engineering question - slip shaft prerotator

[Adam H]

I'm thinking about using a slip shaft prerotator and I'm wondering about what to use for the shaft. I'm sure I'm not the first person who has cringed at the thought of the inner shaft deforming due to torque and jamming inside of the outer square tube. I know that a small diameter, solid round drive shaft is much weaker than say a 4" diameter, thin-walled round shaft. So, here is the question, would, say a 1.25" thin walled square tube sliding into another similarly sized tube be stronger and more resistant to deforming and thereby jamming than the more traditional .5" solid square bar sliding into a similarly sized hollow square tube?

[scottessex]

I would use the small diameter square tubing, just make sure it is 4130 CroMoly
and not the cheap junk from the home depot.
Just my $.02

[Mike Schallmann]

Ive been using a slip shaft for my prerotator for the past 10 years--my U-Joints are 3/8" impact U joints. The shafts are 4130 Tubing. The outer shaft is 1/2" x.058 and the inner shaft is 3/8 x.062 wall --got them through Aircraft Spruce --the U-Joints I got at Sears --they are the ball socket type --I pressed out all of the pins and replaced them with GR8 bolts --never had a problem with them --I put a center bearing in the assembly so the shaft wont whip --this requires making two shorte slip shaft assemblies --I believe I have a pic on my website--

[Adam H]

Mike, I'm not seeing those sizes. http://www.aircraftspruce.com/catalog/mepages/4130square.php

[Timchick]

Adam,
Are you going to use a right angle gear box for the prerotator? If so, which one?

[Adam H]

Tim,

I'm not planning on using a gearbox, instead I'm going to copy a design that a guy in our club came up with several years ago. It uses a v-belt and a traditional rubber wheel to get the power to the slip shaft. I'll be at the airport tomorrow and I'll take a few pictures then. I like it because I was test flying another club member's gyro with this system on it a few weeks ago. It has the older, heavier extruded aluminum blades on it, and they are by far the heaviest blades I've ever tried to lift. Anyway, the gyro never got off the ground due to severe stick shake, but I was amazed at how well the prerotator got those heavy blades up to speed. It was not only quick, but very smooth with no vibration whatsoever.

[Timchick]

I've seen a couple slip shafts on Subaru engines with the wheel pressing against a flat plate on the mast side of the engine but I didn't know how a shaft system would work on a Rotax. The only one I've seen that didn't use a 90 degree gearbox was Chuck Beaty's setup.

[Adam H]

The gyro that I was talking about has a 532 on it. It will make sense when I post the pictures tomorrow.

[Brent_Brown]

Mad Man Mike what kind of RRPM do you get with that set up?

[Mike Schallmann]

You are correct about the tubing sizes --my design criteral called for the dimensions I stated --but in reality I was not able to get those sizes so I actually used the .049 wall tubing so -- USE THE .049 wall tubing and have as much overlap as possible---mine overlap nearly the full length-less the head movemeny from fulll fwd to full aft--

RRPM is limited to the size of the Disc driving the prerotator. Mine is probably a poor setup and has a lot of loss (slippage-due to the 90 contact area )-but I still can hit 200rrpm if I press it --I usually got to 125 or so then begin my take off roll -and ease the throttle forward -I leave the prerotator engaged until it hits approx 175 then I disengage--and nail the throttle to full --liftoff occurs in a moment or so -- RRPM jumps to 275-300 almost instantly --then hits its normal 330 or so RPM--

[Brent_Brown]

That sounds good. I am thinking about using the same set up on the 2.2 soob going on the twinstarr.

[Adam H]

Here are some pictures of the setup that I'm trying to copy. It's hard to see from the pictures, but the cable to engage the prerotator pulls the rubber wheel down to contact the drum. The rest of the set up pivots, since there is only about an 1/8" between the wheel and drum, it doesn't need to pivot very much. The attachment points on the top of the motor are to handle the torque of the unit when engaged. I'm not sure how I can adapt this design since both of the engines I would want to try this on are air cooled and don't have nice, evenly spaced attachment points like the 532 shown here.

[Timchick]

Cool. Wasn't the Brock KB-3 prerotator setup similar to that?

[Passin' Thru]

Cool. Wasn't the Brock KB-3 prerotator setup similar to that?
Yes, it was quite simular. He jokingly refered to it as the "Corvair fan arrangement".
.

[Brent_Brown]

I don't see see how you can do this on a air head. To many parts for me. A big plate under the prop and wounderflextube up to the head works fine and looks better. not sore about the weight.

[Alan_Cheatham]

Does anyone know of a source for aluminum bodied, ball bearing V-belt pulleys like those shown in the pictures further up the thread?

[ckurz7000]

Are you guys familiar with the prerotator setup in an MT03? It's reliable and gets the rotor up to a maximum of 270 rrpm. It uses a pneumatically actuated slip clutch and a 90° gearbox. The small driving pinion at the top of the mast is also raised pneumatically to engage the gear which is mounted on the rotor head.

-- Chris.

[Alan_Cheatham]

The small driving pinion at the top of the mast is also raised pneumatically to engage the gear which is mounted on the rotor head.

Doesn't look like the drive pinion is raised pneumatically, looks like a standard Bendix unit. What is pneumatically operated is the rotor brake and I would suppose if the pneumatic pressure were high enough it would also pivot the head forward, which would be a nice feature for ground handling. The specs on the MT-03 also say it has a pneumatically adjustable in-flight trim, and that would make sense because if the pressure were reversed in the pneumatic cylinder it would act as a trim spring. Clever.

[Timchick]

Are you guys familiar with the prerotator setup in an MT03? It's reliable and gets the rotor up to a maximum of 270 rrpm. It uses a pneumatically actuated slip clutch and a 90° gearbox. The small driving pinion at the top of the mast is also raised pneumatically to engage the gear which is mounted on the rotor head.

-- Chris.

I haven't seen any close-up photos of the MT03 prerotator setup. Anyone have some?

[OuterMarker]

Adam....check out the setup Kevin is using.....we will be at the hanger this weekend.

[dabkb2]

The pic in post 12 is close to a KB3, but it has a disk instead of a drum. The only pic I have is on a video, no close up but you get the idea http://www.youtube.com/watch?v=prfkBWcc8uo

[Alan_Cheatham]

KB-3 prerotator.

[L_Butler]

Alan,

In the photo you posted it doesn't appear that the pulleys have bearings in them. Plain pulleys are available at any power transmission supplier (Kaman, Motion Ind., Grainger, etc) The idler pulleys with bearings in them are usually available at auto parts suppliers. My local Auto Zone lets me wander around in back until I find what I like. The usual auto parts clerk is useless because without a year and model they are lost.

Larry

[Alan_Cheatham]

Alan,
In the photo you posted it doesn't appear that the pulleys have bearings in them.


Sorry Larry, I was talking about the pulleys in the pictures on the first page. I would like to find a source for them, otherwise I will need to machine them myself.

[dabkb2]

I didn't like the twist in the V belt, so I changed the bendix from a straight through to a 2 gear that put the rotation back. I think Ken thought it would be lighter to twist the belt, over adding a gear, either way all I can get out of it is about 100 rrpm.