Tail heavy W&B on 1970 M20F

[FlyLateNLife]

Looking for input and advice from the experts. 

The M20F I just purchased appears to have an extreme aft CG of 52.83" empty from a weighing done in April this year.  One question is the in item #5 which has the nose being -3.5" forward of the datum when the Service and Maintenance Manual (Rev E 9-1-81, pg 2-5) has the nose gear trunion point at STA -5.  If I'm correct, that change would only adjust the CG to 50.1"...the aft edge of, but within, the CG envelope. 

Does anyone else have a similar situation?  If so, how do you work with it?

How can the CG be reasonably adjusted? I know that any weight aft of the CG is "bad" in this situation and any options to remove aft weight is preferred.  For example, the aircraft has an HF clothesline-type antenna from the top of the cabin to the tail which may have options to replace.

I was hoping to remove the ADF instruments but not until the CG is addressed.

Reasonable suggestions welcome.

I've attached the core of the W&B report (tail number, current owner and weigh stations names not include out of respect)

W&B.pdf

[Ragsf15e]

50 minutes ago, FlyLateNLife said:

Looking for input and advice from the experts. 

The M20F I just purchased appears to have an extreme aft CG of 52.83" empty from a weighing done in April this year.  One question is the in item #5 which has the nose being -3.5" forward of the datum when the Service and Maintenance Manual (Rev E 9-1-81, pg 2-5) has the nose gear trunion point at STA -5.  If I'm correct, that change would only adjust the CG to 50.1"...the aft edge of, but within, the CG envelope. 

Does anyone else have a similar situation?  If so, how do you work with it?

How can the CG be reasonably adjusted? I know that any weight aft of the CG is "bad" in this situation and any options to remove aft weight is preferred.  For example, the aircraft has an HF clothesline-type antenna from the top of the cabin to the tail which may have options to replace.

I was hoping to remove the ADF instruments but not until the CG is addressed.

Reasonable suggestions welcome.

I've attached the core of the W&B report (tail number, current owner and weigh stations names not include out of respect)

W&B.pdf 1.24 MB · 2 downloads

I’d suggest comparing this newly completed W&B to the old one.  My F is 45.2.  I suspect they did the computation wrong on yours or else didn’t level the airplane properly.  It’s often messed up as the manual is a little complicated to follow.  You are not the first person to have a messed up weight and balance form.
 

Mooney’s are pretty hard to get outside the weight and balance envelope, so one being that far aft would be  pretty surprising.

[EricJ]

I think that W&B is suspect and you might consider getting it weighed again.    One potential issue is that it is extremely unusual for both main gear to weigh exactly the same amount.   I think you're also right that the forward arm may be incorrect and the -0.5" sounds potentially correct to me (I don't have an F model manual in front of me, but I weighed a C a few years ago and we used -0.5" for that.)   That and the oddball arm make it suspect enough to consider doing it over.

[47U]

2 hours ago, FlyLateNLife said:

One question is the in item #5 which has the nose being -3.5" forward of the datum when the Service and Maintenance Manual (Rev E 9-1-81, pg 2-5) has the nose gear trunnion point at STA -5.

Certainly there are some things that need to be verified.  Yes, this (older) version of the mx manual (pic below) shows the nose gear trunnion 5” forward of the datum (STA -5), but the nose wheel is forward of the trunnion by a skosh.  If they used the wrong distance from the datum to the nose, then the distance from the datum to the main gear wheels may also be suspect.  I’m not sure how much equipment you’ve got in the avionics bay in the tail, but I think you’ll make some headway once the datum reference distances to the wheels (scales) are correct.

 

[Fly Boomer]

8 hours ago, FlyLateNLife said:

I've attached the core of the W&B report (tail number, current owner and weigh stations names not include out of respect)

Have it re-weighed by a Mooney expert.

[N201MKTurbo]

The first thing I would do is re-measure the reference points and redo the math with the wheel weights from the report to double check the math. 

[Nico1]

Be careful, when I got mine the shop that did the pre-buy and annual re-did the W&B and they used all the wrong numbers to calculate it. Me, totally inexperienced at the time, saw that the W&B looked completely out of whack and did my research. Any sanity check would have raised flags, but lots of people don't think about doing that...

I found they used lots of wrong numbers. So, carefully go through the type certificate data. In my case, I found they were using the empty fuel and arm for a M20K instead of an F. They also used the wrong arm lengths for the main gears.

 

[EricJ]

7 hours ago, N201MKTurbo said:

The first thing I would do is re-measure the reference points and redo the math with the wheel weights from the report to double check the math. 

The airplane needs to be levelled to do that, which is a bit of a pain, but doable.

Usually this is done with plumb bobs and a lot of chalk lines on the hangar floor verified with tape measures and framing squares, etc.   The SMM gives the distance between main and nose landing gear axles as 5' 11 9/16", which is always good to verify since slop in the gear and pucks can move things around.   With the airplane level, drop a plumb bob from the nosegear truss bolts and mark that on the floor, and then 5" toward the tail from that is the datum (which isn't really necessary to mark).   There will likely be a small distance from the nosegear axle (mark on the floor with a plumb bob) to the truss bolt reference line.   On the C model I did that was -0.5", which if measured the same on an F model would be -5.5" from the datum, not -3.5, but that sort of thing definitely needs to be measured on the airplane.   The scales measure the weight at the axles, so you need to know where those are relative to the expected envelope.    If that wasn't done correctly the results won't be accurate.

