A few questions about helicopters

[MadSimon]

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.

 

[FredGarvin]

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.

 

[MadSimon]

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?

 

[FredGarvin]

http://www.copters.com/helo_aero.html
http://www.helicopterpage.com/
http://www.cavalrypilot.com/aerodynamics.html