How planes fly (again!) March 19, 2008
Posted by amrut in Engineering.trackback
You know how some questions never get answered convincingly enough? This is one such question — How does a plane fly? Bernaulli’s Bernoulli’s principle is the wrong explanation for lift. And its been proved so years back. Why is it still taught in schools? The only reason I can think of is that it is just too hard to explain to school teachers that it doesn’t work that way. Not that it is easy to explain to most college professors. Anyway, I assume you are an educated and google trained audience audience and would have questioned the Bernaulli Bernoulli explanation of lift in school / shall google right away and find out why it doesn’t fit.
So, the next best explanation offered is the lovely Coanda effect. Now, I’d prefer to believe this one more. Of course, don’t go by the single line statement of the Coanda effect. The physics behind it is fairly convincing.
Take a look at this site — link here. I quote — “The accepted explanation of lift is the Lanchester-Prandtl hypothesis of flying. It involves, to quote one expert, “a bound vortex, tip vortices and the starting vortex all of which, in plan view, form the ring vortex formed just before takeoff…” Er. Yes. That wonly! The site does a great job of explaining all of that with very simple analogies. Take half an hour off and read it! I insist. Also read accompanied articles.
@ Kaka, Tak, other mech boys out there — what say??
Oh, and how do planes fly? If you ask me, hmm, well, there are a few points to note, its to do with lift, and wing angles, but mostly, the answer completely depends on who is asking. In any case, as long as it flies and as long as we can experimentally find out what flies better and what doesn’t, we can leave it to the theorists to argue on why it flies.
The best explanation of lift i have heard so far is this. U can simulate flow across any solid body by combination of simple flows ( like uniform, vortex ,etc. ). Which on using complex transformation give flow around bodies. The tranformation called Joukowski transformation gives flow around an aerofoil. With a slight angle of attack, the stagnation point on the back edge of the wing doesnt coincide with its tip. Since it is found that the stagnation point is always at the tip , it is assumed that there is a certain amount of circulation which is generated by the wing, pushing the stagnation point to the tip. If u find the lift generated by this circulation, its pretty close to actual value. And considering that it uses assumption of classic fluid mechanics, ( and that it was done in early 1900s ) , its a pretty stud explanation. Till late 1960s ( and even after that ) most commercial airplanes which flew below the speed of sound use this to estimate amount of pay load they could carry. ALso, to conserve vorticity, there must be what is called a “trailing vortex” at the rear end of the plane. This too is an observed phenomenon. Though this explanation is purely mathematical, there are some elements of physical intuition in it, which i think its pretty stud
its got something to do with the wings
Ahaa.. So I guess you guys are determined to ensure that a common CS engineer can’t understand how things fly. So, let me try a CS engineer (lay-man)’s way of explaining stuff. Anyways I am assuming that angle of attack is the angle the aerofoil makes with the air flow direction. Now, I am guessing all these starting vortices etc. dont play a role once the plane has started. But you still need to generate lift to explain the planes rise. So, let’s neglect the starting vortices.
Anyways, from the little I remember, I have heard that once you assume that, on a sharp edge no flow(from the underside or topside) bends towards the aerofoil at the edge i.e the flow leaving a sharp edge is normal to this sharp edge (googling told me that is called the kutta effect), then we can still apply bernaulli’s principle, since in this case, the velocity of flow over the aerofoil is much greater than that of the flow below (for the normal velocity).
I think the place where the stagnation point (zero velocity point?) argument comes in is in saying that the flow there is normal to the surface(the sharp edge here), which I assumed before as a consequence of kutta effect. Sure, they is a lot more to be explained.
Coanda effect, btw, plays a very small role in explaining it – it just says that the flow sticks to the surface- if you had a cylinder for eg, it doesnt explain anything at all.
Now the interesting thing is, though it does sound a little complicated to calculate all this, I am told that the wings thing the Wright brithers
used was already developed before them. But no one before them got it to fly. I’ve heard that the difficulty was in co-ordinating all these lifts, flows etc without precise calculations. This is where the Wright brothers scored. The position of their wings were flexible. They could sort of flap it in some sense (like birds do for eg). This wasnt done by other before them. I guess this is another instance of man imitating nature to overcome his deficiences in understanding the theory behind what he is trying to achieve.
So, what I also wanted to point out is the fact that Bernoulli principle is not altogether a wrong explanation, but just that it is incomplete in some sense. And I don’t understand what you mean by, “It has been proved that it is the wrong explanation of lift”.
On mishres demand, interested people can read through this link, its pretty well explained….
http://www.av8n.com/irro/conformi_e.html
And semi, if u could be more clear, what is “flow leaving a sharp edge is normal to the sharp edge”???
So.. a sharp edge(the trailing edge here i guess) still has a curvature. and for this curvature you have a normal, since this radius of curvature is non-zero (if zero, the statement is obviously true). So, flow of the air leaving the edge, is along this normal, i.e perpendicular to surface of the sharp edge.
But Amrut, one needs to appreciate your cleverness to get as many hits for your blog. Im sure you’ll see a completely disjoint set of people commenting on your blog (now, i might be helping you increase the comment count since there will people who will want to prove me wrong). It is interesting to observe the demographics of those who comment on your blog.
Could not help but commenting on this…
“now, i might be helping you increase the comment count since there will people who will want to prove me wrong”
Sheldon Semi Cooper??? Who would have thought!
Interesting discussion (I think). Forgive me if I do not join in on a technical front, but my experiences in HTW have unfortunately conditioned my mind to shut off when I see/hear the word vortex…
Bernoulli explanation of lift is that air flows for a longer distance on the top of the aerofoil, thus it gains velocity and therefore there is low pressure on top of the aerofoil (Bernoulli effect). The pressure difference from bottom to top of the aerofoil gives the wing a lift force. This explanation of life is widely considered to be wrong. There is low pressure on top though. And there is definitely a lift force on the wing. But it doesnt corelate. Why should air go faster over the top of the aerofoil?? So what if the distance is more??
Spellcheck (Thanks Nama) Bernoulli and not Bernaulli.
And on the other comment, I assure you that flight came much before any convincing explanation of flight came along. In fact, in most mechanical stuff, the first steps moving forward are experimental and then you go and put a theory to it. So its very normal that Wright brothers got it right without much calculation.
Again, on your average CS engineer comment, I must add that most average Mech and Aero engineers would avoid this question (flight?). Because the answer is either too complicated to understand or too complicated to explain. Its not trivial at all. In any explanation of flight, there is a tiny
(or large) leap of faith where you say, ‘okay this means this and thus’. That is where people disagree. Which is why we just say vortex and shake our heads in unison. Right Bofi? Right Venkat?
@ Mahesh — Neat transformation. Wasnt there in Fox, was it?
@ Semi —
. I have posted earlier on science stuff. Remember the dancing girl thingie and on LCA.
Ass.. Did you even bother to read what I said…?
To elaborate on my previous comment, yes, we know that velocity need not be more just because distance is more. The whole comment was about how this comes about because of the kutta effect on a sharp edge.
And reg the Wright brothers, I was trying to point out that it worked because they had an option of varying the angles.
Thirdly, BoFi is not a mech engineer.
Eh! you said Bernoulli is not entirely wrong. So i explained.
And Bofi is my htw team mate and we’ve got a lot of answers correct by saying vortex and nodding heads in unison.
the best explanation would be change in momentum( it is a vector) brought about to the fluid by an arifoil, the reaction force ofcourse is lift and DRAG ofcourse.
Kutta joukowski theorem also says that lift is produced if there is a circulation ( take a loop, calculate int(v.dl) ), the proof ofcoruse comes from simple control volume formulation, the same reason why a rotating cylinder produces lift.
Although Prandtl’s lifting line theory is very convincing for finite wings, the ultimate thing would be change in momentum.
Terry Colons explanation of lift and the Coanda effect is due to my coaching of him as he acknowleges in his blurb. The full explanation can be found by following the liks on my website which is http://www.newfluidtechnology.com
Please excuse the misspelling in the above comment. It was early morning.
Thanks Terry! Your website has some amazing stuff.