Weight and Balance
There are lots of web pages and articles about
weight and balance. Most of them have the same
information. Determining the center of gravity isn’t a hard
process, it really only takes an hour or so to do the initial weighing
and measuring, then a little bit of time at the computer and you are
done. I’ll walk you through what I did, and throw a few
pointers in along the way.
Of course you will need some scales to do the weighing part of your
weight and balance process. You can use the fancy
(expensive) scale sets made just for aircraft, or a bathroom scale that
can handle the weight of the airplane. There is no regulation
requiring a certified scale, even to weigh a certified
aircraft. As long as you or the mechanic performing the
weight and balance are confidant in the accuracy of the scale, it is
If you can get one, a scale set makes weighing a lot easier and
quicker. If you can’t get your hands on one though, you can
use any scale or combination.
If you don’t have a single
scale capable of measuring one of your gear, you can use two scales
with a board between them, so each carries half the weight.
This is what I did to get my measurements.
You will also need a plumb bob and a tape measure. A large
in handy too.
Start by getting your plane loaded correctly. This means
remove all the usable fuel and anything extra in the cockpit that isn’t
part of the required equipment list. Then make sure you have
everything installed. You need to have all the
inspection panels and screws back on before weighing. You
also need to have oil, brake fluid, and any other liquids required for
Next get together the blocks you will need to keep the plane from
rolling, and if required, to lift the tail to a level flight
attitude. Don’t worry about weighing them yet, You can do
that after you figure out exactly what you will be using.
Objective: Measure the weight of the aircraft on
The total of these weights is the total aircraft weight, with
aircraft in level flight attitude.
The first part of the weight and balance process
is weighing. If you have multiple scales, roll the plane onto
them, making sure it
is a level flight attitude, and take the readings.
If you have only one scale, it is a little harder. You have
to roll one main onto the scale, then block the other wheel up to get
level across the main spar, then adjust the tail for level
flight. Step two is to repeat this process for the other
main. Finally you can use the scale to measure the tail or
After you have the weight of the plane, make sure to record the weights
of the blocks you had on top of the scale too. This is
called tare weight, and you will have to subtract it from the final
readings to get the real weight of the plane.
If you are off just a little from level flight, you can let air out of
the tires, or pump them a little fuller as required to get level.
Put as many of the leveling blocks as you can under the
scales. This makes figuring out the actual weights
easier. For me, I had to jack the tail up to level.
So I put the blocks on the floor, then put the scale on top of them,
then finally the tail.
Objective: Measure the distance from the
defined datum line to the center of each gear, with the aircraft in
level flight attitude. Each of these distances is called a
"moment arm", or sometimes just "arm".
The second part of the weight and balance process,
after weighing, is measuring. This part is a little
trickier. You will
have to check the
builder’s or maintenance manual to see if there are any special
instructions for measuring your plane. But you might as well
do the measuring while you have the plane in the flight attitude for
All of the measurements will be taken from an
arbitrary datum line--again, you will have to check the manual to see
where that is for your plane. For me it is measured 70 inches
forward of the leading edge of the wing to a point in front of the
spinner. By using a point in front of the airplane, it makes
the math a lot easier, but may make taking the measurements a little
Before we start, here are a couple of tips that will make it a lot
Use the plumb bob to transfer locations to the floor, and mark them
with a pen. Don’t try to measure on the airplane itself; it
will be way too inaccurate.
Mark the tires and floor to ensure nothing moves while you are making
Make sure you measure parallel to the longitudinal axes of the airplane
to get those moment arms. Here is where that large t-square
is helpful. I dropped a plumb line from the front center of
the longitudinal axis and marked the floor, then repeated the process
for the rear. Next I snapped a chalk line between the two
marks. This gave me a reference line to ensure I was square
and correct when making the other measurements.
Once you have the longitudinal axis line marked on
the floor, it is
time to get the other measurements needed for your weight and
balance. These should include
measurements to the
nose or tail wheel, the mains, and any extra measurements you will
need. Because my datum is located forward of the wing leading
edge, I needed to mark that also.
I dropped a plumb bob down the leading edge of
both wings, marked the floor, and connected the two dots. I
ensured this line was square to the longitudinal axes line with the
large T-square and also the 3,4,5 method (measure three feet down one
line, four feet down the other—the distance between the two marks
should be five feet on the hypotenuse).
I then dropped plum lines from the mains and repeated the checks to
ensure square. Last I dropped the line for the tail wheel.
With all of these points marked on the floor, it is now an
easy matter to measure along the longitudinal axes line
to get the distances from the leading edge marks to the mains and to
the tail wheel.
You can now measure and compute the moment arms for your mains and
by adding or subtracting the measurements in reference to the datum
line. Since my datum line is in front of the nose, measured
from the leading edge of the wing, I had to take the measurement from
the leading edge mark to the tail wheel mark and add that to the 70
inches that Vans used for the datum.
Datum: 70 inches in front of leading edge
Leading edge to tail wheel = 181.06
Tail wheel moment arm: 70 + 181.06 = 251.06
Objective: Find the Center of Gravity
(CG), as a measurement from the datum, or another moment arm.
Calculating the Center of Gravity is the third and
final step in the weight and balance process. Now that you
have all the measurements, you are
ready to figure out the
center of gravity. You can compute the empty CG by
the moment arms for the three wheels with their
weights. Each of these multiplications produces a
Then divide the total moments by the total weight to get
CG. Here is an example:
Hopefully the CG is inside the range approved for your airplane. If
not, you will have to add some weight. If for example, you
have an aft CG, you will want to add weight as far forward as possible
to get back into CG with the smallest weight penalty.
You don’t have to re-weigh the plane to figure this out. You
can figure out the arm for a point well forward (say the firewall) and
calculate the weight needed there. Just add a row to the
above table and you can gradually add weight until you get inside CG.
For example, if I needed the CG to be 77.5, and the firewall had an arm
of 35, I would use the below table and add weight to the firewall until
I got the bottom CG under 77.5.
It took me 4 tries to get it right. I
used 10, 20, 30, 25 and
finally 26 lbs to get:
Finally, to get ready for the initial inspection,
you will have to
compute a few sample weight and balance numbers. My DAR
wanted to see max fwd CG, max weight CG, and normal ops. To
get max fwd CG for the RV-8, I had to use the max weight in the front
baggage compartment, pilot and no fuel. To get max weight CG,
I used 170 lbs pilot, 260 lbs rear seat, 50 lbs front baggage and 8 lbs
rear baggage floor. I computed full fuel and no
fuel. The no fuel put me right at the aft CG limit, and I
still never got to max gross weight even with full fuel.
For more on weight and balance, look at FAA-H-8083-1A, the 97 page FAA Aircraft Weight and Balance Handbook (Adobe pdf file).
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