An actual question my five year old asked me and I couldn’t answer, please help!
In: Physics
Propellers are matched to the engine output and that combo is matched to the aircraft.
A four-bladed prop of otherwise equal dimensions can impart twice the power of its two bladed version but it will be heavier and more expensive.
If you have fewer blades the setup will be lighter but you generally need to make the blades longer and some aircraft are too low to the ground to cope with that, so the designers will give them more blades at a reduced radius/chord length.
One video I remember and is kind of relevant (wind turbines) could maybe help you slightly.
I don’t know how to link on mobile but the video is on YouTube by “Real Engineering” and is called “why do wind turbines have three blades”. There’s also a second video with the same title on the channel called “DOB-Academy studio”. They give you some more info.
The videos should take you around 3 minutes to watch and they can explain the topic much better than me.
Next time you’re in a swimming pool try moving your hand through the water as fast as you can. Change the angle a bit and notice that you push more or less water around. Now try it again with your whole arm under water. You can push more water around but it also requires much more effort.
Propellor optimization (fans are just propellers) involves striking a balance between the amount of fluid you want to move at the energy used to move it. Increasing the surface area of the propellor will move more fluid but require more energy to operate. This can be achieved by increasing either the size of the blades, the number of blades, or both.
A house fan only needs to move relatively little air at high efficiency. A speedboat propeller is going to be optimized to move water. Many airplanes also have the ability to change the angle of their blades, taking a larger or smaller “bite” out of the air without changing the surface area exposed to the medium.
Here’s my best shot at an ELIF:
Generally, the more blades you have (and the larger they are) the slower and quieter the fan will be. It all has to do with resistance. You’ll notice that a lot of helicopters only have four really skinny blades; Super fast, and SUPER loud.
Very interesting question, kid!
Edited to say it wasn’t as hard as I thought to make it simple lol
Single blade propeller is the most efficient. And before you think that’s crazy or impossible, google it. It’s just not practical most of the time.
Some planes actually needed shorter blades, because of the ground clearance for taxing, takeoff, and landing, so you would see them with tri or quad blades.
Late to the party, but I thought I would add a sub-question: If I have a boat with a 100HP motor, what gets me the better upgrade — twin 100HP motors or a single 200HP motor?
Aero engineer here, albeit late in the thread.
While everything I’m reading is true for propeller design, it all boils down to mass flow rate. The true ELI5 answer is you can change the propeller’s overall size while pushing the same amount of air.
Each blade moves some air mass proportional to its surface area. Assume two long blades move about the same amount of air mass as four short blades half the size. Design constraints determine the rest which goes above the ELI5: max prop diameter to the ground, excess power required for performance, engine efficiency, blade tip speed, etc.
for anybody who lives in the NJ/NY area, Liberty Science Center has an awesome exhibit that demonstrates this exactly. You can set up different numbers and shapes of propellers and blow air at it. It’s real cool.
This is a complicated question! There are a few design considerations from the number of blades a fan will have:
**efficiency**: One blade spinning really fast is absolutely the most efficient way of moving air, as it doesn’t hit the disturbed of the blade in “front” of it, however monoprops are difficult to balance (although they do exist). Two blades are normally used on low power engines and fans that value fuel efficiency, however the next consideration is…
**blade loading**: The point of the blades is to “push” the air. If you are pushing a lot of air the blades may not be strong enough, so you can add more. This reduces efficiency, but allows you to move more air.
**rotational speed**: another way to move more air is to spin faster, however if the blades spin too fast the tip may go faster than the air can keep up with. This means you get shockwaves at the tip which makes the fan less efficient, and can damage the blades. You can make the blades shorter to avoid this, but then you would need to add more.
**Not quite ELI5**: The trade off between these factors is complex.
Big slow fans with few blades move air efficiently, but are hard to fit on the front of a plane. They are also harder to slow down and spin up. They also have a low exit velocity of air. If your plane is moving faster than the air can be pushed out the back this causes drag.
Small, fast fans are typically less efficient but can be put on the front of planes without hitting the ground. They can handle more power and are more controllable, and tend to produce less drag.
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