To: sling load
Once again I have to correct you. I have been working in the field of product integrity since 1968 and the title of a person working in that field is a Reliability, Maintainability and Systems Safety Engineer. It is not necessary to be a graduate engineer to work in this field but the title is still the same. Again I must also correct you regarding my being an unlicensed A&P mechanic. I got my P license in the 60s and I got my A license in 1976 and it is still valid. Regarding my comment about lowering the collective at the bottom of an touchdown autorotation I was alluding to the fact that if you did not do this there was a possibility of the blades stalling out and hitting the fuselage.
Also please tell me and the others when and where I stated that retreating blade stall does not exist.
ATTENTION: Sling load did find a statement on the Helicopter History site where I stated that retreating blade stall does no exist. Here is my response which also appears in the weather and other nasties thread.
To: sling load:
You can stop gloating. I put that on the web site in conjunction with a thread on Just Helicopters. It is still my contention that retreating blade stall can best be described as a differential of lift across the disc, which results in the disc flapping back. I have even made these comments on these threads. You can use this description and apply aerodynamic precession or gyroscopic precession but in my mind that is what is happening. The blades are not stalling, they are just generating less lift. If you want to call it retreating blade stall that’s OK with me as that is the accepted term but it still ends up being a differential of lift that causes the flapback / blowback.
I’ll use this illustration which I have used on these threads as well as on Just Helicopters.
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.
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[ 04 December 2001: Message edited by: Lu Zuckerman ]
[ 04 December 2001: Message edited by: Lu Zuckerman ]
[ 04 December 2001: Message edited by: Lu Zuckerman ]
[ 04 December 2001: Message edited by: Lu Zuckerman ]