Helicopter Tail Rotor Design

Torque is applied to helicopter rotors to keep them spinning and generate lift required for flight. Unfortunately, an equal amount of torque is applied back to the airframe. So, while the engine pushes the main rotor to keep it spinning, it also pushes the airframe to spin the opposite direction. To prevent a helicopter from spinning out of control, something must counter this. There are many ways to do this, but the tail rotor has been the most successful. In this article, we explain some considerations when designing a tail rotor.

How does a tail rotor prevent a helicopter from spinning out of control?

Before getting into tail rotor design, let’s discuss what it does. Like the main rotor, the tail rotor spins, creating lift on its blades and therefore thrust. However, tail rotor thrust is mostly lateral, whereas main rotor thrust is vertical (to lift the helicopter). If the "counter torque" from spinning the main rotor is clockwise (CW) when viewed from above, the tail rotor will thrust to the pilot’s right in the diagram below, providing a counterclockwise (CCW) yaw moment and “canceling out” the counter torque from the main rotor.

Like the main rotor, tail rotor blades may be feathered to increase/decrease thrust. Pilots control the feathering of the tail rotor with pedal controls. This is necessary because the main rotor torque changes substantially in different flight conditions, and hence the countering torque from the tail rotor must as well. It also allows the pilot to turn the nose left/right by increasing/decreasing tail rotor thrust above/below the amount required to counter the main rotor torque.

High-level design considerations

Tail rotor aerodynamic behavior can be hard to predict. Despite simulations with carefully constructed computer models of the rotor, there are surprises in flight tests, particularly in conditions where the air is highly disturbed by the main rotor and other components. This designer must account for these uncertainties by leaving margins for error.

Many of the design parameters discussed below have a substantial impact on each other. For example, decreasing the nominal rotor speed will likely necessitate an increase in chord length, diameter or the number of blades. Hence, most parameters cannot be selected in isolation from the others.

As with many design problems, there are multiple goals that are, to some extent, contradictory. For example, we typically want to minimize cost and noise while maximizing efficiency, stability and maneuverability. The ultimate design is a tradeoff highly tailored to the requirements of the aircraft being designed.

Placement

Since the tail rotor’s primary job is to produce yaw moments, it should be located as far back (aft of the CG) as feasible. The yaw moment produced by the tail rotor is the product of the thrust and the distance from the CG, so a tail rotor twice as far aft produces twice as much yaw moment with the same thrust.

Helicopter’s typically have a vertical fin to improve directional (yaw) stability. Like the tail rotor, the vertical fin is optimally placed as far aft of the CG as possible. The result is that the tail rotor typically overlaps the vertical fin, creating an aerodynamic blockage effect. The tail rotor can be laterally separated to some extent, but not nearly enough to eliminate this effect (separating these adds weight and cost). If the tail rotor thrusts to the right, it blows air (induced velocity) to the left. This induced velocity is smaller upstream of the rotor, so the tail rotor will almost always sit laterally to the left side of the vertical fin (from the pilot’s point of view). Of course, if the main rotor spins the opposite direction then the tail rotor will thrust the opposite direction and be optimally mounted on the right side.

Blade size and count

To select the tail rotor size, the maximum thrust required by the tail rotor should be known. Typically, this will be the thrust required for adequate yaw maneuvering in the highest torque scenarios (e.g. max hover altitude or max speed). The thrust required by a tail rotor is much less than that for the main rotor, so the diameter is typically less than one quarter of the main rotor diameter. Additionally, there are geometric limitations to tail rotor diameter to prevent it from striking the ground or other helicopter components (e.g. the main rotor and stabilizer).

As with any rotor, a larger diameter is more efficient in terms of thrust per unit power. However, this comes at the cost of added weight, which the main rotor will need to lift. Additionally, this added weight at the tail of the aircraft can have a substantial effect on the CG—a heavy tail rotor could shift the CG too far aft.

The chord length of the tail rotor blades can be computed as the amount required to achieve the maximum thrust plus an error margin, at the lowest air density. Thrust is roughly proportional to the chord length and hence the chord is highly dependent on rotor speed, diameter and number of blades (all of which contribute substantially to the thrust).

Highly coupled with the decision of rotor diameter and chord length is the number of blades. Tail rotors typically have 2 to 5 blades. More blades lead to better aerodynamic efficiency by reducing the chord length required, increasing aspect ratio and reducing tip loss. However, cost and weight typically increase with the number of blades.

Rotor speed and rotation direction

Like the main rotor, the tail rotor is mechanically linked to the engine output shaft and turns at a speed proportional to the engine and main rotor. A tail rotor typically rotates about 5x faster than the main rotor on the same aircraft. This makes the tip speed—speed at the tip of the blades—comparable to the main rotor. Higher rotor speeds allow for smaller or fewer blades (and therefore lower weight), but typically come with added noise and potential compressibility effects.

Although not fully understood, helicopter makers have found that the forward side of the tail rotor (closer to the pilot) should rotate up (the aft side down). This has to do with aerodynamic interference with the main rotor wake, and experiments have shown substantial reductions in efficiency when flipping the rotation direction, at least in some flight conditions.

Concusion

We provided a very rough outline for how a designer may choose some of the high-level parameters for a tail rotor including the location, diameter, blade chord, number of blades and tip speed. Of course, a real design includes many more details based on careful analysis of the aircraft requirements and goals. We plan to update this page in the future with more details including a table of many of the design parameters for existing tail rotors. We hope that, at this point, you have a basic intuition about how a tail rotor works and the effect of these high level parameters. If you have questions or would like to request addition information, please contact us.