3 ms·

Kind of related: Do the same efficiency dynamics that apply to wind turbines apply to a fan providing propulsion? I.e. if a one-bladed fan is the most efficien

Kind of related: Do the same efficiency dynamics that apply to wind turbines apply to a fan providing propulsion? I.e. if a one-bladed fan is the most efficient for capturing wind, would it also be the most efficient airplane propeller or boat propeller? Obviously efficiency isn't the only consideration, but I am curious and I expect there's going to be some fan experts in this thread...

colechristensen 4y ago
With aircraft you can’t just say “efficient” you have to point out what you’re trying to optimize.
- carabiner 4y ago
  In rotor design it's pretty clear. Thrust/power ratio here, or FM (Figure of Merit).
  - colechristensen 4y ago
    It’s not at all. How much does it weigh? How does it perform at different speeds? Thrust to weight ratio is very important to aircraft.
TylerE 4y ago
Due to much higher rpm, balance is imporant, and your diameter is constrained (you want the propellor as small as possible because you want to A: keep the whole thing underwater and B: Not bang the tips on the bottom.
Hendrikto 4y ago
Seems like 1 blade would theoretically be optimal, but due to requiring a counterweight, a two blade design is superior. [0] [0]: https://aviation.stackexchange.com/questions/23009/what-are-the-advantages-of-more-than-4-propeller-blades https://aviation.stackexchange.com/questions/23009/what-are-...
carabiner 4y ago
Short answer is yes. But you have to compensate by making the rotor blades longer or spinning them faster to get the same thrust. You can't let the net velocity (vector sum of tip velocity + aircraft velocity) reach supersonic or bad things happen. So you have practical considerations that lead to tradeoffs that determine the number of blades. The R22 and Cabri G2 are helicopters with basically the same mission, but one uses a 2-blade rotor, the other uses 3 blades. Different engineering choices, style, preferences.

colechristensen 4y ago

With aircraft you can’t just say “efficient” you have to point out what you’re trying to optimize.

carabiner 4y ago
In rotor design it's pretty clear. Thrust/power ratio here, or FM (Figure of Merit).
- colechristensen 4y ago
  It’s not at all. How much does it weigh? How does it perform at different speeds? Thrust to weight ratio is very important to aircraft.

TylerE 4y ago

Due to much higher rpm, balance is imporant, and your diameter is constrained (you want the propellor as small as possible because you want to A: keep the whole thing underwater and B: Not bang the tips on the bottom.

Hendrikto 4y ago

Seems like 1 blade would theoretically be optimal, but due to requiring a counterweight, a two blade design is superior. [0] [0]: https://aviation.stackexchange.com/questions/23009/what-are-the-advantages-of-more-than-4-propeller-blades https://aviation.stackexchange.com/questions/23009/what-are-...

carabiner 4y ago

Short answer is yes. But you have to compensate by making the rotor blades longer or spinning them faster to get the same thrust. You can't let the net velocity (vector sum of tip velocity + aircraft velocity) reach supersonic or bad things happen. So you have practical considerations that lead to tradeoffs that determine the number of blades. The R22 and Cabri G2 are helicopters with basically the same mission, but one uses a 2-blade rotor, the other uses 3 blades. Different engineering choices, style, preferences.