Rotor Track and Balance

Helicopter rotors spin at high speeds and can cause substantial vibrations capable of destroying components, fatiguing the pilot / crew, and even lead to a fatal crash. To reduce these vibrations, rotors must undergo track and balance periodically. We will discuss various facets of rotor track and balance (RT&B) below. We focus on the main rotor, but analogous discussion applies to the tail rotor. The article is organized as follows.

• Track
• Balance
• When to track and balance
• Detecting problems
• Fixing problems

Track

Track measures the vertical path of all blades on a rotor. When the rotor is spinning, if all blades are at the same height when passing the same location, the rotor is said to be “in track.” If one or more blades is at a different height, the rotor is said to have a split track.

A split track could be caused by manufacturing differences between blades, lack of symmetry in the hub, or simply an installation issue like improper setting of a pitch link or trim tab. Such things cause blades to create more or less lift and hence flap (move vertically) to a different location. This primarily induces vertical vibration with a frequency of once per revolution, although other vibrations may be felt as well.

Ultimately, rotor track and balance procedures aim to minimize vibration. Once vibration-based measurements became predominant, people noticed something surprising. A perfect track often does not minimize vibration. The standard today is to leave a rotor with a split track if it reduces vibration significantly.

It’s not known why a split track can provide a smoother ride. Perhaps it’s compensating for other differences in blade shape (chord, twist, airfoil shape). It could even provide an aerodynamic benefit, such as placing a blade in a better location relative to the turbulent air produced by the blade in front of it.

Balance

Balance refers to the mass and aerodynamic balance in the plane of rotation. If one blade is heavier, has more drag, or is swept more than others, the rotor is out of balance.

An unbalanced rotor typically produces longitudinal and / or lateral vibrations around one cycle per rotor revolution. It may be caused by manufacturing differences among the blades, leading edge erosion over time, or improper installation / setup.

When to track and balance

Manufacturers recommend maintenance intervals for RT&B, just as your car manufacturer recommends oil changes every M months or N miles. Modern helicopters with health usage and monitoring systems (HUMS) can automatically determine when to perform RT&B.

RT&B is also recommended when components of the rotor, hub or swashplate are replaced or modified (e.g. the pitch links, control rods or swashplate). Such work can easily “break” the track or balance. Of course, any time the pilot / crew detect a change in vibration, RT&B should be considered.

Detecting problems

A rotor that is substantially out of track or balance will often be flagged by the pilot. She will notice more 1/rev vibrations in flight, either increasing over time or more than another helicopter she flew of the same model.

Again, a modern helicopter with HUMS can detect these problems as well—these systems continuously measure vibrations in flight and will flag the increased 1/rev vibrations over time.

In the early days, a rotor with a suspect track was measured as shown below. Maintainers would apply a different color to each blade tip. With the helicopter on the ground, rotor spinning, a material would be moved up to one location on the edge of the rotor. Each blade would paint its color on the material. If one or more colors were painted at a different height, the associated blade was adjusted as described later.

Later strobe lights were used, with special markers on blade tips to visually freeze the tip to the human eye. An advantage of this method is that it can be done in flight, not just on the ground. This method is still used today. Of course, both this and the prior method can only get the blades in track, not necessarily minimize vibration.

Nowadays, vibration sensors are frequently used to directly measure vibrations somewhere near the rotor. For the most part, blade adjustments are made to minimize vibrations. This comes with a cost, however. What if one blade is substantially out of track after minimizing vibrations? It may be OK, but it might be a sign of a bad blade or serious issue that needs attention.

Fixing problems

Track (or vertical vibration in general) is typically fixed by adjusting the pitch link of the out of track blade. This changes the blades’ pitch angle and therefore lift. (Pitch links must also be adjusted for proper rotor speed in autorotation, as described here.)

Track may also be fixed by bending a trim tab (see image below) up or down. A trim tab is a flat plate extending from the trailing edge of a portion of the blade. It changes the amount of lift the blade creates at a given pitch angle. Bending the trailing edge down increases lift, causing the blade to flap / cone to a higher location (bending it up decreases lift).

Balance is typically fixed by adding weights to the hub at designated locations. The helicopter manufacturer will typically provide areas where weights can be added / removed, as shown in the image below.

Balance may also be improved by changing the sweep angle of a blade. This means changing how the root of the blade is mounted to the hub, rotating the blade tip forward or aft, as shown in the diagram below. This moves the blade center of mass inward, toward the hub, and forward / aft when the blade is swept forward / aft. Forward sweep destabilizes a blade, so aft sweep is used more often.

Unfortunately, fixes for track often affect balance and vice versa. For example, let’s say we extend a pitch link to increase the lift on a blade and reduce vertical vibration. This blade will now have a larger drag and, even if the vertical vibration is reduced, a new lateral vibration may be created.

Another complexity is that vibrations will vary over the flight envelope. It’s not practical to track and balance a rotor in an infinite number of conditions. Nowadays, measurements will be made in a handful of (level flight) airspeeds, e.g. hover, 60 kts, 90 kts, and 120 kts.

For more detail about the track and balance process, see Helicopter track and balance theory.

Here are details specific to the Safari main rotor system.