Let’s talk gear tension and sag

[phxcobraz]

Hello Mooney folks

Been struggling with some main gear sag on my j-bar M20C. Which obviously adds significant drag. Down lock and tension over center are perfect.  We’ve reviewed the maintenance manuals but the adjustment to tension the up position is a bit ambiguous, if at all possible. A few A&Ps have been a bit puzzled by it. 
When fully up and locked the main tires still hang a few inches below the wing. You can push up on them from there and they will go up much higher but sag back down. I’ve looked at a few others that get them all the way up, with only the brake hanging in the wind.

We’ve adjusted it enough with the gear doors disconnected to make sure they weren’t interfering.

It seems as though the main gear retraction springs have worn. Is this a common replacement part? Or am I missing another adjustment that can be done to help up lock rigging?

 

[N201MKTurbo]

I’ve found the best way is to get both sides as equal as you can. I would start by adjusting them so they both come up perfectly and just kiss the up bumpers in the wheel wells. Make sure all the rods are as equal as possible. Get the rods from the Johnson bar to the rear bell cranks so the two bell cranks are perfectly equal in both the extended and retracted positions. Then adjust the rods from the bell cranks to the legs so they both kiss the up bumpers. Then look at the compression of the springs on the outboard rods. They need to be compressed equally with the gear down. The springs should be a bit more than 1/2 compressed with the gear down. Those springs are what creates the overcenter preload. If those springs are between 1/2 compressed and fully compressed, your preloads will be correct. If your preloads are too low lengthen the rods the same on both sides. If they are too high shorten them both. The only way the preloads will be too high is if you are over compressing the springs which is bad because you will be bowing the rods. Also remember that lowest preload that is in range is OK. This isn’t one of those situations where more is better.

[phxcobraz]

51 minutes ago, N201MKTurbo said:

I’ve found the best way is to get both sides as equal as you can. I would start by adjusting them so they both come up perfectly and just kiss the up bumpers in the wheel wells. Make sure all the rods are as equal as possible. Get the rods from the Johnson bar to the rear bell cranks so the two bell cranks are perfectly equal in both the extended and retracted positions. Then adjust the rods from the bell cranks to the legs so they both kiss the up bumpers. Then look at the compression of the springs on the outboard rods. They need to be compressed equally with the gear down. The springs should be a bit more than 1/2 compressed with the gear down. Those springs are what creates the overcenter preload. If those springs are between 1/2 compressed and fully compressed, your preloads will be correct. If your preloads are too low lengthen the rods the same on both sides. If they are too high shorten them both. The only way the preloads will be too high is if you are over compressing the springs which is bad because you will be bowing the rods. Also remember that lowest preload that is in range is OK. This isn’t one of those situations where more is better.

Alright so it sounds like the sag is a symptom of incorrect preload. So even though the preload is set within torque spec range(and not collapsing) it still can be adjusted more to bring things up. In this case it may be that the preload is set too high. Which reading the manual another symptom of that is excess force needed on the last 1-2in of Johnson bar travel to the down lock position, which I absolutely have.  Locking it down is difficult, especially on jacks. 
All things being equal it sounds like adjust everything so it is even and looks correct, then make fine adjustments to preload until all gears are within spec but not too high to start moving things down again. 

[phxcobraz]

Also if I am reading the manual correctly, it might mean the nose gear pre-load is set incorrectly and probably forcing the main gear to not hit their full retract before the nose is all the way up. 

[N201MKTurbo]

41 minutes ago, phxcobraz said:

Alright so it sounds like the sag is a symptom of incorrect preload. So even though the preload is set within torque spec range(and not collapsing) it still can be adjusted more to bring things up. In this case it may be that the preload is set too high. Which reading the manual another symptom of that is excess force needed on the last 1-2in of Johnson bar travel to the down lock position, which I absolutely have.  Locking it down is difficult, especially on jacks. 
All things being equal it sounds like adjust everything so it is even and looks correct, then make fine adjustments to preload until all gears are within spec but not too high to start moving things down again. 

The sag has nothing to do with the preload. The preload is the last thing you should check. Everybody obsesses about the preload. It is just a check that you did everything else OK. 

 

38 minutes ago, phxcobraz said:

Also if I am reading the manual correctly, it might mean the nose gear pre-load is set incorrectly and probably forcing the main gear to not hit their full retract before the nose is all the way up. 

