# Hover Craft Office

Abstract of A Planning Method for Amphibious Hovercrafts

• Lift System

• There are two types of Hovercrafts. One is the single engine type ( integrated system ) and the other is the twin engine type ( non-integrated system ). In both the single and the twin engine types, lifting air is supplied to the space between the ground ( or the water ) and the bottom of the craft. This space surrounded with skirt is called air cushion. Hovercraftt is supported by pressurerized air in this space. In the single engine type, pressurerized flow by a fan is divided into lifting and propulsion by dividing plate closely located behind of the fan. So fan blads are affected by atomospheric pressure and cushion pressure during revolution, the characteristic of the fan is differ from its static characteristic. It is very difficult to estimate the characteristic of the fan operating in these circumstances. Then, an abstract of design method for lifting system of the twin engine type is described as follows. In the twin engine type, a set of a engine and a fan is commonly used as a lifting system and the other set of a engine and a propeller is used as a propulsion device.

The cushion pressure, the volume flow rate and the lift power are obtained by

(1): Pc=W/S
(2): Qc=C Dc h sqrt(2 Pc/Ro)
(3): Lc=Pc Qc

where Pc, Qc, Lc, W, S, C, Dc, h and Ro replesent the cushion pressure, the volume flow rate, the lift power, the all up weight, the cushion area, the cushion perimeter, the discharge coefficient, the hover height and the density of air, respectively. And the cushion area means the horizontal area that is surrounded with skirt hem line, and the cushion perimeter means the peripheal length of the cushion area.
From these three equations, we can estimate the static performance of the hovercraft under consideration. The hovering characteristic replesenting these equations must be combined with the fan characteristic.
The fan characteristic is represented as

(4): Pf=Pf(nf, Qf)
(5): Qf=Qf(nf, Qf)
(6): Lf=Lf(nf, Qf)

where Pf, Qf, Lf and nf denote the static pressure, the volume flow rate, the shaft horse power and the rotational speed of fan, respectively. You had better not put dynamic pressure of the fan in a thought. Because of the loss between fan and cushion, the dynamic pressure does not fully recovery to the static pressure.
The hovering characteristic obtained by eq.(1) to eq.(3) and the fan characteristic obtained by eq.(4) to (6) are related by

(7): Pc=Pf
(8): Qc=Qf
(9): Lc=If Lf

where If denotes the fan efficiency.
The combined characteristics of the craft and the fan can be obtained by solving the equations from (7) to (9). The equations from (7) to (9) can be solved graphicall, using the craft characteristic curves representing the equations (1) to (3) and the fan characteristic curves representing the equations (4) to (6).
The results as examples are shown in Figs. (1) and (2). In these Figs., the several points of intersection of two characteristic curves indicate the matching points of the craft and the fan.
To obtain the over-all hovering performance in which all the characteristics of the craft, the fan and the engine are taken into account, we superpose the combined characteristic curve of the craft and fan with the characteristic curve of the engine in the similar way.
The result as an example is shown in Fig. (3). In Fig. (3), the intersection of two curves indicates the matching point value where maximum hover-height is attained.
We have used here the relation

(10): Lf=It Le

where It and Le denote the transmission efficiency and the shaft horse power of the engine.

Figure (1) shows the relation between pressure and volue flow rate. : Vertical axis shows the pressure ( mmWG) and horizontal axis shows the volume flow rate ( m^3/min ).

Figure (2) shows the relation between power and volume flow rate. : Vertical axis shows the power ( ps ) and horizontal axis shows the volume flow rate ( m^3/min ).

Figure (3) shows the relation between power and revolution of fan and engine. : Vertical axis shows the power ( ps ) and horizontal axis shows the revolution ( RPM ) of fan and engine.

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