Helicopter Vibrations

Helicopters vibrate more than fixed wing aircraft. Inconsistent, periodic forces on the main rotor are the primary culprit. In this article, we’ll discuss the origin of these vibrations, their impact, and how to mitigate them.

Sources of vibration

All working components on a helicopter produce some amount of vibration. This includes the following, ordered from most problematic to least problematic.

1. Main rotor
2. Tail rotor
3. Engine
4. Drive system

The impact of a given source depends on where you measure. On the tail of a helicopter, vibrations are typically dominated by tail rotor sources. However, the main rotor is the dominant source on most of the fuselage, including the pilot(s) and passenger(s).

Main rotor

The main rotor is typically composed of 2 to 5 blades. As one of these blades spins, its airspeed changes as shown in the diagram below. All blades maintain a constant speed due to the rotation/spinning of the main rotor. However, as they spin a full circle, they transition between areas where this movement is with and against the motion of the fuselage through the air.

For example, consider a helicopter hovering with a headwind. If the rotor spins counterclockwise (viewed from above), then when a blade is on the right side of the helicopter it advances into the wind and feels a larger airspeed. On the left side of the helicopter the blade moves with the wind and feels a smaller airspeed.

This variation in airspeed causes a variation in the aerodynamic forces on the blade. The cyclic controls on the main rotor further change the aerodynamic forces. If there are 4 blades, then 4 cycles of these forces will be felt by the helicopter each time the rotor spins one circle. This causes what are called “4 per rev” vibrations to be felt in the helicopter, denoted 4/rev. As you might imagine, if there are N blades, this phenomenon causes N per rev (N/rev) vibrations.

Main rotors typically spin faster than 250 revolutions per minute, meaning a 4/rev vibration would have a frequency above 4*250/60, or 17 Hz. That’s 17 vibrations per second, almost like a buzzing.

As you might imagine, the strength of these N per rev vibrations varies with speed. They are worst in transverse flow and at high speeds.

While N per rev vibrations typically dominate the main rotor, other frequencies can play a large role. This is important when a rotor is out of balance or track. In these situations a 1/rev vibration may dominate N/rev vibration. These vibrations are much lower frequency, around 300/60 or 5 Hz, which can be annoying to pilots / passengers.

Tail rotor

The variation in aerodynamic forces we discussed for main rotor blades also apply to tail rotor blades. One addition is the main rotor wake impinging on the tail rotor. That wake further exaggerates aerodynamic changes as a tail rotor blade spins.

Tail rotors typically spin about 5x more revolutions per minute (RPM)—if a main rotor spins at 300 RPM the tail rotor will spin closer to 1500 RPM. This means the vibration frequencies are also about 5x larger.

Compared to the main rotor, tail rotor blades and forces are smaller—there’s less force driving tail rotor vibrations. Furthermore, the tail rotor is located on the end of a tail boom, further from the fuselage and passengers. These facts help explain why main rotor vibrations dominate.

Engine and drive system

Helicopters typically employ turboshaft engines that produce high frequency vibrations. These engines turn a series of shafts and gearboxes which produce their own high frequency vibrations. The net result of all of these sources should be small if they are installed and aligned properly.

Impacts of helicopter vibration

Vibrations give rise to a number of issues listed below.

• Passenger and pilot discomfort and health
• Passenger and pilot fatigue
• Reduced lifespan of components
• Sudden and dangerous component damage (oil leaks, cracks, …)
• Cabin noise
• If extreme, it can harm flying qualities

Human factors

Vibrations can be annoying and even dangerous to humans, both physically and psychologically. Vibrations fatigue pilots, increasing mistakes which can potentially be deadly. Vibrations annoy passengers who are often paying top dollar for a helicopter ride.

Many have studied the impact of vibrations on humans. The impact is highly dependent on frequency. Lower frequencies around 1/rev tend to excite the stomach area, while N/rev frequencies are more prone to impact the shoulders and neck. Frequencies above N/rev can excite the eyeballs, skull and jaw.

The plot below shows the levels of vibration detectable (green), annoying (yellow), and intolerable (red) as a function of vibration frequency. This is according to a famous study by Goldman in 1948. Helicopter 1/rev frequencies are typically in the grey rectangle on the left, while N/rev frequencies often fall in the purple rectangle further to the right.

Vibration impact on components

Vibrations are not just problematic for passengers. Vibrations increase helicopter maintenance costs. A helicopter with lower vibration levels will be more cost effective.

Vibrations accelerate wear and tear on many components, causing abrasive wear, fretting, or even galling between mating surfaces. Over time, fatigue failures cause cracks to develop and propagate. Components and even surfaces must be tested, maintained and replaced earlier with higher vibrations.

It’s not just mechanical components. Even electronic equipment and instruments are impacted. Vibrations can cause inaccurate readings, malfunction, or even failure. In the case of critical flight data and navigation systems, the impact can be deadly.

Mitigating vibration

Vibration mitigation is a factor throughout helicopter design. The number of main rotor blades is critical—more blades tend to cancel each other out and reduce vibration. The shape of these blades and the stiffness/damping of them, along with all other structures, may be tailored for vibration. Even with extreme care and attention, there will be unexpected vibrations once a prototype is assembled. Passive and/or active devices are typically added to reduce these vibrations. After the design is finalized and a helicopter is delivered, periodic maintenance is required to keep vibrations down—a rotor out of track/balance will produce excess 1/rev vibrations.

Design concerns

Each component has resonant frequencies it tends to vibrate at, just like a guitar string. Many components also drive vibrations at other frequencies, e.g. the engine, drive system, and rotors. Engineers carefully consider all these frequencies when designing a helicopter to reduce the chance of troublesome resonances. This is difficult work and never completely successful, hence the need for add-on vibration suppression.

Passive devices to reduce helicopter vibration

Just like the suspension on your car, connections between helicopter components may be softened to reduce the transmission of vibration. For example, mounts connecting the engine to the airframe are engineered to isolate most engine vibration. Likewise for the drive system. The most critical component to isolate is the main rotor. The path from the blades to the cabin includes a number of materials engineered to absorb/isolate rotor vibration.

Important devices employed in helicopters are tuned vibration absorbers (TVAs) and tuned mass dampers (TMDs). Such a device with a resonant frequency of X Hz would be attached to a location on the helicopter with problematic X Hz vibration. The device shakes at the problem frequency but with the opposite direction to effectively cancel out the vibration.

Active devices

Active devices generally include sensors to measure vibrations, software to compute desirable counteractions, and actuators to execute the actions.

The actions taken may include electromagnetic forces, hydraulic actuators or piezoelectric actuators. A special case of this, termed active rotor control, controls blade shape (potentially including flaps) or pitch to reduce rotor vibration.

Other active devices include ATVAs. These behave like the TVAs described above, but the resonant frequency may be actively changed by the computer (during flight), based on measured vibrations and special algorithms.