weight and balance question

Our club needs to do a weight and balance on our Grob 103 after refinishing and some other mods. We are wanting to use calibrated/certified scales, but the cost of having it weighed or purchasing a set of scales is quite high.. What have others done to get their W&B completed yet not break the bank? We are in the San Antonio area.

From reading FAA circular 8083-1a and Part 91, it isn't even clear that we are required to use certified scales (not that we wouldn't want to). W&B apparently can even be calculated mathematically w/o weighing the aircraft and not every production e.g., Cessna 172s, need be weighed at all (they can weigh 10-20% of the run).

Find a local car club, they may have 4 electronic scales or know someone who does. A good local airplane shop may have portable electronic scales or know someone who does. Remeber, the key is to correctly level the glider on the scales to get main and tail weight, inside a hanger with all the cooling fans off.

BillT

Good idea..... Also check your local aircraft mechanic, good automotive shop (preferably one that does race suspensions/"corner weighting"), etc.

Another option is do a local "consortium" of airports/glider clubs on buying your own scales.

On Monday, June 27, 2016 at 8:30:22 PM UTC-5, Duster wrote:
Our club needs to do a weight and balance on our Grob 103 after refinishing and some other mods. We are wanting to use calibrated/certified scales, but the cost of having it weighed or purchasing a set of scales is quite high. What have others done to get their W&B completed yet not break the bank? We are in the San Antonio area.

From reading FAA circular 8083-1a and Part 91, it isn't even clear that we are required to use certified scales (not that we wouldn't want to). W&B apparently can even be calculated mathematically w/o weighing the aircraft and not every production e.g., Cessna 172s, need be weighed at all (they can weigh 10-20% of the run).

I bought a 660 lb floor scale off Ebay for $50 plus shipping to W&B on my own glider. That's too small for your Grob, but there are higher capacity scales on Ebay or you can double up with two of the 660 lbs scales. Just a quick look on Ebay yielded a 5000lb digital floor scale for $499.

At 14:57 28 June 2016, WB wrote:
On Monday, June 27, 2016 at 8:30:22 PM UTC-5, Duster wrote:
Our club needs to do a weight and balance on our Grob 103 after
refinishi=
ng and some other mods. We are wanting to use calibrated/certified
scales,
=
but the cost of having it weighed or purchasing a set of scales is quite
hi=
gh. What have others done to get their W&B completed yet not break the
bank=
? We are in the San Antonio area.
=20
From reading FAA circular 8083-1a and Part 91, it isn't even clear that
w=
e are required to use certified scales (not that we wouldn't want to).
W&B
=
apparently can even be calculated mathematically w/o weighing the
aircraft
=
and not every production e.g., Cessna 172s, need be weighed at all (they
ca=
n weigh 10-20% of the run).

I bought a 660 lb floor scale off Ebay for $50 plus shipping to W&B on my
o=
wn glider. That's too small for your Grob, but there are higher capacity
sc=
ales on Ebay or you can double up with two of the 660 lbs scales. Just a
qu=
ick look on Ebay yielded a 5000lb digital floor scale for $499.

A cheaper solution would be to find your nearest public weighbridge (not
sure
if they are called something else in the US) and get the trailer weighed
empty
and again with the glider in it, simply subtracting the trailer weight
would
give you the glider weight, as these weighbridges are for commercial use
they should be calibrated and would give you an accurate figure.

On Tuesday, June 28, 2016 at 2:00:08 PM UTC-4, Benedict Smith wrote:
At 14:57 28 June 2016, WB wrote:
On Monday, June 27, 2016 at 8:30:22 PM UTC-5, Duster wrote:
Our club needs to do a weight and balance on our Grob 103 after
refinishi=
ng and some other mods. We are wanting to use calibrated/certified
scales,
=
but the cost of having it weighed or purchasing a set of scales is quite
hi=
gh. What have others done to get their W&B completed yet not break the
bank=
? We are in the San Antonio area.
=20
From reading FAA circular 8083-1a and Part 91, it isn't even clear that
w=
e are required to use certified scales (not that we wouldn't want to).
W&B
=
apparently can even be calculated mathematically w/o weighing the
aircraft
=
and not every production e.g., Cessna 172s, need be weighed at all (they
ca=
n weigh 10-20% of the run).

