Aileron Heim bearings

[0TreeLemur]

While hanging out at the airport Monday (Clear - 70F) giving our M20J a belly rub and applying some Tri-Flow, I noticed that the Heim bearings connecting to the ailerons are worn and have some slop.  I'm talking about the connection from the push rod end to the aileron.  Looking at the parts manual (p.2, sheet 27-10-00, 2003), it looks like there is a manufactured push rod from the bell crank to the aileron with two Heim bearing ends, (1 ea.) P/N M34-14MPB and (1 ea.) P/N M44-14MPB.

How much slop is allowable?   Is there a procedure to check it?

For estimation purposes, if they need replacement, how many hours would it take an A&P to remove, replace the bearings, and reinstall one side?  Looks like a pretty tough job.

Thx.

Fred

[EricJ]

I'm going to be doing this on my airplane in the next week or so.    There is a tolerance spec in the M20J SMM, but I don't know what it is off the top of my head.   There is some slop allowed, but it's not a lot. 

One complication on that particular rod (and it's not the only one like this) is that the factory elected to make some of the ends held in by roll pins rather than screw in.   Those are more difficult to replace.    I bought a couple spare complete rods from BAS, and one of them has threaded bearings on each end rather than having a roll-pin bearing on one end.   On my airplane the roll-pin bearing is the one that has the most slop, so this is a nice development.   I think some of the later J models got the improved configuration with threaded bearings on each end, which makes replacement much easier.

In any case, if you don't have one already the M20J Service Maintenance Manual is available in the download area here.   There are specs in there regarding slops tolerances.   It's kind of hard to find, but it's in there.

[N201MKTurbo]

 Slop in those rod ends will not affect your handling in any way. The ailerons are aerodynamically loaded when in flight. That rod is in tension because of the lift of the aileron. Go ahead and change them if it makes you happy, but it won’t change anything. 

[0TreeLemur]

1 hour ago, N201MKTurbo said:

Slop in those rod ends will not affect your handling in any way.

Agree.  She handles fine.  

I'm more thinking about failure.  Suppose they are the original 40+ year old parts, the slop is significant.  At some point one of those could fail, which I imagine would cause me to have a Really Bad Day.

[EricJ]

Just now, 0TreeLemur said:

Agree.  She handles fine.  

I'm more thinking about failure.  Suppose they are the original 40+ year old parts, the slop is significant.  At some point one of those could fail, which I imagine would cause me to have a Really Bad Day.

They're unlikely to fail in a way that would be catastrophic without giving you some warning first, partly by increased slop.   That said, the slop creates dead spots in the controls, which also aren't catastrophic, but are suboptimal.   On my airplane one aileron and the rudder both have more slop than I'd like.   The aileron slop seems to be mostly limited to the rod end at the aileron.    The rudder slop in my airplane appears to be a stackup of small amounts of accumulated slop through all of the rod ends in the system (there are a bunch), but mostly at the last one connected to the rudder.   My plan is to take a crack at both of those this month.

[N201MKTurbo]

4 minutes ago, 0TreeLemur said:

Agree.  She handles fine.  

I'm more thinking about failure.  Suppose they are the original 40+ year old parts, the slop is significant.  At some point one of those could fail, which I imagine would cause me to have a Really Bad Day.

So, what is the failure mode of those rod ends? What I have seen is they are pretty robust. I have seen the ball seize from lack of lubrication and the ball started rotating on the bolt. It would have to wear clean through the bolt before it would come apart. The slop you are seeing is probably the ring around the ball wearing. It has probably worn a few thousandths. it would need to wear a few hundred thousandths before it would come loose. 

[N201MKTurbo]

5 minutes ago, EricJ said:

They're unlikely to fail in a way that would be catastrophic without giving you some warning first, partly by increased slop.   That said, the slop creates dead spots in the controls, which also aren't catastrophic, but are suboptimal.   On my airplane one aileron and the rudder both have more slop than I'd like.   The aileron slop seems to be mostly limited to the rod end at the aileron.    The rudder slop in my airplane appears to be a stackup of small amounts of accumulated slop through all of the rod ends in the system (there are a bunch), but mostly at the last one connected to the rudder.   My plan is to take a crack at both of those this month.

