To: Joe Pilot et al.
The reason I am starting to make sense is summed up in the story of the young man that thought his father was so stupid, and when the boy reached 21 years of age, he was amazed at how smart his father got in six years.
It must be understood that the major reason for so much antagonism from UK and OZ pilots and their not agreeing with my input on these threads is due to the fact that your training regarding flight theory is totally different from how it is taught in the USA. With that said, I find it difficult to understand why so many UK and OZ pilots quote books and theories postulated by people like Ray Prouty and other American text book writers. How do you reconcile the differences in aerodynamic and physical laws as taught in the US and the UK?
Regarding my quoting the FAA Handbook, the quoted statement should have been preceded with; …The relative wind encountered by the advancing blade is increased by the forward speed of the helicopter while the relative wind speed acting on the retreating blade is reduced by the helicopters forward speed. Therefore, as a result of the relative wind speed, the advancing blade side of the rotor produces more lift than the retreating blade side. This situation is described as disymmetry of lift. Then comes the paragraph I quoted above. The automatic feathering they alluded to is pitch coupling.
If you have an argument, take it up with the FAA because I don’t agree with their explanation. I believe disymmetry of lift, if it exists, results in blow back. If you look at the rotor system that has disymmetry of lift across the rotor disc it appears that it is basically the same as that of retreating blade stall but a much milder version. That’s what I think. The FAA states that the helicopter would roll left under the stated conditions. That is the first stage of retreating blade stall. If the left roll were not countered the disc would blowback, and you might ask, what would cause the disc to blow back. It will blow back for the same reason the disc blows back with retreating blade stall. The culprit (are you ready for this?) is gyroscopic precession.
One of the reasons I disagree with the FAA is that they address individual subjects in an isolated manner much like the way helicopter aerodynamics is taught. They allude to the fact that the blades are automatically feathered (pitch coupling) but they neglect to discuss one very important point. That point is that in order to get the helicopter to fly through the relative wind that will create disymmetry of lift you have to move the cyclic forward which will decrease the pitch on the advancing blade and increase the pitch on the retreating blade thus tilting the disc and at the same time result in eliminating disymmetry of lift. There are other perturbing forces out there that will cause a helicopter to roll right or left one of, which is transverse, flow effect.
Let the race begin.
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The Cat
[This message has been edited by Lu Zuckerman (edited 21 January 2001).]
[This message has been edited by Lu Zuckerman (edited 21 January 2001).]