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Pilot Tools

Center of Gravity Calculator

Weigh your plane at the wheels, enter the distances from the spinner, and see where your CG actually sits versus the target on your plans. Supports inches/mm, ounces/grams, and fractions like 3 1/4 .

Use at your own risk

This tool is provided as a convenience only. The Lake Sawyer Hawks RC Club is not liable or responsible for the results it produces or for any damage or injury resulting from its use. Always physically verify your aircraft's balance against the manufacturer's or plan's specifications before flight.

Calculator

A = spinner  ·  B = main wheels  ·  D = tail wheel  ·  C = target CG  ·  ● = your CG

Fractions welcome in any field: 3 1/4, 5/8, or plain decimals.

Enter your measurements to see results.

Log in to save your aircraft for next time. Your entries are remembered on this device either way.

Weighing & Measuring Guide

Equipment

  • Digital scale(s) — one is sufficient (see Method 3 below); two or three make the job faster. Kitchen or postal scales suit most sport models; a bathroom scale handles giant scale. Example scales
  • Rigid board or straightedge — spans both main wheels across a single scale (Method 2).
  • Support blocks — matched to the height of the scale platform, used to substitute for a second scale and to hold the aircraft in level flight attitude.
  • Tape measure or ruler — for the A → B, A → D, and A → C distances.
  • Manufacturer's plans or manual — for the recommended CG location and acceptable range.

Weighing setups

Any of the setups below produces the same result. Whichever you use, keep the fuselage reference line level (dashed line) for every reading — raise the lower scale or block until it is.

B D
Method 1 — Two scales. One scale under the main wheels (B), a second under the tail or nose wheel (D). Raise the lower scale on a block until the fuselage is level, then record both readings.
board (tare) B
Method 2 — Single scale under the main gear. A rigid board spans both main wheels onto one scale, giving the full main-gear weight (B) in one reading. Zero (tare) the scale with the board in place, or subtract the board's weight.
B
Method 3, Step 1 — One scale, main wheels first. Scale under the mains (B); a block of exactly the same height as the scale platform supports the tail. Record the main-wheel reading.
D
Method 3, Step 2 — Swap and weigh the tail. Move the block under the mains and the scale (on its riser) under the tail wheel (D). The matched heights keep the attitude identical between the two readings.

Procedure

  1. Prepare the aircraft in ready-to-fly condition. Flight battery installed in its normal position; hatches, cowl, and canopy in place.
  2. Level the aircraft in flight attitude. Raise a taildragger's tail until the fuselage reference line is level and support it there — an aircraft weighed sitting tail-low will not balance correctly. Verify with a small spirit level.
  3. Weigh at the wheels. Record the main-wheel weight (B) and the tail- or nose-wheel weight (D) using one of the setups shown above, keeping the aircraft level for every reading.
  4. Measure the horizontal distances from the spinner (A). A → B to the main-wheel contact point and A → D to the other wheel's contact point, measured horizontally along the fuselage — never diagonally down to the ground.
  5. Determine the target CG (C) from your plans. If the plans reference the wing's leading edge, convert it: A → C = spinner-to-leading-edge distance + specified distance behind the leading edge.
  6. Enter the values and review the result. The red dot marks your measured CG, the orange dashed line the target, and the green band your tolerance. Adjust battery position or ballast, re-weigh, and repeat until the dot sits within the band — then confirm the balance physically before flight.

The calculation is the standard moment-balance equation: the CG is the weight-averaged position of the two support points. The tolerance band defaults to ±1/4 inch — adjust it to match the CG range on your plans.