11–20 of 104 posts
0: https://news.ycombinator.com/item?id=33949895
1: https://www.mby.com/gear/sharrow-mx-1-tipless-propeller-1101...
I'd imagine that, just like in other industries, there are tons of metrics (or counter-metrics) that are also important. Some that come to mind are efficiency, noise, vibration, cost, maintenance, stall characteristics, RPM requirements, weight, etc etc. From some fiddling with RC quads, I remember that increasing the number of blades does increase thrust, but at the cost of efficiency and several other parameters.
Not that there isn't some improvements that can be made like have been done with improved materials(less weight), computer design and so on in more complex engines.
Somewhat related thread [0] from last month about the "tipless" Sharrow MX-1 prop [1] which achieves efficiency gains of between 9-15% over comparable 3-blade propeller designs. They started out working on an aircraft propeller but pivoted to a watercraft prop. 0: https://news.ycombinator.com/item?id=33949895 1: https://www.mby.com/gear/sharrow-mx-1-tipless-propeller-1101...
Why are they only using odd numbers of blades?
I know that in wind turbines they are favored because it reduces the effect of the blade crossing the tower. When one of the blades in a three blade wind turbine crosses over the tower the force on it is reduced (since the wind is already slowed by the presence of the tower). The other two blades are in maximum loading, but balance much of their forces against each other reducing the bending moment on the turbine shaft.
A 777 engine has 22 fan blades, twice this thing.
At what point does a prop become a fan? A 777 engine has 22 fan blades, twice this thing.
I don't know, because I think the whole purpose of a prop is to be a fan.
Why are they only using odd numbers of blades?
Easier to balance; if everything is even, you need to balance two blades to counter one misbehaviour, and do so perfectly to avoid introducing new misbehaviours. (They’re prime for resonance reasons. I don’t remember what, specifically.)
The hand-waving about resonance also pervades that reddit "explanation" thread which doesn't explain anything about what prime or odd numbers of blades have to do with resonance in a single propeller.