Originally Posted by
paco
Some considerations:
Incorrect loading can also affect performance. The effects on power and range are mostly due to the angle of the fuselage, with some contribution from horizontal stabiliser download. Any range data in the flight manual is usually conservatively placed at the worst C of G position.
Aft C of G: The nose is up more, so the fuselage is pointed more squarely into the relative wind, for lower cruise drag. Also, the horizontal stabiliser is hardly working, its download is small, so the range is furthest. On the other hand, the main rotor must be flapped down to get the high speed thrust, with a risk of bending the main rotor shaft.
Neutral C of G: The nose is down about 3° further than it would be above. There is little change in performance, but the main rotors will not be down so much at the front.
Forward C of G: The nose is down, so the fuselage has its top exposed to the free stream, requiring higher thrust, and there is a download from the horizontal stabiliser that looks like weight to the main rotor. This is usually the slowest and the worst for range, and the least comfortable for passengers. However, it is generally most stable.
I do not have any source to back this up, but with the NH90(at least the high cabin version), I think theres is a clear gain with being close to the aft limit, perhaps about 5 knots at MCP or so or maybe slightly more. This on a clean A/C withouth a lot of fun stuff mounted external.
When having the cargo in front, which in most cases do not put the CG even close to the forward limit but around the middle we clearly loose indicated airspeed.
Same, A/C same load but repositioned to get to aft CG, and stopping the automatic transfer to stay at that aft CG, there is a clear speed gain.
I also tried this with the Superpuma but the gain was not that high, as I remember it. Perhaps possbile to detect but not much more.
The NH90 also fly with the bottom of the fuselage quite flat, and any forward CG increase the (rear) stabilisator downforce, which in turn cost both induced drag and use up more of the main rotor power.
Paco, You trained me for my ATPL (Thank You Sir, for that!) so I will send any aerodynamic questions direct to You Paco
