Seat rises and falls with gear swing?

[Mcstealth]

I was looking at an F model, and while I was sitting in the plane,  the A/P had me swing the electric gear, and manually lower the gear with the emergency handle for the annual he was performing. The left seat literally sank when the gear was lowered,  and raised when the gear came up. I will say .25"  conservative number. I made the A/P aware of this. He said the seat shared a support with the gear? 

Is this correct? Is something broken? Do your seats flex?

Thanks

David

[N201MKTurbo]

My plane did that when I bought it. The rear support for the actuator was in upside down. It could also be up or down limit switches. Either way, there is something wrong with the actuator rigging.

[PT20J]

5 minutes ago, Mcstealth said:

I was looking at an F model, and while I was sitting in the plane,  the A/P had me swing the electric gear, and manually lower the gear with the emergency handle for the annual he was performing. The left seat literally sank when the gear was lowered,  and raised when the gear came up. I will say .25"  conservative number. I made the A/P aware of this. He said the seat shared a support with the gear? 

Is this correct? Is something broken? Do your seats flex?

Thanks

David

I agree with @N201MKTurbo. A careful inspection should be made underneath. Someone posted a picture a while back where the structure was cracked.

[Mcstealth]

17 minutes ago, PT20J said:

I agree with @N201MKTurbo. A careful inspection should be made underneath. Someone posted a picture a while back where the structure was cracked.

Where do I look? Up through the wheel wells?

[Ron McBride]

Look thru the belly pans.   This was typical on my f with the 40:1 gears.   When I had the 20:1 installed it almost stopped, in fact the first time I flew it, I thought that there was a gear malfunction.   Top gun did the gears and rerigged the gear.   

[Mcstealth]

3 hours ago, Ron McBride said:

Look thru the belly pans.   This was typical on my f with the 40:1 gears.   When I had the 20:1 installed it almost stopped, in fact the first time I flew it, I thought that there was a gear malfunction.   Top gun did the gears and rerigged the gear.   

Okay. Please educate me. What was the purpose of the lower gears? 

Edited August 6, 2023 by Mcstealth

[Vance Harral]

4 hours ago, N201MKTurbo said:

Either way, there is something wrong with the actuator rigging.

Well, hang on a minute...

We've had this debate before in another thread, I guess it's not resolved.  But I maintain that it's completely normal for the seat pan to flex a little when the gear locks into place on extension, and to correspondingly un-flex when it's raised.  Our airplane has been doing this for the entire 20 years we've owned it, both with 20:1 and 40:1 gears in the actuator.  The gear is correctly rigged per the service manual, and the belly pan is not cracked.  Other Mooneys I've flown in exhibit this behavior as well.

If you look at the attached photo of the actuator installed in the belly, it makes sense why this is true.  In the photo, the motor is on the right, and the jackscrew assembly is on the left (red anodized structure).  In the orientation of this photo, the rod that drives the nose gear is at the top of the jackscrew assembly, and the rod that drives the mains is at the bottom (both rod ends are out of the frame).  When the gear are extended, the jackscrew assembly experiences compression, due to the springs on the nose and main gears.

Look at how the jackscrew assembly mounts to the airframe to hold it in place.  It is riveted to the belly pan directly under the pilot's seat.  Accordingly, when the assembly is put in compression by the gear springs on extension, that force is translated to some degree to the belly pan.  Unless the belly pan is perfectly rigid - which it's not even when new - the pilot will feel some degree of "push" in the seat.

Obviously there are reasonable limits.  It's one thing for the belly pan to gently flex a small amount and give the pilot a love tap, when the gear is extended due to the springs compressing.  Quite another for the pilot to get a huge kick in the a** if the springs are bottoming out and the compression to flex the belly pan to the point of cracking.  I can also believe that the specific geometries of individual airframes are such that some exhibit less butt-push than others.  But bottom line, I don't think the mere presence of seat pan flex is unquestionable evidence something is wrong.

[Vance Harral]

27 minutes ago, Mcstealth said:

What was the purpose of the lower gears?

20:1 gearing causes more force between the individual gears, subjecting them to greater wear over time.  Eventually they wear to the point of jamming, and if they do, the emergency extension system isn't going to help you.  This is discussed in a Mooney Service Bulletin, https://www.mooney.com/wp-content/uploads/2020/12/SBM20-190B.pdf

Later model Mooneys with higher gear speeds and more weight/drag (due to additional gear doors) used actuators with 40:1 gears.  The idea is that retrofitting these higher ratio gears to older actuators reduces the inter-gear force, allowing them to last much longer.

