Loose prop on Cherokee 140?

[Electric]

I rent Cherokee 140 from a local airport. A few weeks ago, when I was pushing it back into its parking spot, I noticed the prop moves back and fourth along its rotation axis, maybe an 8th of an inch. It is snug enough to pass the preflight inspection, but when enough force is applied to push the airplane back, it clearly moves.

I told the owner about the occurrence. Two weeks later, I notice the same issue, accept now the prop is even firmer, but it still moves.

So based on this story, two questions:

1) Is the airplane safe to fly in such condition?
2) Can this condition cause fatigue cracks if not fixed promptly?

Thanks in advance?

 

[Henning]

Was it the prop moving, or the entire crankshaft? There is typically some slack between the thrust faces of the main bearings and crank, probably not 1/8 of an inch though.

 

[SoloEqs]

It's normal -- especially on smaller, opposed-cylinder engines -- for there to be a little play along the crankshaft centerline. Without this play, when the engine gets hot and the metal expands, you might have some problems.

 

[Tom-D]

It's called crankshaft end play. There is a maximum measurement given in the overhaul manual for your engine.

look it up.

1/8th inch? the measurement is pretty big, but I don't know if it is that big.

 

[Electric]

It might be less than 1/8th of an inch, I couldn't really measure it, and I don't remember if only the prop moves. Both times when I noticed this, the engine was hot (just landed). Also, it took at least 30lbs of force to get it to move. (again, pushing the airplane back)

But the responses sound pretty reassuring that it's normal, thanks guys!

 

[Henning]

Yeah, sounds like a normal end play situation, good that you noticed it though.

 

[Dan Thomas]

Even .010" feels and sounds like a lot. It goes "clunk."

Dan

 

[kgruber]

Use the towbar.

 

[Skylane81E]

Even .010" feels and sounds like a lot. It goes "clunk."

Dan

Yep, if it seems excessive nothing wrong with having it checked out

 

[Tom-D]

Yep, if it seems excessive nothing wrong with having it checked out

I could not find the measurement in the Lycomig direct drive engine overhaul manual, crank items are items in the 500 block and only give bearing clearances.

 

[Henning]

I could not find the measurement in the Lycomig direct drive engine overhaul manual, crank items are items in the 500 block and only give bearing clearances.

No thrust bearing clearance given?

 

[Tom-D]

No thrust bearing clearance given?

I can't see it.

 

[Henning]

I can't see it.

Hmmm, odd should be around .030 max I would think.

 

[mtuomi]

Is there a thrust bearing for that engine?

 

[Henning]

Is there a thrust bearing for that engine?

The front main has thrust flanges IIRC.

 

[bnt83]

Is there a thrust bearing for that engine?

Of course there is a thrust bearing in the O-320 just no thrust bearing insert.

 

[dell30rb]

What everyone else said.

A slight movement, and a little clunk when you push or pull on the prop is normal.

 

[Skylane81E]

I could not find the measurement in the Lycomig direct drive engine overhaul manual, crank items are items in the 500 block and only give bearing clearances.

Lyc might call it crankshaft end play

I'm going from memory here so...

 

[Silvaire]

The only end clearance checks or specs I see when assembling a Lycoming are to ensure that there is no clearance on the forward thrust surface when the crank is pulled all the way forward. That check is made to ensure the crank slinger is s not bearing on the case. In my O-320 manual it instructs you to measure that if either the crankshaft or case have been replaced. If you have one look in section 7, page 49, par 7-70.

I also have an overhaul manual for Vantage/Superior XP360 and it has a table listing saying Main Bearing (end clearance) as being 9-16 thou. Not sure if that's what they are talking about as the rest of the text in that manual is mostly cut and past from Lycoming.

In my Continental O-200 overhaul manual it lists Crankshaft to thrust washers - End clearance as 4-15 thou (new parts) with a max allowable of 25 thou

 

[Electric]

Use the towbar.

If the prop is used to push the airplane forward when it's flying, what's wrong with me using it to push the airplane back?

 

[Skylane81E]

If the prop is used to push the airplane forward when it's flying, what's wrong with me using it to push the airplane back?

Usually I'm with you, never push on the tips of the props though.

And if I wanted to play devils advocate I'd ask if your engine has a thrust bearing for being pushed backwards

 

[VA Aviator]

Usually I'm with you, never push on the tips of the props though.

And if I wanted to play devils advocate I'd ask if your engine has a thrust bearing for being pushed backwards

The prop would encounter this sort of load while windmilling. How much? Not sure. But I would imagine it's more than the shove you give it to get it in the hangar.

And it can probably take a lot more than one of those flimsy tubular steel excuses for a towbar...

 

[Skylane81E]

The prop would encounter this sort of load while windmilling. How much? Not sure. But I would imagine it's more than the shove you give it to get it in the hangar.

And it can probably take a lot more than one of those flimsy tubular steel excuses for a towbar...

No doubt

 

[Henning]

Usually I'm with you, never push on the tips of the props though.

And if I wanted to play devils advocate I'd ask if your engine has a thrust bearing for being pushed backwards

Yes it does, hence it being able to accept a tractor or pusher prop.

 

[Dan Thomas]

I checked the Lycoming Direct Drive O/H manual today, and it gave the crankshaft thrust face clearance as .0075" max. That's when being assembled with new parts. It's enough to be felt.

Dan

 

[Dan Thomas]

If the prop is used to push the airplane forward when it's flying, what's wrong with me using it to push the airplane back?

Push near the propeller hub, not well out on the blades. That propeller, like a helicopter's rotor, is held straight by centrifugal forces when the engine is at higher RPM and can take the thrust being generated. In fact, if you look at a propeller's side profile, you'll see that the blade's major mass is a bit forward of the centerline of the hub. The centrifugal force generated pulls the blade back a bit toward the centerline, and the thrust pushes it forward, so the net is very little bending in flight.

The Cessna 152's "gullwing" propeller was an extreme example of that theory in practice. It generated too much bending load in the prop blade roots and caused cracking, making necessary an AD for NDI every 1000 hours.

The centrifugal pull on a blade at redline can be as high as 50 tons, according to one textbook. The blade weight, its CG radius and RPM all figure into that, of course. Some physics major here could probably work it out.


Dan