An actual question my five year old asked me and I couldn’t answer, please help!
In: Physics
It’s always a balance.
Number of blades:
* Fewer blades = More efficient shoveling of air because of turbulence (air swirls) created by other blades reduces efficiency. Usually simpler to make.
* More blades = More stable because the force is spread out over more blades and shovels more air compared to how long the propellers are.
Propeller tips breaking sound barrier is bad (because of lots of turbulence). The longer the propeller the faster the tips go compared to the center. But having too short blades means more loss of energy at the blade tips:
* Longer blades = Better at generating lift, shoveling more air at lower speeds. Longer propellers also less stable and vibrate more.
* Smaller blades = Allows higher top speeds since the propeller can go much faster without breaking the soundbarrier with the wingtips. More stable.
So basicly.WW1 airplane: We can’t make so good engines. So we’re gonna go with efficient short two-bladed propellers because that gives is the most thrust for our weak engines.
WW2 airplane: We gots a lot better engines now. But two-bladed propellers can’t shovel enough air to take our planes as fast as we’re going to go. So we’re going to go with 4 short blades!
Helicopter: We gotta generate lots of lift. So we’re going to go with longer and slower rotating blades!
Modern helicopter: Uh. Those blades aren’t generating enough lift. MORE BLADES! More blades is harder to make, but more stable too.
Modern turboprop: Too noisy! We’re making special 6 bladed propellers that are much quieter. And computer power and advanced materials allows us to make them special advanced shapes that generate even less noise and more power. So now they look more like ship propellers. But for air! Still kinda short blades because we gotta go fast!
Ship propellers: Water dense yo. So we gotta make blades short (or they’ll break!) but we make them much wider to shovel a lot of water backwards.
P.S: Jet engines work entirely differently, even if they do have fans at the front they’re for compressing air into the engine, not generating thrust.
P.P.S: For ceiling fan. You’re moving lots of air, but you want to do it slowly and silently. So lots of wide blades. How long the blades are depends on how mobile you want the fan to be. Big fan = more air silently. Small fan = Noisier, but more mobile.
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