Dissymmetry of Lift

The asymmetry

Imagine a rotor turning at 400 RPM, with a blade tip speed of about 400 knots. In a stationary hover, every blade meets the air at 400 kt regardless of where it is in the disc. Lift is symmetric.

Now the helicopter accelerates forward at 100 knots. The advancing blade is still spinning at 400 kt, but its airspeed relative to the air is now 400 + 100 = 500 kt. The retreating blade, which is moving in the opposite direction relative to the airframe, sees 400 − 100 = 300 kt.

Lift on a rotating airfoil scales roughly with airspeed squared. A blade meeting 500 kt of air produces dramatically more lift than the same blade meeting 300 kt of air. Without compensation, the advancing side would lift up, the retreating side would drop, and the helicopter would roll uncontrollably onto its retreating-blade side.

Blade flapping — the mechanical fix

Articulated and semi-rigid rotor systems allow each blade to flap — to move up and down vertically about a hinge or hub. This freedom is the key to the solution:

• The advancing blade, producing excess lift, flaps up.
• As it flaps up, its relative wind angle changes — the upward motion of the blade adds to the downward induced flow, which reduces the blade's angle of attack relative to the air it's actually meeting.
• Lower angle of attack means less lift. The flapping continues until lift on the advancing side equals lift on the retreating side.
• Symmetrically, the retreating blade flaps down, which increases its effective angle of attack and recovers some of the lift it lost from the lower airspeed.

The result: equal lift across the disc, even though the airspeeds at advancing and retreating sides differ wildly. The rotor is constantly flapping in forward flight — the disc is always slightly tilted, with the advancing side a hair higher than the retreating side.

Why the disc tilts back ("blowback")

The advancing blade reaches its peak angle of attack at the 9 o'clock position (left side of the disc, looking down on a CCW rotor) and its peak airspeed there. Maximum flapping-up motion happens 90° later in rotation — at the 12 o'clock (front of the disc), thanks to gyroscopic precession.

The result is that the front of the disc rides higher than the rear in forward flight — the disc tilts backward as airspeed increases. This is called blowback. Pilots compensate with forward cyclic, which is part of why every helicopter requires a slight forward stick position to hold a steady forward speed (along with the deliberate forward tilt of the disc that produces thrust in the first place).

Limits of the fix

Blade flapping equalizes lift up to a point, but it has limits. As airspeed increases:

• The retreating blade has to flap down farther to get enough angle of attack to keep up.
• Eventually the retreating blade's angle of attack approaches the stalling angle.
• Beyond a critical airspeed, the retreating blade stalls — that's retreating blade stall, the high-speed limit of every helicopter.

This is why every helicopter has a VNE (never-exceed speed) above which RBS becomes likely. Dissymmetry of lift and retreating blade stall are the same problem at different intensities.

Different rotor systems, same solution

Fully articulated rotors (3+ blades, Bell 206/407, Sikorsky family) have individual flapping hinges on each blade.

Semi-rigid rotors (2 blades, Bell 47/206, Robinson R22/R44) use a teetering hinge — when one blade flaps up, the opposite blade flaps down by the same amount, like a seesaw.

Rigid rotors (BO-105, Lynx, some EC variants) eliminate flapping hinges entirely — the blades are stiff enough to handle dissymmetry through bending alone. These designs allow more aggressive maneuvering but at the cost of higher airframe stress.

All three solve the same problem. The aerodynamic principle — flap up where lift is high, flap down where lift is low — is universal. Only the mechanical implementation differs.