Lillienthal

[asra]

I haven't been on the Forum much lately, but this evening I was taking a look at the US Forum, and there was Birdy causing havoc as usual with the pundits rabbiting on about "ground referencing" and other phenomena as an explanation for his observation that more power is required when travelling down wind at low level. By now most of us have been brow beaten into believing the experts' claim that down wind/upwind it makes no difference which way you turn as you're just travelling in a parcel of air, etc, etc.Those of you with long memories will recall that quite a while ago I did a post entitled "Lillienthal discovered it" talking about the great aviation pioneer's discovery of a crucial piece of information vital to the safety of low level aviators.Unfortunately I couldn't find it, apparently it hasn't even been archived. Perhaps someone cleverer than me can resurrect it.Lillienthal discovered that as the wind near the ground moves along it actually rises at about 4 degrees (average). So when you take off into the wind it's actually giving you a 4 degree additional lift over and above what you would get in still air. The converse is also true, that soggy climb you get if you ignore the very gentle downwind component is due to the air sinking at 4 degrees. Now that's an 8 degree difference!If you don't believe him, go out and try it!John EvansThink logically and do things well, think laterally and do things better.

[birdy]

John, mate, i'm hardly one to argue coz i know jack sh&* bout wot your say'n, but from wot i can decifer, your say'n that air near the ground is rising. Realy? all of it? Wot about the old, "wot goes up must cum down" thingy? If ALL air near the ground was go'n up, we'd sufforcate pretty quick. It's gota be go'n down sumwhere. And its not like we always TO into rising air.I take it that 4 degrees is an angle and not temp.Sorry mate, i can't by that one.Theres definatly a difference, but i don't think thats it.Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[asra]

I'm following you on this one John. It would explain the apparent disproportionate effort and time expended after taking off downwind and flying just off the ground waiting waiting waiting for some energy reserves before taking on normal flight.Pete

[Brian]

John, the 'experts' are correct when all things are 'right', in other words when the air is doing what it is supposed to do. As Birdy I and many other's know, the air changes in doing whatever it is supposed to do. Now, there are some days when the air is reasonably constant and you can do whatever you want to do close to the ground [under 100 feet]and everything is sweet, then there are the times when certain things happen and it can be near impossible to do a downwind turn [under 100 feet]. You can expereince a low ASI reading, up wind or downwind at straight and level with increased power [18 liters per hour instead of 15.5 LPH] sometimes a power setting of 22 LPH is needed to keep the gyro airborne. This power setting would normally have you climbing very well.There is no doubt in my mind that wirly winds, theremals, turbulence, heat spots etc all cause reactions and the time of day can influence all of the above.It doesnt seem to matter if the air is hot or cool. Anyway, a very interesting topic which hasnt been explained in full as yet !!!Brian

[asra]

It's amazing that this important bit of basic research has been lost as far as most aviators are concerned, just how far up this rising air extends I don't know, Lillienthal's set up was only 10 metres tall. Apparently he linked his vanes together and read their combined deflections as a trace scratched on the surface of rotating smoked drum. It's important to remember that he was experimenting in a very flat part of Germany, he even had to construct his own hill to fly his "hang gliders". Thus the wind he was investigating would have been flowing smoothly over the flat terrain.What we need now is for some inquisitive soul to emulate his work so that it can be confirmed or debunked, any takers?Birdy: Yes that's 4 degrees angle (average). No we won't run out of ground level air, gravity will see to that, remember all that's happening is that the slowed air's pressure rises and pushes the following air up. Sorry you don't like the explanation, read up on Bernouelli.Brian: Yes there is a lot more happening in the air than many of the experts recognise, they seem to only want to talk about air at altitude, when for us at low level their dogma can be dangerous. I am very suspicious about people who account for a crash as due to "ground referencing", particularly when they ignore the effect of tailwind gusts on airspeed.John EvansThink logically and do things well, think laterally and do things better.

[Echo]

John, I typed in " Otto Lilienthal " into google & it came up with some interesting info , it would take a bit of spare time to go thru it all,though.

[asra]

The more I learn about the pioneers of flight the more I stand in awe of them. I never was much into history at school, but now many aspects of it fascinate me.We somehow seem to think of ourselves as more enlightened than those guys, I don't think so. We have quite a few Australians to be proud of, Hargrave, Hinkler, Kingsford Smith,Ulm,Harry Hawker,Harold Gatty, to name a few.There Eric, that should keep you occupied if you Google them!John EvansThink logically and do things well, think laterally and do things better.

[asra]

Dont forget the Black Fella with his boomerang !He had the idea long before the white fella was even born.I think each successive generation wastes a lot of time believingthat they have to invent the wheel and prove to themselves howand why it works.Robert DunnMackay. Qld.Growing old is good while it lasts.

