Lu, I have taken the liberty of recsuing some more of your theories on RBS from "Ditching" and have posted then here for you:
Lu says: First of all, let’s disregard compressibility as retreating blade stall can occur under several different sets of conditions. I agree that there will be a lift differential. At this point our opinions start to differ. The roll will occur prior to the disc rising over the nose. At the onset of retreating blade stall there is a pronounced vibration followed by a roll to the left (American design). If at this time the pilot has not taken corrective action the disc will not just rise over the nose it will flap (blow) back quite violently and in some cases the blades will contact the tail boom (cone).
The major difference between us is how we view the reasons for roll and blow (flap) back. You say that is a pure aerodynamic response and I agree that aerodynamics is involved. As I have stated previously this is an alternate to gyroscopic theory and I can accept that. However, I believe that the differential of lift or, differential of forces acting on the disc which has gyroscopic characteristics causing it to precess. The roll is aerodynamic but it is the precursor to the flap back, which I believe is the result of gyroscopic precession.
The same condition exists but to a lesser degree when transitioning and the helicopter rolls and the disc blows back. Under these conditions the pilot can take corrective action by applying forward cyclic.
In retreating blade stall the retreating blade(s) are generating less lift than the advancing blade(s).
This lift differential is a perturbing force on the right side of the disc and is acting upward causing the disc to respond as a gyro rotor and the resultant of this upward force is the tilting of the disc up over the nose and down over the tail. The differential of lift will cause the helicopter to roll to the left, which is a precursor to the flapping back or blowing back. In this area there is a pronounced vibration just prior to the blow / flap back.
In both cases there is a differential of lift causing the disc to tilt up over the nose and down over the tail. In one case, the helicopter rolls right and in the other the helicopter rolls left. The roll is the result of aerodynamic forces while the flapback / blowback is the function of lift differential across the disc and the upward perturbing force results in a gyroscopic response.
In the case of the actions resulting from transverse flow effect the movement of the helicopter and the rotor is benign and the pilot can counter these actions with cyclic input. In the case of retreating blade stall the roll is pronounced and there is a lot of vibration. If by that time the pilot has not taken corrective action the disc will flapback violently and most likely chop off the tail boom.
Hopefully you can see that the actions taking place are similar if not identical. The corrective actions by the pilot are different and there is a major difference in what happens to the rotor relative to its’ response. Each condition should be taught in the flight training program and it should be obvious to any pilot that he / she will not experience retreating blade stall at translational lift speeds and therefore, understand the corrective actions that must be undertaken for both situations.
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The Cat
To Baranfin:
The blade does not actually stall at the tips, it spreads out from the root first due to relative airflow (root area is moving much slower than tip).
