POINTS OF DETAIL

With all tapered wings on models, the tip chord should err on the generous side to avoid tip stalling caused by scale effects and laminar separation. The aerofoil section at the tip should normally be thinner than at the root, for the same reason.

Whatever the planform of the model, serious losses occur if there are gaps through the wing at any point These often do appear in flight, particularly on large sailplanes, where the wings flex and work slightly apart at rigging joints. Through such gaps the air flows from the high to the low pressure side of the wing, creating turbulence and reducing lift Control gaps have similar effects. All such leakages should be carefully sealed.

6.6 WING TIPS

Compared with aspect ratio and the general planform and twist of the wing, wing tips are of small importance in terms of drag saving although, if a wing has a very bad tip, the resulting disturbance of airflow may cause tip stalling. In the case of a radio controlled aircraft, aileron control may be affected. In general, however, the difference in performance between a model with a good tip shape and a poorish one will be barely detectable in flight There may be something to be gained by trial and experiment, but probably not very much. Practical aircraft wings must end in tips of some kind, and wherever there is a difference of air pressure above and below a wing (or tailplane, fin, forewing etc.) a vortex will form at or near the tip. There will be a drag penalty. The greater the relative difference of pressure (i. e. the higher the Cl) the more severe the penalty will be. One of the reasons why biplanes and triplanes, and other types of multiplane, are relatively inefficient in terms of drag relative to lift is that instead of the two tip vortices of a monoplane wing, there are four, six or more.