Copyright © A. Filippone (1999-2003). All Rights Reserved.

Rotary Wing Aerodynamics

Modes of Operation


Listed below are the most important systems falling in the category of rotary aerodynamics. Other systems not treated here include fans, anemometers and self-spinning devices. In the list are also excluded all interference cases (for example, ducted propellers and counter-rotating props)

The modes of operation of an open airscrew (propeller, helicopter rotor, wind turbine, auto-gyro) depend essentially on the direction of the thrust and on the movement of the airscrew with respect to the undisturbed fluid.

Propellers and helicopter rotors belong to the class of directly- powered rotary-wing configurations, whereas the wind turbine is essentially an auto-gyro, e.g. a machine that derives energy through its motion.

Fig. 1 is a sketch of the fundamental aerodynamic modes of operation of some systems. D denotes the rotor disk.


Figure 1: Some common modes of operation

The Propeller

Propellers for aeronautic and marine propulsion work on the same principle, but are characterized by very different geometries and speed ranges.

Combinations used in aeronautics include counter-rotating propellers, tandem propellers, tilt rotors, etc. For marine propellers see Andersen, 1995. Efficient computational methods are reviewed by Kerwin, 1990.


The Helicopter

A helicopter rotor operates in several different states and speeds, and therefore might by the most complicated airscrew of all. A rotor in hover is a propeller at zero advance ratio; a rotor in ascent/ descent has a positive or negative advance ratio; a rotor in forward flight is intrinsically unbalanced due to a rolling moment created by the asymmetry of the loading.

Due to the high tip speeds dynamic stall on the outboard part of the blade may lead to shock pulse.

Sikorski S-55

Figure 2: Helicopter Sikorski S-55

Typical performance data of a rotorcraft include:

  • Maximum rotor disk loading
  • Maximum advance ratio
  • Maximum speed in forward flight or descent
  • Maximum lateral speed
  • Rate of climb (vertical and forward)
  • Maximum normal acceleration
  • Rotor efficiency
  • Hover ceiling in-ground (IGE) or out-of-ground (OGE)
  • Service ceiling

The Wind Turbine

A wind turbine (or windmill) is an energy conversion system based on the operation of an airscrew in free rotating motion. The sign of the thrust, torque and power are reversed, when compared with a propulsion system (ex. propeller). There are several other differences, though. One is related to the rotational speed of the rotor, that is somewhat fixed by the value of therotor solidity.

Rotors of high solidity are slow, rotors of small solidity (having a few blades) are fast. The problems of the wind turbine are many (rotor configuration, aerodynamic control systems, static and dynamic stall, yaw dynamics, aeroelasticity, airfoil selection criteria, etc.); their solution has created its own branch of aerodynamics.

Wind Farm

Figure 3: Photo courtesy of


The Autogyro

The autogyro is a freely rotating airscrew used, as the helicopter, for sustentation. The rotor must be placed at a small pitch angle to be able to auto-rotate during the forward flight. This makes the autogyro a windmill working at a very high yaw angle.

The autogyro is longitudinally more stable than the helicopter, because the rolling moment created by the unbalanced effect of power is absent, and because there are no trim changes to work with.

In spite of these advantages, the autogyro is the most unsuccessful rotary-wing system ever devised, with poor endurance, low maximum speed, and low take-off weight.

A review of the aerodynamics of autorotating systems (Lanchester propeller, finned missile, etc.) is available in Lugt, 1983.


The term turbomachinery includes a wide array of propulsion systems or components thereof: axial compressors, gas turbines, water turbines, centrifugal pumps, and more. A broad classification can be made between axial and centrifugal machines.

Axial machines fall in the category of rotary wing systems, since they can be reduced to a cascade of airfoils, and modeled with enough approximation by using methods discussed in this chapter. Centrifugal machines operate in a fully three- dimensional state, and approximate methods of their own have been developed for this purpose.

The Vortex Ring State

An airscrew is said to be in the vortex ring state of operation when it strongly interacts with its own slipstream. This typically occurs on propellers in fast backward motion, helicopter rotors in descent and manoevre near the ground, wind turbines with low wind speed regimes, as shown in Fig. 1 above.

In all these cases the fluid passing through the airscrew returns upstream around the tips, thus creating a strongly turbulent flow field, that is also very noisy. The vortex ring state is a particular case of blade-vortex interaction (BVI). The analysis is therefore fairly complicated.

Selected References

  • Glauert H. Airplane Propellers, Volume 4, Div. L, in Aerodynamic Theory, edited by Durand W.F., Dover ed. 1943.

  • Andersen P,Breslin JP. Hydrodynamics of Ship Propellers, Cambridge Univ. Press, 1995.

  • Stepniewski WZ, Keys CN. Rotary-Wing Aerodynamics, Dover Publ, Inc., New York, 1984.

  • Gostelow JP. Cascade Aerodynamics, Pergamon Press, 1984.

Full List of References

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Copyright © A. Filippone (1999-2003). All Rights Reserved.