A destructive instability known as ground resonance can occur when a helicopter is on the ground.
It’s caused by resonance between an oscillation in the main rotor and an oscillation of the helicopter on it’s landing gear,
typically in a poorly maintained helicopter.
The NTSB has recorded 34 incidents in the US since 1990.
Military helicopters and unreported mild issues are not included in that number.
In this article we'll explain the causes of ground resonance.
Links to a few videos are included as well.
So what causes this destructive resonance?
We’ll start with a high-level explanation here, but provide more details further below.
The process typically starts with a helicopter rocking on its landing gear, e.g. by rolling over a bump on one side or having an awkward landing.
This rocking is more likely to occur on a wheeled landing gear with a suspension, particularly one that’s not well maintained with insufficient damping or tire pressure.
This rocking causes the main rotor hub to effectively move back/forth.
As the hub accelerates one direction, inertial forces push the blades to “bunch up” in the opposite direction.
Normally this would be very small and negligible.
However, if the frequency of the fuselage rocking on the landing gear coincides with the natural frequency of this blade motion,
the two oscillations can feed each other and grow to the extent seen in the above videos.
If the main rotor is close enough to operating speed, the pilot can take off to stop ground resonance.
Without the helicopter rocking on its landing gear, the oscillations will subside.
Of course, at lower rotor speeds this is not feasible and the pilot will typically cut power and reduce rotor speed in hopes of stopping the resonance.
Depending on the rotor design, blades may have “in-plane hinges” to relieve the root of the blade from extreme bending moments.
These hinges facilitate the “bunching up” motion we described, but typically have dampers to prevent excessive lead/lag motion.
If these blade dampers are not properly maintained, a helicopter will be more susceptible to ground resonance.
More detail on Ground Resonance
We’ll try to provide a better understanding of ground resonance here.
However, we’ll keep it at the “intuitive” level.
For a more mathematical treatment the classical reference is Coleman and Feingold (1958).
There’s a certain frequency that a helicopter tends to rock on it’s landing gear.
If you were to stand beside a helicopter and give a quick push/release at this frequency—say twice a second—you’d get the maximum roll motion from the helicopter.
If you push it more or less than twice a second it’s harder to move it.
The same is true of your car: if you stand beside it and push it intermittently you’ll find a certain frequency that maximizes movement.
Like the helicopter on it’s landing gear, the main rotor itself has frequencies at which it’s blades tend to move.
For example, if you were to push the tip of one blade up and release it, it would subsequently vibrate or ring up/down at a certain frequency.
Similarly, if you were to push the blade tip in the plane of the rotor (without turning the rotor) it would vibrate at another frequency.
Likewise if you were to twist the tip of the rotor blade, at yet another frequency.
These are called independent modes of the blades.
There’s also “rotor modes” associated with coordinated motion of all the blades together.
There are many such modes, each with its own natural frequency.
The most well known is the roughly 1/rev cyclic flap mode.
This is what’s mostly controlled by the pilot’s
cyclic control to pitch, roll and control speed.
For ground resonance, it’s the “first, cyclic in-plane rotor mode” that’s of concern.
In-plane here means the dominant blade motion is in the plane of the rotor (as opposed to flapping or twisting the blades).
First means the lowest frequency in-plane mode.
Cyclic means that the phase of each blade in it’s periodic motion is offset from other blades on the rotor by an amount equal to their azimuth offset.
For example, adjacent blades on a 4-bladed rotor are 90 degrees offset in azimuth, hence they are 90 degrees offset in phase for a cyclic mode.
If you’re new to this concept of rotor modes, this is probably confusing.
Don't worry, that's expected and we’ll provide a diagram and further discussion of this first cyclic mode below.
The image below shows the in-plane deflection of the main rotor blades associated with a cyclic in-plane mode with “half per rev frequency.”
Half per rev frequency means that the mode completes half of an oscillation in one rotor revolution.
In other words, it takes two rotor revolutions to complete an entire oscillation.
Going left to right, each snapshot corresponds to a 90degree turn of the main rotor from the prior snapshot.
For example, the first snapshot has blade1 pointing down (aft), the next snapshot is after blade1 has turned 90 degrees counterclockwise (CCW) and points to the right.
Due to the half per rev frequency, the blades are in the same position in the first and last snapshots, which are two revs (720 degrees) separated.
The tail of the helicopter is down in the picture (up is forward).
You can see at the first time point the aft blade (blade1) is lagging, aft of “straight.”
Lagging means it’s behind it’s undeflected position in the (CCW) direction of its rotational motion.
The blade over the nose of the aircraft is leading ahead of its undeflected/straight position.
The other two blades are undeflected/straight (neither leading or lagging).
A short time later blade1 advances to the right side of the helicopter.
At this time the blade is still lagging, but not as much as before—it’s 90 degrees into its 720 degree cycle.
The next blade forward is now up in the picture and leading slightly.
You can check that any two adjacent blades are 90 degrees offset in their 720deg oscillation at all times (this is the definition of a cyclic mode).
You can check to see that each blade completes a full cycle after two rotor revolutions—the bottom-right picture matches the top-left picture and blade1 is aft.
This mode is called cyclic because, at any time (any picture below), each blade is offset by 90 degrees in phase from its neighbors, which are 90 degrees offset in azimuth.
The frequency of rotor modes, including this cyclic in-plane mode, changes with rotor speed.
Hence, a helicopter that’s not prone to ground resonance at operating rotor speed may have problems at significantly smaller or larger rotor speeds.
Resonance below about 40% of operating rotor speed is normally acceptable.
There’s less energy in the rotor/system to spiral out of control, and the rotor wouldn’t normally sustain this speed&mash;in a startup/shutdown the rotor speed will ramp/transition through this range, but not maintain such speeds.
Resonance above 120% of operating speed is also considered acceptable because such a speed can and should be avoided, especially on the ground.
Rotors that are stiffer in-plane—the frequency of the in-plane motion is higher than 1/rev—are not susceptible to ground resonance.
This generally excludes 2-bladed teetering rotors.