Chopa is definately right - I have known this to be true for years, and we demonstrate it to students during the course with us. I also have not definately understood why.
My understanding is at a basic level. I believe that if you were to 'suspend' 2 identical helicopters at a height (but one heavier than the other), then drop them - the following would happen...
They would initially fall at the same rate. As drag increases, the heavier one would be slowed less by it, and develop a HIGHER Rod. This would cause the Nr to rise more than the light heli, and therefore cause the pilot to raise the lever more to contain it.
This means, for the same Nr, the heavy acft has a greater pitch angle. This puts the blades in a more efficient area, and reduces the RoD - so that it is less than the light helicopter.
Is this correct? - I always thought it was. In a Cobra, the RoD difference between min fuel and max fuel, at 70 kts in Auto, and recommended Nr, is almost 500fpm.
Like chopa, I would love to know why (if it isn't why I said)
Tourist - Chopa is right - sorry. I don't know why either!