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PostPosted: Mon Feb 13, 2006 9:23 am
by Shannon
Karl cracks I found on the flange portions the plates were in addition to cracks radiating outward from the hub area. Over time these cracks could have merged causing a total failure. All the original plates I've looked at with the exception of one had cracks.


PostPosted: Mon Feb 13, 2006 1:28 pm
by flyalaz
Hi Shannon,

I remember when I first got my Lazair, I needed a lot of information and that besides you helping me by sending me your original manuals, the topic of cracked spinner plates was hot on the menu. The guy I bought the plane from had never installed them while he had the plane, though they had been installed at one time.

The set that I had were already drilled, and cut, and after cleaning them off decided to do a fluorescent liquid penetrant test on them. This was before I blended and polished them because those operations would have obscured any defects. Besides small surface defects, they both came up clean under the UV light, and at that point I decided to use them. I also used a 3/16" thick plate on top of the prop to help keep any local distortion from warping the spinner plate.

I think that the old stacked bi-props having only two bolt holes had the tendancy to kind of fold the plate along that axis and cause the plate to crack from stress. The Prince props have four, as you know, and I think that helps even the stresses out. I also deburred all holes, top and bottom, inside and out, to remove sharp edges, which was never done to them before.

I have never seen how the stacked props actually sit on the plate, but I remember seeing one picture of a destroyed backplate where the destruction followed exactly the line of the prop. It looked like the prop was riding on the plate right along that crack. Is it possible that this is true, and that flight stresses and prop movement forcing on the plate is the direct cause?

My experience here is very limited, but do you know of any that have cracked on a Prince prop, and were they reused plates like mine?


PostPosted: Mon Feb 13, 2006 9:30 pm
by Shannon
I do not know of any original plates cracking with P-tips. Most everyone I know of who switched to P-tips have replaced the plates.


PostPosted: Mon Feb 13, 2006 10:13 pm
by flyalaz
Will do!

I think I will chuck the whole assembly in my drill press at a low speed first just to check how everything lines up. I wouldn't want to find out the hard way that the new installation somehow went cockeyed on me!
Now that everything is apart, I will once again go over the backplates and verify everything. It's good to know that none (?) have cracked on a Prince prop.
I will be neediing a couple more for my second rebuild in the future anyway, so I will probably get a couple of extra at the same time.


PostPosted: Wed Feb 15, 2006 9:17 am
by Guest
For the average Joe... I say switch to solid plates... Yes if you do all the checking, threading, rechecking... that should work too... but... something this simple to upgrade so inexpensively... I say upgrade for most of us.

I do like hearing about the details of the other way though... great job.


PostPosted: Wed Feb 15, 2006 1:26 pm
by flyalaz
Hi Bill,

The backplates aren't just solid plate, unless someone has made his own. Even the new ones that I have seen are machined. Take a look in the Rotax rebuild topic, and take a look at the picture George Curtis sent in. You'll see it's not a solid chunk of metal. Maybe George can tell us what type of fastener he'll use?

Due to all the previous problems with spinner plates, I think most people include a peek at them during pre-flight, regardless of whether they are new or not.
I believe self tapping fasteners have their place in various other industries, but not on an airplane. Every time you take one of those fasteners out and then reinstall it, a little more metal is gouged out simply because they cut themselves in. I am not saying it doesn't work, because it does, but it's not what I prefer to do. Drilling and tapping are very easy to do, and eliminates the question of whether or not the self tapping fasteners helped cause the cracks.

As far as checking and rechecking go, I know that if something were to go wrong with it, and my prop$ were damaged, I would be out of flying for the season. I like flying waay too much to let inattention to detail ground me, and if it takes me a while longer to get set up, I reap the benefits in the long run by being able to fly with a little more confidence. I like reducing the pucker factor wherever I find it! Hee hee! One day I'll get new ones and any remaining doubts about these old plates will be gone, but for now these are fine and I'll keep using them.

When I started this thread, I was just curious to see if anyone else had a way of putting a new spinner on an already drilled backplate. I know it seems pretty simple, but if there is a better method out there, I'm still looking!

Blue skies,

PostPosted: Wed Feb 15, 2006 1:54 pm
by lazairiii

I, like Shannon and hopefully everyone else, will absolutely not use a self tapping screw on these spinners. Mine are drilled, tapped and sealed with Blue LockTite. I use a silver cad plated AN screw with a nylon washer underneath the head. Click on the picture and zoom in for a closer look. You will also notice I'm a firm believer in safety wire. I've heard too many storied of velocity stacks and carbs coming loose and causing poor engine performance.

Good call Karl. No reason to compromise on anything. Think long term, live long term.

George Curtis

PostPosted: Wed Feb 15, 2006 4:01 pm
by flyalaz
Hi George,

Drill and tap is definitely the way to go! I decided to use flush screws on mine just to give it a little more streamlining. I reasoned that the the countersunk holes in the spinner would be just as effective as washers in this application. The plastic would have to split very wide for it to pull out, or the entire countersink rip out a plug of material for it to come off, both conditions would be visible on pre-flight inspection. The only thing to take care of is not to over tighten the screws and crush the plastic.

Nice installations. I particularly like your throttle cable bracket. I've been wanting to that to mine since I first saw it, just been a little hesitant to drill into the case. How deep did you drill? The material is 1/8" extrusion?


PostPosted: Wed Feb 15, 2006 6:01 pm
by lazairiii

You don't have to drill very deep. Maybe 1/4"-5/16" or so. There is already a hole or two drilled in that spot anyway, so look at it and use one of them for this application, then drill, tap, and use LockTite and drilled headed bolts and safety wire. In this pic you will get a good idea of how I made these from 6061-T6 1 1/2" extruded angle. You will see that when the nuts are tightned down on the cable ends, these cables are excallent and have no way to slip. You end up with positive throttle inputs every time. The pic also shows you the back/side of the spinner plate.


PostPosted: Wed Feb 15, 2006 6:05 pm
by lazairiii
Another view...