Estimate lift & power from rotor blade specifications

Hi,

Long time lurker, but first time poster.

I'm interested in designing a large scale tandem rotor and I've been attempting to estimate the power requirements for a given flying weight and airframe.

I've been using the following formula to estimate the thrust generated for a given rotor diameter and induced power.

T = 6400 * P^(2/3) * D^(2/3)

Where T = thrust in KG, P = Power in Watts, D = Diameter in Metres.

Unfortunately I cannot remember when I read about this estimation, but throwing some statistics at it form real world helicopter models it would seem to be plausible.

This formula is all fine but obviously it is unbounded, unlike the real world. A given rotor cannot generate infinite thrust for an infinite power - the real world has physical limits. So if I know the details of a rotor blade, how can I estimate the real world thrust?

For example, if I had a 3 blade rotor with 2,500mm diameter, 70mm chord, reflex S profile and a maximum head speed of 850rpm, how would I determine the max thrust? For a fixed head speed, thrust will be a function of the blade pitch, but at some point the blades shall stall and the thrust shall drop with increasing pitch.

I'm hoping that someone would be kind enough to work me through some formulae, or point me in the direction of a text with relevant information. Most of the text I have read so far are quite in depth maths, and formulae depend on parameters that I cannot obtain. Model rotor blades are rarely specified for anything more than length, chord and max rpm.

As a separate question, how does rotor overlap of a tandem rotor effect the thrust and power requirements? If one has a tandem rotor with two non-overlapping rotors one could assume that half the engine power goes to each rotor and the above formula would still hold true, but I think that this shall fall down when a portion of the thrust is generated from two overlapping rotors.

Thank you.

-Tim

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