Tor

I had this link saved for once a day needed:
http://www.flyingcirkus.com/forum/m_..._/tm.htm#83718

Should have copied and saved it to my harddrive. [&o]

Anyway, someone posted how to find the CG by putting a weight under each of the 3 wheels and the calculate the location of the CG. Seemed like a much better methode for a teoretically oriented guy like myself, than stagger about with an bulky model.

Help, recap needed.

flatlandmike

My Feedback: (17)

i copied this if its what your lookin for

This method is how I go about checking CG on a plane that weighs too much to put on a CG machine. This is generally the same way full scale planes are balanced. You can’t go lifting a 747 by your fingers!


First, have the plane assembled as RTF, but minus fuel. Find a place where you can put the nose of the plane up against a wall. Take a level and prop up the tail wheel until the fuselage is sitting level. You will probably have to take the hatch off in most cases to find a suitable area to set the level on.

Once the plane is sitting level, place masking tape under the area where the main wheels are touching the surface and draw a line where the wheels contact the tape. Make an index mark where the center of the tailwheel falls in the same manner. This mark will be on whatever you propped the tail up with. (I use magazines). It’s also a good time to measure from the wall, to the point where the recommended CG is. Save that number for later.

What you are looking for at this point is the distance to each wheel from the tip of the spinner. (which should be touching the wall at this point) I use a long straightedge to get the distances

As an example, let’s say you come up with theses numbers:
LH main – 22�
RH main – 22�
Tailwheel – 70�

Now, place a scale under each wheel individually. You will have to re-adjust the height of the other wheels each time to keep the plane sitting level. Add more magazines as necessary. (If you have three scales you can get the weights at the same time but make sure the plane is level)
Don’t forget to re-install the hatch before you weigh each wheel. You will wind up with three weights, one for each wheel.

Let’s say you come up with these numbers
LH main – 9 lbs
RH main – 9 lbs
Tailwheel – 5 lbs

Now its just a simple math problem to find where the CG sits right now on the plane.

Weight x ARM (distance) = Moment
22x9=198 in-lbs
22x9=198 in-lbs
70x5=350 in-lbs

Now add the total of all the moments and the total of all the weights.
Weights 9+9+5 = 23lbs
moments 198+198+350 = 746

Then divide the total moments by the total weights.
746 divided by 23 = 32.43

That number 32.43 is "in inches" how far back from the tip of the spinner to where the plane balances right now.
Compare that to the distance that you measured earlier to the recommended CG location.
If you are off one way or another adjust items then recalculate the weights. The distances (or ARM's) arent going to change so you already have those numbers.

Tor

YES! flatlandmike, your da' man!!

I was thinking this methode should work in reverse aswell.. first figure out what the weight should be on each wheel, then since the weight on the mains should be the same, you will know the weight of just the tail wheel... Put a weight under the tail and move the gear around untill the number on the weight is right... voila, with out having to lift the plane even once...

What do you think?

flatlandmike

My Feedback: (17)

Glad i could help

norcal tom

My Feedback: (1)

That's how we learned to balance full scale aircraft in A/P school We used a world models cub with kitchen scales. You do have to make sure that there is no breeze though. It doesn't take much to make the wing lift and give you a false reading.

Martian Spy

My Feedback: (6)

It seems as though I could get a good start place on lateral balancing by ensuring that both mains weigh the same. Assuming the mains are equal distances from the middle of the fuse, would this center the lateral CG in the middle of the fuse?

Joe McBride

My Feedback: (12)

Invader,

Have a friend lift your prop and you lift under the tailpost. If you start wings level the heavy wing will drop. Do that test a few times until you figure you've got an accurate reading.

Then go fly it. Look for our video on trimming coming later this week for the dynamic test.

SJ

Tor

Looking forward to the video...

Another great resource on trimming is this essay by Peter Goldsmith who competed in the TOC for 10 years.
http://www.flagstaffflyers.com/image...thTrimming.pdf

Dan767

My Feedback: (19)

If you decide to balance your plane this way, here's a little more info.

When you raise the tail of your plane, the mains go aft, so you'll need to put the mains on the forward part of the scale to keep it centered. Use the largest (longest?) bubble level you can for better accuracy. Convert all the weights to ounces and tenths. Convert fractions to the nearest tenth or hundredth. I used a calculator with a paper print out to keep track of my numbers. Make a mark on the highest point of the fuse where you'd like the cg to be. This will help in minimizing parallax errors and make it easier to measure.
The scale should be facing the wingtip or aft, so you can read the numbers. You may have to "chock" the wheels to keep the plane from moving away from the wall. When you weigh the tail, the entire scale has to be supported, or use the tare function and leave the scale on the floor.

I bought my scale on ebay. Much cheaper. 36lb capacity for less than $30, shipped.

Happy Flying

wgeffon

My Feedback: (4)

Once you measure the Level distance to each wheel you can weigh the plane away from the wall. Of course the main thing is having it level when you take the reading for weight.

Dangerous Dan

My Feedback: (1)

I know this is and old thread but have a question. If I have a plane that I like where the cg is and want to remove weight from the nose and then shift weight forward that is behind the cg can I just put the tail wheel on a Schafer without worrying about level take reading remove weight from nose then shift weight aft of cg till the reading is the same?