Checking for a loose prop

[Bruce Tuncks]

It is not usually possible to tell if the prop is loose on the ground because the forces you can apply are small compared with what the engine delivers. So I wonder if putting some spots of a hard material (eg polyester resin) as a fillet between the prop and the driver flange would indicate if movement was in fact taking place. The idea being that an unbroken fillet would indicate no movement.

 

This is such an obvious idea, along the lines of the paint spot on nuts and bolts, that it must have been tried before... does anybody know if it works?

 

.... Bruce

 

 

[Steve Donald]

It is not usually possible to tell if the prop is loose on the ground because the forces you can apply are small compared with what the engine delivers. So I wonder if putting some spots of a hard material (eg polyester resin) as a fillet between the prop and the driver flange would indicate if movement was in fact taking place. The idea being that an unbroken fillet would indicate no movement.This is such an obvious idea, along the lines of the paint spot on nuts and bolts, that it must have been tried before... does anybody know if it works?

.... Bruce

NO sorry it wont there is so much that affects your prop, they not only get loose but can get to tight as well , temp , moisture, expansion ect, you just have to keep a regular check of the tension as well as tracking and balance your engine and airframe will appreciate it, as well as your wallet cheers

 

 

[Bruce Tuncks]

Thanks Steve, but what if the prop has already been checked for balance and tracking and bolt tensions? If the drive-bushes are a tiny bit loose then I reckon it could still be moving. In fact I'm pretty sure I know of a prop which was like that.

 

...Bruce

 

 

[Guest Maj Millard]

Your drive bushes should not be too loose, if they are replace them. Regardless, movement or looseness should be elimated by the contact between the rear face of the prop and the front face of the prop hub. This is the tension and contact that is maintained by correctly torqueing the prop bolts.

 

If movement between the two is allowed, there will be marks that appear like burn marks, or fretting corrosion that indicates movement.

 

The propellor is the highest stressed component on the aeroplane, it must be maintained correctly. Any failure can occur rapidly and be catostropic in the worse case.

 

The second most stressed component by the way is the crankshaft, and the third the pilot when the prop stops or departs........................................................Maj...

 

 

[Bruce Tuncks]

I certainly agree the drive bushes shouldn't be loose, although just how tight is tight enough would be hard to quantify.

 

But I don't agree that there is enough friction between the prop and the driver face to prevent movement. If this were so, the tightness of the bushes wouldn't matter. Perhaps heavier wood would enable higher bolt forces and enable this to be the case.

 

But I have seen an Auster which had a double-sided sandpaper disk between the prop and driver, obviously to increase the friction there.

 

This prop had no driver bushes but was of a heavier wood than out Jabiru props. The owner was replacing the sandpaper disk because the old one had been worn shiny smooth. This could only have arisen from relative movement between the prop and driver. I didn't take notice of the bolt tensions used at the time, but I reckon they would be more than we use.

 

It would be interesting to see some high-speed photos of a Jabiru prop under power. During the power stroke, the hub is apparently noticeably rotated ahead of the tips.

 

And there are about 7000 G's of centrifugal force on the tips, so I completely agree that the prop is the highest stressed part.

 

regards, Bruce