Standing Waves

Questions about training in general, syllabus', requirements etc
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Tower
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Standing Waves

Postby Tower » Sat Oct 28, 2006 1:14 pm

Sorry guys, but what is a standing wave, and what causes it? Do we get them up jere on the highveld?
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Fairy Flycatcher
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Postby Fairy Flycatcher » Sat Oct 28, 2006 3:41 pm

The best book on the weather to use in Microlighting is Dennis Pagen's "Understanding the Sky". And if you want to know more about South African weather and its affect on our small flying machines, get hold of Johan Anderson.

Essentially, waves are caused by strong winds (average wind speed of at least 15kt), hitting a very large obstacle perpendicular(such as a mountain), and with the general direction of the wind remaining the same, but the general strength increasing with altitude.

The air has to be fairly stable as well.

A quick, basic illustration below.

The wave often causes rotor turbulence under its crests. Sometimes, this is accompanied by a roll-clouds, which looks a bit like broken-up cumulous clouds churning around, but often the rotors are just blue-sky turbulence.

You might not see any cloud with wave activity, but lenticular clouds (photo attached), is the surest sign of wave.

Wave tends to spread out and get weaker further from the mountain which generated it.

The illustration is not quite correct though, because a little round mountain on its own is not likely to cause any wave - its just to give you an image to visualise from.

Gliders use wave activity (especially in August in the Drakensberg), to achieve heights in excess of 40 000'!

Flying in the wave is no problem, (its wide-spread smooth areas of lift and sink) but the rotors below the waves are extreemly turbulent and can be very dangerous. They extend to about the same height of the mountain, downwind.

The best way to picture anything in the air, is to imagine a it as water. A pond is still, even with rocks and logs and stuff in, but a river can be violent. Air is a fluid and behave much the same way.

I don't know of wave activity on the reef. The air tends to be pretty undstable, and that is not good for wave formation. When you have the heavy winter-inversions, and maybe strong Southerly winds on the Magaliesberg though, I would imagine its possible, but then I doubt that even that will cause sustained wave, as the upper wind is mainly a Westerly, and I don't think the wind-direction will be sustained to altitudes allowing for wave.

Best advice for anyone, anywhere, is never, ever, ever! fly in the lee of a mountain when there is strong wind. The stronger the wind, the further away from behind the mountain, and the higher. And until you know the area where you are well enough to safely do otherwise, don't fly when there are lennies in the sky.
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gertcoetzee
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Postby gertcoetzee » Sun Oct 29, 2006 1:25 pm

Tower,

You must not believe everything you read on this forum. Fairy's explanation, although superb in the use of superlatives, is quite confusing. Rather look at these pictures, and all will be much clearer. Her pictures may be quite colourful, but in the first one, she obviously directed the camera over dem mexicans just as they did a sink, and the second picture must have been the instructions she gave dem mexicans, no wonder they didn't do de lift when she took de picture.
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Tower
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Postby Tower » Sun Oct 29, 2006 5:49 pm

Thanks Fairy, very informative, and I agree with you, dont think we will get too many of those up here, as you said you need a constant sustained wind which we hardly ever have. we do get rotor turbulance though especially when aproaching the magaliesberg and the steep ridges around harties. :)

Gert, I hit one of dem mexicans last week, will cost me a new nose wheel axle, he was cunningly disguised as a hole in the grass, still looking for his mates!!!!! :evil: :evil: :twisted: :twisted:

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