Originally Posted by
tottigol
Nailed it!
He has not really "Nailed" it. the "Unless" statement is still hogwash.
To have the CG in front of the nose of the helicopter, you would need a MRB sticking over nose, the portion beyond the nose, weighing more than the entire helicopter behind it.
To keep this sensible and simple, The Helicopter CG should be directly under the Main Rotor Mast centroid both Longitudinally and Laterally. That's the optimum ideal position giving 100% MR flight control in all directions.
However, practically speaking, there has to be some loading of the helicopter which would cause the CG to move around the rotor centroid. It cannot move that much. Even in a large helicopter like an EC225 it can only move 0.5m longitudinally.
There are limits to how far the CG can move away from the rotor centroid before effective flight control is compromised.
If the TRGBX and its blades depart the airframe, the CG will make a significant move forward. It is highly likely in ALL helicopters that this will result in a significant compromise of the forward CG limit and effective aft cyclic to counter the forward movement of the CG will reach the aft stop before the pitch moment forward can be cancelled. The results.....well not pretty.
The Datum, as someone has already posted, is simply a point in space to facilitate all calculations to be in the positive range. Usually it is quite a way out in front of the helicopter nose to facilitate the fitment of long PITOT probes during certification. For simplicity, it remains there. Therefore, the limits for longitudinal CG are expressed in positive numbers and to keep the theme at the EC225 4.4m to 4.9m. This actually means, behind the datum. Not in front of the rotor centroid.
I know 99% of Rotor heads know this but to clear up the incessant garbage postings by the odd individual who seems confused....enjoy!