To: Kyrilian
It's almost midnight so I can't be responsible for typos or technical errors.
Yes, I stated that the web site was pro Robinson and in searching my files I found a second identical web site. One is sponsored by a Robinson Dealer in Connecticut and the other is sponsored by a Robinson dealer in British Columbia. The web site was developed and is maintained by the Robinson Helicopter Association. Now, I don't think you can get any more pro Robinson than that. I would also assume that they get a great deal of technical support from the Robinson training departmen but that is only an assumption.
I recently read an FAA handbook on Rotor Craft Flying and that manual has the same errors that you will find in some of the major text book on helicopter aerodynamics.
If you had read one of my previous posts that addressed the teaching of helicopter aerodynamics it goes something like this.
First the discuss Bernouli. Then, they discuss Venturi. This is followed by fixed wing aerodynamics and when the students understand how a wing works they start talking about autogyros and how the advancing blade flaps up due to the relative wind and the retreating blade flaps down because its' attached (similar to a Bell).
The attach a great deal of importance to this action and once the students understand that, the go into helicopter aerodynamics but they are still talking about autogyros.
The error in teaching this way is analogous to taking a movie film and addressing the overall activity that is taking place based on one cell of that film strip. It is true that the blade wants to flap up due to the relative wind and assuming you have a Bell blade the opposite blade flaps down. Of course, they never talk about a fully articulated rotor because the blades are not attached and the argument falls apart.
Now, we get to the flapping up due to the relative wind on the advancing blade. At the time they tell you the blade is flapping up, it is actually flapping down over the nose as this is how you develop forward flight. If the blade does try to flap up and the pitch link/teeter / coning hinge are not coincident then you have a restoring force caused by pitch coupling. However,the aerodynamic effect is minimal as it can't react in the time it takes to go from the right side of the helicopter to over the nose. Now, what is really happening is that the aerodynamic forces cause a lift differential across the disc and the disc will be displaced slightly by gyroscopic precession unless corrected for by the pilot who moves his cyclic stick accordingly. The flapping you addressed is minimal at the most.
The action of the delta effect has no effect on the phase angle as it is fixed by the laws of physics. Here is something you can work out. Lets say that your theory is correct and the delta effect actually alters the phase angle. At some point in time the pitch horn/pitch link combination will be in a direct line with the cone hinge which means that there is no pitch coupling. Now, what happens to the phase angle?
Back to the 18 degree offset. You may have noticed that I defined the offset as the number of degrees the pitch horn must travel in relation to the blades which are disposed laterally and the swashplate is fully tipped down over the nose. The difference is approximately 18 degrees of rotation ahead of the longitudinal axis of the helicopter.
If what you said about pitch coupling altering the phase angle is true, it must occur in this 18 degree arc at a tip speed of 599 feet per second or, 408 MPH. Please explain how it happens in this distance and at that speed.
Relative to transverse flow this is discussed above in another post. Once you get above 20 knots this goes away.
Another point that has been brought up by several pilots is that theory doesn't always reflect reality. I have to agree as the text books that define helicopter aerodynamics always define it under specified conditions. That is why most designs have to be altered when the helicopter is first flown. The text books will in effect introduce the student to the engineering theories that will put him into the ballpark when he gets to design helicopters or any thing else for that mater.
Here is an example of an engineer that had limited knowledge of aerodynamic theory. When I was in tech rep training at Sikorsky I was assigned to the hydraulics tes lab.
One day, I saw one of the hydraulic engineers working on a mockup of a rudemetary collective control system. Attached to the collective stick was a hydraulic cylinder that would raise the collective stick until it was fully up and then the hydraulic cylinder which was spring loaded lowered the collective stick to the down position. I asked him what this was and he said that the hydraulic cylinder was actuated by the hydraulic system as the rotor was spinning up and the internal spring was being loaded. I asked him at what rotor speed the system was actuated up. He replied that it was really at what point the hydraulic system started to build up pressure which was 1200 rpm on the pump. He further stated that the system was designed so that if the engine lost oil pressure or shut down the cylinder would automaticly pull the collective down putting the helicopter into auto rotation taking the responsibility away from the pilot. I told him that it wouldn't work because at 1200 repm of pump speed the rotor blades were not fully aerodynamic and would flap up and stall contacting the tail cone. He had absolutely no idea about what I was talking about because he was a hydraulic engineer and not an aero engineer. He didn't even cordinate with the flight control or aero groups. The next day, the rig was gone and I think the engineer was also gone.
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The Cat