Parasite drag

11.1 THE IMPORTANCE OF PARASITE DRAG

As shown by Figure 4.10, parasite drag is a major problem for the designer of high speed models, racers and cross country or multi-task sailplanes. It is very much less important for free-flight and other duration models. The old controversy (in the days of ‘8 ounce’ unlimited rubber Wakefield duration models) between advocates of streamlined and ‘slabsided’ fuselages was partly based on a misunderstanding of this. The streamlined fuselage model gained very little in the glide, and only a little more in the faster part of the climb, from its lower drag coefficient To build a refined, streamlined fuselage always added some weight The rubber motor weight was then usually reduced, so sacrificing climb performance. In general, the same still applies, although with rubber quantities limited as they usually are, there may be a little extra weight to spare for structures and nothing is lost by refining the shape of the parasitic components (except the time taken in building them). The engine powered duration model, climbing at high speed with flaps up and at low Cl, gains more in the climb by a good fuselage design, and will not suffer for it in the glide.

There is hardly any model aircraft that could not be improved to some extent by greater attention to parasite drag. It is easy to recommend smooth and polished surfaces, sealing all gaps, burying all protuberances, such as control horns, dowel ends, rubber bands, etc. removing struts, and retracting undercarriages. The general principles are clear, but it is often very much less simple to achieve such perfection from the engineering point of view.

Any part of a model which does not contribute directly to the lift or which is not absolutely essential to control and stability should either be removed or buried inside so that the air does not flow over it. Where some component, such as the fuselage, wheel strut, engine, etc. simply must exist it should be of minimal cross section, faired, smoothed and polished. On power models, because of the disturbance caused by the propeller, flow over the fuselage is usually turbulent Little is to be gained by designing such a fuselage for laminar flow under power, although this does not mean the fuselage shape should be clumsy. Ordinates for the basic form of a streamlined body should be taken from Appendix 3. Depending on the length and cross sectional area (which should always be as small as possible compatible with good shape), the low drag body ordinates may be scaled up or down to give the plan and side view of the fuselage. It is hardly ever possible to retain the perfect form, but it should be regarded as the ideal and departures from it should be as small as possible. Probably the most likely alteration will be to simplify and extend the tail cone as suggested in Figure 11.1b, to make construction easier. This will have slight effects on drag. Protuberances such as cockpit canopies are undesirable from the aerodynamic point of view but if they are required they should be as low as possible and carefully faired. Where such things as silencers must protrude, they should be of streamlined form and carefully aligned with the average airflow, allowing as far as possible for the fact that the flow over trie fuselage itself is not straight On ‘duration’ models, propellers should fold or feather on the glide.