How can you improve helicopter stability in forward flight?

Helicopters are versatile and agile vehicles that can perform a variety of missions, from rescue and transport to surveillance and combat. However, they also face some challenges in achieving stability and control, especially in forward flight. In this article, you will learn about some of the factors that affect helicopter stability in forward flight, and some of the methods that aerospace engineers use to improve it.

1 Aerodynamic forces

One of the main sources of instability in forward flight is the aerodynamic forces acting on the rotor blades. As the helicopter moves forward, the advancing blade (the one that moves in the same direction as the flight) experiences a higher relative wind speed and lift than the retreating blade (the one that moves opposite to the flight). This creates a phenomenon called dissymmetry of lift, which causes the rotor to tilt and create a rolling moment on the helicopter. To counteract this, the blades are designed to flap and feather (change their pitch angle) cyclically, so that the lift is more balanced across the rotor disk. However, this also introduces other effects, such as gyroscopic precession and blade flapping dynamics, that can affect the stability and response of the helicopter.

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J. Wayne Trimmer
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StabAug -A stability augmentation system (SAS) is another type of automatic flight control system; however, instead of maintaining the aircraft required altitude or flight path, the SAS will move the aircraft control surfaces to damp unacceptable motions. SAS automatically stabilizes the aircraft in one or more axes.

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2 Control inputs

Another factor that affects helicopter stability in forward flight is the control inputs from the pilot or the autopilot. The helicopter has four main controls: the collective pitch, which changes the pitch angle of all blades simultaneously and controls the vertical lift; the cyclic pitch, which changes the pitch angle of each blade individually and controls the lateral and longitudinal tilt of the rotor; the tail rotor, which counteracts the torque from the main rotor and controls the yaw; and the throttle, which controls the engine power and the rotor speed. These controls are interrelated and require coordination and feedback to achieve a stable and desired flight path. For example, increasing the collective pitch will increase the lift, but also the torque and the drag, which will require more throttle and tail rotor input to maintain the speed and heading.

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3 Stability augmentation systems

To assist the pilot or the autopilot in maintaining stability and control in forward flight, helicopters often employ stability augmentation systems (SAS), which are electronic devices that sense the motion and attitude of the helicopter and provide corrective signals to the control actuators. SAS can be classified into two types: attitude SAS, which maintains a constant attitude or orientation of the helicopter; and rate SAS, which dampens the rotational rates or oscillations of the helicopter. SAS can improve the handling qualities, performance, and safety of the helicopter, especially in turbulent or gusty conditions, or when performing complex maneuvers.

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Alexandros (Alex) Orfanakos

NATO Officer
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Actually I disagree with the classification. SAS is an outer loop (meaning that the corrections applied to the rotors can't be felt in the pilot's flight controls) system used to dampen oscillations in pitch, roll, yaw and collective. The system used to maintain attitude is pitch and roll TRIM. TRIM can be also used to maintain the yaw and collective positions in order to minimize pilot efforts. TRIM actuators can be used in conjunction with Autopilot systems. TRIM is an inner loop system.

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4 Rotor design

Aerospace engineers use rotor design modifications to improve helicopter stability in forward flight. Increasing the number of blades can reduce vibration, noise, and power consumption, while also increasing lift and thrust. Rigid or semi-rigid rotors can reduce blade flapping and bending, improving the responsiveness and maneuverability of the helicopter. Hingeless or bearingless rotors eliminate the mechanical joints and bearings that connect the blades to the hub, reducing weight, friction, and maintenance. Variable-speed rotors adjust the rotor speed according to flight condition, optimizing efficiency, performance, and stability. Coaxial or intermeshing rotors cancel out torque from each other, eliminating the need for a tail rotor and reducing drag, noise, and vibration.

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Henry Prestwood

Prompt Engineer/Solidity/AWS/Web3 Developer
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Main Rotor Challenges The main rotor generates destabilizing forces on the helicopter. As speed increases, the advancing blade has higher airspeed than the retreating one, creating lift imbalance. This lift dissymmetry causes the rotor disc to oscillate, inducing vibrations in the fuselage and instability during forward flight. Dynamic stall at high speeds adds pitch and roll moments, potentially causing hazardous oscillations. Additionally, tail rotor adjustments are needed to counter main rotor torque, adding to stability complexities.

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Frank Rodríguez Paradas

PIC Bell412/B206/A109 - SIC C500/550/BE350. (Autónomo) ICAO COMMERCIAL PIC B412/B206/A109/ MI35 - ICAO COMERCIAL SIC C500/C550/B350
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La sustentación es un tema muy complejo e interesante por demás , la disimetría de sustentación de los helicópteros es la fuerza generada sobre un cuerpo que se desplaza a través de un fluido, tomando en consideración la velocidad de la pala que avanza y la reducción en la pala que retrocede así como la de dirección perpendicular a la dirección de la corriente incidente.

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Frank Rodríguez Paradas

PIC Bell412/B206/A109 - SIC C500/550/BE350. (Autónomo) ICAO COMMERCIAL PIC B412/B206/A109/ MI35 - ICAO COMERCIAL SIC C500/C550/B350
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La sustentación es un tema muy complejo e interesante por demás , la disimetría de sustentación de los helicópteros es la fuerza generada sobre un cuerpo que se desplaza a través de un fluido, tomando en consideración la velocidad de la pala que avanza y la reducción en la pala que retrocede así como la de dirección perpendicular a la dirección de la corriente incidente.

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5 Computational methods

Aerospace engineers use computational methods to analyze and improve helicopter stability in forward flight. These methods involve developing mathematical models and simulations of the rotor and fuselage dynamics, applying control theory and optimization techniques to design control laws, and using artificial intelligence and machine learning to develop and train the helicopter systems. Through understanding and applying these methods, you can enhance the stability, robustness, adaptability, decision-making, and autonomy of your helicopter, resulting in a more enjoyable experience.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Ajit Tiwari

Aviation Consultant
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We know that the helicopter is dynamically unstable in forward flight. The topic also needs to dwell upon the tail stabiliser which prevents the fuselage from following through when a gust of wind causes the disc to flap back.

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How can you improve helicopter stability in forward flight?

1

2

3

4

5

6

1 Aerodynamic forces

2 Control inputs

3 Stability augmentation systems

4 Rotor design

5 Computational methods

6 Here’s what else to consider

Aerospace Engineering

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How can you improve helicopter stability in forward flight?

1

2

3

4

5

6

1 Aerodynamic forces

2 Control inputs

3 Stability augmentation systems

4 Rotor design

5 Computational methods

6 Here’s what else to consider

Aerospace Engineering

Rate this article

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