STRUCTURAL PROBLEMS

From the engineering point of view, high a. r. wings are thinner and narrower at the roots, where stresses are high. There is less space in the wing for spars, fittings, servo mounts,* etc., while the long span, relative to wing area, increases the bending moments. Wing tips are more vulnerable, because more likely to hit the ground in a low turn, yet they need to be lightly built to reduce inertia in roll and yaw. During building, the long, narrow wing requires greater care, since not only are warps more likely to develop, because of the greater length, but their effect in flight is greater because of the steep lift curve slope and added sensitivity to smaller angular changes.

In spite of such problems, for all models which are expected to soar in weak upcurrents or to climb on low power, the high aspect ratio wing is essential. The inevitable penalty in terms of extra structural weight and higher wing loading is repaid, for the radio controlled sailplane, since high wing loading aids the glide at speed for penetration. For ‘duration’ models too great a weight penalty is not acceptable because of the large influence of weight on the rate of climb. Even so, if the high a. r. wing can be built down to the minimum weight required for a contest model, the performance improvement on the glide will be large providing that the Reynolds number is not too low for the aerofoil section.

Aerobatic models must not be over-sensitive in pitch, and must be quick in the roll. A high aspect ratio is most undesirable for such models, although as will be mentioned again in Chapter 13, long, narrow ailerons are superior to short, broad ones of the same area. For pylon racers and speed models, aspect ratio is relatively unimportant from the drag point of view, and as with the aerobatic type, sensitivity in pitch, especially at the turns, is undesirable since it may lead to high speed stalling and an inefficient, wavering flight caused by small over-corrections by the operator.