To: All
In the Sikorsky Blue Book (Sikorsky Helicopter Flight Theory for Pilots and Mechanics) they discuss the aerodynamic theory proposed by members of the forum that are based in the UK and several of its’ Colonies and it is incomplete agreement with them. In this theory the pitch is changed in the blades cyclically and as pitch is removed from the blades they dive and if pitch is added to the blades they climb. If they didn’t carry the theory beyond this point then it would be true that Lu Zuckerman is full of crap. However they didn’t stop there as several pages later they added the following:
“We have stated that by means of cyclic pitch change, we make the blades climb from point A to point B and then dive from point B to point A. In doing so we effectively tilt the rotor in the direction of desired flight. In order for the blades to pass through points A and B it is obvious that the blades must flap up and down on a hinge.
One might be led to think that at point A the blades are at their lowest flapping point they would also be at their lowest pitch. And that at point B where they are at their highest flapping, they would also be at their highest pitch. If only aerodynamic considerations were involved in this statement it could be considered to be true. A Rotor system however has the qualifications necessary to take on certain properties of the gyroscope.
While we are not necessarily interested in the gyroscopes’ property of rigidity in space, we are interested in its’ property of precession.
Gyroscopic precession is an inherent quality of rotating bodies in which an applied force is manifested 90-degrees in the direction of rotation from the point where the force is applied.
Considering a rotating disc turning in a counter clockwise as shown in the illustration a downward force to the side of the disc would cause the disc to tilt downward 90-degrees in rotation from where the force was applied. Thus, to achieve a forward rotor tilt in a counter clockwise rotor system the force causing the blades to flap downward over the nose must be applied to the rotor on the right side of the helicopter while the force causing the blades to flap upward over the tail must be applied on the left side of the helicopter. 90-degrees of rotation from where the forces were applied, the blades will flap to their highest and lowest position. The text goes on describing the mechanics of the system that input pitch in the blades at the specific time in their period of rotation.
This is what is being taught at the Sikorsky Service School and based on what is contained in my Bell and Boeing helicopter training manuals it is also taught in those schools as well.
Based on the text in the Blue Book if you only read certain pages then The UK and OZ types could claim victory. However if you read several more pages in that same text you will see that gyroscopic precession is what is taught in the factory schools in the United States. If some of you might remember that on a long ago post I asked several members from the UK how they reconciled the different approaches to teaching the subject when they attended a US sponsored school. From what I understand the subject is not even covered in the Robinson schools.