[Mcstealth]

Now that, was an answer. 

[N201MKTurbo]

1 hour ago, Vance Harral said:

Well, hang on a minute...

We've had this debate before in another thread, I guess it's not resolved.  But I maintain that it's completely normal for the seat pan to flex a little when the gear locks into place on extension, and to correspondingly un-flex when it's raised.  Our airplane has been doing this for the entire 20 years we've owned it, both with 20:1 and 40:1 gears in the actuator.  The gear is correctly rigged per the service manual, and the belly pan is not cracked.  Other Mooneys I've flown in exhibit this behavior as well.

If you look at the attached photo of the actuator installed in the belly, it makes sense why this is true.  In the photo, the motor is on the right, and the jackscrew assembly is on the left (red anodized structure).  In the orientation of this photo, the rod that drives the nose gear is at the top of the jackscrew assembly, and the rod that drives the mains is at the bottom (both rod ends are out of the frame).  When the gear are extended, the jackscrew assembly experiences compression, due to the springs on the nose and main gears.

Look at how the jackscrew assembly mounts to the airframe to hold it in place.  It is riveted to the belly pan directly under the pilot's seat.  Accordingly, when the assembly is put in compression by the gear springs on extension, that force is translated to some degree to the belly pan.  Unless the belly pan is perfectly rigid - which it's not even when new - the pilot will feel some degree of "push" in the seat.

Obviously there are reasonable limits.  It's one thing for the belly pan to gently flex a small amount and give the pilot a love tap, when the gear is extended due to the springs compressing.  Quite another for the pilot to get a huge kick in the a** if the springs are bottoming out and the compression to flex the belly pan to the point of cracking.  I can also believe that the specific geometries of individual airframes are such that some exhibit less butt-push than others.  But bottom line, I don't think the mere presence of seat pan flex is unquestionable evidence something is wrong.

You are correct about everything you said. The actuator does have to compress the springs in the actuator rods. And if it compleatly compresses the springs, something has to give, it will either bend the floor, or the rods themselves. That assumes the problem is on the gear down side. There are no tests like the preload tools for the gear up position. If the gear is over retracting, there is nothing to stop it except the floor bending. It puts the main gear actuator rods in tension, and they will take a tremendous force without damage. The nose actuators will be in compression while retracting, but the nose wheel still has about an inch to go before it will hit the wheel well when the mains hit the bumper in their wheel wells, so the nose is never a problem. Except, you may bend the gear doors if you over retract it.

I still contend that a properly rigged landing gear will not bend the floor. Especially, not 1/4 inch.

[gmonnig]

I definitely think some seat flex is totally normal. My plane does it. With 6000+hrs on it, everything has been checked and rechecked, no issues at all. 

[MikeOH]

Well, my F has 5800 hours and does NOT have any seat flex.  It would most certainly bug me if it started doing that!

[chriscalandro]

That really doesn’t seem like a thing that should flex. 

[Vance Harral]

14 hours ago, N201MKTurbo said:

That assumes the problem is on the gear down side. There are no tests like the preload tools for the gear up position. If the gear is over retracting, there is nothing to stop it except the floor bending.

That's a good point.  I guess it's not clear to me from the OP whether the movement is occurring at the end of extension and beginning of retraction (which I would consider normal), or at the end of retraction and beginning of extension (which I agree is more disconcerting).

Per the service manual, the mains should not actually touch the bumper pad in the retracted position, so significant witness marks on the bumper pads might indicate a problem.  Other things could hang up too, including the gear doors, though I'd think the gear doors themselves would flex before any significant flex in the belly pan as retraction completes.

[Vance Harral]

2 hours ago, MikeOH said:

It would most certainly bug me if it started doing that!

It will most certainly bug me if mine stops doing that!    I actually think of the love tap on my butt as an extra "gear down and locked" indicator.

[Hank]

My C always thumped nicely when the gear went down. I could also feel them come up. This was good for 7 years and a bit.

