Incrementally updating weight and balance...

Michael Burbidge

Well Known Member

• Jan 18, 2017

• #1

I'm going from a Sky-Tec LS starter to a B&C starter. The B&C starter is about 3.5 lbs. heavier.

Unfortunately my fuel tanks are full. To do a from scratch weight and balance would require that I empty the tanks! Ugh!

It seems that mathematically you could calculate a new weight and balance using the change in weight and position.

Is that true? Is that legal?

Thanks,
Michael-

 

R

rstory

Member

Patron

• Jan 18, 2017

• #2

Here is the link to the FAA's handbook. It looks like page 7-5 should take care of what you need.

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-1.pdf

 

az_gila

Well Known Member

• Jan 18, 2017

• #3

Michael Burbidge said:

....

Is that true? Is that legal?

Thanks,
Michael-

Click to expand...

Yes and yes.

Make sure you get the correct location of the center of gravity for both starters to put that into the formula as well, not just the weight change.

 

Michael Burbidge

Well Known Member

• Jan 18, 2017

• #4

Thanks!

Great thanks for the answers and reference!

 

F

Fast Eddie B

Member

• Jan 18, 2017

• #5

Incrementally is normally how it's done.

An example from my Sky Arrow:

As an aside, shows the gains possible from switching to a lithium-iron battery!

 

az_gila

Well Known Member

• Jan 18, 2017

• #6

Fast Eddie B said:

Incrementally is normally how it's done.

An example from my Sky Arrow:

As an aside, shows the gains possible from switching to a lithium-iron battery!

Click to expand...

And although no-one has mentioned it yet, do it in an Excel spreadsheet to keep the mathematical calculations correct...

And I see the image above was created in Excel.

 

Michael Burbidge

Well Known Member

• Jan 18, 2017

• #7

In Excel

My original W&B is in Excel, so I can just go from there.

The B&C starter shows the CG for the starter in the drawings. The Sky-Tec does not. I'll just figure that out myself with some experimenting.

Michael-

 

az_gila

Well Known Member

• Jan 18, 2017

• #8

Michael Burbidge said:

My original W&B is in Excel, so I can just go from there.

The B&C starter shows the CG for the starter in the drawings. The Sky-Tec does not. I'll just figure that out myself with some experimenting.

Michael-

Click to expand...

This CG location came from a Sky-Tec supplementary installation document -

Weight and Balance Calculation
Fore-Aft CG: Half the distance between the front and rear starter mounting holes (starter datum)

http://lib.store.yahoo.net/lib/stylespilotshop/sky-tec-starter-install.pdf

 

Michael Burbidge

Well Known Member

• Jan 18, 2017

• #9

Thanks!

I figured it was somewhere. I didn't see that document. Thanks!
Michael-

 

Bubblehead

Well Known Member

• Jan 19, 2017

• #10

Is there really 3.5 lbs difference? I checked the B&C website but could not find the weight of their starters.

 

Michael Burbidge

Well Known Member

• Jan 19, 2017

• #11

2.4

So it looks more like 2.4 lbs difference.