Weak bolts

[facthunter]

I don't think anyone's suggesting that.. The thread could be anything but it is 8 MM as the Motor is metric and that size is determined as suitable. The actual difference is very slight in diameter but the UNF thead is finer... Nev.

[Thruster88]

4 hours ago, old man emu said:

Stands to reason. The bolts are pulling the prop boss and hub together by creating forces acting in a longitudinal direction. At the same time there is that transverse force created by the rotation of the propellers matter. Over the length of time that the engine is running, per flight, minor variations in the speed of rotation can cause microscopic amounts of movement between the propeller boss and the hub. That little bit of "rubbing" creates the heat that facthunter has mentioned.

If the propeller is moving against the drive flange and that friction is creating heat the said propeller will soon depart the aircraft.  

[old man emu]

1 hour ago, Thruster88 said:

Because the prop flange on a rotax has M8 METRIC, YES METRIC THREADS IN THE FLANGE. do keep up.

Not relevant to what I was saying. I was replying to a comment about castellated nuts being retained by cotter pins, but that the cotter pins only stop a loose nut falling off. By the time the cotter pin has saves the nut, the clamping function of the nut/bolt combination has gone with the wind. I was suggesting that if one was to use bolts with drilled heads and shank holes as well as nuts with corner drilled jam nuts, then safety wired the heads of the bolts and the nuts at the other side  you would be doubly secure, hence "belt and braces"

 

 

[APenNameAndThatA]

7 hours ago, old man emu said:

Stands to reason. The bolts are pulling the prop boss and hub together by creating forces acting in a longitudinal direction. At the same time there is that transverse force created by the rotation of the propellers matter. Over the length of time that the engine is running, per flight, minor variations in the speed of rotation can cause microscopic amounts of movement between the propeller boss and the hub. That little bit of "rubbing" creates the heat that facthunter has mentioned.

That sounds ludicrous, but it you can provide a reference of some sort I’ll stand corrected. 

[old man emu]

5 hours ago, APenNameAndThatA said:

That sounds ludicrous, but it you can provide a reference of some sort I’ll stand corrected. 

Isaiah 55:8-9

[APenNameAndThatA]

29 minutes ago, old man emu said:

Isaiah 55:8-9

In the past, you have said that your theories were better than Einsteins because your theories could be tested. This is a new high. Congratulations. 

[Bruce Tuncks]

OME had it right. The variations in rotational speed come primarily from the pulsations from the power strokes. 4 cylinders are worse than 6. The worst I have ever come across was a single cylinder 4 stroke model plane engine. Just imagine...  power for every 45 degrees in 720 degrees.

Apparently with high-speed photos, you could see the prop bent one way on the power strokes and (less so ) the other way for the rest of the cycle.

Here, the prop acted as the flywheel. It sure would on a big plane engine too.

[APenNameAndThatA]

3 minutes ago, Bruce Tuncks said:

OME had it right. The variations in rotational speed come primarily from the pulsations from the power strokes. 4 cylinders are worse than 6. The worst I have ever come across was a single cylinder 4 stroke model plane engine. Just imagine...  power for every 45 degrees in 720 degrees.

Apparently with high-speed photos, you could see the prop bent one way on the power strokes and (less so ) the other way for the rest of the cycle.

Here, the prop acted as the flywheel. It sure would on a big plane engine too.

But does that heat up the hub? 

[Bruce Tuncks]

yes it would if there is any movement permitted. The slightest looseness, which is not discernible on the ground, will provide the relative motion and energy = force times distance . The energy from this friction will become heat.

This is what caused the prop hub to go burnt black on our first Jabiru.

I reckon we were lucky in that the flywheel bolts stayed good. The lesson is...  Make sure your prop is on tight, and as I said, an indicator blob will help. The dried blob will crack if there is relative movement.

[Bruce Tuncks]

There could be some lesser heating just from the flexing of the prop, but this is beyond me to work out.

[turboplanner]

8 minutes ago, Bruce Tuncks said:

There could be some lesser heating just from the flexing of the prop, but this is beyond me to work out.

Come on, you can do it, split the sectors................

[old man emu]

1 hour ago, Bruce Tuncks said:

The variations in rotational speed come primarily from the pulsations from the power strokes.

I missed that source of vibration. I was thinking of the propeller producing Lift longitudinally. All other things being equal, the amount of Lift is dependent on air density. What happens when a plane flies through an "air pocket"? Air density decreases; Lift decreases and the plane moves away from its steady state path. Then it flies out of the "air pocket", air becomes denser and Lift increases. 

