Electric Propeller Size Calculator
Enter the motor rating, battery cell count and motor Kv. The calculator gives a sensible starting propeller plus lighter-load and higher-pitch alternatives.
How the estimate is made
The calculator estimates loaded motor RPM from Kv and battery voltage, then compares common diameter/pitch combinations using the traditional propeller power relationship of diameter, pitch and RPM. Aircraft type shifts the preferred pitch-to-diameter ratio. It is intended to get you close enough to choose a sensible first prop, not to replace a wattmeter.
TMFA TOOLS
Centre of Gravity Calculator
Calculate the Mean Aerodynamic Chord (MAC) and a practical starting CG position for your RC model.
Your wing and calculated MAC
The diagram updates as you enter your measurements. The green line shows the calculated Mean Aerodynamic Chord (MAC); the red marker shows the selected CG position.
What is the MAC?
MAC means Mean Aerodynamic Chord. It is a calculated chord length that represents the aerodynamic average of the whole wing. On a tapered or swept wing, it is not simply the average of the root and tip chord.
The calculator finds both the length of the MAC and where its leading edge falls along the wing. Your chosen CG percentage is then measured back from that MAC leading edge. Negative sweep (forward sweep) is supported; the MAC leading edge can therefore lie forward of the wing-root leading edge.
How to measure your wing
- Root chord: measure from the leading edge to the trailing edge at the wing root, alongside the fuselage.
- Tip chord: measure from the leading edge to the trailing edge at the wing tip.
- Half span: measure from the aircraft centreline to the wing tip.
- Leading-edge sweep: enter the leading-edge sweep angle. Use a positive angle for sweep-back, a negative angle for forward sweep, and 0° for a straight leading edge.
Tip: You do not need to calculate the MAC yourself. Enter the four wing measurements and the calculator does the geometry for you.
1Wing measurements
Units
2CG setting
CG position as a percentage of MAC
Starting point: 25% MAC is a commonly used initial balance point for many conventional RC sport and trainer models. Where the manufacturer gives a CG position, use that value instead.
3Results
Mean Aerodynamic Chord (MAC)—
MAC leading edge
from root leading edge—
Centre of gravity
from root leading edge—
CG as % of MAC—
Why choose a particular CG percentage?
A more forward CG generally gives a model more pitch stability and a more forgiving feel, but can require more elevator input. Moving the CG aft usually makes the model more responsive in pitch, but reduces the margin of stability. Aft is not automatically better.
Values such as 25%, 28%, 30% and 33% MAC are useful starting points for comparison, not universal answers. If the manufacturer gives a CG position, that figure should take precedence.
Aircraft characteristics
Tail moment arm is measured from approximately the wing aerodynamic centre to the horizontal tail aerodynamic centre.
Calculated aircraft characteristics
Wing area—
Wing aspect ratio—
Tail volume ratio—
Wing loading—
Estimated stability
Preliminary guide only
Forward / more stableAft / less stable
Selected CG Estimated neutral point
Estimated neutral point—
Estimated static margin—
Enter the horizontal tail area and tail moment arm above to estimate the neutral point and static margin.
What the numbers mean: tail volume describes how strongly the tail can influence pitch stability; aspect ratio affects the wing's lift characteristics; wing loading is mainly a performance indicator rather than a CG-setting calculation. The neutral-point estimate uses simplified assumptions and should be treated as an educational guide, not a flight-safety guarantee.
How to use the result
Use the Centre of gravity figure in the Results box as your measurement from the wing root leading edge. For swept or forward-swept wings, the calculator first locates the MAC leading edge and then measures the chosen CG percentage along the MAC. A negative result for the MAC leading-edge offset means the MAC leading edge lies forward of the root leading edge.
The diagram is there to help you see where the calculated MAC and CG sit on the wing. For a real model, transfer the measurement carefully to both sides of the wing or use the manufacturer's stated balance point.
If your model has a published CG position, that manufacturer's figure should take precedence over this calculator.
ImportantThis is a geometry-based starting point, not a flight-safety guarantee. Always check the model manufacturer's instructions and perform a proper balance and test flight.
