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It seems that the Giant Scale trend in modeling has caused a rebirth in scratch building. Also, many more modelers are designing their own airframes and building from their own plans. This is not so surprising since there are still not very many kits to choose from (remember folks, this was written in 1981--RN). Additionally, I suppose that the Giant Scale movement has really caught the attention of the senior modelers who were brought up on elaborate and elegant stick frameworks (as opposed to slab-side fuselages, foam wings, etc.).
However, the art of designing and scratch-building requires somewhat more knowledge of aerodynamics than one would need in assembling a kit. Fortunately the large size of the models makes them more tolerant of the physical laws that govern flight. For example, wing loading does not appear to be as critical as for smaller models, and flight characteristics are often more gentle even for similar subjects.
The earliest subjects chosen for giant modeling were light private plans which were good flyers in full scale. Now that heavy, high-performance military aircraft are becoming more popular, modelers are again wrestling with the problems of power, weight and wing loading. What I'm suggesting is that a little washout in the wingtips is your best insurance against tip stalls. After all, washout is used in full-scale airplanes too.
For those of you who are just now designing and scratch building for the first time, I would like to discuss stalls and the effect of washout. This will be a brief and very non-technical discussion. When the wing is flying at a high angle of attack and the aircraft speed is reduced to a point when the wing stops lifting, causing a stall, the nose drops and the plane picks up speed until the wing starts lifting again and the plane recovers. If the plane stalls straight ahead, then the altitude lost depends on the aircraft configuration and wing loading. If, on the other hand, the plane stalls by dropping a wing tip and doing a snap roll, then a lot more altitude will be lost before a recovery can be made. Since stalls often happen on landing approaches, there is little margin for error.
Washout is intended to keep the wings level during a stall so that you can have a higher survival rate. Washout is a deliberate twist in both wing panels such that the tips have a lower angle of attack than the root. Thus, when the center section of the wing stalls, the tips are still lifting and the airplanes nose falls straight ahead.
To achieve this washout the trailing edge is usually raise progressively from the root outward to the tip. Figure 1 shows this when siting from the tip. Three to five degrees is generally used, with the greater amount being appropriate for the higher loaded models (especially if they have tapered wing planforms). Figure 2 illustrates the gradual increase in twist from root to tip when viewed from behind.
A common way of producing washout is to warp the covered wing over a pot of boiling water. For iron-on film covered wings, the structure can be twisted while the covering is being shrunk. On some designs the washout can be held in place by struts and flying wires. However, the most secure method is to build the washout right into the framework by placing shims under the trailing edge of the wing as the wing is being build.
This simplified discussion should leave you with the
message that washout is almost always beneficial in a model. In some
heavily loaded high performance designs it is essential in order to prevent
vicious tip stalls. Even though giant models by their nature fly better
than standard-sized models, washout is still advisable to make life easier
for the pilot and to help preserve the investment.