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On looking through a book entitled"Yacht Racing. The Aerodynamics of Sails" by Dr Manfred Curry I found a reference to the work of Lilienthal in discovering that the wind rises at about 4 degrees..Curry discusses this in the context of observations and experiments he did with Albatros. We have all seen birds soar for ages without any wing movement, I had always assumed this was due to them getting ridge lift, but it seems the Albatros hangs there over the ocean with an angle of attack of 3 degrees negative into an average of 4 degrees rising wind to have a resultant AOA of 1 degree!What's this got to do with gyros you may well ask? If you draw the forces involved out, as we are so fond of doing, to explain the wonders of autorotation, you will note that there is actually a forward component acting on the wing. The question then arises as to whether or not Cierva and his contemporaries used this information to investigate autorotation. Curry's book was originally published in 1929.I don't think we realise how smart our forbears were!John EvansThink logically and do things well, think laterally and do things better.
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I think I do John..I know you're a good deal smarter than me..Cheers,Nick.
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Before we pat ourselves on the back for the knowledge of our forbears we had better decide which side we were on. The Dutch discovered that the wind went down about 3 or 4 degrees due to ground friction. All their windmalls had slightly angled heads to allow for this.Tim McClure
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Tim, can you build a head to compensate for the 3 or 4 degrees wind down, might help those muster's keep on top of things
!You never know how far you can go, till you get there !
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This is most intrigueing! Surely they couldn't have both discovered the opposite thing!Tim, you mention friction, which I understand is the explanation for this phenomenon. My understanding is that when the wind is slowed by friction the pressure rises, and so pushes the following air mass up. Think of it as the opposite of what happens in a venturi, where air is accellarated resulting in a pressure drop.So which way are these windmill heads tilted? I will try to find out.You've got me a bit worried Nick, some people might hold it against me if you think of me as a 'forebear'.A Palestinian friend of one of the kids was confused when I asked if she knew much about her forbears. She asked what forebears were. I couldn't resist telling her they were the ones who came after the three bears.John EvansThink logically and do things well, think laterally and do things better.
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See if I can post a picture of a Dutch windmill.Image Insert:
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A little bit of trivia.Some interesting specs. of the above MillTower = 40 ft tall (four stories)Foundation = 2 metres wide X 1 metre deep concrete ring.Sails = 80 ft diameter X four blades with aerodynamic helical twist."Cap" weighs eleven tons.The sails and tail pole also weigh eleven tons.The Cap is rotated to face the wind by turning a capstan wheel at the rear.Internal shafts and gears are wood.One such mill has operated for over 2 centries.Tim McClure
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Tim,Are you sure it wasn't just due to expedience in their construction technique for the mill tower? You might ask if this is a factor why is it not applied to the wind generators on the power grid?The mill looks a masterpiece of engineering, it would be interesting to see one.John,Aren't the albatross generally surfing down the front of a wave and in surface effect most of the time, hence the air is always rising up at 4 degrees?Nick.
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Nick I do not know but I would suspect that air slowing due to friction only happens close to the ground. I would also expect that the wind generators are set up very high to avoid the slowing air.Perhaps someone with more knowledge on the subject will chime in.Tim McClure
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That's a great illustration of a Dutch windmill Tim, what's that flag it's flying?. This is certainly an interesting one! Dare I say that the Dutch might have got it wrong, or as Nick points out built them that way for a different reason.Certainly Curry attributes this discovery to Lilienthal, and I don't think there was a lot of systematic study of aerodynamics prior to his activities. The apparatus he used consisted of a stack of vanes, the highest being at 10 metres, recording on a revolving drum.Nick,you mention surface effect, or ground effect supporting the albatros, my understanding is that ground effect only works up to about one and a half wing spans, the albatros would be cruising well above that.I dicussed this information with a couple of GA CFIs, it was news to them.Point to consider, is this why in a downwing T/O or landing performance is always much worse than expected when the measured wind and the IAS are taken into account.Here's where I really stick my neck out, is this why a low level down wind turn really is dangerous because we are entering sink? I have no idea how high this phenomenon extends. ?? Time to check it for ourselves.John EvansThink logically and do things well, think laterally and do things better.
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John, the first "Australian" Dutch windmill was built at Sydney Cove in 1797 on the rise that is now called Kings Cross. (it is visible on the horizon in William Blake's c1800 painting of Sydney Cove at the museum of Sydney)Apparently Sydney eventually had 36 such windmills and there were about 200 Australia wide before steam power made them redundent.This particular one was built in Australia by a Dutch immigrant to complement his restraunt business, 100 Klms from Albany in WA. Perhaps that is why the Australian flag.Image Insert:
85.7 KBRef. Australian Owner Builder - Mar 2004Tim McClure
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Tim,Nice PictureJohn,It has been a while since I've seen smoke in a paddock with a grass fire, what happens there? I've never observed it that keenly.Nick.
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Nick, that would be an interesting experiment. It would have to be done with a cold "smoke generator" though rather than a fire, for obvious reasons.Tim McClure
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Know any apiarists?Don't tell me to mind my own bees wax...
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We could always copy Lilienthal, he had, according to the illustration in Curry's book, 5 linked vanes mounted on a mast 10 metres high and recording their movements on a revolving drum. The average direction of the wind was 4 degrees up, but varied between extremes of +16 and -9 degrees.This finding was independant of wind speed, direction, season and time of day.The recording drum he used would have been a smoked drum with a stylus scratching the movements in the soot. This is the sort of apparatus early physiologists used in their investigations John EvansThink logically and do things well, think laterally and do things better.
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