DO-335 prop shaft

[John Rist]

Lee,
The 8" Cox prop shaft came today and it is grate!

I will be starting to build a 24" wing span DO-335 soon.  This shaft will allow me to mount the motor inboard achieving better CG.

IF you can dream it Lee can make it!  Thanks for quick service and a beautiful shaft.

PS short shaft is from original DO-335 that was to small and is being redesigined and rebuilt.

[Tim Wescott]

Is that solid or tubular?

You realize that you're replicating the original Pfiel layout, with a shaft-driven rear prop from a mid-mounted motor, yes?

I can't wait to see the plane.

[John Rist]

Is that solid or tubular?

You realize that you're replicating the original Pfiel layout, with a shaft-driven rear prop from a mid-mounted motor, yes?

I can't wait to see the plane.

Shaft is solid aluminum.  The total rotating mass is not all that bad because the shaft is not that big in diameter.  It of course will robe some HP to turn the rig but with two TDs the plane has excess HP.  I will time the fuel so that the rear enging quits first and the airplane should fly great on one engine - Just like the full size aircraft did.  The only problem with the setup (as I learned on the first stab at a DO335) you can not run the back engine by itself on the ground.  No cooling air and it gets smoking HOT.

[James Lee]

John
Glad to help!!   Enjoy
Thanks
Jim

[John Rist]

Lee,

I need to remove weight from my DO-335, especially from the rear.  Can the shaft you made for me (pictured above) be made hollow by cutting off the ends and inserting into a hollow aluminum tub.  Or perhaps make it out of magnesium.

Any thoughts on the subject would be appreciated.

[Tim Wescott]

I'm not Lee, but strength-wise a thin-wall hollow tube should be fine, and much lighter.

Something that slips neatly over what you have, with 1/32" walls "feels" right to me, if it's made out of a decent high-strength alloy (2024?  I'm not sure what's available in tubes).  It'd probably work to epoxy it on if you used the right glue (meaning: not what's on your model airplane workbench).  Bigger diameter would be better, probably up to 1/2" OD, if you don't mind throwing away what you have and starting from scratch.

You'll still have the weight of the bearing, fittings and prop -- you may find that the prop hanging out back there is as much of an issue as anything else.

[James Lee]

John
Yes, it could be done with a tube...   However, I agree with Tim in that, the bearing and prop are probably the bigger weight problem....   Plus, after doing the fittings necessary to get a solid end on the tube for the bearing and prop drive, the weight lost is partially gained back....     I will try to find some alloy tube and see if it is a possibility.
Thanks
Jim Lee

[John Rist]

John
Yes, it could be done with a tube...   However, I agree with Tim in that, the bearing and prop are probably the bigger weight problem....   Plus, after doing the fittings necessary to get a solid end on the tube for the bearing and prop drive, the weight lost is partially gained back....     I will try to find some alloy tube and see if it is a possibility.
Thanks
Jim Lee

I am stuck with the weight of the prop and bearing.  I realize the weight savings in the shaft would be small.  However every gram out of the tail is a 2 gram loss in the noise weight.
 
How about machining it out of magnesium?  I found a source for a 1/2" x 12" rod.  Would require turning rod to size and then do the marching on the ends.  I believe mag is about a 1/3 of the weight of aluminum.

PS I may make the shaft 3/4" longer, moving the rear motor closer to the CG.

[Tim Wescott]

Before I went to magnesium I'd do a little design study and see just how far forward I could get that rear motor.  If I could mount them backplate-to-backplate on the same bearers I would, with the fuel tank on the inboard side. (I'm assuming that you're thinking of this for your new plane).

If you want to risk it run a smaller bearing, but let Jim's judgment steer you on that.

Using a solid rod for the shaft offends my sense of thrift, because the stuff in the middle of a shaft or beam really isn't contributing much to the strength or stiffness of the thing.  My sense of thrift can get me into a lot of trouble, sometimes.

McMaster-Carr has a wide selection of thin-wall 2024 tube (I got all interested in this, for no good reason whatsoever).  It's at their usual "oh, aluminum isn't gold?" sort of prices, but it's available.

[John Rist]

It's not possible to mount both engines on the same beams.  They are on different center lines.  The rear motor is mounted inboard so that the slope of the shaft pushes the tail in (noise out) for line tension.  Best I can do is place rear engine on CG.  The trade off is longer shaft.  It is already at 8".  I had thought of using a boat flex shaft drive but I believe it would be too draggy.  You run a flex shaft in a tube.  The current set-up has worked well.  I just want to move the rear engine 1" forward and remove some weight from the shaft.

[John Rist]

New shaft arrived and it is spot on good.  Price was right also.  It weigh less and is 1" longer than the old one.  Thanks Lee!

[Tim Wescott]

Where'd the coupler come from?  I'm kind of a fan of oddball airplanes, and Dick Sarpoulis's Hanriot-Bische 110 has had my eye for years.

[John Rist]

Where'd the coupler come from?  I'm kind of a fan of oddball airplanes, and Dick Sarpoulis's Hanriot-Bische 110 has had my eye for years.

I am not sure where the coupler came from.  It was left over from my RC boating days.  It probably came from a 40 to 60 size boat.  Boating hardware is getting hard to find.