Fuselage Length, Lfus

This is along the fuselage axis, measuring the length of the fuselage from the tip of the nose cone to the tip of the tail cone (which is unlikely to be on the axis). This is not the same as the aircraft length, L, shown in Figure 3.49a.

1.23.2 Fineness Ratio, FR

This is the ratio of fuselage length to average diameter, FR = L/Dave. A good value for commercial transport aircraft design is from 8 to 10.

1.23.3 Fuselage Upsweep Angle

In general, the fuselage aft end incorporates an upsweep (Figure 3.49b) for ground clearance at rotation on takeoff. The upsweep angle is measured from the fuse­lage axis to the mean line of aft fuselage height. It may not be a straight line if the upsweep is curved like a banana; in that case, it is segmented to smaller straight lines. The rotation clearance angle is kept to 12 to 16 deg; however, the slope of the bottom mould line depends on the undercarriage position and height. Rear-loading aircraft have a high wing with the undercarriage located close to the fuselage belly. Therefore, the upsweep angle for this type of design is high. The upsweep angle can be seen in the elevation plane of a three-view drawing. There is significant varia­tion in the upsweep angle among designs. A higher upsweep angle leads to more separation and, hence, more drag.

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