To: Joe Pilot et Al.
First of all when I wrote that bit about the father and son I was visualizing you as the son. In other words sooner or later you would come to realize that the father was correct. As long as you stay in the UK or OZ you will always be right and I will be perceived as an idiot. However if you ever have to come the USA for training say at Sikorsky and they start talking about theory of flight they will have to put you in restraints because based on how you respond to me on this forum you will most likely become violent.
Now I’m going to tell you a story about the life cycle of a helicopter blade or should I better use the term rotational cycle as that would be proper English. What I am about to say is applicable to every counterclockwise rotor system with the exception of a Robinson and a Westland Lynx. It applies also to clockwise rotation blade systems but the clock positions of 9:00 and 3:00 are switched, otherwise everything is the same. All numbers denoting pitch settings are applicable to this description and do not apply to a specific helicopter. Another point, when I use the term maximum pitch change relative to collective I am not addressing rate of change as that is constant as it reflects the angular deflection of the swashplate. There are several other disclaimers one of, which is cyclic pitch change to counter tail rotor propeller effect, is not considered. Another is that the collective pitch once set will not change to compensate for translational lift. We will use a four-blade rotor for simplification.
1) The rotor system is at rest. We measure the basic collective pitch setting at the root of the blade and it is 6-degrees with the cyclic in neutral.
2) The collective range is set at 18-degrees.
3) The rotor is started and the helicopter is brought to a hover. If we could measure the collective pitch setting in a hover it is 18-degrees on all blades as they rotate.
4) The pilot pushes forward cyclic. If we could stop the rotor with the blades disposed over the longitudinal and lateral axes the pitch readings would be 3:00= 12-degrees, 12:00= 18-degrees, 9:00= 24-degrees and 6:00= 18-degrees.
5) At these respective positions 3:00 and 9:00 the blades have the maximum variation relative to the basic collective setting of 18-degrees and rate of change has nothing to do with it.
6) When the blades move from the respective positions of 3:00 and 9:00 the pitch will start to change until the blades are disposed over the longitudinal center line of the helicopter and at that time they will be at the full collective position of 18-degrees. They have increased and decreased pitch respectively.
Now try to visualize this. If what you say is true about the advancing blade flapping up and the retreating blade flapping down due to changing aerodynamic loads then for gods sake please explain how or what forces are involved to tip the disc down over the nose. You say the advancing blade flaps up and the retreating blade flaps down. How much is the flap up and flap down and why won’t the delta three effect cancel this movement. Nobody ever tells you how much, he or she just says it happens and the rest is left up to your imagination.
Another point to ponder, if the advancing blade is increasing pitch between 3:00 and 12:00 and the retreating blade is decreasing pitch between 9:00 and 6:00 how can the aerodynamic forces cause the disc to tilt down over the nose and up over the tail? The way you explain it the disc would rise over the nose and down over the tail if it were pure aerodynamics. Yet, when the pilot pushes forward cyclic the disc tips down over the nose. The next time you fly your helicopter, watch the tip path when you move the cyclic in any direction. If when you address flapping up and down in forward flight and the rotor disc tips back this phenomenon also results from an increased lift on one side of the disc causing the disc to blow back(FAA definition) the blowing back is caused by the gyroscopic precession effect.
In the United States where they respect the laws of physics and the laws as they apply to gyroscopic phenomenon they would look at it in this way. When the pilot first pushed forward cyclic he changed the pitch relationship between the advancing side and the retreating side. This creates a differential of lift due to the pitch differences and this equates to a force that is applied to a spinning rotor. The lift differential is just like the perturbing force on a gyroscope and the application of the greater force is on the left side of the disc. And like all good gyros the response will be 90-degrees later in the direction of rotation. Once the forces are stabilized (at the time the cyclic stopped moving) the perturbing force is removed and since the rotor exhibits the gyroscopic characteristic it will remain rigid in that position unless the pilot moves his cyclic or some aerodynamic generated force perturbs the rotor. In this case read Transverse flow effect or a lift imbalance cause during retreating blade stall or the lift imbalance resulting in blowback as described above.
I have said this many times and it still goes un- noticed. Do not look at the spinning rotor as individual blades. The disc moves as a unit and is a composite of the blades made up of that disc. The only time blades act individually and fly out of track the delta 3 pitch coupling will return the wayward blade.
One last thing, when you all went through pilot training 101 you must have been taught how the instruments on your panel work. How did they address gyroscopic precession as it applied to the specific gyro operated instruments? What you rely on for your compasses, your attitude indicator and your turn indicator and some not mentioned all work using gyros and they all work under exactly the same rules that govern your rotorheads.
I think if you cut back on the trifle and the spotted dick your system might clear up so that you can accept an idea that is foreign to you.
Here is another thought and that is the AH 56 Cheyenne was not controlled directly by the pilot when he moved his cyclic. When he did move the cyclic it actuated the servo and the output of the servo was coupled to a spring. The spring was in turn linked to the swashplate which was in turn linked to a very powerful gyroscope mounted on top of the rotor head. When the pilot made the perturbing force via his servo the gyro responded to its’ maximum deflection 90-degrees later and it was linked to the pitch horns on the blade and they responded 90-degrees later at least they did most of the time. Sometimes they would respond early or late and on two occasions they responded with such fury that the blades impacted the fuselage. On one ship it killed the pilot and on the other it destroyed a wind tunnel at Ames Labs in San Francisco, California.
Guess who designed the blades? Ray Prouty.
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The Cat
[This message has been edited by Lu Zuckerman (edited 22 January 2001).]
[This message has been edited by Lu Zuckerman (edited 22 January 2001).]