Rotor blade size calculations

[bobo67]

Main Question or Discussion Point

What calculation is the to find the amount a helicopter (or flying device using rotor blades) can lift? I would of thought I would need to take in to consideration the RPM of the blades, the size of the blades and maybe the angle. I was hoping there would be some calculation telling me how much it can lift.
Thanks

 

[jmatejka]

I would think the first consideration of "can lift calculations" would be engine horsepower or equivalent units.

Once you know what the engine puts out, you can figure how to transfer it to thrust. Almost all helecopters have variable pitch blades, for lift you could say the collective pitch verses cyclic.(think of the variable pitch blades as your adjustable transmission, (too much pitch too much work trying to be done), and the engine bogs down and strays from optimal power output.

Most turbine helecopters set the engine to a "set" optimal RPM. From this set RPM, you adjust the pitch of the blades.

Loooonnngg story short, find the engine horsepower and calculate lift from that, (after you subtract the weight of the helecopter). Because of induced drag losses on the rotor blades, etc,etc,etc,etc, I would guess less than 75% of what the engine(s) put out is available for lift.

 

[FredGarvin]

There is no straightforward calculation because the real world numbers take into account a lot more factors than just the engine max power and the blade lift. There are a lot of other limiting factors like structural limits on the fuselage and the blades/rotor hub them selves which may limit the max lift available. There's also the issue of whether in a hover or in forward flight, etc...

Not to mention that all things change at different elevations and temperatures and humidity. That is why a very large section of most aircraft operator's manuals have charts on allowable performance at certain conditions. If you're looking to look at a real aircraft, look in it's operator's manual. Any number you calculate won't be in the ball park.

 

[bobo67]

Thanks very much for your help. It made it a lot clearer.
Thanks