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2 IMPACT LOAD IN WAVES
A WIG flies just over sea surface, impact load in waves during different phases like takeoff, landing
and cruising should be considered. However research shows that the most serious impact happens
when WIG land on rough sea. So it is the main problem to discuss in this paper.

3 THEORY FOR IMPACT LOAD IN WAVES
It's lucky that there are many research results of seaplane that is effective to be used as reference in
WIG study. According to experience of seaplane, impact load is the most serious load that a WIG
should sustain. Furthermore, it is the most serious situation when step of WIG hull impact on sea
surface symmetrically. Von-Karman and Wagner's impact theory of wedge has been used in research of
seaplane'.''.
The main difference between WIG and seaplane is that WIG is required to sustain higher sea state in
operation, including its landing. Furthermore, WIG always flies close over sea surface. It is more
frequent to encounter waves during flight. This situation must be considered in the research. In this
paper, correction about three aspects based on Wagner's theory is considered as follows:
(1) Wave effect on impact load must be considered accurately.
(2) Change of aerodynamic lift will affect impact load remarkably, so it must be considered in the

(3) At relative low speed, the effect of hull bilge immersion should be taken into account.

3.1 Basic Assumptions and Coordinate System
3.1.1 Basic assumptions
(1) WIG hull is regarded as a rigid body.
(2) Inertial force is governing while buoyancy and viscous can be neglected during landing course.
(3) Due to the time is short enough from the moment of beginning landing to the biggest overload
reached, velocity parallel to keel can be regarded as a constant.
(4) Hull rotation, which doesn't affect impact load much, can be neglected in analysis when WIG
landing, that is to say trim angle is regarded as a constant at that timei5].

3. I. 2 Coordinate system
Oxy is a fixed coordinate system where ox is an axis settled on undisturbed free surface along the
direction of velocity and oy is vertical downward. 0,s 5 is moving coordinate where 0,s is parallel to
keel with positive direction forward and 0, c is an axis vertical to keel with positive direction upward,
the origin 0, is at stern.The coordinate systems are shown in figurel.
3.2 Motion Equations and Impact Load

3.2.1 Motion equations

Forces acted on WIG are shown in figure 2. Basal equations for landing are listed as follow:

W
-xCN = F,
g