To: SPS
In my email to you as the operator of a web site in New Zealand I asked a question regarding how they teach helicopter theory in your part of the world. Obviously by the questions posed by you in this thread it is obvious that they use the same instruction syllabus in New Zealand as they do in the UK.
On another thread in the Rotorheads forum I got chopped to pieces when I expounded on my theory about gyroscopic precession only to find out that in the UK they discount gyroscopic precession as the moving force in changing the disc tip path plane. Now, if I give an answer to your queries you must understand that I am speaking from my point of view which is based on 51 years of exposure to helicopter flight theory as taught in the United States.
The Robinson helicopter has a low inertia rotor system. What this means is that it has a low amount of stored or potential energy available to carry the rotor through in the event of a power failure. The low inertia has nothing to do with gyroscopic precession. The blade system has very low weight as compared to other helicopters but it has a rotational speed ranging from 495 to 530 RPM, which contributes to one of the gyroscopic characteristics of rigidity in space. Even with this rigidity it does not take a great deal of mechanical force to perturb the disc and thus introducing the other gyroscopic characteristics, which is precession. Look at the Robinson swashplate. The lateral inputs are very close to the centerline of the swashplate and as such do not offer very much mechanical leverage. The fore and aft input is at the front and back of the swashplate and as such have a slight mechanical advantage over lateral input but not by much.
You have to understand that rigidity and precession are characteristics of a Gyro and are separate elements. By applying a mechanical perturbing force the rotor will precess but it doesn’t effect the rigidity in space it simply changes the tip path and the rigidity in space is maintained. Rigidity means that the disc will stay where you place it and precession means that you can command a new position for the disc.
That is the difference in the two methods of teaching rotor theory. The UK method addresses individual blades and not a disc like a gyro. In the states they teach that the disc is like a spinning rotor on a gyroscope and as such has the characteristics of a gyroscope rotor.
Regarding using the cyclic as a lever to change disc attitude in space this is how it is done on autogyros that do not have articulating rotorheads. The gyro rotor also has rigidity is space but the disc can be moved with mechanical force exerted by a pilot with no help from a hydraulic boost system.
On larger rotor systems they use hydraulic boost to effect control. The main reason for the boost is not to input control forces to the rotor but they are there to resist the feedback forces generated during cyclic pitch changes as the blades rotate. Blades as such want to climb or dive depending on the input and they also have large centrifugal twisting moments that want to return the blades to a flat pitch condition.
Here is something you can try. On a larger helicopter that uses a constant pressure variable delivery hydraulic pump as the pressure developer bring the rotor up to speed and the pump will be developing full pressure. Move the cyclic rapidly in a circle and you will see that the pump pressure will drop to a very low level as long as the cyclic movement continues. the pressure reading when you do this test is a reflection of the forces necessary to perform the test. Do the same test when in a hover or when slowly flying forward and the pressure level should be higher than when you performed the test on the ground. The higher pressure reflects the force necessary to not only effect pitch change but to resist the feedback forces. The feedback forces will never be higher than the force necessary to resist those forces. That way the feed back forces will always be stopped by the hydraulic boost system.
Recently Robinson installed a hydraulic boost system on the R44. There was no change in rotor configuration or the force necessary to introduce pitch change; they installed the boost system, as a means of canceling the feed back forces that were reflected in the cyclic stick.
Regarding larger helicopters that you would think would require a boost system to overcome feedback forces you only have to look at the Kaman Sea Sprite or, the K-MAX. They effect the precession of the rotorhead by the use of small servo flaps on the blades. The pilot moves the servo flaps with out hydraulic boost and the servo flaps change the disc position via precession. A very small perturbing force moves a very high mass rotor system. Any feed back forces that are generated by the servo flaps are very small and can be easily countered by the pilot.
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The Cat
[This message has been edited by Lu Zuckerman (edited 22 December 2000).]
[This message has been edited by Lu Zuckerman (edited 22 December 2000).]