The cushion pressure, the volume flow rate and the lift power are obtained by
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
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
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
where It and Le denote the transmission efficiency and the shaft horse power of the engine.
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