I'm still skeptical of the main gear weighing the same, too.   That generally doesn't happen.    I think it just needs to be done over.

[N201MKTurbo]

1 hour ago, EricJ said:

The airplane needs to be levelled to do that, which is a bit of a pain, but doable.

Usually this is done with plumb bobs and a lot of chalk lines on the hangar floor verified with tape measures and framing squares, etc.   The SMM gives the distance between main and nose landing gear axles as 5' 11 9/16", which is always good to verify since slop in the gear and pucks can move things around.   With the airplane level, drop a plumb bob from the nosegear truss bolts and mark that on the floor, and then 5" toward the tail from that is the datum (which isn't really necessary to mark).   There will likely be a small distance from the nosegear axle (mark on the floor with a plumb bob) to the truss bolt reference line.   On the C model I did that was -0.5", which if measured the same on an F model would be -5.5" from the datum, not -3.5, but that sort of thing definitely needs to be measured on the airplane.   The scales measure the weight at the axles, so you need to know where those are relative to the expected envelope.    If that wasn't done correctly the results won't be accurate.

I'm still skeptical of the main gear weighing the same, too.   That generally doesn't happen.    I think it just needs to be done over.

Indeed, I’ve done it a few times. I sometimes need to put a spruce catalog under the mains.

[PT20J]

9 hours ago, N201MKTurbo said:

The first thing I would do is re-measure the reference points and redo the math with the wheel weights from the report to double check the math. 

THIS^^^^ weights are usually right but you have to follow the procedure in the service manual and often mechanics unfamiliar with Mooneys use a generic procedure. 

[M20F]

19 hours ago, Ragsf15e said:

I’d suggest comparing this newly completed W&B to the old one.  My F is 45.2.  I suspect they did the computation wrong on yours or else didn’t level the airplane properly.  It’s often messed up as the manual is a little complicated to follow.  You are not the first person to have a messed up weight and balance form.
 

This.   There is an 8 page thread going on that ends up with how Bob Hoover doing rolls has something to do with landing gear.  In the early part of the thread though some useful factoids on how weighing Mooney’s is tricky. 

[N204TA]

Put me in the camp of being suspect of the weighing procedure.  
 

The aircraft needs to be level with both front seats fully forward.  More importantly, your form shows that the main wheels are 76” aft of the datum.  As @EricJ mentioned, the maintenance manual shows that the center of the mains are just less than 6 feet (72 inches) aft of the center of the nose wheel.  Since the nose wheel is forward of the datum, the mains should be less than 72” aft of the datum by the about the same distance that the nose is forward.

I suspect that the nose wheel arm is more like -5.5 and the mains are closer to 67 inches aft of the datum than 76 which would give you a more realistic CG around 46”

Edited July 30, 2024 by N204TA

[EricJ]

55 minutes ago, N204TA said:

Put me in the camp of being suspect of the weighing procedure.  
 

The aircraft needs to be level with both front seats fully forward.  More importantly, your form shows that the main wheels are 76” aft of the datum.  As @EricJ mentioned, the maintenance manual shows that the center of the mains are just less than 6 feet (72 inches) aft of the center of the nose wheel.  Since the nose wheel is forward of the datum, the mains should be less than 72” aft of the datum by the about the same distance that the nose is forward.

I suspect that the nose wheel arm is more like -5.5 and the mains are closer to 67 inches aft of the datum than 76 which would give you a more realistic CG around 46”

Good catch.   Looking at it now that sheet appears to be full of errors.

Line 1 says the datum, station 0, is 3.5 inches behind the nose gear support bolts.    According to the manual it should be 5".

Line 3 says the main gear are 76" aft of the datum, but...

Line 4 says the main and nose gear are 76" apart.     1, 3, and 4 can't all be true unless the nose wheel axle is 5" behind the truss bolts, which doesn't sound possible.   3 and 4 can't even both be true unless the nose gear axle is the datum, which it is not.

Line 5 winds up being a restatement of line 1, which is not true.

I still think the whole thing just needs to be redone.

[N204TA]

I just looked at the data for my 68 F as it was computed by the factory.

The nose wheel is 5.4 inches forward of the datum and the mains are 66.9 inches aft.

How about that for my SWAG above.  

 

[FlyLateNLife]

All,

Thank you for the insight and recommendations.  I'll attack this problem in a two step approach:

1) As a temporary measure, I've re-computed the CG using the weights from the most recent weighing and the arms provided by N204TA to arrive at a CG of 45.8"....much more realistic.  I'll use this while I fly with my instructor to obtain my insurance-mandated hours and also for my flights when solo or with one other person until the annual.

2) I'll request a re-weigh at the end of the annual in September and I'll make sure to look over the numbers closely.

Thanks again for the excellent inputs and advice.