Weight and balance question.

My plane, a 175B has an O-360 engine and a constant speed prop. I had
the plane weighed and and about to begin the calculations of finding the CG,
and it's measurement behind the datum. I'm a bit rusty there and would
appreciate what ever help you could do. Here is what I have.

While plane is leveled according to specs
Left main gear 644 pounds
Right main gear 631 pounds
Nose gear 528 pounds
Total, 1803 pounds.

I'm not real sure how they get the CG while the left and right weights are
different. Do they average them? I don't have the measurements for the gear
with me at the moment, I intend to use the standard values for the plane
rather than measing my particular plane.

What further information do I need? The shop is going to calculate it as
well, but I want to understand it as I do it. Thanks in advance for your
help.
Wayne

In a previous article, "Wayne" said:
While plane is leveled according to specs
Left main gear 644 pounds
Right main gear 631 pounds
Nose gear 528 pounds
Total, 1803 pounds.

I'm not real sure how they get the CG while the left and right weights are
different. Do they average them? I don't have the measurements for the gear

No need, since I'm assuming both gear are the same distance back from the
datum. All the fact that there is a diffence tells you is that the c/g
will be slightly off the centreline of the aircraft.

with me at the moment, I intend to use the standard values for the plane
rather than measing my particular plane.

Yeah, that should be it. It's pretty simple, take the three weights,
multiply each one by the distance from the datum to that scale, add those
numbers together, and divide by the total weight.



--
Paul Tomblin http://xcski.com/blogs/pt/
I think I've finally worked out why the Irish drink Guinness. It's
to lubricate their throat so they can speak their own language.
-- David P.

left and right gear have the same moment arm, so add the weights and
multiply by the moment arm.

I believe the shop needs to verify the weights, and do the calculations and
endorse the completed empty weight and balance entry and provide a list of
equipment installed (and moment arm location) that are included in the
"empty weight"

BT

"Wayne" wrote in message
...
My plane, a 175B has an O-360 engine and a constant speed prop. I had
the plane weighed and and about to begin the calculations of finding the
CG,
and it's measurement behind the datum. I'm a bit rusty there and would
appreciate what ever help you could do. Here is what I have.

While plane is leveled according to specs
Left main gear 644 pounds
Right main gear 631 pounds
Nose gear 528 pounds
Total, 1803 pounds.

I'm not real sure how they get the CG while the left and right weights are
different. Do they average them? I don't have the measurements for the
gear
with me at the moment, I intend to use the standard values for the plane
rather than measing my particular plane.

What further information do I need? The shop is going to calculate it
as
well, but I want to understand it as I do it. Thanks in advance for your
help.
Wayne

For a perfect w&b you'd measure the position of the wheels relative to
whatever reference place is selected for the rest of the plane. In the
absence of this information you will probably use the position that the
original w&b used for wheel location. You add the main gear together, not
average them. They should be in the same location (for/aft). Did you
level the plane laterally? That could easily explain the difference.
Cushions, equipment, bent gear, side loaded scales are also possibilities.

Maybe the most important part is to identify in detail what equipment was
in the plane at the time of the weighing and it's location. If it was
moveable, both weight and location needs to be noted. Exact
fuel state should be noted also. Plus or minus a gallon is not good.
Note the oil quantity.

A good example of the detail needed -- in what notch were the seats?
What was in the pockets in the seats? Was the towbar in the plane and
exactly where was it? Did you have any control locks installed? Where
were they? How much did they weigh? Everything that is not a permanent
non-moveable part needs to be identified and measured. This would be
a good time to make sure that everything that is installed is documented.
No extra stuff like mounts for portable GPS's, handheld radios, or headsets.

Obviously, it is much cleaner to do a weight and balance with NO loose
stuff in the plane and absolutely empty tanks.




In article , "Wayne"
wrote:
My plane, a 175B has an O-360 engine and a constant speed prop. I had
the plane weighed and and about to begin the calculations of finding the CG,
and it's measurement behind the datum. I'm a bit rusty there and would
appreciate what ever help you could do. Here is what I have.

While plane is leveled according to specs
Left main gear 644 pounds
Right main gear 631 pounds
Nose gear 528 pounds
Total, 1803 pounds.

I'm not real sure how they get the CG while the left and right weights are
different. Do they average them? I don't have the measurements for the gear
with me at the moment, I intend to use the standard values for the plane
rather than measing my particular plane.

What further information do I need? The shop is going to calculate it as
well, but I want to understand it as I do it. Thanks in advance for your
help.
Wayne

A good example of the detail needed -- in what notch were the seats?
What was in the pockets in the seats? Was the towbar in the plane and
exactly where was it? Did you have any control locks installed? Where
were they? How much did they weigh? Everything that is not a permanent
non-moveable part needs to be identified and measured. This would be
a good time to make sure that everything that is installed is documented.
No extra stuff like mounts for portable GPS's, handheld radios, or
headsets.

Seats were full forward as indicated in the service manual. Towbar, and
POH were in the plan. Stripped everything out.

Obviously, it is much cleaner to do a weight and balance with NO loose
stuff in the plane and absolutely empty tanks.

Not so sure that is true. Some must me empty, others can be calculated
if full. At least that's what the director of maint said.

I found out that the nose wheel has a -0.7 arm and the mains have a
+58.0

I think I have everything now. As you all said though, I have to list all
the equipment as weighed.

Wayne

(cddb) wrote in message link.net...


Obviously, it is much cleaner to do a weight and balance with NO loose
stuff in the plane and absolutely empty tanks.





Tanks should be "empty" in accordance with manufacturer's
Unusable Fuel figures. A 172 has something like 4 gallons unusable, so
the tanks get completely drained and two gallons added to each side.
Then we weigh the airplane. The dipstick is calibrated with Zero being
the unusable fuel level when the airplane is on level ground.
Unusable Fuel is that amount of fuel that won't flow out of the
tank when the airplane is in its max climb attitude or lowest nose
attitude, with full flap if applicable. It will depend where the fuel
outlet in the tank is located, and the fuel flowing to the rear or
front may uncover the port if run below unusable levels. Dead engine
on approach. Seen it a couple of times. Not by me, though. Not yet.

Dan