The aileron rods will never cause dead spots in the aileron control. They are always held in tension and or compression while flying. the only rod ends that can make dead spots are the ones in the rods from the center bellcrank to the yokes.

I was ferring a Mooney a while back with a loose yoke. It had huge dead spot in the control response. It was annoying to fly. I fixed it after the second leg.

[0TreeLemur]

4 minutes ago, EricJ said:

They're unlikely to fail in a way that would be catastrophic without giving you some warning first, partly by increased slop. 

Good to know. 

4 minutes ago, EricJ said:

 That said, the slop creates dead spots in the controls, which also aren't catastrophic, but are suboptimal. 

Agree.   Might make that good 'ol KFC-150 work harder than it should.

 

[EricJ]

Just now, N201MKTurbo said:

The aileron rods will never cause dead spots in the aileron control. They are always held in tension and or compression while flying. the only rod ends that can make dead spots are the ones in the rods from the center bellcrank to the yokes.

The transition from tension to compression is the dead spot.

[0TreeLemur]

2 minutes ago, N201MKTurbo said:

The aileron rods will never cause dead spots in the aileron control.

So maybe not overworking the A/P either.  Good to know.

Here's the deal.  It just displeases me when I'm doing a preflight and I start to move a control surface and I hear a "thunk" sound. 

[N201MKTurbo]

11 minutes ago, EricJ said:

The transition from tension to compression is the dead spot.

That's the point the never go from tension to compression. They are always being loaded by the lift of the aileron.

[N201MKTurbo]

15 minutes ago, 0TreeLemur said:

So maybe not overworking the A/P either.  Good to know.

Here's the deal.  It just displeases me when I'm doing a preflight and I start to move a control surface and I hear a "thunk" sound. 

I have no problem with you changing them, but they are not a safety or handling issue.

[EricJ]

29 minutes ago, N201MKTurbo said:

That's the point the never go from tension to compression. They are always being loaded by the lift of the aileron.

I don't think that's true through the full travel of the controls.   As the aileron is raised by the controls the pushrod at the aileron will transition from tension to compression at some point, and the more slop there is in the rod ends the more lost motion there will be at that transition from tension to compression (aka dead spot).

[PT20J]

The lost motion in the various rod ends will add to create a dead zone when the control wheel is moved. The down deflected aileron has a negative hinge moment and will put the push pull tubes in compression. The up deflected aileron has a positive hinge moment and will put it's tubes in tension. From Perkins and Hage, Airplane Performance, Stability and Control:

[N201MKTurbo]

1 hour ago, EricJ said:

I don't think that's true through the full travel of the controls.   As the aileron is raised by the controls the pushrod at the aileron will transition from tension to compression at some point, and the more slop there is in the rod ends the more lost motion there will be at that transition from tension to compression (aka dead spot).

Doesn’t happen. There is about 120 Lbs of force between the two ailerons. It varies a bit as you move the controls through the range of motion, but it never goes to zero.

[EricJ]

2 minutes ago, N201MKTurbo said:

Doesn’t happen. There is about 120 Lbs of force between the two ailerons. It varies a bit as you move the controls through the range of motion, but it never goes to zero.

So an aileron disconnecting at it's local rod end will result in the aileron going to full up deflection under aero load?   That sounds unlikely to me.  Otherwise it takes compression on the rod at the aileron to move it to its full up deflection.

[N201MKTurbo]

14 minutes ago, EricJ said:

So an aileron disconnecting at its local rod end will result in the aileron going to full up deflection under aero load?   That sounds unlikely to me.  Otherwise it takes compression on the rod at the aileron to move it to its full up deflection.

Yes, it would go up until lift of the aileron equaled the drag from it going up into the wind. It would probably hit the up stop. 

Are you saying the aileron makes no lift? It is about 6 sq feet. The wing makes about 10 Lbs of lift per sq foot. So the aileron is being pushed up with about 60 Lbs of force. A little more when they are down and a little less when they are up.

