# Helicopter Basics: How does lead-lag of blades actually help in compensating for increase/decrease in speed due to flapping?

I understand the following -

1. Due to dissymmetry of lift, flapping up and down occurs.
2. Since angular momentum has to be conserved, the speed of blade increases/decreases during flapping up/down motion.
3. To compensate for this blade leads front by a certain degree in the rotor plane, in advancing half and lags by a certain degree in retreating half.

My question is - what does the "compensation means"? Even after rotating the blade front by 1-2 degrees (leading), the speed of blade will remain the same after it is reached its maximum lead angle. So, what happens after that?

Is it the case that lead/lag helps in absorbing the load/vibrations by allowing some degree of freedom for blade to move by few angles?

• None of these is true. The advancing/receding blade moves faster/slower relative to air so they need to have more/less angle of attack to generate balanced amount of lift. Flapping doesn't happen in the up/down or forward/backward dimension, but in the axial dimension of the blade. Commented May 26, 2020 at 16:13
• @user3528438: Yes, I understand the blade flaps up and down, thereby changing the angle of attack. What do you mean by "axial dimension of the blade"? I understand flapping happens perpendicular to the hub-plane. Where am I wrong? Commented May 26, 2020 at 16:17
• No flapping is up and down vertically on the flapping hinge axis, which is horizontal and perpendicular to the blade axis. On a two blade system the flapping hinge is the teetering hinge in the center of the hub. The axial motion is blade incidence as set by the swash plate. The blade flapping up and down is simply the blade following the path determined by the incidence set by the swash plate as it moves, with determines the overall rotor disc plane. With an articulated rotor, the blade flapping range of motion is the rotor's tilt range of motion. Commented May 26, 2020 at 17:37