Helicopter Retreating Blade Stall

In this article, we discuss retreating blade stall. Retreating blade stall occurs when a portion of a helicopter main rotor stalls in high speed flight. We cover the cause of retreating blade stall, contributing factors, and side effects.

The cause of retreating blade stall

Think about the tip of a helicopter rotor blade. It moves relative to the surrounding air due to the rotation of the rotor. The faster the rotor rotates, the faster the blade moves through the air. This airspeed facilitates the creation of lift on the blade, which keeps the helicopter aloft.

The airspeed varies along the length of the blade. The inboard portion of the blade—near the hub—gets less airspeed from rotor rotation, as shown in the diagram below. In hover, the airspeed at a section of a blade is the product of the rotor angular velocity and the distance of the section from the hub.

When a helicopter flies forward, these airspeeds must be added to the forward speed of the helicopter (depicted in blue below). When rotation moves a blade in the same direction as the helicopter (the right side of the rotor in the diagrams) it is called an advancing blade. A blade on the other side (left) moves aft and is said to be retreating.

The diagram above shows the total airspeed with red arrows, found by adding the arrow due to rotation (green) to the arrow associated with helicopter forward speed (blue). The advancing blade (on the right side) has a high total airspeed—the helicopter forward speed adds to the rotational speed at the section. The retreating blade (on the left side) ends up with a lower airspeed because the forward speed subtracts from the rotational speed.

The faster the helicopter flies (giving a longer blue arrow), the shorter the red arrow becomes on the retreating side. In normal conditions, blade pitch increases on the retreating side, providing more lift per unit airspeed (to maintain lift comparable to the advancing blade). However, at a high enough speed, the red arrow (i.e. the total airspeed) on the retreating blade will be so small that further increasing the pitch of the retreating blade no longer helps—the blade has stalled. This is retreating blade stall.

Notice that the lack of airspeed gets worse further inboard on the retreating blade. Since the airspeed due to rotor rotation shrinks inboard, there is a point where the total airspeed is actually zero and, inboard of that point, the air actually flows in the reverse direction over the blade (trailing edge to leading edge). This is called reverse flow and is shown in the diagram below.

Airfoil view of the problem

A cross-section of a blade is called an airfoil. The diagram below shows a representative airfoil shape. Blade (and wing) aerodynamics are often estimated by analyzing a set of airfoils along the length of a blade (or wing). It’s all about finding the air velocity relative to the airfoil, i.e. the speed and direction of the "relative wind." The angle between the airfoil centerline and the air velocity is an important value called the angle of attack and shown below.

The lift generated by an airfoil is proportional to the angle of attack and the square of the airspeed). However, this is true only up to a certain angle of attack, called the critical angle of attack (typically around 15 to 20 degrees). Increasing the angle of attack past the critical value no longer increases lift. A wing or airfoil is said to have stalled when the angle of attack reaches this value.

In forward flight, helicopter blades flap and feather (via the action of a swashplate) to increase the angle of attack of retreating blades. This allows retreating blades (with smaller airspeed) to generate lift comparable to advancing blades.

The faster a helicopter flies, the more feathering is applied to achieve suitable angle of attack and lift on retreating blades. Inboard portions of the retreating blade will stall at lower helicopter speeds, since the local airspeed is much smaller there, as shown in the prior section. The helicopter may continue to accelerate as outer portions of the blade can generate suitable lift. Eventually, at higher speeds, most of the retreating blade will have stalled or be in reverse flow. At some point no more lift can be generated and retreating blade stall has occurred.

Problems caused by retreating blade stall

You may ask: why does it matter if the retreating blade stalls? After all, when the blade advances it will have a large airspeed, capable of generating excess lift. The problem is that excess lift on the advancing side will not be balanced by the retreating side, which creates too much aft flapping. We'll describe this further below.

Helicopter blades are flexible, and lift causes them to bend upward. If there’s more lift on one side of the rotor, blades will accelerate upward more on that side. This vertical movement of blades relative to one another is called flapping and is always present in forward flight.

In the case of retreating blade stall, the upward accelerating blades on the advancing side end up in a relatively high position over the nose of the helicopter. Normally, a pilot could keep this under control with forward cyclic—increasing the pitch (feathering) of the retreating blades so that they accelerate up to a desired height over the tail. (Increased pitch of these blades allows them to generate more lift per unit airspeed.) In retreating blade stall, this control is lost—increasing the pitch of retreating blades no longer increases lift.

In addition to creating large vibrations capable of damaging the helicopter, the rotor disk effectively tilts aft. This creates a nose up pitch moment and prevents the rotor from providing propulsive force (forward) for higher speed flight.

In short, retreating blade stall causes

• increased vibration at the blade pass frequency (N/rev for a helicopter with N blades),
• aft flapping causing the helicopter to pitch nose up, and
• lateral flapping (stalled side down) and a roll in the same direction.

Other factors

Retreating blade stall occurs more easily with

• high gross weight,
• high G maneuvering (e.g. in a coordinated turn),
• low air density (high altitude),
• low rotor speed,
• high drag (e.g. with weapons or other equipment mounted on the helicopter),
• a forward cyclic input, and
• high turbulence.

All of these items can be understood via blade angle of attack and airspeed. At high gross weight, or in high G maneuvering, blades operate at larger angles of attack to produce more thrust. In high drag scenarios, the rotor must produce more propulsive force, requiring larger angles of attack (and forward cyclic/flapping). In lower air density (high altitude and/or hot air), or with lower rotor speed, blades must operate at larger angles of attack to produce the same thrust as they would in normal conditions. In all of these cases, blades start at higher angles of attack and higher pitch / feathering angles. This causes them to reach the critical angle of attack sooner, at a lower airspeed.

A forward or right cyclic input can exacerbate retreating blade stall as well. Forward cyclic directly increases the feathering and hence the angle of attack of the retreating blade, potentially transitioning it past the critical angle of attack.

How a pilot gets out of retreating blade stall

When a pilot detects retreating blade stall, the goal should be to stop the stall as soon as possible to prevent damage. Attempting to use cyclic to fix the pitch and roll can worsen the situation. This first step should be reducing collective, which immediately reduces the angle of attack of all blades. (This can lead to a loss of altitude, but that should be OK—a helicopter should not be flown at high speed and low altitude.) Any roll and pitch excursion due to retreating blade stall should start to subside, and then the pilot can start to use the cyclic to regain attitude, but aiming for a lower airspeed.

Increasing rotor speed is not a solution

It’s tempting to think that higher rotor speeds are a solution to retreating blade stall. Indeed, higher rotor speeds increase the airspeed of the retreating blade and delay stall. In the diagram we provided, the green arrows lengthen and hence the red (total airspeed) arrows lengthen. Retreating blade stall is fixed, but a new problem is created on the advancing side of the rotor. With higher rotor speed and high forward flight speed, the advancing blade tip can near the speed of sound. At this point, drag and twist effects on advancing blades become problematic. So higher rotor speeds delay retreating blade stall, but create other problems. Manufacturers select a rotor speed to balance all these issues and best fit the goals of the particular helicopter.