Why edgewise modes are lightly damped
When a blade moves flapwise, its motion changes the angle of attack along the whole blade: moving into the wind raises lift, moving away lowers it, and that change opposes the motion. That is aerodynamic damping, and in normal operation it is large. When a blade moves edgewise, the motion is mostly along the chord, the angle of attack changes little, and the lift opposes the motion only weakly. Edgewise modes therefore rely on the structure's own damping, which for a composite blade is small.
The consequence is that an edgewise vibration, once started, decays slowly, and under some conditions does not decay at all. Three sets of conditions matter.
Stall-induced vibration in operation
When part of the blade operates in stall, at high wind speeds on stall-regulated machines or transiently on pitch-regulated ones, the relationship between motion and lift can reverse: the aerodynamic force feeds the motion instead of opposing it. The blade's own edgewise mode is then excited, at its natural frequency, and grows until structural damping or the controller limits it. Modern pitch-regulated turbines avoid deep stall in normal operation, but ice on the leading edge, heavy erosion, a pitch fault or sustained operation at an unusual angle can put sections into stall that the design did not intend.
Standstill and idling
A parked or idling rotor at a large inflow angle, when the turbine has yawed away from the wind or is in a storm shutdown, can see vortex-induced and stall-induced vibrations of the stationary blade. Long blades on modern turbines, with low edgewise frequencies and low structural damping, are more exposed than the previous generation. In our experience standstill vibrations are under-recognised because nobody expects a stopped turbine to be loading its blades, and the SCADA record of a parked turbine is rarely examined.
Forced excitation
Gravity loads a rotating blade edgewise once per revolution, and that 1P load is the largest single fatigue contributor to the trailing edge regardless of vibration. On top of it, rotor imbalance from ice accretion, a repair that added mass to one blade, or pitch misalignment between blades adds edgewise excitation at the rotor frequency and its multiples. Where a multiple sits close to the edgewise natural frequency, the response is large.
What edgewise vibration does to the blade
Edgewise bending puts the leading edge and the trailing edge alternately in tension and compression. The trailing edge is the part of the structure least able to bear that: a long adhesive joint, thick and variable, often with voids from manufacture, inside a narrow cavity that no inspection reaches easily.
- Trailing edge bond line fatigue. The dominant consequence. Cyclic edgewise load opens voids in the adhesive, and the crack grows along the joint towards maximum chord, where the edgewise moment is highest. The signature and diagnosis are set out in trailing edge cracks.
- Transverse cracks at maximum chord. Where the trailing edge has opened, the shell panel behind it carries load it was not designed for, and transverse cracks follow.
- Panel buckling. A thin sandwich panel in edgewise compression buckles locally, and repeated buckling debonds the skin from the core.
- Root fatigue. Edgewise moments at the root are carried by the same bolted joint as flapwise, and an edgewise vibration adds cycles the root design counted differently.
None of these appear immediately. A blade that has been vibrating edgewise for a season shows its history a year later as a trailing edge that has opened between inspections.
What SCADA shows, and what it does not
Standard SCADA rarely records blade vibration directly. What it does record can still show an edgewise problem, if anyone looks:
- Nacelle or tower vibration alarms that cluster at particular wind speeds, yaw errors or turbine states. Edgewise blade vibration transmits to the nacelle through the hub.
- Power curve scatter and pitch activity at high wind, which show sections entering stall.
- Rotor imbalance indicators where the turbine has them, and pitch angle differences between blades.
- The record of the turbine when parked: yaw position relative to wind direction during storm shutdowns, and any vibration alarms raised while stopped.
What SCADA does not show is the blade's own edgewise motion. Root bending moment sensors, where fitted, and blade-mounted accelerometers see it directly; the trade-offs of fitting them are covered in blade condition monitoring systems. Without sensors, the most reliable indicator remains the inspection record: a trailing edge that opens faster on one turbine, one wind sector or one blade type than the fleet average is an edgewise question until proven otherwise.
What an owner can do
- Ask the manufacturer what edgewise vibration protection the turbine has: dampers in the blade, a controller strategy that detects and reacts to edgewise oscillation, and rules for parking and idling in storms.
- Check the storm shutdown procedure against the manufacturer's recommended yaw and pitch positions, and confirm the turbine actually reaches them.
- Treat ice, heavy erosion and pitch faults as edgewise risks, not only as production losses.
- Trend trailing edge findings by turbine, wind sector and blade, and treat an outlier as a diagnostic finding.
- After a suspected episode, inspect the trailing edge at maximum chord internally, because that is where the evidence is.
Frequently asked questions
What is edgewise vibration of a wind turbine blade?
Oscillation of the blade in the rotor plane, along the chord, at the blade's edgewise natural frequency. It is lightly damped aerodynamically because chordwise motion changes the angle of attack very little, so it decays slowly and under stall conditions can grow.
Can a parked turbine suffer edgewise vibration?
Yes. A stationary blade at a large inflow angle, when the turbine has yawed away from the wind in a storm, can see vortex-induced and stall-induced vibrations. Long modern blades are more exposed than older ones.
What damage does edgewise vibration cause?
Mainly trailing edge bond line fatigue and the transverse cracks and panel buckling that follow once the trailing edge has opened, concentrated near maximum chord. It also adds fatigue cycles at the root.
How do I know if my turbines have an edgewise vibration problem?
Vibration alarms that cluster by wind speed, yaw error or turbine state; power curve scatter at high wind; and above all trailing edge findings that grow faster on particular turbines or blades than the fleet average.
How Apex Wind can help
We are an independent blade engineering consultancy in Denmark, independent of every turbine and blade manufacturer, and we sell no sensors, dampers or monitoring products. When trailing edge damage is recurring and the question is whether edgewise vibration is behind it, our blade failure investigation reads the damage, the SCADA record and the operating history together. Where a blade design or a modification raises an edgewise question, our blade design and structural assessment addresses it at the source. If you have a trailing edge that keeps opening, contact us.