Weather Vane

[joegina]

How do you make a weathervane? Forget design etc, what I want to know is what properties does it need to enable it to turn into the wind. Is it to do with weight or aerodynamics or anything else?
Thanks, Johnny

[Buenchico]

Take a vertical plate of metal (of any shape) and place a vertical pivot at any point where the area of the plate on one side of the pivot is greater than the area on the other side.

When placed in the wind, the end with the greater area will have a greater pressure on it, resulting in it being pushed away. Consequently, the smaller end turns into the wind.

Diagrams here:
http://www.madsci.org/posts/archives/dec99/944 415555.Eg.r.html

Chris

[heathfield]

A link in the above link mentions 'equal mass'. This means you should add a weight to the smaller end of the vane so that the whole thing is equally balanced at the pivoting point. This makes it easer for the vane to turn, making it more effective in light winds.

[mibn2cweus]

The weather vane seeks alignment with the wind by virtue of a distinct imbalance in aerodynamic resistance between the ends of the vane with respect to the axis of rotation. The end of the weather vane with the least aerodynamic resistance is turned into the wind by the predominating force of the wind on the opposite end.

Weather vanes indicate the direction of the wind in the horizontal plane using a vertically oriented surface that extends primarily along the horizontal dimension. This surface must be free to pivot without restriction around a vertical axis and should lie within the plane of the axis of rotation. The surface area is effectively multiplied by leverage gained with distance from the axis of rotation. Stable alignment with wind direction is achieved by positioning the axis of rotation an appropriate distance forward from the vanes center of aerodynamic resistance. Surfaces that are not congruent with the direction of air flow introduce potential instability to wind vane performance. A weather vane should be placed where other structures do not significantly modify the direction of the prevailing wind.

Weight does not directly modify the directionality of a weather vane however increasing the mass increases inertia which dampens and slows the response time to changing wind direction. Weight added to balance the vane with respect to the pivot point can reduce wandering of the pivot point, reduce friction between stationary and rotating surfaces and thereby improve sensitivity.

The aerodynamic principles that induce a weather vane to point into the wind can be used to explain related aerodynamic behaviors in a variety of objects that interact with the air, from flying frogs to rockets. For example . . .

weather cocking