SAILPLANE FUSELAGES

In designing fuselages for sailplanes, some laminar flow may be expected over the front portion, perhaps as far back as the wing. This suggests that the 60% laminar low drag bodies given in Appendix 3 should always be used for the nose at least Apart from very low drag, the advantage of these bodies, designed by Young, is that even when the fuselage is at a slight angle to the local airflow, when the model is yawed or when it flies at different angles of attack, the drag is not increased. The Young bodies (as opposed to old bodies) have a low drag range analogous to the low drag bucket of NACA ‘6’ series aerofoils. Cockpit canopies, access hatches etc. should fit closely and be free from steps or humps. Taking a hint from the full-sized sailplane built in 1975-9 by Gary Sutherland in Australia, a complete nose cone of‘Young’ form was used on Australian contest model sailplanes in 1982 and since copied widely. Laminar flow is thus almost assured. Aft of the point where the boundary layer becomes turbulent, skin drag will be high. It is the practice on most full-sized sailplanes to contract the cross section of the fuselage, producing a ‘pod and boom’ or tadpole shape. (See, for example, Figs. 4.6 & 4.7). This reduces the area of skin exposed to the tuibulent boundary layer. The gain is not very large and can easily be outweighed if the contraction is too sharp. This can cause flow separation. The effects are particularly bad if the fuselage upsets the airflow over the wing roots. Some well-known full-sized sailplanes suffer from this problem. The pilot can hear, at low speeds, the flow breaking away from the wing and fuselage just aft of the cockpit area, with quite noticeable effects on sinking speed. This is particularly likely when the

sailplane is in a turn, as when thermalling, since there is nearly always some slight slip or skid, causing cross flows. For this, reason the high wing position is probably better all round than the mid-wing mounting, which is ideal for high speed flight For models, there is only small advantage in the tadpole shape unless the fuselage cross section in front has to be increased to provide internal space for radio gear, etc. For free flight sailplanes the ‘stick’ type fuselage is best. However, for a radio sailplane, a low drag body should be used for the front ‘pod’, and after the contraction, a minimum tail boom of round section is all that is required to carry the tail.