I have noticed propeller aircraft with propellers having from two blades to as many as five. Why different numbers of blades?

About the only reasons that makes sense to me might be that some blade numbers create more screw action for greater speed or acceleration - perhaps for fighter aircraft. Some blade numbers create significantly smoother rides - probably a silly reason?? And some create significantly better fuel efficiency.

If any of these supposition reasons is accurate, why?

quote:

Originally posted by Naphtali:
I have noticed propeller aircraft with propellers having from two blades to as many as five. Why different numbers of blades?

About the only reasons that makes sense to me might be that some blade numbers create more screw action for greater speed or acceleration - perhaps for fighter aircraft. Some blade numbers create significantly smoother rides - probably a silly reason?? And some create significantly better fuel efficiency.

If any of these supposition reasons is accurate, why?

The basics:

More blades equals smaller diameter for given area equals improved ground clearance.

More blades quieter than fewer blades.

More area equals more power absorption equals more thrust.

Increasing diameter increases tip speed which decreases efficiency. Slowing a big prop requires a bigger gearbox which equals more weight. Pi*D*RPM will get you tip speed. Must stay way under 700 mph (approximately transonic)

There is free software on the net you can download to experiment.

Don't forget the single bladed propellers as well:


As with a lot of other things in life there is danger in over-simplification to explain a concept as invariably it gets misunderstood by someone but here are the 2 main issues-

-> Propellers work "best" when flowing in undisturbed air.

-> The closer to the speed of sound, the worse they work


So much of propeller design is big juggling act with having enough propeller surface area to transfer the horsepower while keeping the tip speed below supersonic, as fishman points out.


If you are interested in aviation history of various aircraft, here is a story that I find somewhat interesting-

I had a friend who was a P47/P51 pilot in the 353rd Fighter Group. One of the things the 353rd FG was known for was being deeply involved in the development of ground attack techniques from the beginning. The CO, Glenn Duncan was also a big proponent of attacking the Luftwaffe on the ground at their airfields.

If anyone does research about the P47, one of the changes that significantly improved performance was a switch in propellers. So here is my story...

I had emailed my friend this pic once when the topic of robustness in the P47 came up:



This was a plane that hit the ground in Italy while doing a strafing run, and then made it back 130 miles to its home base.

I received a comment back that one time Col Duncan had also hit the ground during a strafing run, and while flying back home he was experimenting with his plane and discovered that performance was actually better. After he got back to base he had his mechanics cut the bent ends off the prop and he flew it again, verifying that performance was indeed better with shorter blades. I have always been curious if this was what precipitated research into the later version "paddle blade" propellers on the P47.

That Thunderbolt is one tough airplane they were amazing how much damage they could take.
My brother-in-law flew F100 in nam and came back more than once with bamboo stuck in the undercarrage he flew close air support for the grunts

Had not heard of a single bladed prop!