Then I moved, found a good IA for annuals with the 2nd one I used. After three annuals, he retired. The next guy was totally incompetent, and raising / lowering the gear made huge amounts of noise, large thumps and make the yoke twitch pretty hard. Joey Cole checked everything, re-rigged the gear (adding that I was lucky that it came down at KDNN). Because bozo had replaced the aileron rod ends when I asked him not to (because he said he did not have travek boards), Joey also rerigged the flight controls--ailerons weren't too bad, but the tail was way out of tolerance in the "down" direction.

Ever since then, there's only a light feeling when the gear goes up and down. While I miss the thump, I no longer have to explain to passengers that the gear is now up or down.

Please have your gear checked by an experienced Mooney mechanic with the proper tools. You'll like the result!

[MikeOH]

Here's the way I look at this:

1) If the seat is moving, "something" is flexing.

2) The "something" flexin is more than likely made out of aluminum

3) Aluminum has NO fatigue life limit.

[chriscalandro]

7 minutes ago, MikeOH said:

Here's the way I look at this:

1) If the seat is moving, "something" is flexing.

2) The "something" flexin is more than likely made out of aluminum

3) Aluminum has NO fatigue life limit.

While that may be true, designing anything that flexes aluminum like that as either a normal direct funtion, or indirect function is bad design. 

Edited August 6, 2023 by chriscalandro

[MikeOH]

3 minutes ago, chriscalandro said:

While that may be true, designing anything that flexes aluminum like that as either a normal direct funtion, or indirect function is bad design. 

Ugh, that was EXACTLY my point!

[MikeOH]

Perhaps there is confusion over the term "fatigue life limit."

A material like most steels HAS a fatigue life limit.  Stay below that limit and you can 'flex' steel indefinitely; it will NEVER crack (at least not from fatigue due to flexing).  A paper clip is a great example; flex it with a normal thickness of paper and it will last forever. Bend it back and forth severely and you can break it in half after 10-20 'flexes.'

Aluminum, on the other hand, has NO such limit.  In other words, no matter how tiny the bending eventually the aluminum WILL crack due to flexing fatigue.  Example from aviation is the Aloha Airlines 'sunroof' flight: due to the much higher number of press/depress cycles their many short trips caused, the aluminum fatigue cracked!  I'll bet it was a lot less motion than the 1/4" one poster has mentioned!

[chriscalandro]

3 hours ago, MikeOH said:

Perhaps there is confusion over the term "fatigue life limit."

A material like most steels HAS a fatigue life limit.  Stay below that limit and you can 'flex' steel indefinitely; it will NEVER crack (at least not from fatigue due to flexing).  A paper clip is a great example; flex it with a normal thickness of paper and it will last forever. Bend it back and forth severely and you can break it in half after 10-20 'flexes.'

Aluminum, on the other hand, has NO such limit.  In other words, no matter how tiny the bending eventually the aluminum WILL crack due to flexing fatigue.  Example from aviation is the Aloha Airlines 'sunroof' flight: due to the much higher number of press/depress cycles their many short trips caused, the aluminum fatigue cracked!  I'll bet it was a lot less motion than the 1/4" one poster has mentioned!

Ah. Got it. I’m an electrical guy with very little metalworking knowledge. 
 

 

[Mcstealth]

On 8/6/2023 at 3:24 PM, Vance Harral said:

That's a good point.  I guess it's not clear to me from the OP whether the movement is occurring at the end of extension and beginning of retraction (which I would consider normal), 

There was movement in both directions, in both actions either up or down. 

I felt the most movement, both ways, when the gear reached its full extension or retraction. 

[Vance Harral]

3 hours ago, Mcstealth said:

I felt the most movement, both ways, when the gear reached its full extension or retraction. 

That's different than my experience, so not sure what to tell you.  I agree it warrants putting the airplane on jacks, pulling the belly panels and taking a look at what's actually happening.

[Mcstealth]

I was in Dugosh yesterday. I asked both David and his main AP about the movement with the seat. They both said yes, completely normal. You can see the flex happening they said. They did say to check for cracks at annual in that support beam. 

[MikeOH]

15 hours ago, Mcstealth said:

I was in Dugosh yesterday. I asked both David and his main AP about the movement with the seat. They both said yes, completely normal. You can see the flex happening they said. They did say to check for cracks at annual in that support beam. 

Hmm, "completely normal" as in they see it on some Mooneys and aren't worried about it, or "completely normal" as in it's in ALL Mooneys?