North Star. Rubber Wakefield competition model.
Quote: "Martin Dilly describes the development of Jack North's highly successful Wakefield Model.
THIS model is one of a number that I (RJ North) have built, in an attempt to produce a design, suitable for Wakefield competitions, but which was simple, stable and free from any defects which might make it unreliable. The necessity for reliability is obvious and now that the old idea of unlimited fly-off times has been abandoned, there need be no compromise with reliability for the sake of maximum performance in a hypothetical fly-off. While building this series of models, I have gained some insight into the factors which influence the performance of this type of model and it seems worthwhile to briefly discuss these factors.
Theoretically, the performance of a model should be better if the aspect ratio of the wing and the diameter of the propeller are increased. We all know that increasing either brings practical problems of an engineering kind, but I felt that these could be solved if it were shown to be desirable to do so. Therefore, I did not limit my early investigations to practical shapes and sizes.
An increase of aspect ratio with a fixed wing area results in a decrease of the airfoil chord, and sections of small chord are more subject to boundary-layer separation effects, which tend to reduce the efficiency of model wings. However, different sections behave very differently in this respect, so it was decided to find a wing section which was satisfactory in small chord sizes and then increase the aspect ratio as far as practicable.
Theoretically, a large propeller diameter improves the efficiency of propulsion, but as excessive torque may be required to drive it, this torque may be difficult to control at the beginning of the flight. Further, the long blades result in a large CG movement after folding. Also as, of course, this immediately follows the stage of the climb when the model trim is decided principally by the glide trim requirements after folding, therefore, before folding there will be an unusually large nose-down pitching moment. As the modeller has it, the model will be under-elevated.
The series of models was based on a single design of fuselage, with a wing mount which would accommodate any reasonably sized wing; while the tail was the same for all configurations, since it could only affect the performance in a trivial way. The propellers could be changed on these models using the same noseblocks, a shaft and hub design which made this job a matter of a few minutes, having been developed and thoroughly tested, some years ago.
Peter Scarbrow already had a wing which produced a considerable increase in performance when it was used in place of my original 41 x 5-1/4 in design. So in the winter of 1958 he and I built a number of further wings based on his original 50 x 4-1/2 in planform, but differing somewhat in size and airfoil section. As it turned out, none of these was any improvement on Scarbrow's original choice!
Initially, a number of propellers were tried varying from 18 to 24 inches in diameter, and the simple result emerged - the larger the better!
It sounds simple enough to say that these combinations were tested. This all hinged on the weather being satisfactory, and we were very fortunate that winter in having a run of calm Sundays without complication of thermal conditions. It has not happened since. The other factor, besides the weather, which could affect the tests, was variability in rubber. This subject was covered at some length last month and I shall merely remark that the torque tester was used to select a number of suitable motors. They were used with care, and the experimental flights were planned so that rubber variations had no effect on our conclusions.
By this time the Eliminators had arrived so I assembled the two best model combinations from the parts I had made. At that stage, none of the models had pylons and in test flights, before the first eliminator, both models were crashed due to spiral dives developing after they were hit by gusts of wind. This difficulty had not appeared previously, although many flights in strong winds had taken place. It illustrates the fact that a contest is required to bring out the worst in a model! However, a model was assembled from the pieces and it did well enough in the contest, comparing favourably with the best in the London Area. Before the next eliminator the best model was repaired and I did rather better, although once more the odd spiral dive appeared, fortunately at high altitude, and the model levelled out before hitting the ground.
Two measures were taken to avoid the reappearance of this trouble - a small pylon was fitted, which to date has been completely effective, and various changes in CG position were tried. This showed that however stable the model might be in most respects with various GG positions, the transition from climb to glide was hopeless unless the CG was over the wing trailing edge as shown on the plan..."
North Star, Model Aircraft, March 1961.
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