Page 142 - Theory and Design of Air Cushion Craft

Problems concerning ACV/SES take-off 125

(RJW)
0.06
o Boeing Corporation
n Bell Textron Regressive curve
A Aerojet General
0.04

0.02

4 Fr

Fig. 3.38 Skirt drag of ACV running on ice as a function of Fr.

third to one-fifth of normal design speed. The physical phenomenon of take-off is
therefore considered here and some comments on craft optimization presented.
When craft speed increases, at Fr of about 0.38 the craft begins to ride between two
wave peaks located at the bow and stern respectively. The midship portion of the craft
is then located at a wave hollow and a large outflow of cushion air blowing up water
spray is clearly observed in this region, as shown in Figs 3.18(c) and 3.39. This in turn
reduces the air gap below the bow and stern, which in the present case with wave
peaks located at the bow and stern seals, would result in contact of water with the
planing surface of the seals and present a new source of drag acting on the craft.
This condition was investigated by MARIC by towing tank model experiments. The
surface profile was obtained with aid of a periscope and photography [28].
Seal drag consists of two parts. One part is the induced wave drag of the seals and
the other is frictional drag acting on the planing surfaces. A large amount of induced
wave drag can be built up when the seals are deeply immersed in water and the plan-
ing surfaces contact at large angles of attack.
The skirt-induced wave is also superimposed on the wave system induced by internal
cushion pressure and constitutes secondary drag. In the case of poorly designed seals
or skirts, the peak drag at Fr = 0.38 may be larger than that at Fr = 0.56 (main resistance
hump speed). Meanwhile, transverse stability will most probably also decrease.
A craft will tend to pitch bow down when the craft has a rigid stern seal (such as
fixed planing plate with a large angle of attack or a balanced rigid stern seal) and a
relatively flexible bow seal. The craft will most probably be running at a large yawing
angle as well, due to poor course stability. The operator of the ACV or SES will be
obliged to use the rudder more frequently.
The forces arising from these situations are complicated and quite large in magni-
tude. Meanwhile the ship may be difficult to control, the propulsion engines are over-
loaded and a lot of water spray is blown off from the air cushion and flies around the
craft, interfering with the driver's vision, making handling of the craft even more dif-
ficult. Operation would probably become very complicated if the sea were rough
rather than the calm conditions considered in this chapter. Such phenomena are the
features of a craft failing to accelerate successfully through secondary hump speed.
Meanwhile, if the thrust of the propellers is larger than the resistance of the craft,