[telf]

I never did see the purpose of a bent stick that you can't throw away........until now!telfFlying - The knack lies in learning how to throw yourself at the ground and miss. (Douglas Adams-The Hitch-Hiker's Guide to the Galaxy)

[birdy]

John, if all the air your fly'n in when your go'n DW is sink'n, and all the air your fly'n in go'n UW is riseing, why don't we just stay in the riseing lanes?I know, thats a stupid conclusion, but it cums from a stupid theory.All the air can't be go'n up at 4*, sums gota be cumn down to replace it. Its called convection.This bloke with his flag pole, did he move it round to different areas or only test it in one or two spots? I know that if i set up one of these poles at either end of me strip ere, one would , 'on average,' show riseing air and the other would show sink'n air.[ IOW, theres nearly always an upper at one end and a sink at the other. One end would have an 'average' .... degrees of riseing air.]Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[Brian]

Another interesting thing with wind is that at daybreak, you might find dead calm on the ground right up to say 400 feet AGL. At 450 feet AGL you could find a wind at say 20 knots. As the morning goes by, the wind level will keep dropping down till at 10.00 am, the wind could well be right down to ground level !!Brian

[asra]

Gee Dave, I thought you'd be all clued up on Bernoulli by now, even to the point of telling me I spelt his name wrong.Daniel Bernoulli (1700- 1782) was a professor at Basel University in Switzerland. Bernoulli's principle or law states that the higher the speed of a flowing fluid or gas, the lower the pressure. As the speed decreases the pressure increases.I know that's probably counter intuitive, but it doesn't stop it being right!Bernoulli's law is one of the explanations of why air flow over a wing creates lift, it's actually acting as one side of a venturi. The venturi is why a carburettor works, the accelarated air through the narrowed venturi creates a lower pressure, which sucks the fuel up through the jets. [If you're going to reject the professor's law you'd better get rid of those carbys on the Rotax and fit fuel injection[]].If you now look at the second bit of his law, the bit about pressure increasing as the speed slows down you can see that the nice wind velocity gradient the man on the US Forum drew for you clearly demonstrates the obvious, the wind slows as you approach the ground.Remember he is working in wind tunnels where they go to a lot of trouble to control the wind direction, also it is contained in the wind tunnel. So unless Bernoulli got it wrong the pressure has to be higher, in other words the energy in the wind has been converted to pressure, now unless that gas is contained somehow, it has no choice but to expand, so it pushes the following bits of air upwards.I have no idea how many times, or in how many places Lillienthal tried this, but I do know that he is credited with making this discovery.Remember he was working in a very flat place, and investigating steady winds, so the terrain would have had little impact on what he found and I presume he would not have had the thermal activity that you are familiar with.Anyway, there's not much point arguing about it, what we need is someone to go out and demonstrate that it is either true or untrue. What Lillienthal discovered is explainable by Bernoulli and we all depend on him being right when we fly.John EvansThink logically and do things well, think laterally and do things better.

[birdy]

Brian, wind ere usualy do'nt get to ground level till bout 10 am in winter, and its usualy weaker on the ground coz of the gradient/ surface friction.I remember Taken off at sparrows one morn'n and it was dead still, at bout 50' i flew into a tail wind and it had trouble climbn, then as i broke bout 150' i hit a head wind and shot up to 200' in no time. Was curious with these different wind levels and held me breath and climbed more. When i hit 300 odd feet it was calm again[ gps said so].Within an hour the second level [ easterly] had overpowered the lot and was wipn up the dust.Theres been plenty o times i'v been headn back to camp and noticed the fire smoke with a 200' S bend init as it rose.John, pressure i can sorta understand, but not rise'n air. Maybe there is higher pressure air close to the surface over n above the regular alt pressure variations, but i can't see this 'very slightly' more dence air do anythn noticable to a gyro.BTW, flat terrain don't equate to smooth air or wind. Sum of me roughest days has been over flat, unchanging country, open or scruby.Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[asra]

John I personally think you have the answer in your posts. Birdy think about the effect when travelling with the wind. You will find that right at ground level because of the friction therein that the wind effect will be slightly slower than at 100 ft. Because its slower the upper wind will tend to want to ride over and to ride over the lower wind it will need to rise slightly. We were told of this effect in fW trainingTedStrongly opposed to the retriction of free speech

[birdy]

Your all still say'n ALL the air rises at low level.The air thats 'rides over' Ted, where did it cum from?. It had to cum from higher up to start with, which means it had to decend first. Or are you say'n we never fly through this sinking air at low level?.Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.