GROUND EFFECT VEHICLE


With the development of aerodynamics, we can easily design an aircraft which can be operated under low altitude ( WING IN GROUND ).


                     Clark Y airfoil


Bernoulli theory:

The most common explanation of the concept of lift is based upon the Bernoulli equation, an equation that relates the pressures and velocites acting along the surface of a wing. What this equation says, in simple terms, is that the sum of the pressures acting on a body is a constant. This sum consists of two types of pressures: 1) the static pressure, or the atmospheric pressure at any point in a flowfield, and 2) the dynamic pressure, or the pressure created by the motion of a body through the air. Since dynamic pressure is a function of the velocity of the flow, the Bernoulli equation relates the sum of pressures to the velocity of the flow past the body. So what this equation tells us is that as velocity increases, pressure decreases and vice versa.

To understand why the flow velocity changes, we introduce a second relation called the Continuity equation. What this relationship tells us is that the velocity at which a flow passes through an area is directly related to the size of that area. For example, if you blow through a straw, the air will come out at a certain speed. If you then blow in with the same strength but now squeeze the end of the straw, the air will come out faster.

So how do these equations relate to our two-dimensional airfoil? Look again at the Clark Y and notice that an airfoil is a curved shape. While the bottom is relatively flat, the top surface is thicker and more curved. Thus, when air passes over an airfoil, that flow over the top is squeezed into a smaller area than that airflow passing the lower surface. The Continuity equation tells us that a flow squeezed into a smaller area must go faster, and the Bernoulli equation tells us that when a flow moves faster, it creates a lower pressure. Thus, a higher pressure exists on the lower surface of an airfoil and a lower pressure on the upper surface. Whenever such a pressure difference exists in nature, a force is created in the direction of the lower pressure (since pressure is defined as force per unit area). Think of it as the upper surface being sucked upward. This upward force, of course, is lift. It is this theory that appears in most aerodynamic textbooks, albeit sometimes with incorrect assumptions applied and conclusions drawn.


To understand what ground effect is and how it functions, we first need to take a step back and explain some aerodynamic properties of an airplane wing. When producing lift, a wing generates strong swirling masses of air off both its wingtips. As discussed in a previous question on the creation of lift, a wing generates lift because there is a lower pressure on its upper surface than on its lower surface. This difference in pressure creates lift, but the penalty is that the higher pressure flow beneath the wing tries to flow around the wingtip to the lower pressure region above the wing. This motion creates what is called a wingtip vortex. As the wing moves forward, this vortex remains, and therefore trails behind the wing. For this reason, the vortex is usually referred to as a trailing vortex. One trailing vortex is created off each wingtip, and they spin in opposite directions as illustrated below

                                 Creation of trailing vortices due to a difference in pressure above and below a lifting surface

While trailing vortices are the price one must pay for generating lift, their primary effect is to deflect the flow behind the wing downward. This induced component of velocity is called downwash, and it reduces the amount of lift produced by the wing. In order to make up for that lost lift, the wing must go to a higher angle of attack, and this increase in angle of attack increases the drag generated by the wing. We call this form of drag induced drag because it is "induced" by the process of creating lift.

                     Effect of downwash in decreasing lift and increasing drag


That having been said, let us now explore what happens when an aircraft flies very close to the ground. The phenomenon is most often observed when an airplane is landing, and pilots often describe a feeling of "floating" or "riding on a cushion of air" that forms between the wing and the ground. The effect of this behavior is to increase the lift of the wing and make it more difficult to land.

However, there is no "cushion of air" holding the plane up and making it "float." What happens in reality is that the ground partially blocks the trailing vortices and decreases the amount of downwash generated by the wing. This reduction in downwash increases the effective angle of attack of the wing so that it creates more lift and less drag than it would otherwise. This phenomenon is what we call ground effect, as shown below.

                                       Ground effect and its influence on trailing vortices


An additional bonus of ground effect that becomes more significant as speed increases is called ram pressure. As the distance between the wing and ground decreases, the incoming air is "rammed" in between the two surfaces and becomes more compressed. This effect increases the pressure on the lower surface of the wing to create additional lift.

As you might expect, the impact of ground effect increases the closer to the ground that a wing operates. As indicated in the plot shown below, ground effect typically does not exist when a plane operates more than one wingspan above the surface. At an altitude of 1/10 wingspan, however, induced drag is decreased by half.

                          Decrease in induced drag due to ground effect


When all of these benefits are taken into account, we find that a vehicle operating in ground effect has the potential to be much more efficient than an aircraft operating at high altitude. The aerodynamic efficiency of an aircraft is expressed through a quantity called the lift-to-drag ratio, or L/D. In steady, level, non-accelerating flight, a plane's lift is equal to its weight, and the amount of thrust required is equal to the drag it produces. Therefore, the L/D ratio is a measure of the weight that can be carried for a given amount of thrust. The higher the L/D, the more efficient the vehicle. Typical L/D values for conventional, subsonic aircraft are on the order of 15 to 20. By comparison, a ground effect vehicle could, in theory, achieve L/D ratios closer to 25 or 30.

Though ground effect has been known since the early days of flight, most pilots regarded it as nothing more than a nuisance that changed the flying qualities of their aircraft during takeoff and landing. Nevertheless, many researchers soon realized that this phenomenon could be exploited to produce a new class of highly efficient craft known as WIG vehicles.




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