To: Helimutt
Corriolis force (conservation of angular momentum) is the force that causes the blades to lead and lag. The change in the blade mass is often diagrammed out showing one blade in various angular positions relative to the flapping hinges. The diagram shows the relationship of the blade mass at each position relative to the rotational axis. The diagram only shows one blade flapping up and the other blade is in the pure radial position and this can’t be. Another means of explaining this phenomenon is to have you picture an ice skater that brings her arms closer to her body and as a result speeds up in rotation and when she moves her arms outwards slows down. Speeding up and slowing down is due to her changing he center of mass.
Regarding what your instructor stated about the small change in blade mass CG not being sufficient to cause the leading and lagging I think he is not considering the blade mass due to centrifugal force. On a large helicopter the blades have a very large weight due to centrifugal force and could weigh in excess of 70,000 pounds at the lead lag hinge. So at this weight it does not take a very large change in mass CG to exert the energy to move the blades about the lead lag hinge.
Now, this is the way I learned it at Sikorsky over 45 years ago. It is difficult to explain without drawing diagrams but here goes.
First, something to consider: Discounting mast tilt to compensate for tail rotor drift or forward tilt on a Sikorsky mast that adds forward cyclic, visualize the disc as being horizontal and rotating about a fixed axis. This is the drive axis. In order to have lead and lag you must have an offset hinge. On the Robinson the cone hinges are the offset hinges. For arguments sake the rotor head does not move relative to the mast (difficult if you think of a Robinson head. Assume that the blades are coned up at the flapping hinge when you pull in collective. This will form a “V” with a flat bottom. Now lets look back at the explanation above that describes the blade in the flapped condition(s) and showing the blade mass CG moving closer to the center of drive creating forces that cause the blade to speed up or in other words, lead. This is totally false, as the blades do not lead when in a hover discounting tilt due to tail rotor compensation.
The blades lead and lag only when you introduce cyclic pitch change. Lets go back to the “V” with the flat bottom. When cyclic pitch is made the disc will tilt in the respective direction. This will make one leg of the flat-bottomed “V” move up and the other down in relation to the rotor head. Initially we discussed the drive axis. That was when you had a flat-bottomed “V”. Once the cyclic input was made the disc was displaced and now the blades are rotating about the driven axis. Due to the laws of conservation of angular momentum the blade wants to move forward from its’ position about the drive axis to the position scribed by the rotation about the driven axis. Since the blade can’t move linearly because the lead lag hinge anchors it, it will do what it can under the restrained condition. The tip of the blade will move forward on the advancing side. Now, when that same blade is on the longitudinal axis the forces diminish and the blade goes to the radial position and it wants to stay in that position. So, when it passes over the longitudinal axis, the blade will tend to hang back or actually move slower than it is being driven due to the change in mass CG position, which is lower. This is per the Ice skater illustration. It will hang back until it is over the left side of the helicopter and then return to the radial position. If you could look down on the tilted disc a four-blade rotor system would look something like a peace symbol.
While my wife and I went out for Chinese the other guys chimed in and covered droop as it applies to the rotor and the engine but there is another type of static droop and it applies to the drooping of the ailerons on a wing. When I worked on the CL604 and the Regional Jet flight control systems I discovered that after the flight control system is rigged and the Power Control Units (Servos) are turned on the pilot valves in both aileron servos are biased to move the ailerons down from the rigged position. I believe this was to both provide a bit more of lift and that aerodynamic forces would tend to move the ailerons to the rigged position and stiffen the system because the servos were always trying to get them to extend to the drooped position.
To:212man
Regarding your comment below:
Regarding the coriolis effect, don't forget that it is more concerned with the flapping of the blade in forward flight ie the advancing blade rises, so speeds up then slows down as it flaps down on the retreating side. Hookes joint effect has a similar effect as do periodic drag changes.
Hope this tallies with your understanding.
The advancing blade flaps down and the retreating blade flaps up. According to your descrioption you would be flying backwards. Your comparison to a hookes joint is valid. That is why they have constant velocity joints on a front drive car.
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The Cat