What Are the Key Differences Between Propellers and Rotors in Aircraft Design?

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Discussion Overview

The discussion revolves around the differences between propellers and rotors in aircraft design, focusing on their appearance, functionality, and design characteristics. Participants explore the implications of these differences in terms of aerodynamics and mechanical design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants note that propellers and rotors are fundamentally different, primarily in their operational contexts and design purposes.
  • One participant suggests that the shape of rotor blades may be influenced by centrifugal force, raising questions about their design compared to propellers.
  • Another participant discusses the droop of rotor blades while stationary, indicating that their large aspect ratios necessitate different design considerations compared to propellers.
  • Concerns are raised about the lack of twist in rotor blades compared to propellers, with some attributing this to the requirements of collective and cyclic control in helicopters.
  • A participant mentions that while there is some twist in helicopter blades, it is less pronounced than in propellers, citing specific examples like the Chinook's negative twist.
  • There is a suggestion that the difference in RPM between propellers and rotor blades may contribute to the observed differences in design.

Areas of Agreement / Disagreement

Participants express varying views on the design characteristics of propellers and rotors, with no consensus reached on the implications of these differences. The discussion remains open-ended with multiple perspectives presented.

Contextual Notes

Participants reference specific design features such as aspect ratio, twist, and control mechanisms, but do not fully resolve the implications of these features on performance or efficiency.

mtworkowski@o
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I was thinking about the difference between the appearance of a propeller on a plane and a rotor on a helicopter. What is the consensus?
 
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Two different things that live on two separate worlds. They are only similar in appearance and the fact that they move air. What exactly is it you are wondering about?
 
I'm thinking these things look different because they're doing different things. I know a propeller is moving air horizontally, but is a copter blade shaped that way because centrifugal force is holding it out?
 
If you are referring to their droop while stationary, then you are correct. Because they have such large aspect ratios, to make a rotor blade stiff like a prop you would have to make it extremely thick ad heavy. It would be completely impractical.
 
I'm also not seeing the twist that I see on a prop. I know that collective/cyclic controll needs that compromise but is that the reason the twist is not there?
 
mtworkowski@o said:
I'm also not seeing the twist that I see on a prop. I know that collective/cyclic controll needs that compromise but is that the reason the twist is not there?

HAH! Good observation. I never noticed that until your post!

Its probably because the change in speed due to RPM is much lower than a prop from center to blade tip. -but that's just a guess.
 
I'll buy that!
 
mtworkowski@o said:
I'm also not seeing the twist that I see on a prop. I know that collective/cyclic controll needs that compromise but is that the reason the twist is not there?
There is a slight twist in helicopter blades, but not a huge twist like in a prop. For example, my beloved Chinook has a negative 12° twist towards the tips. The twist is there to accommodate the large length of the rotor and the resulting change in lift as you go from the hub to the tip. A prop is less like a real wing than a rotor blade. A prop, while can be feathered and adjusted, is less adjustable than a rotor blade which is why there is a huge twist in them, to get the max thrust they can get in their design condition. That is why a helicopter is a rotary-wing aircraft and not a prop driven aircraft.
 
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