Crowfly (oz2006)

About this Plan

Crowfly. Wingspan 32 in flying wing glider. Aeromodeller July 1946.

Quote: "INTEREST in the development of successful Flying Wing types in this country has been accelerated by such contests as the Handley Page Trophy and the prospects of more events for Tailless Model Aircraft. On the Continent there has been for many years a nucleus of enthusiasts conducting research in this branch of model aeronautics, who in their turn have been unaware of British work in this sphere. Nevertheless in spite of awakening enthusiasm there has as yet beim little literature published on the subject, and certainly no attempt to publish any concise description of the sundry variants that have been developed.

Perhaps a note of warning is necessary at the outset. The experimenter may be tempted to devote his research to small all-balsa solid gliders, which in their way are interesting and instructive, but one must not be misled by the behaviour of such models, which in the main have what approximates to a flat plate airfoil and very narrow chords, not that this last is, in itself, neccessarily a bad thing.

Equally, experiments with models of too large a size are apt to be costly and limit the amount of research carried out in a given time. Probably a span of three to four feet is the best compromise, it was on a model of this size that the writer gained much valuable data; this machine, the 'Crowfly,' is reproduced in plan form and illustrated in this publication.

Airfoil Sections: The absence of the controlling effect of orthodox tail surfaces gives rise to immediate problems of stability, and this may be met, in the first instance, by a wise choice of airfoil section. There are a number of proven sections which have a high degree of stability, among which are those with reflex trailing edges such as RAF 30, RAF 33 and CLARK Y H, which are available as Airfoil Section Sheets. Using these as a basis the keen experimenter may like to develop his own section. Given the requisite apparatus, there is a lot of satisfaction in so doing, otherwise it is better to rely on a proven section ; in any event many of the so-called 'original' sections can be recognised as very near relations to existing types.

Wing Form: There is a surprisingly wide range of wing forms that may be adopted, and protagonists claim successes with many varied shapes. However, stability needs force the conclusion that a measure of sweepback or sweepforward is essential. The sweptback wing form has proven the most popular basis for experiments to date. Degree of sweepback may be varied within certain limits of say 15° and 30°, or may be stated as a total sweepback equal to one chord width, or one mean chord width in the case of tapered wings, as the effective minimum. The sweepback can be accentuated by 5° to 10° at the tips for approximately 10 percent of the span which adds considerably to the stabilizing effect of the tips.

With these types there seems no logical reason to increase the normal dihedral of from one to one and a half inches per foot of semi-span. The dihedral may vary as in orthodox model aircraft, by underslung weight or parasol wing combination being included in the design, giving pendulum stability.

To aid stability it is desirable that the outboard sections should be washed out, for an average airfoil there will be a difference of 5° to 6° between the angle of incidence of the inboard and outboard panels. This can be achieved by building it in or by having adjustable outer panels that can be varied for experiments. Some authorities believe that a definite negative incidence is preferable for these panels, particularly where anhedral tips are used. Adjustable built out ailerons are employed on some designs, and certainly give an added measure of control. Normal lift section in the centre, with increased reflexing of the trailing edge per rib spacing to the tips, is the means of obtaining washout on the 'Crowfly' and has proved very successful... "

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