Helicopter Transverse Flow

In this article, we discuss transverse flow around a helicopter. Transverse flow describes a variation in the downwash across a rotor, in the direction of helicopter flight. For example, in low speed forward flight, the aft portion of the rotor is engulfed in more downwash than the forward portion of the rotor, which flies through relatively cleaner air.

Background

Helicopters create lift by accelerating air downward. The downward flow of air around a helicopter rotor is often referred to as downwash or induced velocity.

A rotor must work harder when operating in its downwash. The downward flow of air at the rotor reduces angle of attack, requiring the blades to pitch higher and create excess induced drag. It’s much like a canoer paddling upstream.

In hover, the amount of downwash is essentially the same in the front and back halves of the rotor. Likewise, it’s the same in the left and right halves. When a helicopter moves forward, however, the front portion of the rotor enters cleaner air and experiences less downwash. This variation in downwash across the rotor is transverse flow and is depicted below.

Wind tunnel experiments have shown essentially zero downwash at the leading edge of a rotor and double the average value at the trailing edge, at moderate forward speeds.

Side effects

As a helicopter transitions from hover to forward flight, transverse flow will ramp in quickly. The reduction in downwash on the leading portion of the rotor results in higher angles of attack and increased lift. Increased lift accelerates blades upward, and they reach a peak upward displacement (called flapping) a short time later, around the side of the helicopter. For more information about this offset between peak lift and peak flapping, lookup “phase lag.” (For a technical derivation, see our article on flap dynamics.)

The converse occurs in the aft portion of the rotor. Extra downwash reduces angle of attack and lift, causing blades to flap down on the right side.

For a rotor spinning counterclockwise (viewed from above), this means the blades will be flapped up on the left side and create a right wing down roll moment known as transverse roll (or inflow roll). (For a clockwise spinning rotor, this would create a left wing down roll moment.) Pilots use lateral cyclic to counter this effect and keep the helicopter level.

In addition to lift, higher angle of attack creates higher blade drag. Each blade undergoes a significant variation in aerodynamic forces from the back to the front of the rotor, which vibrates the helicopter. A rotor with N blades will produce an N/rev vibration (N vibrations per rotor revolution), very noticeable to the pilot and occupants.

The effects are typically most significant around 10 to 25 knots airspeed, where the variation in downwash is the greatest. At slightly higher airspeeds the entire rotor gets the benefit of cleaner air and the total downwash decreases. This is called effective translational lift (ETL) and makes the rotor more efficient.

All directions

Although we’ve emphasized forward flight, transverse flow occurs with flight in any direction. The attitude change (and cyclic correction) depends on the direction of flight. For example, when flying right, increased lift on the right side of the rotor flaps blades up over the nose of the aircraft (for a counterclockwise rotor). Forward cyclic is required to counter the nose up pitch moment. Similar vibrations occur as well.

Wind

Transverse flow is brought about by airspeed, not groundspeed. On a windy day (10+ knots), it’s possible to experience transverse flow before liftoff. With a headwind (and a counterclockwise spinning rotor), a pilot will need more left cyclic to lift off into a stable hover.