 

Apply that thought to the propeller at the same time. It seems logical that going through the changes in air density would result in changes in the Lift (i.e. Thrust) forces it creates. Surely that must induce fore and aft vibration that expresses itself at the Boss/hub interface as rubbing. And rubbing induces heat.

[facthunter]

Ther's NO such thing as an airpocket. It's a bad layman's term.   Hot air over  a runway will have more effect on air density. Nev

[old man emu]

2 minutes ago, facthunter said:

Ther's NO such thing as an airpocket.

Well, what do you call it when you are flying along, fat; dumb, and happy and your aircraft suddenly drops 50 ft then jumps back to where it started?  Or same fat; dumb, and happiness and your aircraft suddenly lifts or drops?

 

When everyone gets the gist of what is being discussed, can't we use layman's terms for the sake of brevity? Or do we have to go through the minutiae of the endochronic properties of resublimated Thiotimoline ?

[facthunter]

Just helping you in case some fella from Pprune drops by and thinks WE know nuthin. Nev

[old man emu]

We might know nuthin', but Pprune fellas know f all.

[facthunter]

Not true Just ask them for an expert opinion. .  Nev

[Yenn]

A castellated nut does not stop the nut departing from the bolt. It stops the nut rotating and loosening, so is effective at stopping prop to hub relative movement.

[old man emu]

BUT:

After correct torqueing, may be necessary to turn the nut again up to 30 degrees (in either direction) to locate the hole. With six notches spaced at 60-degree intervals, the castellated nut can only be locked where a notch corresponds to the hole. As fine-tuning the torque is not possible, castellated nuts are better suited to low-torque applications. 

 

Is it usual to have a specified torque value for bolting a prop to a hub?

[facthunter]

Yes to that question. There are 3 positions/turn where the split pin can be inserted. Where it needs to be precise have a pocketful of nuts to try a few or an oilstone which will enable you to vary the length slightly to get it to the right figure.  You can do this with adjustable wheel bearings too. Nev

[Bruce Tuncks]

thanks nev for that tip about altering nuts for adjusting the torque .

I agree about air pockets....  they are actually areas of lift and sink, thermals and sinking air. flying though them with a winged plane will cause extra lift and downwards "lift " as they are encountered.  Yes, flying into a sharp-edged bit of sink can seem like you have flown into a vacuum, but that is not the case.

Thermals have been extensively studied, mainly in Germany, and one of the surprising things, for me anyway, was that the air is more buoyant in a thermal if it has water vapour in it. ( molecular wt of water vapour is 18, while nitrogen is 28 and oxygen 32 ).

We see this effect on some summer days when the cu are only over the areas of scrub, where the deep roots are sucking water.

[Bruce Tuncks]

thanks nev for that tip about altering nuts for adjusting the torque .

I agree about air pockets....  they are actually areas of lift and sink, thermals and sinking air. flying though them with a winged plane will cause extra lift and downwards "lift " as they are encountered.  Yes, flying into a sharp-edged bit of sink can seem like you have flown into a vacuum, but that is not the case.

Thermals have been extensively studied, mainly in Germany, and one of the surprising things, for me anyway, was that the air is more bouyant 

[Bruce Tuncks]

Sorry about the mucked up posting....  I left the thing to look up how to spell bouyant, then had trouble getting back to the start again.

[facthunter]

There's actually 6 positions so the adjustment is smaller. I was remembering you never have to take a lot off and the original machine marks tell you that you are removing it evenly.

 Re the lift You are certainly getting more under a Cu cloud and they form over rivers lakes and trees. Trees are water pumps.

     I also managed to keep a c-150 in the air for 6 hours when shark spotting by riding the rising air currents just in from the shoreline.  A frontal rotor is more intense than most other weather , but a sea breeze caused windchange can be  very rough also,,as the wind reverses. Nev

Edited September 30, 2022 by facthunter

[APenNameAndThatA]

15 hours ago, Bruce Tuncks said:

yes it would if there is any movement permitted. The slightest looseness, which is not discernible on the ground, will provide the relative motion and energy = force times distance . The energy from this friction will become heat.

This is what caused the prop hub to go burnt black on our first Jabiru.

I reckon we were lucky in that the flywheel bolts stayed good. The lesson is...  Make sure your prop is on tight, and as I said, an indicator blob will help. The dried blob will crack if there is relative movement.

That is an abnormal situation. I’ll take your word for the blackness occurring because of heat, but I would have thought that any blackness would have occurred because of mechanical wear, not due to heat. 

 

The idea that two pieces of metal could be fixed to each other and move about enough against each other to cause heating just amazes me. I have never heard of such a thing.