Fans As Actuators
Fans may be used as primary actuators for mobile robots intended to move without direct physical contact with a surface (floor, ceiling, etc). In particular, applications may include flying and hovering agents such as planes, helicopters, blimps, and hovercraft.
It is important to note that fans operate much differently than wheels or legs as they produce motion as a "force source". Without direct physical contact to a surface or ground, this will result in an acceleration. To command the position of the agent, one will need to devise a double integrating controller.
Aside from typical input power specifications (voltage / current), the physical performance of a fan is measured by the fan's max flow rate, and max pressure. Flow rate is the measure of volume of fluid (air) through a given area in a fixed amount of time. Pressure is the measure of force per fixed unit area.
Most Common Types of Fans
For more information, this link gives a good overview including pressure/volume comparison graphs.
This type of fan uses a fixed blade that produces airflow parallel to the direction of rotation. This fan design can be seen in many cooling applications, as it is designed to maximize the volume of air moved.
A centrifugal fan is constructed with an impeller inside of a rigid housing and resembles a paddle wheel. Airflow is input through the center of the impellor and is output at a single radial vent. This type of fan is designed to increase the air pressure at its output, and is used in many blowing and inflating applications
Driving and Controlling
Driving a prepackaged fan is simply a matter of controlling the motor. Most fans will be driven by brushed DC or brushless DC motors. In many applications, fans are low power, and can be driven by prepackaged H-bridge circuits or with a single MOSFET. Note that if the fan you have selected is unidirectional, there is no advantage to using an H-bridge over a single MOSFET.
Most computer cooling fans are designed to minimize EMI, and hence are actuated by brushless DC motors. Inside, they have their own brushless DC motor controller that adjusts the speed of the fan as a function of the input voltage. Although the controller accepts variable input voltages, the controller only functions for a single polarity and hence drives the fan in a single direction. From experimentation, it appears that the motor is not harmed by applying a reversely biased voltage.
Important Practical Considerations
Most prepackaged fans are optimized to spin in a single direction. In particular, both the physical shape and orientation of the blades as well as the controller for the electric motor are designed to allow for unidirectional operation. Even if the controller may be hacked to allow for bidirectional motion, the thrust will most likely be asymmetric.
To attain true bidirectional motion, one will typically have to select a bidirectional motor and propeller blade separately (avoid prepackaged fans). R/C airplane and helicopter hobby sites typically offer good starting points for such applications.
Time Constant (Phase Lag)
One of the most significant practical limitations of using a fan as an actuator is the lag when accelerating the fan. Accelerating from one velocity to another velocity (increasing/decreasing thrust) physically takes time. The amount of time is typically proportional to the difference in velocities and can be on the order of seconds.
This phase lag limitation is due to the inherent design of many prepackaged fans as they are built to provide a constant velocity with no acceleration. (They are hence actuated by a motor with a small torque) This represents a typically blowing/venting application.
The phase lag of a fan is an important consideration for applications using fans to control their motion as this will increase the complexity of the controller.