Move CG

[Kerrville]

I just got some updated avionics and along with it a new weight and balance.  I am not thrilled by how forward the CG is on my M20J. It is useable, but I generally will need to throw a couple cases of oil in the back, especially when I am flying solo.  I want to find some kind of useful avionic that installs in the tailcone to move the CG aft. Any ideas?

[toto]

That’s strange. New solid-state avionics are usually lighter, not heavier. I would expect the CG to be going backward instead of forward.

[Paul Thomas]

36 minutes ago, Kerrville said:

I just got some updated avionics and along with it a new weight and balance.  I am not thrilled by how forward the CG is on my M20J. It is useable, but I generally will need to throw a couple cases of oil in the back, especially when I am flying solo.  I want to find some kind of useful avionic that installs in the tailcone to move the CG aft. Any ideas?

Are you such the W&B is accurate? It seem almost impossible to have a CG issue is most J model unless you get absurd with scenarios.

[LANCECASPER]

1 hour ago, Kerrville said:

I just got some updated avionics and along with it a new weight and balance.  I am not thrilled by how forward the CG is on my M20J. It is useable, but I generally will need to throw a couple cases of oil in the back, especially when I am flying solo.  I want to find some kind of useful avionic that installs in the tailcone to move the CG aft. Any ideas?

Newer lighter avionics, plus removing old wiring, the vacuum pump - if you did, etc., would move the CG rear-ward not forward, unless you had a lot of the boxes in the tailcone, which didn't usually happen on older airplanes.

[Kerrville]

There were a lot of large boxes in the tailcone, storm scope, dme, and something else.  We weighed the components of the Brittan Autopilot, and they were surprisingly light.  My CG did indeed move aft, but it is still forward of where I would like it.  Sitting empty it is around 39.5. I would prefer it to be at least 41.  I did not remove the vacuum pump as I changed navigation and radios, but I have this issue, if it aint broke don't fix it, so I didn't replace my vacuum system yet.  I will probably do this when the old vacuum pump finally fails.  It's going strong right now.  

[PT20J]

Make sure they followed the procedure in the POH exactly. Large CG shifts are usually an error.

[Ragsf15e]

I have a 3 blade prop and still have a 45.2 empty cg.  Im finding it difficult to see a 39.5?  Possibly a mistake was made prior to your recent upgrade.  Have you started at the factory WB and checked all the changes?

[Kerrville]

It's a few inches better than the last WB.  The airplane has a past damage history where the tailcone was replaced. Maybe got an extra light one?

[Andy95W]

2 hours ago, Kerrville said:

It's a few inches better than the last WB.  The airplane has a past damage history where the tailcone was replaced. Maybe got an extra light one?

More likely that someone made an arithmetic error 20 or 30 years ago.

[Sabremech]

Might be time to put it on the scales and get an accurate weight. 
 

[skydvrboy]

You know it's pretty straight forward to put a scale under each wheel and find out if there's an error.  My A&P did this for me after installing my JPI 900 (without asking, but he told me if I didn't like the results we wouldn't log it).  Turns out there was a calculation error, they forgot to subtract the weight when they pulled some avionics back in the 90's and I gained about 5 lbs of useful load.    I know it normally goes the other way, but it will give you the actual number instead of just continuing to add rounding errors.

[takair]

I have found errors almost anytime I check someone’s closely enough.  They carry forward and compound.  As Sabermech says, you can get it weighed.  For my own airplane I compiled all the data for over 50 years into a new spreadsheet to really clean things up.  Just yesterday I was updating a weight and balance for a job I did for a customer and broke my spreadsheet when I was trying to make aesthetic improvements…..I didn’t notice until the last minute…..was just close enough to escape immediate attention but would have been a problem down the line….

[EricJ]

It does sound like there was a mistake in a WnB calculation somewhere.    I just did a new WnB for somebody and went back to the initial factory WnB and did a spreadsheet with all of the changes since there.    There were two errors, and miraculously they cancelled each other...well, the error was made on an equipment installation that was later removed, and the removal was done in such a way that the error went away with it.    It's not too difficult to make those errors, so they happen fairly frequently.   It's also not too difficult to use a spreadsheet and go back and check all the WnB changes.

And +1 that a re-weigh is also an option.   Find a mechanic that has a decent set of scales and knows how to do it properly.   Paint shops seem to do this routinely.

 

[LANCECASPER]

4 hours ago, Kerrville said:

It's a few inches better than the last WB.  The airplane has a past damage history where the tailcone was replaced. Maybe got an extra light one?

The Mooney Factory Service Center does re-weighs and if there's anyone that knows how to do it, it's them.

[Kerrville]

We did re-weigh it with certified scales. These things are incredible, they adjust the weight based on your position on the surface of the planet earth!

[Aerodon]

19 hours ago, Kerrville said:

I just got some updated avionics and along with it a new weight and balance.  I am not thrilled by how forward the CG is on my M20J. It is useable, but I generally will need to throw a couple cases of oil in the back, especially when I am flying solo.  I want to find some kind of useful avionic that installs in the tailcone to move the CG aft. Any ideas?