[N201MKTurbo]

1 hour ago, PT20J said:

The lost motion in the various rod ends will add to create a dead zone when the control wheel is moved. The down deflected aileron has a negative hinge moment and will put the push pull tubes in compression. The up deflected aileron has a positive hinge moment and will put it's tubes in tension. From Perkins and Hage, Airplane Performance, Stability and Control:

These charts are assuming zero angle of attack. Our wings have a positive angle of attack. The text says the hinge moment is highly influenced by angle of attack. In all cases the wing has a positive angle of attack and the tail has a negative angle of attack.

[Ragsf15e]

2 hours ago, N201MKTurbo said:

These charts are assuming zero angle of attack. Our wings have a positive angle of attack. The text says the hinge moment is highly influenced by angle of attack. In all cases the wing has a positive angle of attack and the tail has a negative angle of attack.

I just love watching you, @EricJ and @PT20J talk about stuff like this.  Always interesting, civil, educational and well documented when possible. Now I just gotta watch and munch my popcorn.

[good2eat]

Makes a guy appreciate scientific/technical minds.

Hell, I can’t even get through the one page without going “huh?”

Fun reading the smart people conversations.

[MikeOH]

Okay, I'm just a dumb electronic engineer, not aeronautical....but I'm just not following why an aileron won't just 'trail' along the wing contour if disconnected, regardless of wing AOA.  I didn't think there was any aileron area in FRONT of the hinge (besides the counterweight).  I struggle that the delta-P at the trailing edge is sufficient to drive the aileron up against the up-stop and dynamic pressure from the airstream (both above and below the wing)

What explanation do the experts have, please?

[1980Mooney]

Here is a 2018 topic on ailerons floating up above the flaps during level flight.  Some had ailerons rising in flight as much as 1 inch above the flaps on both sides in level flight.

As @jetdriven explained (and like @N201MKTurbo said):

• "So you have 16.75lb per sq ft of wing loading. Those ailerons are 3-4 sqft. So they have 50-67 lb of force lifting them upwards in flight.

  Either start replacing stuff to get the slop out, or rig them lower so they fly level in cruise. Check the ailerons with rigging boards, while holding both up with 20-50lb of force, to be sure the flaps aren't 2 degrees down instead of the ailerons looking high."

• The Service Manual says to adjust the ailerons  0 to -2 degrees down while on the ground (droop below the flaps when fully retracted).  For the same reason as stated above, any clearance/slop in the heim bearings or turnbuckles will be taken up in aileron rise during flight due to lift on the ailerons.

 

[MikeOH]

I believe wing loading to be an average; likely much lower at the trailing edge/ailerons.  I'm still skeptical the delta-P is large enough to drive the aileron up against the up-stop if disconnected as N201MKTurbo postulated; I think the airflow would overcome the small force due to delta-P.

[0TreeLemur]

Lift is predominantly a by-product of streamline curvature.  That's why lift generation mostly happens near the leading edge where the airfoil is most curved.   At a cruising angle of attack with the ailerons in trail the streamlines in their vicinity are pretty much straight.  They will therefore produce no lift for that reason. 

I agree with @MikeOH that at the trailing edge of the wing or aileron the pressure is single-valued but there is a little pressure difference on average between the top and bottom, which will push the aileron up a little until its deflection creates and equal but opposite downward force.  If appropriately balanced it shouldn't oscillate.

I just convinced myself that if a Heim bearing on one aileron failed, there would likely not be dire consequences.  Provided that the linkage to the other aileron remained intact a pilot would still have about 1/2 the normal roll authority. 

Thanks for the entertaining discussion!

 

 

Edited February 8, 2024 by 0TreeLemur

[EricJ]

6 minutes ago, MikeOH said:

I believe wing loading to be an average; likely much lower at the trailing edge/ailerons.  I'm still skeptical the delta-P is large enough to drive the aileron up against the up-stop if disconnected as N201MKTurbo postulated; I think the airflow would overcome the small force due to delta-P.

I tend to agree.   The center of pressure and center of lift are well forward of the ailerons.    Deflecting a surface into an airstream creates a pretty big delta-P by changing the pressure on both sides of the surface, which is usually the desired outcome of deflecting the surface.   The zero-force-applied position would be expected to be in trail where the pressure on both sides is equalized.   I wouldn't expect that to be at the up stop in level flight, especially with any significant airspeed. 

I was thinking it'd be great if Blue On Top were around to comment, but it turns out he did: