kaz3g Posted January 28, 2012 Share Posted January 28, 2012 Nev said: The wing works by moving air. If it didn't move the air there would be no lift (or drag), which are a reaction to what it's doing. Perhaps more particularly, Nev the movement that occurs is by way of a change in direction of the air. Velocity is a vector so it has both speed and direction. A wing changes the direction of the air molecules flowing past it and therefore their velocity. A change in velocity of the mass of air means the mass of air accelerates (usually beneath the wing) and F=m x a. The vertical component of the reaction to this force is the lift generated by the wing (Newton's 3rd Law). A flow of air will not bend/curve unless the pressure on the inside of the curve is less than the pressure on the outside. That seems to rely on the air molecules above arriving at the end of the wing at the same time as those travelling under the wing meaning they are more strung out and hence less densely packed than their colleagues. I think boundary layer/viscosity theorems provide better answers to the spoon under the tap thing. The NASA links in my earlier post also contain lots of internal links which are really interesting to follow if you are into this sort of stuff. Great thing about most of these discussions is they are all about the argument because no-one seems to be able to satisfy ALL the rest that they are correct. kaz Link to comment Share on other sites More sharing options...
Powerin Posted January 28, 2012 Share Posted January 28, 2012 The effect Maj was trying to name is the Coanda Effect Old Man Emu Yes, thanks OME! Maj, you knew what you were talking about, you just had the wrong name and I couldn't think of what it was either. Just knew it wasn't Coriolis No, that's incorrect. The Coanda Effect only occurs within similar fluids eg gas/gas or liquid/liquid. It does not happen with dissimilar fluids eg gas/liquid. I knew someone would pull me up on it . Yes, water curving against a spoon is caused by surface tension not Coanda. But I reckon it's still a good visualisation of the effect. Link to comment Share on other sites More sharing options...
coljones Posted January 28, 2012 Share Posted January 28, 2012 And yes, I can't see how a venturi can exist in an open (ie above a wing) system. It can. Think of it as a venturi with an infinite width throat. Like water, wind can't move out of the way instantly and thus provides a laminar barrier to moving air closer to the airfoil. If the airfoil shape didn't add lift perhaps we could replace them, propellors and racing boat mainsaile with flat planks. A simpler explanation might be "majik". This sometimes helps when I get a head ache thinking about some of this stuff. cheers Col Link to comment Share on other sites More sharing options...
Deskpilot Posted January 28, 2012 Share Posted January 28, 2012 Hey Winsor, I had one of those kites as well. Made a great sound to. As you say, I never seen another one either. Link to comment Share on other sites More sharing options...
Powerin Posted January 28, 2012 Share Posted January 28, 2012 If the airfoil shape didn't add lift perhaps we could replace them, propellors and racing boat mainsaile with flat planks. A simpler explanation might be "majik". This sometimes helps when I get a head ache thinking about some of this stuff. A flat plank will fly....just not as efficiently as a curved aerofoil. One example is a paper aeroplane. A ceiling fan with just flat blades works fine and shifts air. However I'm perfectly willing to settle for the "majik" explanation as the alternative is trying to understand fluid dynamics and bound vortices etc etc. Link to comment Share on other sites More sharing options...
facthunter Posted January 29, 2012 Share Posted January 29, 2012 Probably the realm of designers rather than pilots. You can't derive the stall speed of a design without consideration of wing area, aspect ratio and lift/drag coefficients etc ( Even there I have to state that the stall speed is in straight unaccelerated flight, and we all know that the stall point relates to an angle, not a speed), for a particular airfoil. Most of our designs still use pre-WW2 aerodynamics. That's the speed envelope we are in. Nev Link to comment Share on other sites More sharing options...
old man emu Posted January 29, 2012 Share Posted January 29, 2012 One thing that has been worrying me after I looked at the smoke streamline diagrams was "How do you change the speed of the air without applying a Force to it?" Newton's First Law: A body travelling at a uniform velocity will maintain that velocity unless it it acted upon by an external force." I just could not see where this external force was coming from. Then I remembered the Law of Conservation of Momentum. "The Total Momentum of a system after a collision is equal to the Total Momentum of the system before the collision" I realised that this change in momentum was the source of the velocity change of the air. Major contributed a bit with the Coanda Effect, which leads to the Bournoulli Theorem. Add a touch of Coriolas Effect and I can come up with Old Man Emu's "Theory of Lift Generation by an Aerofoil" This theory will explain how the velocity of a mass of air is altered when it meets an aerofoil shape; how that change in velocity results in the Coanda Effect upon the airstream, and how the combination of the flow of the airstream and the curvature of the aerofoil produces a pressure variation around the aerofoil. It is going to take me a couple of evenings to get this all down on paper, so I hope I can publish something by the end of the week. Old Man Emu Link to comment Share on other sites More sharing options...
fly_tornado Posted January 29, 2012 Author Share Posted January 29, 2012 It looks like the lower airflow is trapped by the wing, I hadn't really thought about that prior to seeing that video. 1 Link to comment Share on other sites More sharing options...
old man emu Posted January 29, 2012 Share Posted January 29, 2012 Nah, FT! There's no trapping of air. The air passing below the wing has slowed due to the change in momentum. You really have to think hard about what those videos are actually showing you. It will all become clear when I publish my theory (I'm being serious here, chaps.) OME Link to comment Share on other sites More sharing options...
facthunter Posted January 29, 2012 Share Posted January 29, 2012 Don't keep us waiting too long. The excitement is killing me. Pressure and change of path/ position are totally interrelated. Newton will cover all that happens. You don't have to make it any more complicated that that. To make a particle of air change position you have to have a pressure difference, same as for electricity flow you need a potential difference. Wind in the atmosphere is an example on a large scale, but the earths rotation modifies it a little. Nev 1 Link to comment Share on other sites More sharing options...
turboplanner Posted January 29, 2012 Share Posted January 29, 2012 There is a very detailed thread on this subject. One of the weaknesses of the forum culture is file structure, and so many posts which are exceptional reference material just pass into the ether and people start again making the same mistakes, missing the same points or continuing valuable knowledge - it's a shame such a valuable resource is lost. 1 Link to comment Share on other sites More sharing options...
farri Posted January 30, 2012 Share Posted January 30, 2012 I agree that it is a shame if any valuable resource is lost, however, it appears to me, that on this paticular subject, ( Lift ) , what we have so far are theories only, therefore, who`s theory do we choose as a valuable resource ? Though it is an interesting subject for discussion, how lift is truly generated is of most value to an aeronautical engineer and aircraft designer. What the pilot realy needs to know most, is, how to fly the aircraft successfully. Frank. 1 Link to comment Share on other sites More sharing options...
facthunter Posted January 30, 2012 Share Posted January 30, 2012 Know it's Characteristics, Vices Capabilities, Limitations. You don't have to know precisely how the wing developes lift, but know when it does and when it doesn't; Of course the more you know the more you can do, and you have got to talk about something when you are around the bar. I'm just very interested in physics, and I think I would be a lot less of a pilot if I had not had that interest. Nev 1 Link to comment Share on other sites More sharing options...
old man emu Posted January 31, 2012 Share Posted January 31, 2012 Have a read of this: http://www.newfluidtechnology.com.au/THE_COANDA_EFFECT_AND_LIFT.pdf It will put the cat amongst the pigeons. OME Link to comment Share on other sites More sharing options...
facthunter Posted January 31, 2012 Share Posted January 31, 2012 I don't have a problem with it. Ive long ceased to believe bernoulli is relevent with wings. Anderson has written a lot on aerodynamics. Nev 1 Link to comment Share on other sites More sharing options...
farri Posted February 1, 2012 Share Posted February 1, 2012 I learnt BAK from the Captain Trevor Thom manuals, passed the GA exam and went on to teach " The Bernoulli Theory ", ( When the velocity of a fluid is increased, the static pressure is decreased ). Though I could never really see ( and still can`t ) how the wing could create a venturi effect, in the open air, it was the accpted syllabus for pilot training and how a wing generates lift. This is my dilemma!... I retired from instructing several years ago but I dare say the Bernoulli theory is still being taught and if anyone asked me today how lift is generated, I would explain as I did then, that lift is generated by decreasing the static pressure over the top surface of the wing.( Of course this is only part of the explanation). I have no problem with the Coanda Effect, ( I`ve been aware of it for a while ), or any other theory that can be proved to work, but in my opinion, for the student pilot who is trying to get his or her head around the subject matter, so as to pass their RAA. , BAK. exam, all this conflicting information and theory is detrimental. As I said in my previous post, who`s theory will we choose as a valuable resource, to instruct future recreational pilots, for the types of RAA registered aircraft we fly? Frank. 1 Link to comment Share on other sites More sharing options...
Guest pookemon Posted February 1, 2012 Share Posted February 1, 2012 Add a touch of Coriolas Effect and I can come up with Old Man Emu's "Theory of Lift Generation by an Aerofoil" Adding the Coriolas Effect will only decide which way your theory will turn as it goes down the toilet. Link to comment Share on other sites More sharing options...
old man emu Posted February 1, 2012 Share Posted February 1, 2012 It is really hard to come up with an explanation for the observable facts. Have a look at this snapshot: The wing is stationary in the middle of a wind tunnel; air flows past it from left to right. A little ways upstream of the wing (near the left edge of the figure) there are a number of smoke injectors. Seven of them are on all the time, injecting thin streams of purple smoke. The smoke is carried past the wing by the airflow, making visible stream lines. In addition, there are five injectors between each pair of purple injectors. They create pulsed streams. The smoke is turned on for 10 milliseconds out of every 20. In the figure, the blue smoke was injected starting 70 milliseconds ago, the green smoke was injected starting 50 milliseconds ago, the orange smoke was injected starting 30 milliseconds ago, and the red smoke was injected starting 10 milliseconds ago. The injection of the red smoke was ending just as the snapshot was taken. The snapshot shows that the streams that pass over the top of the wing are travelling faster than the streams that are passing underneath the wing. The $64 question is: What causes the stream of air to change its velocity? The only answer I can think of is based on the Law of Conservation of Momentum. As a result of the collision between the air and the leading edge of the wing, there is a change in the velocity of the air on either side of the wing. This explanation requires a page or two of algebra to explain, so I will do that later on. The snapshot also shows that the distance between succeeding "blocks" of air is greater over the top of the wing than below it. This means that the volume represented by those spaces, is greater than in the air in general, so the pressure is reduced. Likewise,under the wing, there is a reduction in the distance between the spaces, meaning that the volume is smaller, and hence the pressure is greater. This indicates that there is a pressure differential between the upper and lower sides of the wing. I've discounted the Coanda Effect, because this only exists when a stream of high velocity air is injected into a slower stream of air. There is no means of injecting a high speed stream of air into the air flowing around a wing in the average GA or RAA airplane. The Bernoulli effect doesn't seem to be involved either. OME Link to comment Share on other sites More sharing options...
facthunter Posted February 1, 2012 Share Posted February 1, 2012 On very large aircraft the wing will produce a "lenticular" cloud over the top of it, in the right conditions, indicating that the air there has cooled. This can only be caused by an adiabatic pressure drop. In normal circumstances the top of the wing produces most of the lift. Keep considering that air will not flow in a curved path , change direction, speed up or slow down without a change of pressure acting to cause it. Nev Link to comment Share on other sites More sharing options...
winsor68 Posted February 1, 2012 Share Posted February 1, 2012 I don't have a problem with it. Ive long ceased to believe bernoulli is relevent with wings. Anderson has written a lot on aerodynamics. Nev HERETIC!!! lol... Amazed me how something so simple can be complicated so much... it is what it looks like. I guess back in the early days flying was seen as some sort of magic so they couldn't keep it so simple.... Link to comment Share on other sites More sharing options...
fly_tornado Posted February 1, 2012 Author Share Posted February 1, 2012 I always thought those clouds you see over the top of the wing are caused by the water not being as easily decompressed by the wing. Does anyone know if the clouds can occur under a wing? Link to comment Share on other sites More sharing options...
facthunter Posted February 1, 2012 Share Posted February 1, 2012 If I want lift, I'll take downwash over hogwash any day. Lot's of this stuff becomes a point scoring in conversations thing, or a question to trip you up at a job interview or some theory exam. We are probably more confused than we ever were, but there is actually more knowledge out there than ever before. That knowledge is for the designers to use, and the planes they make are for pilots to use. No they don't Ft..... Nev 1 Link to comment Share on other sites More sharing options...
fly_tornado Posted February 1, 2012 Author Share Posted February 1, 2012 Funny thing as a kid, I stuck my hand out the window of a car and lift seemed obvious. 1 Link to comment Share on other sites More sharing options...
winsor68 Posted February 2, 2012 Share Posted February 2, 2012 Downwash... KISS principle... Enough... Go fly. lol Link to comment Share on other sites More sharing options...
farri Posted February 2, 2012 Share Posted February 2, 2012 If I want lift, I'll take downwash over hogwash any day.Nev If I want lift, I go and jump in the Drifter and up I go! For the purpose of the discussion, let`s accept that as the the article on the " Coanda Effect " suggests, the Bernoulli Principal is incorrect. ( And in fact,it may well be ). This theory has been used to teach many pilots for a long time and if it is such a critical issue to the pilot, then we must have a hell of a lot of misguided, dangerous pilots, out there! As far as I`m concerned, regardless of the type of aircraft being flown, it is the skill and ability of the pilot to understand what is required in flying the aircraft correctly, that is the critical factor, not whether the Bernoulli principal is correct or not. I believe there is more chance of reducing accidents, if instructors put greater emphases on the physical control of the aircraft, rather than who`s theory it is that`s creating the lift. Frank. 3 Link to comment Share on other sites More sharing options...
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