minibirdpilot,
I haven't read the article to know what it said, but I do know that nothing has changed on the S-76 models to make the noise different. The S76A had variable RPM, and it worked well, but when the MGW rose as power was added to later models, the rpm was kept at the upper boundary to keep the speed up and reduce the blade stall factor.
This thread is interesting, but shows the flaws of the socratic method, where a question and answer session leads to debates that try to find out if witches are made of wood...(apologies to Monty Python!)
Some funny things Adam has said (this is not criticizing you, Adamfrish, just pointing out how much strange belief exists in the land of the regular people):
Has anyone experimented with reducing rotor RPM at altitude? I assume this could safely be done in any helicopter given enough height. It seems quite logical to do so, yet that's not part of the way heli's are operated
Yes, and it really makes the helo stall and go out of control, just like we knew it would. However, this didn't seem to be what the customer wanted, so we didn't deliver it that way.
Lift is a product of disc loading.
This proved to be a mystery, since we thought the blades lifted the helicopter, and when we removed the blades, the disk performed badly.
helicopters stress out components and are expensive to use simply because they're flown at the limit all the time
We had thought they were expensive because of all those pesky machined parts that are flight critical, so we were working on that, but now we find that if we just gave the parts valium everything would be easier.
Adam, it seems that most of the beliefs in your worldview about helicopters comes from a hodge-podge of press releases and TV shows, and a strong belief that a few quick phrases explain how a rotor works. I strongly suggest that you drop out of the Hummingbird press release school of engineering and just buckle down with a good Prouty or Padfield or Stepneiwski for a while, because most of the starting places for your concepts are not only wrong, they are dead wrong.
Here are some rotor basics:
The blades do the lifting, and they want speed. This means that you must mix blade area (number times width times length) and RPM (tip speed) properly so that there is enough lift to make the helo go.
The best blade design is not one design point, because hover and high speed cruise conflict. The hover where the blade should work a bunch, at maybe 3/4 of its maximum lift so that the power it eats is smaller for the lift it produces. High speed where the blade should be working less, where you would like the balde to be at 1/3 of its max lift. This paradox makes a variable rpm attractive, where you slow down the rotor RPM at hover, and speed RPM up at high speed cruise flight.
The rotor disk determines the size of the air package the blades move, and the bigger the disk, the less power the rotor needs in a hover. This means big, slow rotors are better for hover efficiency, which is why Harriers and tiltrotors eat so much power. It is also why older piston helos had big rotors (the 8300 lb piston H-19 had the same rotor diameter as the 21,000 lb turbine Black Hawk!) Turbines throw power away, because the weight of a somewhat bigger turbine engine isn't that much, realtive to the weight of a bigger rotor.
Areodynamics excites us, but weight, structures and dynamics is what designs a flying machine. The "best" rotor and blade is seldom chosen, but the lightest, strongest one almost always wins. It is like going to the movies, we watch Scarlett Johanssen, but we bring the girl next door. Mundane aspects like weight, cost to build, vibration suppression end up dominating the designer's life. Variable rpm sounds cool, until you try to match the vast band of rotor frequencies (which tickle the whole aircraft) to the structure, and find that you can't permit a 40% rotor rpm range without beefing up the tailcone so much the advantage is lost.
Helo designers aren't stupid, their "mistakes" that you notice are testaments to what you don't know about designing helicopters. The matchbook school of engineering says there are two or three simple rules, the real world says there are hundreds of comprimises that make a helicopter.
Here is a crash course on helo design with thanks to Lakshmi Sankar of Georgia Tech:
www.ae.gatech.edu/people/lsankar/AE6070.Spring2004/Part1.ppt www.ae.gatech.edu/people/lsankar/AE6070.Spring2004/Part2.ppt
www.ae.gatech.edu/people/lsankar/AE6070.Spring2004/Part3.ppt
also a touch of the same from a micro-lite site:
http://halfdome.arc.nasa.gov/publica...ung_AIAA02.pdf