Especially on a manual gear Mooney, you have to get the nose gear adjusted first. You should rig the nose gear with the mains disconnected. Then rig the mains. I could write a whole dissertation about doing the nose gear on a manual gear Mooney. Most I’ve worked on have been messed up. Usually the two rods are unequally loaded. One does all the extension and one does all the retraction instead of working together. You do need to shoot for a preload on a manual gear nose wheel. This is because the manual nose gear doesn’t have springs and the preload adjustment is very sensitive. That’s why they have eccentrics in the front pivots. A few thousandths makes a difference.

[47U]

12 hours ago, phxcobraz said:

Been struggling with some main gear sag on my j-bar M20C.

Almost 15 years ago when I did my first annual inspection (correction, 100 hr, the IA does the annual), I determined my preloads were all quite low.  Out of spec even.  I followed the mx manual rigging prodecure and got the preloads into spec, and my gear was hanging low just as you describe.  Since there’s no ‘up travel’ adjustment, I started looking for slop in the linkage.  There’s a number of possible wear points and I wrote down the part numbers for bolts and bushings and called Dan at LASAR.  After describing my problem, he agreed worn hardware was my issue and sourced it for me, about .3 amu as I remember.  That fixed my problem of the gear hanging low.

Good luck…

[phxcobraz]

9 hours ago, 47U said:

Almost 15 years ago when I did my first annual inspection (correction, 100 hr, the IA does the annual), I determined my preloads were all quite low.  Out of spec even.  I followed the mx manual rigging prodecure and got the preloads into spec, and my gear was hanging low just as you describe.  Since there’s no ‘up travel’ adjustment, I started looking for slop in the linkage.  There’s a number of possible wear points and I wrote down the part numbers for bolts and bushings and called Dan at LASAR.  After describing my problem, he agreed worn hardware was my issue and sourced it for me, about .3 amu as I remember.  That fixed my problem of the gear hanging low.

Good luck…

We thought this could be part of the problem too. There is some play in the bushings at a few places, not sure how much is acceptable.

[N201MKTurbo]

3 minutes ago, phxcobraz said:

We thought this could be part of the problem too. There is some play in the bushings at a few places, not sure how much is acceptable.

The best way is to remove the links for the gear doors so they hang down, and the belly panels. Feel the different joints while you push the wheel up and down with your hand while they are retracted.

[phxcobraz]

21 hours ago, N201MKTurbo said:

The sag has nothing to do with the preload. The preload is the last thing you should check. Everybody obsesses about the preload. It is just a check that you did everything else OK. 

 

Especially on a manual gear Mooney, you have to get the nose gear adjusted first. You should rig the nose gear with the mains disconnected. Then rig the mains. I could write a whole dissertation about doing the nose gear on a manual gear Mooney. Most I’ve worked on have been messed up. Usually the two rods are unequally loaded. One does all the extension and one does all the retraction instead of working together. You do need to shoot for a preload on a manual gear nose wheel. This is because the manual nose gear doesn’t have springs and the preload adjustment is very sensitive. That’s why they have eccentrics in the front pivots. A few thousandths makes a difference.

It very well could be that when it was adjusted last to correct some slop in the nose gear, it threw the rest out of wack. Makes a lot of sense. It flew significantly faster before that was adjusted a few years back. Good place to start.

11 hours ago, 47U said:

Almost 15 years ago when I did my first annual inspection (correction, 100 hr, the IA does the annual), I determined my preloads were all quite low.  Out of spec even.  I followed the mx manual rigging prodecure and got the preloads into spec, and my gear was hanging low just as you describe.  Since there’s no ‘up travel’ adjustment, I started looking for slop in the linkage.  There’s a number of possible wear points and I wrote down the part numbers for bolts and bushings and called Dan at LASAR.  After describing my problem, he agreed worn hardware was my issue and sourced it for me, about .3 amu as I remember.  That fixed my problem of the gear hanging low.

Good luck…

Just walked through the parts manual and noted a few bolts/bushings I know were sloppy when we had it up on jacks. I will see about ordering those and a few others when I have another closer look at it, even if just for piece of mind and to tighten it up once done adjusting everything. Looks like Lasar has the majority in stock.