Helicopter Aerodynamics: Understanding How Helicopters Fly

Key Takeaways

• Helicopters take advantage of free stream flow along a rotor blade to produce lift and thrust.

• The blades on a helicopter’s main rotor have an angle of attack, which plays the same role as a wing in an airplane.

• The tail rotor is responsible for stabilizing the helicopter so that it does not rotate under torque from the rotor.

The correspondence between helicopter aerodynamics and airplane aerodynamics spans beyond the need for free stream flow across an airfoil. Helicopter aerodynamics involves the same forces that arise in airplane aerodynamics, but these forces arise in different ways due to fluid flow across the aircraft. In this article, we’ll look more at the basics of how a helicopter generates its lift and thrust with only a single main rotor as well as how the design of the rotor influences helicopter aerodynamics.

Overview of Helicopter Aerodynamics

All helicopters have two rotors that generate the lift and thrust required to steer the aircraft as well as stabilize the helicopter against unwanted rotation. Attached to the engine are the main rotor blades, which rotate against the surrounding air to produce a flow along each rotor blade. Technically, a helicopter’s rotor blades are a set of airfoils, and they can produce lift in the same way as the wing on a fixed-wing aircraft.

A helicopter’s main rotor interacts with the surrounding airflow to manipulate the main aerodynamic forces in the following manner:

• Lift: As the rotor blade spins, airflow across the bottom of the rotor blade produces lift to counteract gravity.
• Gravity: Obviously a helicopter does not manipulate gravity, but by exerting just enough lift to counteract gravity, the helicopter can hover at a fixed altitude.
• Thrust: Unlike fixed-wing aircraft or jets, thrust is not produced by the engine directly. Instead, the rotor is tilted, which orients the lift vector away from the vertical direction.
• Drag: As the helicopter moves, airflow across the body creates drag due to the formation of a boundary layer.

It should be clear as to the function of the main rotor: to provide lift and thrust, depending on the relative orientation of the rotor blades and the body. We can now dig a bit deeper into the function provided by each of these elements in helicopter aerodynamics.

Angle of Attack and Tilt on the Rotor Blades

The role of the rotor is two-fold: it converts lift into thrust and it needs to generate lift. The former is accomplished by tilting the rotor using the cyclic pitch control while the latter is determined by the angle of attack of the rotor blades and the length of each rotor blade. Larger blades, faster rotation, and an appropriate angle of attack can produce maximal lift on the helicopter during flight.

During flight, the oncoming free air stream will imbalance the lift provided by the rotor, which will create a rolling motion. This is balanced by designing the rotor to have flapping blades, meaning the blades can naturally tilt in response to an imbalance in lift.

The hinge on the rotor blade is designed to balance lift across the span of the rotor blades

Tail Rotor and Torque

The tail rotor plays an important role in helicopter aerodynamics. As the engine causes the main rotor to spin, each angular impulse generates a torque on the main rotor. According to Newton’s third law, the same angular impulse and torque is exerted against the engine and the body of the helicopter, but in the opposite direction. The tail rotor compensates for this additional torque generated by the main rotor and keeps the heading of the helicopter fixed. Without the tail rotor, the helicopter’s body would have a tendency to spin in the opposite direction as the main rotor.

These rotor blades have a shallow attack angle so that they can produce some torque on the helicopter and balance the counter-rotation of the body during flight

The tail rotor serves a secondary purpose: to allow the pilot to rotate the aircraft about its vertical axis of orientation. The tail rotor blades have an angle of attack that creates a region of low-speed, high-pressure airflow along the flat side of the rotor. This high-pressure region generates some torque about the helicopter’s rotor axis, causing the helicopter to rotate. By modulating the speed of this rotor, the pilot can rotate or steer the helicopter as needed.

Designing the Appropriate Helicopter Aerodynamics Parameters

The flight parameters listed above (tilt, angle of attack, and tail rotor speed) all need to be designed based on the expected airflow across the main rotor during rotation as well as the torque required to stabilize the aircraft during flight. CFD simulations across the rotor elements can be used to determine lift, while flow across the body during flight determines drag. Taking these two simulations together, a systems designer can determine which of these parameters might need to be adjusted to ensure minimal drag and maximal thrust within the limits of the engine capabilities and rotor speed.

Since rotor speed and orientation are critical to determining the other important parameters governing helicopter flight, their effects on fluid flow across the rotor are important to analyze. A RANS model can be used to predict the transition from laminar to turbulent flow during rotation. To prevent flow separation along the top side of the rotor for excessive attack angle, the transition to turbulent flow should be analyzed and balanced against the available rotor speed, tilt, span, and shape to ensure maximal lift is achieved during flight.

Systems engineers that study helicopter aerodynamics for aircraft design can use the fluid dynamics analysis and simulation tools in Omnis 3D Solver from Cadence to build and run their CFD simulations. Modern numerical approaches used in aerodynamics simulations, turbulent and laminar flow simulations, reduced fluid flow models, and much more can be implemented in Cadence’s simulation tools.