M20K CG

[ArtVandelay]

Decades ago a shop offered to weigh my first Mooney, a 231. It came out with similar results. I just ripped l it up as you should do with this one and hopefully you didn’t need a new one from any changes. But if so just have them do a revised wt & bal from calculating. I’ve been through major panel updates and always calculated the revised wt& bal.

Ditto…I also went through the entire history of W&B, found something like 20 errors or omissions, but they all balanced out to within 2 lbs. It’s an exercise I recommend every owner should do.

[Schllc]

33 minutes ago, Z W said:

Sheets like the one posted here have made me reluctant to let the plane see the scales...

This is an interesting statement, and not the first time someone has said something similar. 
Are you concerned that you will exceed the structural limits of the plane and have an accident as a result, or that an FAA official will ramp check your W&B?

it’s rhetorical, and not meant to pick you you.  It’s actually a serious question for all owners. 
I know that useful load is always the first or second question asked about an aircraft, but let’s be honest, if you want any particular model, you already know the range it will fall in, and +/-20 pounds isn’t going to make a difference on your decision to purchase. 
personally, I would much rather have a super stable CG envelope like a Mooney, than more useful load that has to be carefully managed for safety.

[Shadrach]

12 hours ago, kortopates said:

So given the erroneous weight and bal is 3.5 years old i am still scratching my head to understand how the pilot has been doing his or her obligatory weight and balance pre-flight check. Although we all know the answer. sigh…


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To be fair, almost any Mooney is well with in CG limits if placarded weight limitations are followed. I can fault him for not recognizing the miscalculation earlier, but experience suggests that in almost any Mooney, out of CG loading is nearly impossible without doing something obviously stupid. 

[Crawfish]

Little late to the conversation but my 1979 M20K CG is 43.9 only real change is a GTN650 and added a fire extinguisher. No built in O2. 

She has been weighed twice in the past 10 years, came out to within .5# on both weights, done by independent shops. Although on the first weighing it gain 15 pounds from the calculated W&B from the factory. 

Cheers! 
  Crawfish 

[Z W]

1 hour ago, Schllc said:

Are you concerned that you will exceed the structural limits of the plane and have an accident as a result, or that an FAA official will ramp check your W&B?

More worried that it won't be done correctly, the numbers will be wrong and unreliable, and the logs will be inaccurate. 

[Marc_B]

20 hours ago, druidjaidan said:

However, it should be a 5in arm, not 15in. And the main gear arm is measured from the nose gear as a reference, so I think it should be 66in not 56in.

@druidjaidan Agreed!  The page should be in your POH regarding how to calculate the W&B from scales.  The two numbers to use are distance from Nose wheel trunion to datum (POH shows constant at 5) and the distance between main and nose wheel center axles.  For comparison, the numbers from my last actual weight measurement were -5.3 and +64.  So your scale weights are actually probably accurate, but your calculations are horribly off as they are not referencing the correct arms.  Measurements forward of the datum point are negative.  Aft of datum are positive.  You could easily go measure this next time you're at the hangar to see where you likely are with CG...but I think you should go back to the place that calculated it and insist that they reprint it for the actual arms...no need to pay someone else to fix their mistake.  Make sure you bring your POH showing the proper calculations.

Here is the page of note from my POH (M20K Encore):

[Marc_B]

Here's the W&B pages for a M20K 231 S/N 25-0001 thru 25-0446.  Same idea, measure gear trunion to datum or use constant 5.  Mark a line for the nose wheel center axle and mark a line between the main gear center axles and measure between the lines

231WB.pdf

[atpdave]

@druidjaidan, in the W&B sheet you show in the third post, I think all the arms are about 10 inches too far forward. Just looking at dimensions, weighing and gear rigging info in the SMM, they should be around -5.3 from the zero datum for the nose and +66.3 for the mains. Assuming the weights (and my calculations and assumptions) are accurate, the CG for your plane should be around 41.5 (no fuel) and 42.8 (full fuel).

[Fly Boomer]

5 hours ago, atpdave said:

@druidjaidan, in the W&B sheet you show in the third post, I think all the arms are about 10 inches too far forward. Just looking at dimensions, weighing and gear rigging info in the SMM, they should be around -5.3 from the zero datum for the nose and +66.3 for the mains. Assuming the weights (and my calculations and assumptions) are accurate, the CG for your plane should be around 41.5 (no fuel) and 42.8 (full fuel).

And the reference datum is 5 inches aft of the nose gear.  Are those two numbers related?

[atpdave]

21 hours ago, Fly Boomer said:

And the reference datum is 5 inches aft of the nose gear.  Are those two numbers related?

Yes. The slight difference is mostly due to the trunnion radius. Compare how the plumb lines are set in the SMM (weighing vs. rigging).

I wonder why the distance between nose and main axles in @Marc_B's W&B is 2.3 inches less than that shown in the M20K dimensions figure I attached.

[Fly Boomer]

1 hour ago, atpdave said:

Yes. The slight difference is mostly due to the trunnion radius. Compare how the plumb lines are set in the SMM (weighing vs. rigging).

I wonder why the distance between nose and main axles in @Marc_B's W&B is 2.3 inches less than that shown in the M20K dimensions figure I attached.

I didn't really deduce how it could apply to these calculations, but I was just thinking that the difference between -5 inches and +5 inches is 10.  So if all those numbers are off by 10 inches, it could have to do with what was used as the datum.

I may have posted that before coffee.

[Phil EF]

My 231 came out of the factory in 1980 with cg 42.68. 
Adding O2 moved it to 44 and with all subsequent changes never went less than 43.8 on the cg. 

[Marc_B]

On 12/18/2023 at 4:16 PM, atpdave said:

I wonder why the distance between nose and main axles in @Marc_B's W&B is 2.3 inches less than that shown in the M20K dimensions figure I attached.

My guess is a combination of rigging, individual difference from aircraft to aircraft due to manufacturing differences, and exact accuracy of measurements.  I'll have to say though, that when I had my nose gear shock discs replaced that it was surprising the difference that they made in the wheel camber (shop had to flip collar).  And as evidenced by OP, certainly can see how inaccuracies with the datum reference measurements can cause large changes in the CG.  Fortunately I'm pretty much in the middle of the CG range.

I'll have to measure wheel to wheel and see if it the sum of my measurements from fore to aft.