I bought a 660 lb floor scale off Ebay for $50 plus shipping to W&B on my
o=
wn glider. That's too small for your Grob, but there are higher capacity
sc=
ales on Ebay or you can double up with two of the 660 lbs scales. Just a
qu=
ick look on Ebay yielded a 5000lb digital floor scale for $499.

A cheaper solution would be to find your nearest public weighbridge (not
sure
if they are called something else in the US) and get the trailer weighed
empty
and again with the glider in it, simply subtracting the trailer weight
would
give you the glider weight, as these weighbridges are for commercial use
they should be calibrated and would give you an accurate figure.

It's not so much a question of empty weight, but where the CG is located, So weighing the glider in a trailer is useless.

On Tuesday, June 28, 2016 at 2:54:01 PM UTC-5, wrote:

It's not so much a question of empty weight, but where the CG is located, So weighing the glider in a trailer is useless.

Ah, but if you know the total weight of the glider, you can then use a calibrated (or verified) bathroom type scale under the wheel that has the low weight on it, and from that determine the weight on the "heavy" wheel, measure moment arms (or trust factory measurements) and calculate C of G location.

So, I respectfully disagree with your conclusion that weighing the glider in the trailer is "useless". It is just the first step to determining empty weight and C of G.

You could also weigh each part separately, sum them to get total weight, then get the weight on the light wheel with the plane assembled and properly leveled, and calculate C of G location.

But, I agree it is far easier to get an appropriate set of scales, and weigh the assembled airplane!

Steve Leonard

On Wednesday, June 29, 2016 at 10:57:53 AM UTC-4, Steve Leonard wrote:
On Tuesday, June 28, 2016 at 2:54:01 PM UTC-5, wrote:

It's not so much a question of empty weight, but where the CG is located, So weighing the glider in a trailer is useless.

Ah, but if you know the total weight of the glider, you can then use a calibrated (or verified) bathroom type scale under the wheel that has the low weight on it, and from that determine the weight on the "heavy" wheel, measure moment arms (or trust factory measurements) and calculate C of G location.

So, I respectfully disagree with your conclusion that weighing the glider in the trailer is "useless". It is just the first step to determining empty weight and C of G.

You could also weigh each part separately, sum them to get total weight, then get the weight on the light wheel with the plane assembled and properly leveled, and calculate C of G location.

But, I agree it is far easier to get an appropriate set of scales, and weigh the assembled airplane!

Steve Leonard

Further to Steve's point.
A quick and dirty safety check is to level the glider and weigh the tail with an accurate scale.
Comparing the tail weight to what is shown on the last measured weight and balance will show whether there is something to be concerned about from a safety point of view.
This can be done with the bathroom scale we all have.
It is not a substitute for the real thing but it will show if you have a likely problem.
I did this when my first '20 was really hairy to fly after I bought it. This disclosed the 13 pounds of undocumented tail weight.
UH

FWIW having just completed a W&B for a Grob recently. I'd be more concerned about making sure you have all of the geometry measurements correct including (especially) leveling the glider per the W&B instructions in the manual. A couple CM of error in a dimension (e.g. datum to the tail weighing location) has a lot more impact than 1 lb error in the scale itself. If you have concerns about the scale, it's pretty easy to do some simple calibrations/tests yourself. A plastic 55 gallon drum filled to the reference mark with water (for example) gives you a pretty good indication. Don't forget to adjust for temperature in the specific weight of water.

We have been able to rent a suitable scale here in Houston. San Antonio has some rental places according to Google.