What is Ground Effect?

The principle of Wing-in-ground-effect or (W.I.G) has been known for a long time. Ground effect is a natural phenomenon, as an aerofoil approaches the ground its lifting ability increases and the drag reduces. The lift to drag ratio increases as the pattern of circulation around the wing changes. This pattern of air around the wing causes a cushion on the underside to develop. The Radacraft has been developed as a purpose built craft to operate in the ground effect region to maximize the efficiencies of operation that can be returned to the operator in cash terms.

In the world today there are basically four (4) designs of Ground effect craft, the Ekranoplan, which is basically an aircraft configuration. The Jorg, which is similar to the Ekranoplan except that it has tandem wings. The Flarecraft (or Lippisch) which is a Reverse Delta wing configuration. The Radacraft is completely unique in its configuration and application. One of the goals of all Ground effect configurations is to create a craft with the highest possible lift over drag coefficient, to maximize the advantages of the type.

To explain lift over drag. Those of us that have ever stuck a hand out of the window of a traveling car and tilted it slightly upwards would have experienced both the tendency for the hand to lift and also try to move backwards. The first effect is called lift, the second is called drag. In this scenario, the hand is mirroring the effect of an aerofoil, or in other words, copying the effect of a wing. To obtain a Lift to drag Co-efficient, the aerodynamisists attach scales to measure how much lift is being created (the hand trying to go up) and scales to measure how much drag is being induced (the hand trying to be pushed backwards). Expressing the lift over drag as a number then tells us how much power will be required to fly, and so how efficient the craft is. The major reason for building craft to operate in Ground Effect is to obtain the highest possible Lift over Drag ratio so as to reduce power requirements and the subsequent costs.

To explain Ground Effect in its simplest form; the closer the wing is to the ground, the greater the amount of lift developed and the less drag induced, because of the interaction of the air flow around the wing with the ground surface. The same applies over water. The overall effect is proportional to the height of the wing above "ground" and the average width of the wing expressed as Height over Chord.

As can be seen from the graph, (click image for larger view) the larger the craft, the more efficient it will become because the Height over Chord ratio will become lower when operating at the same altitude as a smaller craft.

Figure 1. this demonstrates that the ultimate goal is a craft with the highest possible Lift over drag ratio to make the craft most efficient.

Figure 2. demonstrates the surrounding air flows of a wing in free flight and in Ground Effect. Operating the craft in Ground Effect has the same effect as having a much larger wing area but without the actual physical structure, weight or drag associated with it.

Figure 3. shows a computer simulation of a conventional wing profile both in ground effect and free flight. By comparing the total Cl (Co-efficient of lift) of both, it can be seen that the same wing in Ground Effect has an increase in lift of approximately 75%. You can also see that the pressure below the wing has increased dramatically, this is called the dynamic air cushion.

A WIG craft, like a Hovercraft, rides on a cushion of air. The difference is that a Hovercraft generates a static air cushion (by blowing air into a bag) whereas a WIG craft has a free ride on a cushion that is created by its own forward movement. There are also lots of other advantages of WIG craft over Hovercraft, not the least being operational altitude.

When determining the optimum performance (altitude) for a Wing-in-ground-effect craft, Figure 1. gives good rule of thumb figures to work with. Eg. Radacraft G-35 is most efficient at between 0.5 and 1 metres above the surface due to it's aspect ratio and chord length.

As a rule of thumb, Ground Effect is accepted as being up to 1 1/2 times wing span, expressed as height above surface. However, as demonstrated in Figure 1; at heights greater than half the chord length the craft tends to be less efficient.

From a regulatory point of view, the maximum Ground Effect height is seen as the service ceiling (the height above which the craft will not sustain flight under its own power). If the craft can operate full time above this height, it legally becomes an aeroplane and must meet all of the associated regulations. As examples of this, Radacraft G-35 with a wingspan of 6.5 metres and has a service ceiling of 9 metres (30 feet) approximately. The C-850 has a projected wingspan of 8.5 metres; it's service ceiling therefore will be 12 metres (40 feet) approximately.

In its simplest explanation the Radacraft could be considered as a Hovercraft with wings, by this we mean that it has all the flexibility of a Hovercraft without the inefficiencies of operation or the restrictions of effect created on the craft by the surface.

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