Unducted fan propellers reach peak efficiency at about 85%. What happens to efficiency if the operation pushes tip speeds higher beyond that point?

• A. Efficiency continues to increase
• B. Efficiency remains at peak value
• C. Efficiency decreases
• D. Efficiency becomes unpredictable

Answer: (C)

Open-rotor efficiency isn’t simply higher with faster tip speeds. It climbs as tip speed increases up to a peak around 0.85 Mach, where more of the input power goes into useful thrust. Beyond that point, compressibility and wave effects become dominant: shock waves form near the blade tips, drag and energy losses rise, and flow can separate. These losses waste energy that used to contribute to thrust, so the thrust gained per unit of input power doesn’t keep up. The result is a drop in overall efficiency as tip speeds push past the peak. That’s why pushing tip speeds higher beyond about 85% of the speed of sound leads to lower efficiency.