Spinner for A75

[jesse]

After the latest repairs I've done I really started to take a close look at the prop as I was starting to wonder if perhaps excess vibration was the source of much of the cracks I fixed.

While watching the prop spin at idle I became suspicious of a couple of things. The first was the prop track. It came in at 3/16" which is outside the 1/8" tolerance. Some messing around with paper cardstock and I was able to get the track to less than 1/16".

The next thing I noticed was how the spinner was nowhere near straight and you could see lots of motion in it. The spinner is a "skull cap" style with a rather simple mounting bracket underneath it

I started to suspect the mounting bracket was bent and tried to straighten it which I finally decided was a waste of my time. I ordered a new one and very carefully installed it making sure not to tweak it. Started it up and the spinner was even worse than before in it's motion. You can just see that it's not tracking in center with the prop.

I finally just removed the spinner entirely and ran it and I've never seen the prop look so smooth. Flew it and it was really smooth as well.

So the question is...has anyone managed to install one of these spinners and actually have the damn thing spin "straight" or does someone have a method for trying to bend that damn bracket "straight". Or is there a better spinner I can use?

I could just leave it no spinner but it looks a little odd...

 

[SteveR]

I used to have that same spinner on my A-75. It came off and went through the prop (in front of my parents and Granddad, the day they learned I had bought a plane). As they stood by the runway watching me make a low pass (100'AGL or so), parts were literally falling out of the sky, off the plane. It was a glorious moment.

Anyway...since then, I've had no spinner on the Piet. Looks more "antiquey" anyway, and it allows one to inspect and easily torque the prop nuts.

 

[jesse]

I used to have that same spinner on my A-75. It came off and went through the prop (in front of my parents and Granddad, the day they learned I had bought a plane). As they stood by the runway watching me make a low pass (100'AGL or so), parts were literally falling out of the sky, off the plane. It was a glorious moment.

Anyway...since then, I've had no spinner on the Piet. Looks more "antiquey" anyway, and it allows one to inspect and easily torque the prop nuts.

You're the second person that has told me they've had that spinner come off. I guess I'll just live without it. It's a really poor design.

I was hoping there was a better A75 spinner setup out there.

 

[CJones]

Are you sure the spinner itself is balanced? Can you put it on a drill motor or something and spin it up to see if it feels balanced at speed? Might be able to glue some lead fishing weights on it somewhere to get it balanced out better.

 

[jesse]

Are you sure the spinner itself is balanced? Can you put it on a drill motor or something and spin it up to see if it feels balanced at speed? Might be able to glue some lead fishing weights on it somewhere to get it balanced out better.

Well the real issue is that the mount's are never straight. You can see it moving all over hell when you run it with just the mount on there and no spinner. Imagine that mount bolted to the prop. Then imagine running the engine and looking at that one single hole that the spinner screws into. The amount of side to side motion there is BIG TIME. It is quite easy for that mount to get tweaked a bit as you torque down the prop nuts on top of it (which I did everything I could to prevent and didn't see occur). but there's just lots of room for error.

 

[Dan Thomas]

An out-of-track prop can indicate a bent crankshaft, which itself can indicate a previous propstrike, which can cause a crack in the crankshaft.

Ask me how I know...

Dan

 

[jesse]

An out-of-track prop can indicate a bent crankshaft, which itself can indicate a previous propstrike, which can cause a crack in the crankshaft.

Ask me how I know...

Dan

This engine is pretty "new" and I have all the paperwork and bought it from someone I trust. There has not been a prop strike since It was built in 2008. The use of paper cardstock to tweak the prop track a bit is pretty common in the cub community.

Everything is great without that spinner. I just can't figure out how the hell to actually get that cheesy ass mount straight enough.

 

[CJones]

If the mount only has two legs on it like in the pic, is there a way you can make another piece identical to that piece to put it cross-ways so you have four points of contact with the prop? Then use a longer screw through the spinner to get cinched down against both pieces.

 

[SteveR]

You're the second person that has told me they've had that spinner come off. I guess I'll just live without it. It's a really poor design.

I was hoping there was a better A75 spinner setup out there.

I think the problem with this spinner is the clipnut. It is just too flimsy...a sheetmetal screw on a clipnut nut just won't take much torque, so mine was constantly coming loose.

