THE WING ANGLE OF INCIDENCE

For fast models, including cross country and ‘F3B’ sailplanes, it is important that when the model is at high speed, the fuselage should be aligned as closely as possible to the airflow. At low speeds, since parasite drag is less vital, this is not so important, though of course some reduction in drag will result if the fuselage is accurately aligned. Visual judgement of the model in flight is a rough guide but it must be a judgement of the fuselage’s angle relative to the true flight path. With gliders, the flight path is always inclined somewhat downwards, so a glider which, when trimmed, adopts an apparently ‘nose up’ attitude, will actually sink a little more rapidly due to extra drag than one which has the fuselage pointing directly along the path of glide. At high speeds the same applies – a model which, at maximum speed, appears to fly either nose up or down has its fuselage at an inefficient angle of attack to the airflow. It is best in design stages to think of the wing and tail as being fixed to give flight in one desired position, and then the angle of the fuselage is adjusted to this, rather than thinking of the fuselage as fixed with the wings set at some angle of incidence. Unless carefully designed, the flight line will certainly not be direct extension of the fuselage datum line on the plan (see Fig. 1.3). Trimming the model, by adjusting the centre of gravity and altering the relative angles, one to the other, of wing and tail, will determine the angle of attack of the wing. The flight path will then be at the angle to the wing. The fuselage should be set at this angle to give least drag in that condition. If suitable wind tunnel test results are available calculations can be of assistance. By studying the results given in Appendix 2, the Cl at which the model will operate may be found. From the section test results, corrected for aspect ratio and downwash effects, the geometric angle of attack at which this Cl develops may be estimated. If the wing is twisted and tapered, the average value for the wing as a whole should be taken, rather than that at one station, such as the wing root The method is explained with some examples in Appendix 1.

The validity of this method depends on the model in practice being correctly trimmed at the designed Cl, and the wing profile being accurate and reproducing fairly closely the wind tunnel figures. However, small errors of the wing setting angle will not make a great deal of difference in practice. The calculations should be regarded as a safeguard against gross errors in design, and in the workshop the modeller should maintain as high a standard of precision as is practical.

11.5 CANARD FOREPLANE DRAG

Some discussion of potential savings in vortex drag appears in Chapter 12. Canard foreplanes work in relatively undisturbed air and should be designed for laminar flow, to save parasite drag. The wake from the foreplane may turbulate the flow over some of the mainplane. ]5g

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