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J. Wieringa

Abstract

Wind vane motion constants (damping ratio, natural wavelength and decay distance) are derived in a way which can accomodate both mechanical friction and the presence of a propeller. The motion is shown to be insufficiently described by a second order equation because of the way in which the aerodynamic torque changes with angle of attack. This implies that any measurements of vane constants made in the wind tunnel at initial angles of attack above 20° are not representative for the vane. Simple relations between easily measured vane dimensions and motion constants are derived, and vane motion is proved to be independent of the fin area.

The WMO requirement for wind vanes is translated into motion constants and shown to be fulfilled for any vane with a damping ratio of 0.30. For turbulence measurements a certain short-wavelength reliability limit for vane-measured spectra is proposed. Experimental comparison of basic fin configurations shows the inferiority of streamlined and splayed fins.

General vane design rules are given and are applied in the construction of an operational wind vane with a damping ratio of 0.30 and of a fast propeller bivane with an annular fin.

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J. Wieringa

Wind information for use at airports can be called representative if it provides an optimal estimate of wind variations to be expected over the runway. It is shown that a single anemometer at a nonideal but reasonable location will usually provide adequate observational data to allow such a representative wind estimate after proper correction and data handling. It follows from the properties of the surface wind field and of the instruments used to measure it that aeronautical wind averaging periods should be at least 2 min for mean winds and at least 5 s for gusts. Small-scale obstacle effects and measuring height variations can be dealt with by an objective exposure correction model. It is shown that the aggregate wind estimation error cannot be less than 10% because of the influence of spatial distance and reporting time lag. The employment of additional anemometers appears justified only by large-scale terrain influences such as mountains or sea breeze-inducing coastlines.

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J. Wieringa
and
F. X. C. M. van Lindert

Abstract

Double-fin vanes which have their fins at angles >10° with the vane axis may refuse to point into the wind, having instead two symmetric equilibrium positions at angles up to 16° with respect to the wind direction. The dependence of this effect upon vane geometry is investigated in the wind tunnel; this leads to the construction of a slow, well-damped aerodrome wind vane. Tests under natural wind conditions show that when two vanes are used, which are equally well damped but have different natural wavelengths, the difference in measured azimuth variance may attain a factor of 2 for stable stratification and weak winds. Experimental verification is obtained of a suggestion by Protopopov to salvage bad wind vanes by employing them in pairs. The practical usefulness of vane improvement by the investigated techniques and by means of frontal fins, mechanical dampers, etc., is discussed.

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