48.1 INTRODUCTION

A Wing In Ground (WIG) Craft/ Airfoilboat/Ekranoplan utilizes the so-called ground effect, in which lift increases compared to a wing in free flight, if the distance to the ground H measured from the trailing edge is less than 30% of the mean aerodynamic chord. Provided that the aerodynamic arrangement of a WIG craft is properly designed, the drag also is reduced for a certain range of attack angles ? when H tends to zero. In all cases the lift-to-drag ratio (L/D) is increased even if the drag increases. The L/D ratio can be improved by a factor of approximately 2, or even more, at the minimum possible height of flight. For comparison, the L/D ratio of WIG craft can approach the value of 30 in close proximity to the ground whereas the maximum L/D of a transonic airliner is only about 20.

A few samples of utilization of the ground effect can be found in the nature. For instance, it is used by heavy sea birds and flying fish for efficient long-distance flights over water. Ground effect is also known to pilots of seaplanes as a way to increase the range of the aircraft. For instance, the well-known German flying boat DO-X flew a long time under ground effect during the first part of its transatlantic flight in 1929.

One of the earliest WIGs was the Ekranoplan Orlyonok shown in Figure 48.1. It was designed by Dr.