What is rotor damping and how does it affect helicopter stability?

Question

What do we mean by rotor damping (in terms of helicopters)?

How is it affected by air density?

How does helicopter stability depend on rotor damping?

Answer 1

I will try to answer these questions in terms of structural dynamics. Please correct me if I say dumb things on other parts of my answer.

You ask about damping. Damping must be considered when dealing with structural dynamics, or vibratory phenomena. In a helicopter rotor, multiple phenomena can lead to vibrations. For instance :

• Differences in blade properties (mass mostly, but also stiffness)
• Rotor imbalance due to machining uncertainties
• Bearings
• Engine, which may produce torque oscillations of some sort
• Gearbox, which may also produce oscillations (look for gear mesh vibrations)
• Helicopter operations (if you have quick flight trajectory modifications, this may lead to increased or reduced lift needs, which induce torque oscillations in the drive line)

So long story short, multiple phenomena can excite the rotor. As any structure, a helicopter has natural vibration patterns called natural modeshapes, associated with natural frequencies. However, if you excite a component enough on its natural frequency, this may result in a fatigue failure, hence the utmost importance of damping.

As far as I know, the most visible technology used to increase damping on a helicopter rotor are the blade root dampers. Multiple technologies exist, however they seem to damp the same phenomenon : blade edgewise motions or so-called lead-lag motions or hunting motions.

If you want to have a deeper understanding of these issues, have a look at this article for instance. This page may be helpful as well.

The first paper addresses semi-active dampers to control the blade edgewise bending motions in order to reduce the constraints on the hub. Basically, if the blade was mechanically fixed to the hub, large constraints could occur due to vibration loads, leading to for instance fatigue failure. In the configuration presented in the paper and reproduced here, the liaison between the hub and the blade is a cylindrical liaison, allowing in-plane rotational motion. The (stiffness-)damper system is then used to absorb the kinetic energy of the blade in order to limit the loads on the hub, which allows to reduce its overall weight.

The second paper provides some more general concepts on the technologies involved in helicopter rotors. In particular this here is a good schematic of a full articulated rotor. Other rotor technologies exist : semi-rigid or rigid rotors but they use a different strategy for vibration reduction and blade-hub liaison.

Answer 2

Rotor damping refers to aerodynamic and structural forces that counter blade vibratory motion.

Aerodynamic damping forces are proportional to air density. The most important one is the aerodynamic damping of flap motion. As a blade flaps upward, the angle of attack is reduced. This reduces upward aerodynamic force or, in other words, increases a downward aerodynamic force. This phenomenon always opposes blade "flap velocity." It pushes the blade down when it's flapping up and vice versa.

An analogous thing happens with lead-lag motion (within the plane of the rotor). As a blade moves forward drag increases to "push it back." When a blade lags drag decreases to help "push it forward." Unfortunately, this variation in drag is very small and does not dampen lead-lag motion nearly as much as the flap phenomenon described above. It is for this reason that lead-lag damping is problematic, often resulting in mechanical dampers placed near the root of the blade in the lead-lag direction.

Helicopter blades also have structural damping. Let's say you grab the tip of a blade and try to pitch/twist it (and you are extremely strong!). When you release the blade it of course wants to go back to its original, undeflected shape. It may, however, oscillate about that shape like a guitar string. This oscillation would quickly become too small to see, not so much due to aerodynamics or any add-on damper, but because the blade material itself will absorb much of the energy. You can search for "structural damping."