Introduction:

A balanced R/C airplane responds predictably to control inputs, flies straight and level, and is easier to control. One effective way to ensure your R/C airplane is balanced correctly is by using three scales, a method that provides precise measurements of the airplane’s weight distribution. Included in this post will be a spreadsheet that you can download that will do most of the math for you.

Materials Needed:

  • Three identical scales
  • A level working surface
  • Your R/C airplane
  • Calculator (or the spreadsheet)

Step 1: Preparing Your Airplane:

Before you begin the balancing process, ensure your R/C airplane is ready for flight, with all components installed, including the battery, motor, and other electronics. Also, ensure that the fuel tank is empty if your airplane is fuel-powered.

Step 2: Measurements:

You will need two specific measurements done as accurately as possible:

  • Distance between center point of main wheels to nose or tail wheel(s)
    • The way I do this is I put the airplane on a flat surface. I draw a straight line from the center of the main gear wheel hub to the other main gear wheel hub. I then measure the distance from the center of that line (which should be the center point between the main gear) to the center of the nose gear wheel hub
  • Distance of desired CG location from main wheels
    • Since most manuals will give you a desired CG location from the leading edge of the root of the wing, I take this approach. I take a string and a plumbob at the root of the wing at the front and let it point to a spot on the ground. I mark that spot with a marker. I then measure from that spot (towards the line we previously made between the main wheels) to the airplane’s manual’s recommended CG location. I now can measure the distance between the main wheels to our desired CG location accurately.

Step 2: Positioning Your Airplane and Scales:

Place the three scales on a level surface. Position one scale under the nose of the airplane and the other two scales under each wingtip. Ensure the airplane sits level on the scales.

Step 3: Recording Initial Readings:

Record the weight readings from each scale. These readings represent the weight distribution of your airplane along its longitudinal (nose to tail) and lateral (wingtip to wingtip) axes.

Step 4: Calculating the Center of Gravity (CG):

The Center of Gravity (CG) is the point where the airplane balances fore and aft. It is crucial for stable flight. Use this attached spreadsheet do do all this for you.

If you are interested in the math, the formula for calculating the CG is as follows:

  • CG_x is the position of the center of gravity along the airplane’s longitudinal axis,
  • W_nose, W_left, and W_right are the weight readings from the scales under the nose and wings respectively,
  • d_nose, d_left, and d_right are the distances from a reference point (usually the airplane’s nose) to each scale,
  • W_total is the total weight of the airplane, which is the sum of W_nose, W_left, and W_right.

Step 5: Adjusting Weight Distribution:

If the calculated CG does not align with the manufacturer’s recommended CG, adjust the weight distribution by moving components or adding ballast. Recheck the weight on each scale, recalculate the CG, and continue adjusting until the CG aligns with the recommendations.

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