Modes of Operation
Summary
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.
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.
Figure 2: Helicopter Sikorski S-55
Typical performance data of a rotorcraft include:
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.
Figure 3: Photo courtesy of
www.winddata.com
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.
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.
The Helicopter
The Wind Turbine
The Autogyro
Turbomachinery
The Vortex Ring State
Selected References