W&B fun...math's not for everyone (including me).

[EricJ]

1 hour ago, ArtVandelay said:

Are you sure?

Which revision/serial POH do you use?   1220G fits my early J, 24-0077, and it says to drain the oil...although the proper SMM says fill it.

[Hank]

5 hours ago, EricJ said:

That changed in 1975, due to the General Aviation Manufacturer's Association standardizing definitions for Empty Weight, for POHs and the associated pilot W&B calculations.

So, yeah, if you have something older, your W&B probably requires addition of oil in the loading calculations.   Later aircraft include it.

 

5 hours ago, Bob - S50 said:

The POH for our '78 Mooney, step 3 of weighing says to drain the oil.

Huh. My 1970 Owners Manual has no weighing procedure, and makes no mention of how to do so, but does mention multiple times to check the official Weight & Balance Record in the airplane.

[ArtVandelay]

Which revision/serial POH do you use?   1220G fits my early J, 24-0077, and it says to drain the oil...although the proper SMM says fill it.

See pix, it’s rev A

[EricJ]

2 hours ago, ArtVandelay said:

See pix, it’s rev A

Sorry, I meant which manual, not which revision.   That looks like manual #1221, for 1978.   I see that #3203 (1997) says that also.    Mine is #1220, for the 1977 model, which says to drain the oil.    I suspect somebody noticed it didn't agree with the S&M and changed it in the next edition.

 

[Bob - S50]

4 hours ago, ArtVandelay said:

Are you sure?

Yep, I can read.

[ArtVandelay]

Yep, I can read.

Yeah, yours is original.

How did Mooney handle updates, did they send notices to owners, publish a service bulletin, or just do nothing?

[Bob - S50]

6 hours ago, ArtVandelay said:

Yeah, yours is original.

How did Mooney handle updates, did they send notices to owners, publish a service bulletin, or just do nothing?

I don't know.  That's the POH that LASAR gave us when we bought the plane from them in 2013.  The cover says it is for serial numbers 24-0378 to 24-0763.  Ours is 24-0576.

[Pete M]

Maybe I misunderstand but using a positve moment forward of datum will only, incorrectly, increase the moment aft of datum. If i hang a 1000lb weight from the spinner with datum at the firewall and use a positive moment then my result will be a calculated cg well aft of datum. If the datum is forward of the added weight, ie datum at spinner, then i would use a positive number. In the original w&b posted the added prop and spinner should have a negative arm leading to a negative moment. Removing the weight forward of datum results in a positive moment. Feel free to correct me if my thinking is backwards:)

Edited April 14, 2020 by Pete M

[EricJ]

8 hours ago, ArtVandelay said:

Yeah, yours is original.

How did Mooney handle updates, did they send notices to owners, publish a service bulletin, or just do nothing?

It's actually a different manual.   There are different Mooney POH part numbers that are specific to M20J serial numbers.   The one you posted appears to be from #1221.   I think Bob - S50 and I have #1220.   There were quite a few different POHs across M20J production and they differed significantly.

[Bob - S50]

1 hour ago, Pete M said:

Maybe I misunderstand but using a positve moment forward of datum will only, incorrectly, increase the moment aft of datum. If i hang a 1000lb weight from the spinner with datum at the firewall and use a positive moment then my result will be a calculated cg well aft of datum. If the datum is forward of the added weight, ie datum at spinner, then i would use a positive number. In the original w&b posted the added prop and spinner should have a negative arm leading to a negative moment. Removing the weight forward of datum results in a positive moment. Feel free to correct me if my thinking is backwards:)

WARNING - long answer from an engineer.  This is your basic engineering statics class.

Anything forward of the datum point is a negative arm.  If we view the aircraft from the left side, negative arms and the prop will be left of the datum point.  Any weight added in that area will be a positive weight times a negative arm which will result in a negative value indicating a counter-clockwise rotation relative to the datum point.  Any weight added to the right of the datum will be a positive weight times a positive arm which results in a positive value.  That indicates a clockwise rotation relative to the datum point.

If you remove weight, it's a negative weight so you get the opposite results.

CG is calculated by dividing the total moment by the weight.  If you put a fulcrum under the CG the airplane will balance.

It's pretty intuitive that removing the prop will move the CG aft.

Removing a weight from the front of the aircraft will provide a positive moment.  Even though the weight has decreased, the moment has increased.  That's because the CG has moved aft.  So even though the total weight is less, the arm is longer resulting in a larger moment.  Either reducing weight without changing the moment or increasing the moment without changing the weight will move the CG (moment/wt) aft because either a larger top value or a smaller bottom value will result in a larger value for CG.  So for example, if you removed the prop and put it in the cabin, the weight would be the same but the moment would increase and the CG moves aft.  Likewise, if you were able to remove weight that was directly over the datum point (vacuum pump?) the moment would not change but the weight would decrease so the CG would again move aft.

If the moment decreased when you removed the prop (which it doesn't), one change would offset the other and the CG would not move much.

Interestingly, you can pick ANY datum point you want, measure arms and weights, multiply to get moments, and divide by total weight.  While the distance to the CG from the datum point would change, the actual location of the CG of the airplane would calculate out to be in the same physical location.  So they could have picked a datum point of the prop and had nothing but positive values.  But the CG range would be more like 75 to 81 inches instead of 45 to 51 inches.

[Hank]

Great explanation, @Bob - S50.

--signed, a fellow Engineer 

[N9201A]

The POH for our '78 Mooney, step 3 of weighing says to drain the oil.

Interesting. I’m looking at one that defines empty weight as including weight of “unusable fuel and full oil.”

[Hank]

35 minutes ago, N9201A said:

Interesting. I’m looking at one that defines empty weight as including weight of “unusable fuel and full oil.”

All depends upon the year, model and sometimes serial number of the airplane in question.