It seems some have problems dealing with this subject of dissymmetry of lift and flapping. They believe the advancing blade is flapping up in forward flight when it’s obvious the advancing blade is flapping down.
They were taught in regards to the early autogyro that used flapping as their method of correcting dissymmetry of lift; unfortunately, they were never taught that dissymmetry of lift can and is handled in another way in a helicopter by the pilot using the cyclic. Here’s a few quotes to set them on the right path:
“It isn’t any automatic flapping or feathering that equalizes the lift, it is mostly the pilot who stops the pitch and roll by applying the right amount of cyclic correction that balances the forces and moments.
Flapping helps relieve the stresses on the blade and hub, and keeps the structure lighter as a result. To a smaller extent, flapping does allow some automatic balancing of the forces across the disk, as does the blade's pitch-flap and flap-lag coupling (Delta 3 and Alpha 1 coupling).”
Nick Lappos, Question about flapping/feathering
Cyclic Feathering and Flapping
c. Correction of Dissymmetry of Lift for Helicopters. Two questions that seem to best encompass the subject of cyclic feathering versus flapping in helicopter flight are, "When does cyclic feathering correct dissymmetry of lift?" and "When does flapping correct dissymmetry of lift?"
(1) Cyclic feathering (cyclic repositioning) corrects dissymmetry of lift whenever a constant attitude is maintained by the aviator during changing lift patterns that occur during-
c. Rpm changes.
d. Collective pitch changes.
e. Transient gusts, wind shear, or turbulence.
(2) Blade flapping action corrects dissymmetry of lift whenever attitude change results from any of the conditions in (1) (a) through (e) above. When not prevented or corrected by the aviator, blade flapping action (blade flexing or hingeless rotors) will correct dissymmetry of lift in helicopters. Depending on whether airspeed is increased or decreased, this blade flapping action will cause a nose up or nose down attitude change.
(3) When cyclic feathering is preventing and/or correcting dissymmetry of lift, any action at or around the flapping hinge is due to nonaerodynamic causes such as-
(a) Nose-high or nose-low fuselage due to existing C.G.
(b Design shortcomings of rigging between rotor, mast, and fuselage.
(4) When action at or around the flapping hinge is due to nonaerodynamic causes, the aviator's concern is one of awareness for mast bumping, vibrations, C.G. management, and of shifting his item emphasis on daily preflight inspection.
5) Just as action around the knee joint of one's leg may involve kicking, this action could also be used for kneeling, sitting, stepping, or stooping. Therefore action at the knee cannot arbitrarily be labeled "kicking." Similarly, action around a "flapping hinge" should not be arbitrarily related to "dissymmetry of lift" and its correction.
“If the pilot pushes the stick forward, the swashplate is tilted forward. Since the pitch arm from the blade is attached to the swashplate 90° ahead, the blade has its pitch reduced when it is on the right-hand (advancing) side and increased when it is on the left-hand (retreating) side.
When the blade is over the nose or the tail, the forward tilt of the swashplate has no effect on the blade pitch. Cyclic pitch can be used for two purposes: to trim the tip-path plane with respect to the shaft, and/ or to produce control moments for maneuvering.
In the first case, the pilot can mechanically change the angle of attack of the blades by the same amount as the flapping motion would have, thus eliminating the flapping. This adjustment can be used to eliminate all of the flapping, or to leave just enough to balance pitching and rolling moments on the aircraft—such as those due to an offset center of gravity.
In the second case, the pilot deliberately introduces an unbalanced lift distribution to tilt the rotor for maneuvering.
For example, if the helicopter is hovering and the pilot wishes to tilt the nose down to go into forward flight, he pushes the stick forward, causing the swashplate to tilt down in front. The pitch of the blade on the right side is decreased as the left side is increased.
The resultant lift unbalance accelerates the right-hand blade down as it moves toward the nose and the left-hand blade up on its way to the tail. The rotor flaps down over the nose and up over the tail— tilting the rotor-thrust vector forward to produce a nose-down pitching moment about the aircraft’s center of gravity.”
Ray W. Prouty, Helicopter Aerodynamics, “Blade Flapping and Feathering”
Edited by iChris, 17 October 2018 - 11:03.