Sun Nov 1 13:02:13 PST 2020
If there's one thing that all airplanes have in common, it's the importance of the Center of Gravity to how it flies. We've written a little about what a Center of Gravity is elsewhere, and also about why it's important, so we won't go into it too much here.
The short version is this:
On the plans for airplanes CG is marked with this symbol:
All WhistlePig Toys airplanes are marked with a CG symbol.
The first step to checking out a new model airplane is to make sure that it balances at this point.
You can check the balance by holding the plane up with your fingers, and with practice this can work well. But if you want something more precise you can build a gadget that will make it much easier to check and measure the CG accurately.
There are many variations on this gadget. If you want you can purchase them commercially, too. Ours is very simple and works well for little airplanes. It's a fun hour or so in the workshop and it makes a good project for the family.
What we're going to do is take a block of wood and drill two holes in it about 1 1/2 inches apart. We'll take the two pencils and stick them in the holes to make two uprights. We'll put the erasers or the T-pins in the ends of the pencils to make a nice pivot for our airplane to balance on.
Here's a drawing of the gadget. Click on the drawing for a printable PDF.
Got that? Great! Lets's get started.
Cut off a block of 2x4" 6" in length. The length isnt very important - any old scrap of 2x4 will do. It's nice if the block is has square corners, but again that's not very imporant.
Find the center of the block. If your block is square, use the ruler to draw diagonal lines across the block from corner to corner. Where the lines cross is the center.
Drill the hole locations. Using the 7/64" drill bit, drill a hole at each location. The hole doesn't need to be more than about 1/2" deep. You can use a piece of tape around the drill bit as a depth marker.
Insert the pencils in the holes. Carefully twist the wooden end of the pencils in the holes. The erasers should stick up.
Add the balance point. To find the CG, we want as narrow a balance point as possible. One way to do this is to use a T-pin. Another way is to use pencil erasers.
Adjust them so that they are the same height, and that the top of the T-pin, or the edge of the eraser line up.
And that's it!
Locate the CG symbol on your plane. (If your plane is not marked with a CG symbol, the plans or instructions should indicate where it belongs. Mark the location on your plane.)
Decide whether your airplane is easier to balance right side up or upside down. Low wing wing planes usually balance more easily upside down (This is because the CG is above the wings on a high wing plane. When the plane is upside down the CG will be below where the plane is supported), while high-wing planes usually balance more easily right side up.
Place the plane on the balance points, near the CG. One balance point will be on either side of the fuselage.
Let go of the plane. If the plane wants to tip nose down, slide the plane back a little bit. If it wants to tip nose up, slide it forward a little bit.
Be sure that the balance points are at the same place on both sides of the airplane. From the top, the airplane's nose should be pointed straight down the middle of the block and not off to one side:
Check the balance. Look at the plane from the side. Move your eye until the edges of the balance points line up. If the balance point is lined up with the CG mark, that's all you need to do.
If the balance point doesn't line up with the CG:
Check the CG again, and adjust as necessary.
You're done!
A well balanced airplane is a good flying airplane. Making a balance gadget will reward you with many excellent flights. Enjoy!