1 – Calculate the Maximum Speed at the Blade Tip

Use the following formula: Blade tip speed (in m/s) = propeller radius (m) × 2 × π × engine speed (RPS)

With our values, we get: 0.45 × 2 × π × 60 = 170 m/s

Since this is well below the critical limit of 290 m/s, the configuration is considered safe. But for a more precise result, you can also factor in the vehicle’s maximum forward speed.

2 – Factor in the Vehicle’s Maximum Speed

At full speed, the forward motion of the vehicle slightly increases the effective speed at the blade tip. To account for this, use the following formula:

(Actual blade tip speed in m/s)² = (Blade speed from rotation in m/s)² + (Vehicle speed in m/s)²

Using our values: Actual blade tip speed² = 170² + 13.9² = 28,900 + 194 = 29,094

Then take the square root: Actual blade tip speed = √29,094 ≈ 170.6 m/s

This remains well within safe limits.

3 – Take Into Account the Use of a Gear Reducer

Let’s consider another example with different specifications, which initially result in an unsafe configuration:

Propeller diameter: 1 meter (so the radius is 0.5 meters) and engine running at 6000 RPM (which equals 100 RPS)

We’ll use this formula: Blade tip speed (m/s) = propeller radius (m) × 2 × π × (engine RPS ÷ reducer ratio)

Without any reduction: 0.5 × 2 × π × 100 = 314 m/s

This exceeds the safe limit of 290 m/s, so we need to use a reducer to bring the speed down.

Let’s include a 1.1:1 reducer in the calculation. This means the propeller completes one rotation for every 1.1 engine shaft rotations—slightly reducing the final speed:

0.5 × 2 × π × (100 ÷ 1.1) ≈ 285 m/s

This brings the configuration back into a safe range.