Background information
A hovercraft is a vehicle that floats frictionless upon a cushion of air. They are also sometimes known as ground effect machines (GEM), surface effect ships (SES), air cushion vehicles (ACV), and many other names. The basic principle is that high pressure air is trapped in a chamber (the plenum) under the vehicle. For this air to escape, it must squeeze itself between some seal on the vehicle and the ground (or water or snow or ice) surface. This forces the machine to rise above the surface and float practically frictionless. Once the machine is floating, it is guided by rudders, thrust propeller, and thrust port. In addition to this basic type of hovercraft, there is a hovercraft which creates air cushion at low speeds to wing in ground effect (low flying airplane) at moderate forward speeds. This is termed dynamic air cushion. This is typically called a WIG craft or Ekranoplane. (Finnish Toivo J. Kaario built and tested a ground effect machine first in 1932 and continued 1935-1936 he received Finnish Patents 18630 and 26122.) Airfoils (wings) are designed in such a way that the pressure on the underside is higher than that on the upside, thereby generating a lifting force. When the wings approach water or land, the high-pressure zone on the underside acts as an air cushion that sustains the aircraft. With this extra support, it travels faster and farther with no additional energy expended. (It's a well known fact by pilots that flying just a few metres above the sea, the needed power is much less than flying in the free air). WIG crafts are specifically designed with a shorter, wider wingspan, keeping the craft stable and aloft, making better use of ground effect. Here are couple of links related in WIG WIG page, Mauri's WIG project.
My model hovercraft
Many types of hovercraft are underactuated in that they have less inputs than degrees of freedom. In addition, a hovercraft system is nonlinear and non-minimumphase (physically manifested in the need to overcompensate to follow a path, i.e. sideslip). These factors make them difficult to steer accurately and, additionally, creates problems when designing control systems. Hovercraft are used in various forms for ice breaking, Arctic exploration vehicles, military and civilian transport, rescue craft and passenger craft. The principles also apply to spacecraft control and submersible vehicles. Thus, robust control schemes would be of value. I built a small hovercraft in order to perform experiments. The hovercraft is underactuated, it provides only vectored thrust, which complicates the problem of designed a control scheme model hovercraft project.
Design bases of hovercraft design
My hovercraft is constructed of glass fibre and it has been built by the eggshell technique in order to get optional weight and strength. A design parameter is that the lift area should be designed so that the weight to lift area ratio (cushion pressure) is below 70 kp/m2.
Cushion pressure (Pc) = Total weight of the craft
area of cushion
For example, my craft has a final weight (tank totally filled with fuel) of around 183 kp and a lift area of about 6.23 m2. This gives Cushion pressure (Pc) 29.4 kp/m2 (290 Pa). A low height is also desirable to prevent sideward winds from blowing the machine significantly of course. The velocity pressure is calculated as following:Velocity Pressure (Pv)
Pv =
½ r v²
= 0.69 m/s
This is 0.69 m/s is the escape velocity (Ve) of the air where it escapes through the hover-gap at a given cushion Pressure (Pc)The Volume of air lost (Vol) = Escape Velocity (Ve) * Escape Area ( Ae)
Vol = 0.69m³/sec * 1.31m² = 0.9 m³/s
So the craft requires 0.9 m³/s of air at 290 Pa
The calculations above are based on ideal airflow. They have been simplified and take no account of turbulent airflow, frictional losses, or variations in air density.
My craft is so called an integrated craft, which uses a single engine and single fan for lift and thrust. A splitter plate (nonadjustable) inside the air duct directs in my case about 28 % of the air through the hull, where it exits around the hull perimeter behind the skirts to provide lift. The remaining air blows out the back making the thrust. Skirt design, without any doubt, is one of the trickiest portions of the hovercraft design process. There are several skirt designs to choose from. My hovercraft uses the segment skirt. The skirt is constructed of 225 g/m2 neoprene coated nylon. This type of material is used for example making truck blankets. The neoprene coated nylon, makes the segment low stretch and non-porous. Each of 64 segment skirts was sewn together by a domestic sewing machine. The upper part of segment skirt was fastened to the upside hull by the fast connectors and the down part was connected by the cable ties. Fast connectors were used to allow easy removal and replacement of worn and damaged skirts also in rural area. The progres of my fullscale hovercraft execution project, video of Maiden Voyage, Video of GraPool on Tour, video of Maidenvoyageonice.
Trailer
Towing a hovercraft trailer is quite different than towing other trailers for example boat trailer. Hovercraft can be transported of course on flat bed style trailer like car trailer which is large enough for the craft. However there are a few things to take into account to design specially suitable trailer for hovercraft. One of most important design bases for the trailer is that the hovercraft can easily take off and load into the trailer. Most suitable for that purpose is trailer type " fly in and fly off". The fly in and fly off trailer is good to have tipper to make loading and launching hovercraft easier. It is also good to have winch mounted in case engine failure. One very handy thing is also to have a small front wheel mounted in front. Fly-in fly-off trailer,
