A few questions about helicopters

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Discussion Overview

The discussion revolves around the parameters affecting the thrust of helicopter rotors, specifically comparing a 2-blade rotor with an 8-foot diameter to a 4-blade rotor with a 4-foot diameter. The conversation also touches on the implications of rotor design on thrust, including considerations for tip-jet helicopters.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant inquires about the thrust dependency of helicopter rotors based on parameters such as blade number and diameter.
  • Another participant suggests that thrust in a hover is unlikely to be the same for the two rotors, noting that thrust is a function of angular speed squared, assuming constant thrust coefficient and area.
  • The second participant mentions potential aerodynamic effects from the preceding blades, such as tip vortices, which could influence thrust when increasing the number of blades.
  • There is a suggestion that adjustments to blade twist may be necessary to maintain similar lift distribution along the rotor length, particularly for shorter blades.
  • A participant expresses gratitude for the clarification and seeks additional resources for learning about helicopter flying theory.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the two rotors will produce the same thrust, indicating that the discussion remains unresolved with multiple considerations and factors at play.

Contextual Notes

The discussion includes assumptions about rotor design and performance that may not be universally applicable, such as the effects of blade twist and aerodynamic interactions. The inquiry into helicopter flying theory suggests a desire for deeper understanding, but specific limitations in the discussion are not fully explored.

Who May Find This Useful

Individuals interested in helicopter aerodynamics, rotor design, and those seeking foundational knowledge in helicopter flying theory may find this discussion beneficial.

MadSimon
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Hello all, I've got a few questions about helicopter rotors.

On which parameters does the thrust of an helicopter rotor depend? If i have: a 2-blade rotor, 8 feet of diameter. A 4-blade rotor, 4 feet of diameter. Do they have the same thrust? Or the second rotor will have less thrust because of the smaller radius(and less tangential speed)?
And in the case of a tip-jet helicopter? Even reducing the diameter, the tangential speed is about the same, right?

Sorry for the stupid questions and my bad english.

Thanks for any answer.
 
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I am going to assume that you are talking about lift in a hover.

Will they have the same thrust? In all likelyness no. In theory, with Ct and area constant, the thrust is going to be a function of the angular speed squared. So if the required adjustment can be made to the rotor tip speed, then in theory, you should be the same. HOWEVER, when going to 4 blades with the same chord length as the 2 blade, you will run into aerodynamic effects due to the preceeding blades in the form of tip vorticies whose strength is a function of tip speed IIRC. You would also have to look into adjusting the blade twist to attempt to keep the lift distribution along the length similar. A shorter blade will require less twist.

I guess the answer to this question really depends on how in depth you want to get.
 
Thanks for the answer FredGarvin, you cleared my doubts. I think i'll inform myself better on the argument. Does anyone know good links for helicopter flying theory?
 

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