# How does one choose the number of blades and the diameter required for a turboprop propeller?

I am interested in the preliminary design of a regional aircraft that uses turboprops as the propulsion method.

Following considerable research online about propeller design (disk theory, BEM...) I am rather lost on where to start.

To put it short, what parameters must be considered when designing the propeller of a turboprop device?

I am mainly interested in obtaining an idea of how many blades would be required and the diameter of the prop.

I have obtained values for the engine power requirements, and from what I can gather the main reason to have an increased number of blades is due to the requirement of absorbing the power from the engine. The latter therefore implicating an increased solidity ratio.

Is the any standard formula/theory that can provide such answers?

Thanks,

FZ

The basic rules are:

• Make the diameter as large as practical.
• Now you will most likely run into problems with supersonic tips. Reduce RPM and diameter until the tips are below Mach 1.
• Now you will most likely not be able to absorb the maximum power of the engine. Increase blade count and chord until you do.

In the end, there is no simple formula because you need to find a compromise between conflicting demands. Increasing the number of blades will make the hub more complex and reduce propeller efficiency. Increasing chord will increase centrifugal loads and friction drag.

Try to find existing designs with the same disk loading. Your propeller should look very similar - existing designs went through the same design process.

The prop diameter is chosen to provide suitable ground clearance for the desired installation. Then the reduction gearbox ratio is chosen to keep the prop tips subsonic. Then the number of prop blades is chosen to absorb full engine power. Compromises then get made to take advantage of off-the-shelf props in the market and standard gear ratios offered by the engine manufacturer.

• Thank you for your answer. Is there any theory that you know of that determines the number of required blades to absorb the full engine power? Commented Dec 16, 2020 at 8:42