Thorp Air Command - T18.net

I pulled my propeller off (Felix wood prop) for the first time to inspect and retorque, and found the bushings in the bolt holes to be flush with the rear face of the hub. This doesn't look right to me.

The prop extension has counter-bores in it which look like they are meant to receive the bushings and provide mechanical engagement. Also the holes in the spinner backplate are made to the OD of the bushings, and not the OD of the prop bolts.

Any advice? Seems to me that I ought to be able to just remove the bushings and put longer ones in.

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In theory, theory and practice are the same. In practice, they are not.

Contact Sam at Sabre Mfg. http://www.sabermfg.com/ He should be able to answer your questions and provode the right parts.

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Bob Highley
N711SH
SN 835
KLAL

Dick,

My prop flange has bushings that engage with the extension as well as bushing in the extension that engage into the prop. This way the bolts don't see the torsional loads imposed by the cylinders firing. The prop bolts are supposed to be subjected to tensile loads only. Is there any evidence of fretting between the extension the engine flange due to the absense of bushing engagement?

I would suggest sourcing new, proper length bushing and bolts.

Jim

*** admin posted for Jim Mantyla ***

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Forum Administrator

That's what I figured the bushings were fore.

Yes... lots of fretting on the spinner backplate and the associated front face of the prop extension.

I think I'm going to take it all apart again and evaluate further. I was originally going to try to get another year out of the parts, but now I'm concerned about how the wear may have effected the parallelism of the prop extension. If the prop extension is salvageble, I'll get it refaced at a machine shop. Really the spinner backplate should be thrown away, especially if I repair the extension.

The prop will look nice over the fireplace.

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In theory, theory and practice are the same. In practice, they are not.

Dick,
It should be a simple matter to take a light skim cut off the extension if required. It is easy to check on a surface plate and dial indicator. What is the issue with the prop, that it would go over your fireplace?

Jim

I was half joking about the fireplace comment.

There are a couple of things I need to consider before deciding on a path forwared. Through an inquiry to Fred Felix, I learned that the prop was actually designed for an O320-160, so technically it is the wrong prop for an o360. Also, I understand that metal props are preferable to wood on O360's because they have more rotational inertia (flywheel effect). I do find the vibration to be quite bad with the wood prop at the low end of the rpm range, but it smooths out at 900. I wonder if trying to salvage a poor installation might be penny wise and pound foolish.

What are folks using for diameter and pitch on the o360's using fixed metal props? Wait, don't answer that... I'll check the newsletters first.

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In theory, theory and practice are the same. In practice, they are not.

Dick,
If you're considering metal, there's an M76 on the classified section pitched for a 180.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

Correction on my Felix wood prop. I was wrong about the prop being made for an O320-160. I found the response from Fred Felix and the prop was made for "an O360 derated to 168 HP." Not sure what that means, but I intend to find out.

Thx for the tip on the metal prop, Lee. I contacted Dave.

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In theory, theory and practice are the same. In practice, they are not.

Means it has low comp pistons - burn mogas with no booze.

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Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

OK... finally got the drive lugs out of the prop and had a chance to evaluate further. Looks like the prop extension was made from the Thorp drawing. Most of the c'bores in the extension have gone conical. (ie wider at the flange face) A couple are noticeably egged to one side. The face is quite badly fretted. The holes in the prop vary in tightness, a couple being quite loose and egged out.

Here's what I am thinking to rescue the prop extension:
1) take a skim off the face flange, as per Jim's advice above
2) mill the c'bores to 13/16 diameter to make them round again
3) make new drive lugs with a step change in diameter, one side 13/16" press fit into the prop extension, and the other side 3/4" to match the prop c'bores.

Comments?

As an aside, I was surprised to see that the drawings called for the drive lugs to be a loose fit in the extension, ie ~.001 undersize compared to the c'bores. I would have expected drive lugs to be a press fit on the engine side and a "snug" fit on the prop side.

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In theory, theory and practice are the same. In practice, they are not.

I would prefer to have the lugs press fit. If you have the ability to machine "new" parts that's a plus. I believe a majority of us bought our extensions/crush plates/prop bolts from Saber Manufacturing. www.sabermfg.com This is the same mfg. that supplies Spruce.

RB

Well, the price on the Sabre part is pretty darn attractive. I don't think I can execute my prop extension rescue plan (as outlined above) for much less than the cost of a new part. Thx.

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In theory, theory and practice are the same. In practice, they are not.

You will find the Saber part is a work of art. It is nice knowing that are the parts are a match fit and when the noise maker starts to turn all the pieces stay together ! [8D]

RB