This thread is the second attempt at exploring Retreating Blade Stall (RBS). Lu Zuckerman begins by sying:
>>If you believe that the retreating blade has stalled and further believe that individual blades stall and drop out of orbit and fall due to the stall then try to visualize this. Once the blade has stalled and dropped down over the tail (90-degrees later) it still is attached to a spinning rotorhead and must then immediately get back to the commanded tip path. That means that that blade when it is down over the tail it must fly up until it is now the advancing blade and, will end up being down over the nose, as this is the commanded tip path. If you were to look at the disc from the side of the helicopter the disc would scribe an inverted V or be just the opposite of the cone angle. Can you imagine the vibratory forces that would ensue if the blades had to change their position so radically at anywhere from 250 to 500 times per minute?
Now this may be difficult to comprehend but try to visualize the disc as a single entity. The basic premise in helicopter design is to have an equal distribution of lift across the disc. When the pilot moves his cyclic he alters the lift distribution across the disc and the disc tilts in the direction of cyclic movement. You can visualize this as aerodynamic precession and I think of it as gyroscopic precession. In either case there is a differential of lift across the disc.
In the case of retreating blade stall, the retreating side is generating less lift than the advancing side. This causes either a perturbation of the disc if you accept gyroscopic precession or it is a direct aerodynamic lift that responds 90-degrees later and the result is the disc blows back / flaps back.
I put the comment on the web to find out what other people thought on the subject.<<
Respond if you are game.......
