M20J W&B Question

[0TreeLemur]

Howdy.   The previous owner of our J weighed it in 2021.  The log book contains the measured E.W., calculated C.G. & M.A.  I have nothing else.  No prior equipment W&B sheet. 

My A&P did provide an W&B sheet after upgrading the panel earlier this year.  I'm checking his work and it seems that he used an implausible moment arm for a few instruments.   Can someone tell me the moment arm he should have used for (1) panel-mounted instruments, and (2) rack mounted instruments like radios?

Thx.

Fred

[PT20J]

The arm will vary somewhat depending on the dimensions and CG location of the equipment itself. From the original equipment list:

Attitude Gyro 17.46

Directional Gyro 16.8

VSI 18.5

Turn Coordinator 16.5

KLN 90B 22.94

[0TreeLemur]

Thanks Skip.  I estimated that the panel was 18" aft. which is pretty close.   For some reason the W&B update done by my A&P used a number quite different from this.  I'll have him fix it.

I'm wondering what explains the variance?  Everything is mounted on the panel.  Why is the KLN90B almost 5" aft of the others?   Why would the TC be only 16.5, when it is supported by the panel.   The RHS panel in a J is slightly slanted aft outboard, but not that much?

 

[Pinecone]

Look at what Skip posted, each instrument has a CG itself.  So you have to use the CG of the instrument, not the panel distance to calculate the new CG.

The heavy part of the TC is on the end furthest forward.  the heavy part of the KLN-90 is near the panel

[ArtVandelay]

The arm will vary somewhat depending on the dimensions and CG location of the equipment itself. From the original equipment list:
Attitude Gyro 17.46
Directional Gyro 16.8
VSI 18.5
Turn Coordinator 16.5
KLN 90B 22.94

Interesting, my KLN90A in my list shows 13.5 installed, 14.2 when removed.
Nothing in my panel is higher than 21 (GFC 507).

[PT20J]

27 minutes ago, ArtVandelay said:

Interesting, my KLN90A in my list shows 13.5 installed, 14.2 when removed.
Nothing in my panel is higher than 21 (GFC 507).

It likely includes the complete installation including cable and antenna as these were not listed separately. 

[0TreeLemur]

Thought about this a bit more last night.  For a cantilevered panel mount instrument of known weight, the distance from the panel to the c.g. of the instrument creates a moment, which affects the effective moment arm of the instrument.   Because the weight is fixed, and a cantilever mount instrument creates a negative moment, it should effectively reduce the moment arm, never increase it.   This figure shows an example of how adding a cantilevered instrument increases the weight of an aircraft, but decreases the moment arm of the C.G. 

In summary, in an aircraft with the panel installed at a positive station, a cantilevered mount panel instrument should always have a moment arm relative to the datum that is less than the distance from the datum to the panel mounting station.

[DCarlton]

Not to high-jack the thread too far...  if you reweigh an airplane, is there any configuration information provided?  Equipment list, etc.  Or is it just a pure weight, CG, balance data sheet on a given date?  If you get it reweighed, can you deep file your decades old W&B data (or even trash it)?  

[EricJ]

1 hour ago, DCarlton said:

Not to high-jack the thread too far...  if you reweigh an airplane, is there any configuration information provided?  Equipment list, etc.  Or is it just a pure weight, CG, balance data sheet on a given date?  If you get it reweighed, can you deep file your decades old W&B data (or even trash it)?  

It's up to the owner to maintain the equipment list, etc.    Weighing the airplane is a bit independent of that, but a weight taken with an accurate equipment list means things can then be deleted, replaced, etc., with W&B updates done from a more accurate starting place.

If you update your W&B you don't need to keep the old one, but if they're done by analysis (i.e., deleted this, added that, did the math), then having that history can be good to check for errors or whatever.    If you update the W&B from a new scaled weight and the equipment list is accurate, you don't need to keep the old ones.

[OR75]

Download generic POH and the weight and balance table / equipment list will have CG locations for different instruments