Naval Postgraduate School, USA
If the physics of the desired control is to separate the boundary layer on the upper surface, why is the air introduction advantageous? Why not use water injection and avoid undesirable lift fluctuations? Even better, why not induce the boundary-layer separation point, and thereby control its angular motion (a few millimeters), the length of the separation zone, and thus the lift as in the case of thrust controls for missiles? Your comments will be appreciated.
One of the reasons for using air introduction on the ride control foils is to overcome the force limitation cavitation puts on the foils. At the current speeds of high speed craft, most notably catamaran ferries, ride control foils are required to be quite large, thus heavy, to produce the required force without cavitating. Air injection on the surface of the foil gives the ability to greatly adjust the lift of the foil as well as delaying the onset of cavitation. While a movable plate along the foil would give us the ability to control the lift of the foil to a certain extent, it would invariably induce cavitation along the foil. Air injection allows greater control of the cavity along the entire foil with variable flow rates of injection and the ability to use multiple vents along the surface of the foil. By using a large flow rate injected at the leading edge, a large force can be generated to dampen motions in a heavy sea-state, whereas in relatively calm seas, short bursts can be initiated toward the trailing edge, thereby reducing drag when large damping forces are not needed.
The choice of air entrainment instead of using water is being pursued due to the ease of implementing such a system. While it is possible to use the surrounding water as the injection fluid, this would require a more robust support system than that of air injection. One of the aims of this project is to reduce the number of through-hulls below the waterline. Whereas atmospheric pressure can easily be supplied within the strut, water would require a port in the strut or along the hull. It was therefore decided that air would be a more advantageous medium.