If you install a riveted anchor nut or even a rivnut on the steel bracket, you could then use a machine screw and would be able to get a decent amount of torque on it so it would actually stay attached. A locking clipnut with a machine screw/bolt would also work better. Mine is staying on the shelf in the hangar though...not interested in dinging another prop.

http://www.aircraftspruce.com/categories/aircraft_parts/ap/menus/ha/nut.html

The most fail-safe option would be to weld a nut on the back of the steel bracket.

 

[KSCessnaDriver]

You're the second person that has told me they've had that spinner come off. I guess I'll just live without it. It's a really poor design.

I was hoping there was a better A75 spinner setup out there.

I've got a different spinner on my A75 on my EAA biplane. Don't know who made it, but its more pointed

 

[SteveR]

This is what that spinner did to my prop:

 

[jesse]

I think the problem with this spinner is the clipnut. It is just too flimsy...a sheetmetal screw on a clipnut nut just won't take much torque, so mine was constantly coming loose.

If you install a riveted anchor nut or even a rivnut on the steel bracket, you could then use a machine screw and would be able to get a decent amount of torque on it so it would actually stay attached. A locking clipnut with a machine screw/bolt would also work better. Mine is staying on the shelf in the hangar though...not interested in dinging another prop.

http://www.aircraftspruce.com/categories/aircraft_parts/ap/menus/ha/nut.html

The most fail-safe option would be to weld a nut on the back of the steel bracket.

Mine doesn't actually have the clipnut. I have this one:
http://www.aircraftspruce.com/catalog/appages/bracketspinsm.php

Still have the problem with trying to get the damn thing straight.

 

[jhausch]

As they stood by the runway watching me make a low pass (100'AGL or so), parts were literally falling out of the sky, off the plane. It was a glorious moment.

 

[nrpetersen]

Since it is a homebuilt with a wood prop (I assume) & 6 equally spaced bolts holding the prop on, you could layout & make up a "Y" shaped bracket with a better anchor nut system in the center. Use 4130 steel, lay it out with radiussed inside corners, bend it carefully, and put the center hole in after everything is formed, because you will end up with forming errors.

Where did the pictured bracket come from? It seems awfully flimsy to support that spinner.

There is a lot of radial vibration at twice engine rpm (i. e. 80 Hz @ 2400 rpm etc) in any 4 cylinder opposed engine. It is inherent in the layout of the crankshaft and cylinders. That spinner has to be well supported in all radial directions or you will be eventually breaking something in that flimsy bracket, especially with a lightweight wood prop.

 

[Dan Thomas]

Where did the pictured bracket come from? It seems awfully flimsy to support that spinner.

There is a lot of radial vibration at twice engine rpm (i. e. 80 Hz @ 2400 rpm etc) in any 4 cylinder opposed engine. It is inherent in the layout of the crankshaft and cylinders. That spinner has to be well supported in all radial directions or you will be eventually breaking something in that flimsy bracket, especially with a lightweight wood prop.

That bracket is the same bracket that's been on my A-65 since 1946. It's plenty strong enough. The spinner centers itself on the propeller's front clamping plate, and the bracket's job is simply to hold the spinner under a bit of tension to keep it there. Thousands of these things flew for decades without trouble.

It's real easy to over-engineer stuff and make it heavy.

Dan

 

[jhausch]

put the center hole in after everything is formed, because you will end up with forming errors.

Interesting recommendation and overall analysis. When I read the above, however, I could not help but think: "heck, if you're brave enough, drill the centerhole after it's installed and spinning. :wink2:

 

[nrpetersen]

That bracket is the same bracket that's been on my A-65 since 1946. It's plenty strong enough. The spinner centers itself on the propeller's front clamping plate, and the bracket's job is simply to hold the spinner under a bit of tension to keep it there. Thousands of these things flew for decades without trouble.

It's real easy to over-engineer stuff and make it heavy.

Dan

I have a spinner backing plate from a C-85 that is thoroughly cracked out (high cycle fatigue with two large "C" shaped cracks) between the prop blades. The cracks are from flat plate flexing. Obviously the spinner was resonant - probably at ~80 Hz.

It is common for people to assume spinner assembly cracks are due to simple balance problems, but these stresses would not be dynamic. I contend instead they are due to transverse and torsional vibration enhanced by the spinner structure's resonance.

I don't see what provides rigidity in the transverse direction (i. e. bending) on the pictured bracket. If it is oriented properly with respect to the crank throws it may well be adequate. But anything which can be visually off center would be best handled with a little more attention to detail - even if it involves "over-engineering".