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So you say the one's in Avatar could not?
Really like the concept.
On the subject of the fans, using the lift equation as a guide:
Lift = Cl * 1/2rho * S * V^2
where Cl is the coefficient of lift ( a dimensionless number derived from wind tunnel testing)
rho is air density in kilograms/m3
S is surface area in square meters
V is velocity in m/s
So long as the blades are of sufficient surface area and moving at an appropriate velocity and optimized for generating the required lift, the fans in Avatar are more than reasonable. The biggest thing is the design of the blade. All non-symmetrical blades produce some form of lift at flat pitch, depending on the amount of camber, that balde could be producing a lot of lift prior to pulling pitch. To lift my Apache, at 18600-ish pounds, requires a 48ft diameter rotor system spinning at roughly 292rpm. Now, this puts us up there on the torque required, but thats all that's required to lift that sucker and give me a pretty decent rate of climb. So, wow, derailed myself...anyway, i'd say it's not so much the size of the fans that would be the deciding factor, it'd be the overall surface area of the blade available, its operating speed, and the overall coefficient of lift of the blade (or the design of the blade). Blade element theory sucks, and I won't pretend to say i've actually worked that kind of math, but...it's more than reasonable, and considering so much of the stuff in Avatar is explained away by the fact that pandora is a 'low gravity world' its definatlye within all possibility to be pretty accurate.
BradMick
And as a side note: keep in mind that for something like a prop or rotor system to work properly, the tips of the blades can never reach more than the speed of sound. If they begin to break the sound barrier, they start creating supersonic shockwaves behind them, which interferes with the blade behind that, and so on. This creates an instability in the prop and all kinds of bad things start happening. For one thing, you can rip the blades right off of the hub, or you could possibly overpower in one direction and start groundlooping. It's a hairy situation.
Anyway, back on topic: there's no reason not to have a tailrotor on the fan version. It could help in stability. It isn't strictly necessary, but it also doesn't hurt to have it.