Drag due to interference effects

Any intersection between two surfaces such as at the wing-fuselage junction has a disruptive effect on the flow, and extra drag is incurred. Acute angles such as that formed between the wing and fuselage on either high – or low-wing air­craft are worse than oblique angles. A mid-wing position would be better from this aspect, but mid-wing designs introduce structural problems. The cabin crew in a mid-wing airliner might not take kindly to the main spar getting in the way of the drinks trolly.

On a low-wing aircraft, the fuselage can interfere with the pressure distribu­tion on the upper surface of the wing, possibly inducing flow separation. A high-wing configuration is better in this respect, as in this case, the flow on the under-surface is the most affected. The under-surface flow is normally in a favourable pressure gradient, and is thus less likely to separate. The high wing arrangement has a number of disadvantages, however, including problems involved in trying to avoid long undercarriage legs, and adverse interference effects between the wing wake and the tailplane. Notice the very high mount­ing position of the horizontal tail surface on the C-17 (Fig. 10.20), and the BAe 146 (Fig. 6.26). This is necessary, in order to keep the tailplane out of the wake of the wing at high angles of attack.

The wing-fuselage interference effect is largely a manifestation of the gap in the spanwise lift distributing mentioned above (Fig. 4.11), and can be reduced by use of a lifting fuselage as on the MiG-29 (Fig. 4.12), where the interference effect is also reduced by use of a blended wing-fuselage. A blended wing-fuselage was also used on the SR-71 Blackbird spy-plane (Fig. 6.40). In this case, the arrangement has the important advantage that the elimination of sharp junctions reduces the aircraft’s radar signature. Interference effects can also be reduced by means of wing fillets, but this feature is rarely found on modern aircraft.

A more radical solution to the interference problem is to remove most of the junctions by adopting an all-wing configuration as in the B2 ‘Spirit’ (Fig. 4.19). Large slender-delta-winged aircraft lend themselves to a nearly all-wing con­figuration, and such an arrangement was considered at the early stages of the Concorde project. The idea was eliminated because it would have required a very large aircraft, in order to provide sufficient cabin depth, and would have introduced another set of novel features in an already revolutionary design. It was also realised, that passengers would react unfavourably to the idea of having traditional port-holes replaced by overhead fanlights.

The last item in the drag budget is the undercarriage. Despite the consider­able added cost and weight of a retracting undercarriage, the benefits are so great, that fixed undercarriages are rarely used on anything other than small light aircraft. An interesting solution to the problem of undercarriages is that used on the Quickie shown in Fig. 11.9. The canard foreplane has pronounced anhedral, and also serves as the undercarriage legs. The Rutan Vari-Eze shown in Fig. 4.20 uses a retractable nose wheel which is also lifted for parking, as shown. Retracting the nose wheel saves a considerable amount of drag, and the pilot would probably get away with forgetting to lower it on landing; a com­mon error with amateur pilots.