RC Motor KV & Prop Calculator
Enter motor KV, battery cell count, and propeller dimensions to get theoretical max RPM, prop tip speed in m/s and Mach, and an overspeed risk rating. FPV, quad, and fixed-wing. Nothing uploaded.
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Tip speed above 0.65 Mach reduces efficiency and increases noise. Above 0.85 Mach prop integrity may be at risk. Loaded RPM is approximately 80% of no-load.
Learn more: Motor selection and prop matching
Choosing the right KV motor and prop combination
The key constraint is prop tip speed. Above 0.65 Mach (around 220 m/s), efficiency drops sharply and noise increases significantly. Larger props need lower KV motors to keep tip speed safe. A 5" prop on a 2300KV 4S motor sits around 0.55 Mach - acceptable. The same prop on a 2700KV 6S motor would exceed the tip speed limit.
Understanding prop tip speed and Mach numbers
Prop tip speed is the velocity of the propeller tip as it moves through the air, calculated from RPM and diameter. As tip speed approaches the speed of sound (Mach 1 = 343 m/s), compressibility effects cause a sudden increase in drag and turbulence. Efficiency drops dramatically above 0.65 Mach and the prop becomes increasingly noisy.
FAQ
What does motor KV mean?
Motor KV (kilovolts) is the RPM per volt of applied voltage at no load. A 2300KV motor on a 4S battery (14.8V) has a theoretical no-load maximum of 2300 x 14.8 = 34,040 RPM. Under load, typical actual RPM is 75-85% of this figure. Higher KV = faster spinning but less torque; lower KV = more torque but lower max RPM.
How do I choose the right KV motor and prop combination?
The key constraint is prop tip speed. Above 0.65 Mach (around 220 m/s for a standard atmosphere), efficiency drops sharply and noise increases significantly. Larger props need lower KV motors to keep tip speed safe. A 5" prop on a 2300KV 4S motor sits around 0.55 Mach - acceptable. The same prop on a 2700KV 6S motor would exceed the tip speed limit.
What is prop tip speed and why does it matter?
Prop tip speed is the velocity of the propeller tip as it moves through the air, calculated from RPM and diameter. As tip speed approaches the speed of sound (Mach 1 = 343 m/s), compressibility effects cause a sudden increase in drag and turbulence. Efficiency drops dramatically above 0.65 Mach and the prop becomes increasingly noisy.