapio-
Received on Sat Jan 19 2008 - 02:01:42 CET