Are you familiar with the Charlie Weights on a 231/252?  Is there a shelf and holes pre-drilled into the aft bulkhead an a M20J?  Much as I don't like adding useless weight, this is a very effective way of fine tuning your CG.

And for the people contemplating upgrades - consider remote boxes for transponder, traffic, stormscope etc., especially if you already have the avionics shelf in the back.  Not only does it help your CG, I think it makes the overall installation easier because things are not as cramped behind the panel.

 

Aerodon

 

 

[Kerrville]

I will check on that shelf. Here is our worksheet based on the new weigh in. 

[Fritz1]

Once upon a time I had a G-model, W&B was about 10" off, traced it back to a log book entry in 1976 and then did the the arithmetic forward. Something sounds strange here, I would do both, compute forward from original factory W&B and check recent weigh in against the procedure lined out in the service manual

[EricJ]

How it is weighed makes a difference on the numbers, particularly whether the airplane was properly levelled, etc.   Getting all that done and accounting for tare, fuel, etc., can make a difference.   The unusual CG is puzzling.

[Wingover]

1 hour ago, Kerrville said:

I will check on that shelf. Here is our worksheet based on the new weigh in. 

Did they level the plane per instructions by letting air out of the nose tire and then marked the datum correctly? Otherwise it's a waste of time if the nose wheel is not positioned correctly

[atpdave]

6 hours ago, Kerrville said:

I will check on that shelf. Here is our worksheet based on the new weigh in. 

I'm pretty sure it's impossible for Lm/n to be 5 inches greater than Lm/r. I have a page for the nose gear steering adjustment from the a M20K SMM, not an M20J, but I think it's the same. It says that the nose wheel axle centerline should be no more than 0.06 inches forward of the trunnion leading edge, measured with a plumb line. It's interesting that the difference in your diagram is exactly 5 inches, the same number as the trunnion distance from the reference datum. Measuring Lm/n is easy. Measuring Lm/r requires that a plumb line be dropped from the trunnion centerline. If we assume that Lm/n was accurately measured and Lm/r was not measured and is off by approximately five inches,  your actual CG would be somewhere around 44 inches aft of the reference datum. I suggest that Lm/r and Lm/n be re-checked.

Pages from MAN134 SMM M20K.pdf

[EricJ]

1 hour ago, atpdave said:

I'm pretty sure it's impossible for Lm/n to be 5 inches greater than Lm/r. I have a page for the nose gear steering adjustment from the a M20K SMM, not an M20J, but I think it's the same. It says that the nose wheel axle centerline should be no more than 0.06 inches forward of the trunnion leading edge, measured with a plumb line. It's interesting that the difference in your diagram is exactly 5 inches, the same number as the trunnion distance from the reference datum. Measuring Lm/n is easy. Measuring Lm/r requires that a plumb line be dropped from the trunnion centerline. If we assume that Lm/n was accurately measured and Lm/r was not measured and is off by approximately five inches,  your actual CG would be somewhere around 44 inches aft of the reference datum. I suggest that Lm/r and Lm/n be re-checked.

Pages from MAN134 SMM M20K.pdf 79.46 kB · 4 downloads

Good catch!   Lm/n is shown in the M20J SMM as nominally 5' 11 9/16", or 71.5625".   That's pretty close to the 71.5" shown, and it really should be measured due to uncertainty from the nosewheel puck compression, etc.

The same max 0.06" tolerance from the front of the trunnion to the nosewheel axis is given in the M20J manual, so, as expected, it is the same as the M20K.

I agree there shouldn't be 5" difference between Lm/n and Lm/r, especially given the max of 0.06.   That difference will increase if the aircraft isn't properly levelled, though, and the 0.06 spec is also for when the aircraft is properly levelled.   I don't think it can get to be 5" off just from not being levelled, but it is a potential source of error.   It's possible it was 0.5" and a decimal point got moved or something.

That does provide a reasonable explanation for the discrepancy.

Edit:   It's also possible that the 5" from the trunnion to the datum got pre-subtracted to get the 66.5", and then subtracted again in the computation on the sheet.   In any case, it appears that there's a 5" error in the CG computation.

[Kerrville]

I’ll ask the IA if he leveled it. The double 5 seemed suspicious to me as well. I don’t know enough to question it, but with the tolerance that you guys quoted I feel like I may have some ammo here. Thanks. 

[cliffy]

I did one and found a big error on the original factory W&B sheet

They made mistakes also. Don't just blindly trust the first one. 

 

[PeteMc]

16 hours ago, Sabremech said:

Might be time to put it on the scales and get an accurate weight. 

But you still have to hope they did it right.  After my interior was done they had a local shop re-weight and redo the W&B.  When they sent me the numbers I plugged them in and discovered that I could not fly in the plane alone without 200# in the luggage compartment.  

They had used the wrong number or measuring point for the nose wheel (even though I had warned them it was not like a Piper...) so the rest of the math was all wrong.  New one seems similar to the last W&B, but I think at some point I'm going to have it done again to see if a different shop comes up with the same "new" numbers.