To: imabell
I too have taught POF to both mechanics and pilots and I believe they accepted what I put out in these classes. I would well imagine that if you locked my students and your students in a room they would be having the same argument that you and I are having. The basic argument stems from the differing philosophies relative to POF. There are differences in the teaching of POF and the major differences exist between the UK and OZ collectively and my methodology which deals with pure gyroscopic precession as the moving force and the aerodynamic forces that are involved in initiating gyroscopic precession.
I am not saying that you or the guys from the UK are wrong because the theory you teach is more closely allied with what Nick has said because it teaches the engineering principles involved in moving the blades from one point of the disc tip path to another. It is just that my philosophy is more easily understood by US Army mechanics and Warrant officers that were in most cases not better educated than the mechanic students.
Here is the way I would teach it and actually it is the way I teach it. Most likely you will say it is wrong. I say it can be accepted as an alternative theory. I first posted this on Just helicopters last year.
Try to visualize this, or better still, pick up a pencil and paper and draw a picture of a circle. Cut the circle into four equal parts. On the circumference of the circle where one line meets, write the letter N for North. Now, label the other lines East, West and South. North is the direction the helicopter is flying. The existing way of teaching helo aerodynamics is to say that the blade is stalled over West and because the blades have a precession angle of 90 degrees the effect will take place 90 degrees later or, over South. That sounds reasonable, but can you picture a blade that has a tip speed near the speed of sound flying from an in track position at West to a stalled position over the tail cone at South. Now, try and picture that same blade flying from that low position over South to an in track position over the right side at East. Then, because the pilot has the cyclic in a forward position, that same blade, because of precession, will be flying low over the nose or, North and back up to an in track position over West and then, it starts all over again at the rate of say 250 or more times a minute. Try to visualize the vibratory forces involved if the blade in fact acted as an individual unit.
I would suggest that you stop thinking of individual blades flying in a circle. Instead, think of the blades as a solid disc just like the rotor on a gyroscope. If you apply a force to a gyro that is on gimbals, the gyro because of precession will react 90 degrees later in the direction of rotation. The same is true on a helicopter. Cyclic input will change the pitch relationship across the disc and will result in an imbalance of forces. If the pilot pushes the stick forward, the greater force is over the West side of the disc. This upward force (due to precession) will cause the disc to raise over South and drop over North. Aerodynamics plays a minimal part in this action. The change in disc position was caused
by the change in lift forces but the actual movement was caused by the gyroscopic characteristics of the spinning disc.
Blades do not fly to a position, the are moved by gyroscopic forces. If you want to call it blade stall that’s OK. When you enter the realm of blade stall there are ways of getting out of it but once it happens, individual blade do not stall. They simply change the balance of forces across the disc and physics does the rest.