propeller shaft material

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Postby daffy1029 » Mon Aug 16, 2004 6:46 pm

I want to make new prop shafts for my redrives which were designed as a pusher and now are going to be in tractor configuration. So I want to design the new shaft so there is no way it can come off by machining a shoulder on one end. What is the best material to use? Mild steel, 4140, or some grade of stainless steel? The original shaft is .787" dia. by 5.2" long and has a .312" hole through the center of the shaft. The shaft is stationary and the prop rotates on the shaft with ball bearings. The highest prop rpm would be 2800 rpm x 44" diameter, if that matters. It will be driven by a stock Solo 210cc engine at 2.5:1 reduction. This is for my "almost" series III Lazair. The JPX's are coming off. Thanks
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Postby Guest » Tue Aug 31, 2004 7:05 pm

No replies? I am going to go with 4140 for my prop shaft material. I looked in the machinist handbook and this seems to be the best material for this application.
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Postby Guest » Tue Aug 31, 2004 9:43 pm

Don't think anyone here can help you with choosing proper shaft material. I'm sure if anyone knew they would offer an opinion. Is there any chance of using factory made re-drives instead ?
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Postby JPXman » Tue Aug 31, 2004 10:28 pm

I think the spectrum of knowledge on custom redrives is very limited, and the few who fly lazairs with redrives use the commercial versions. I have heard of only two people who have made their own redrives, one has passed and the other is MIA. Myles, you're boldy going where no lazairhead has gone before... I look forward to seeing you zip past me at 120mph :)
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Postby daffy1029 » Wed Sep 01, 2004 6:19 pm

Jpxman, If the prop shafts hold, maybe eh? :ph34r: Gotta keep up to your JPX's! :lol:
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Postby Chappy » Wed Sep 01, 2004 10:35 pm

Well, I can tell you not to build them out of aluminum!

Now, before you you say, "Well, of course not", you should know that back around 1980 several companies did use aluminum prop shafts in their redrives (the Hummer was one). When I designed my redrives in 1980 (for the little Pioneers) I let Frank Allan in Florida, who built the parts for several company's redrives, talk me into using aluminum (2024-T4 IIRC), even though I thought it was a terrible idea. One broke (at a poorly done radius) after only about 15 hours of running, mostly on my test stand, but I had one successful flight on them. Fortunately, I had one back on my test stand when it let loose. I still have the broken prop. I'll try to post a picture here tomorrow.

I rebuilt them with solid steel shafts, and they have run many hundreds of hours. I wish I could remember what alloy I used, but I can't (or more likely never knew). I seem to remember just using some scrap pieces I had in my pile. From the way it machined, I think it was a little better than cold rolled. I knew it wasn't stainless.

Incidentally, my props are 44" pine, three laminates of Murphy and Aims (a local lumberyard - long gone now) #2 better pine glued together with polyester resin - I had run out of Epoxy! They are 23 years old now and doing fine. I painted them with Pactra Formula U model airplane paint, along with my lift struts - and they still look good.

I started out with a 3:1 reduction, but grossly underestimated the amount of prop slippage that would be present (due to such fast turning props in such a slow moving body of air) and built my first props with way too little pitch. So I built a second set of props with more pitch, but it still wasn't enough! I had planned to cut them some if require, too. Because I am not a "wood person", I decided to make new sheaves for the engines and went with a 2.75:1 ratio to speed up my props some, and make up for the under pitch. I believe my first props were 16", the second set 20" pitch. They are non-helical in pitch distribution. Beware - most prop formulas and nomograms will NOT take into account the affects of high prop RPM/slow flying aircraft - and will give you an under pitched prop! BTW, my Pioneer's turn almost 8500 rpm on takeoff! You don't even want to know what the cylinder head temps run!

I probably spent 5 times the work, at least, designing and building my little redrives as I did building my first Lazair. I even signed up for an adult education class to learn how to run a metal turning lathe so I could build my own components. Being able to make many of my own parts is a personal freedom that I now rate right up there with flying my own plane.

There were a few more pictures posted on the Lazair Yahoo forum site of my plane and Pioneer redrives. I guess they are still there.

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Postby daffy1029 » Wed Sep 01, 2004 11:46 pm

Thanks for your input Chappy, I wasn't worried about props coming off for my safety but for the safety of people on the ground. I checked the machinist handbook and it seems 4140 is very good material to use in this application. I am satisfied with my research. I too will put these engines on (Tyler's) test stand and test them before mounting them to my Lazair. I will probably go with a ground adjustible prop because I don't think any one knows what the best pitch for my plane would be at 2.5:1 ratio using stock Solo engines. I will use about 44" diameter prop. I don't want the plane to go any faster but have good take off power and cruise. The reason I didn't use the manufacturers shaft was because it was designed as a pusher on a PPG. By changing to a tractor configuration I saw a weak spot where the propellers could work themselves off the shaft. I am going to machine a shoulder on the shaft to make sure that can't happen. Maybe later I can take pictures and show you how I designed it. The shaft is hollow as well so I might even add a bolt thru the center for added safety. How can you beat that? B)

Thanks again for your input, it's always helpful, Myles
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Postby JPXman » Thu Sep 02, 2004 1:43 am

If I remember correctly from solids analysis, a hollow shaft is actually stronger than a solid shaft - can anyone confirm this? There must be a limit for wall thickness where this is no longer the case though, but i'm more of an electrical dude than a mechanical dude (trying to change that...)

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Postby ozzie » Thu Sep 02, 2004 3:40 am

hi guys just my two cents worth and a story for you all
yes a hollow shaft is stronger than a solid one a hollow shaft has two surfaces to transmit the load over rather than one on a solid shaft. wall thickness is directley related to what loads are being applied, shear compression touque ect.

one day when flying a Thruster two seat prototype fitted with a robyn 440cc twin with a belt reduction drive with a hollow shaft made from alloy.the pully was fitted with two ball race bearings and the shaft was concentric ie the centre of the end wear the prop was was offset to the other end that was clamped with two blocks on the engine mount. so to adjust the belt loosen the blocks and rotate the shaft. the end of the shaft had a step machined on it to stop the shaft pulling forward.
i was doing steep turns with approaches to the stall. i came off the top and stuffed the nose down and just after i hauled it back up the engine went very loud and rpm was off the scale. i just hit the switch recovered from the funny attitude and started back to the strip at about 1300'. this was the first time the prototype had been deadstick(it was only six hrs old and a blown up version of the single seater).
i gave it it"s head so to speak and it settled into a nice glide much better than the single seater with smaller wing. well after playing with different speeds i landed about halfway down the strip. it was only during the round out i noticed that the prop was gone. no windmilling prop means bingo on the glide that explains getting back. so what had happened was that when the bearing part was machined were the bearing face fitted hard up against the shaft it had been machined square. unfortunatley the bearing is radiused here so you end up with a small gap between the shaft and the bearing right were you need contact. it snapped the end clean off(it is a solid piece at this end). we radiused the next and subsiquent shafts and had no problem with them. and the prop, pully and busted bit? never found them looked all around the area. last i saw it it was spinning were it should be, i never saw it depart. i miss being a test pilot (not) OZZIE
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Postby jb88ci » Thu Sep 02, 2004 11:56 am

very cool...now how do i make some for my 185's.....i really think re-drives is a good answer over bigger engines.
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