What You Learned About How Planes Fly Was Probably Wrong
danielmiessler.com
danielmiessler.com
A few planes have been built with the Coanda effect in mind (http://en.wikipedia.org/wiki/Boeing_YC-14), but in general it isn't very important.
There is one thing that's slightly different in this case: the misleading coolness of diagrams of the Bernoulli effect. One can easily see how in the hands of a popular writer, that could get transformed from an interesting optimization for wing cross-sections to "how planes stay up." I was fooled by this till a later age than I'd like to admit. It never occurred to me to ask how planes fly upside down, if the shape of the airfoil is how they generate lift.
You can't say, is it the Bernoulli effect or is it the fact that the wing makes the air behind it go down. Those are both different ways of expressing the same thing. You can't have one without the other. This is why I like the "See How it Flies" discussion.
Actually, you can. It's fairly straight-forward to construct an example where you do get the Bernoulli effect and there is no downwash. What is true is that when you fly you have both, and both contribute.
What's more true is that there are many effects, each of which contributes, many of which are inter-dependent, and all of which are simple in isolation, and complex in interaction and action.
The point about Bernoulli is that it only applies in its naive form in fluid flow where it's effectively a closed system. If you introduce airstreams of varying speeds then all bets are off.
Effectively Bernoulli works because the velocity changes are being caused by the pressure differences. Take a very, very long plate with a hump in small part of it:
____________XXXX____________
As the fluid approaches that faster flowing area, it must accelerate, and the only thing to accelerate it is a pressure gradient. In the video he is using other means to accelerate the fluid flow, so it's different.
At the end of the plate there is no downwash, so the only effect is Bernoulli, and in this experiment you do get a pressure difference between the sides of the plate, and hence "lift".
In the case of the flow around the "elbow" there is no acceleration of the air, hence no Bernoulli effect. Quite the opposite, I would expect a Bernoulli effect to push the "elbow" to the left. However, the air is being sucked around the plate, so the "downwash" effect dominates.
There are some really, really bad explanations in the literature and on the net, often written by people who do one experiment without separating the effects. They go on to teach, and unsurprisingly people get confused.
Impossible, that would violate momentum conservation.
A pressure difference implies a force on the plate, so upward momentum is transferred to the plate. That momentum has to come from somewhere and it can only come from the air, which must move downward.
Look, this is pointless. Static states can isolate the individual effects, and then they all get combined into a dynamic state in varying amounts, and it becomes horribly complicated. People insist on trying to produce and explain overly simplistic models, and others insist on misunderstanding them. I've actually physically done these experiments and I know that what I say is true.
I'm not going to reproduce all the nitpicking tiny details in this forum because it's hard, inappropriate, and people will continually try to pick holes in it. It's the Monty Hall problem all over, and I'm just too tired to care.
There's every chance that your understanding is right in the cases you're considering, but I can't be bothered finding out where our experimental models differ.
I wish I hadn't bothered.
It applies to fluid elements that have the same internal energy. In the case of the airplane wing, the air ahead of the wing is effectively unaffected by the wing's presence, so you can apply the Bernoulli effect to comparing different fluid elements.
Sure, if you compare the air coming out of a hair dryer with air that doesn't, you'll get in trouble, but that has nothing to do with an airplane wing.
In the venturi, the pressure is lower where the speed is higher, that's a fact. But is the pressure lower because the speed is higher, or is the speed higher because the pressure is lower? That question makes no sense, because it depends on how you think about it.
On the one hand, you can say: mass conservation dictates that the fluid must go faster in the narrow part of the tube. If the fluid is to go faster, it must accelerate, so there must be a pressure gradient. Hence, the pressure in the narrow part must be lower.
On the other hand, you can say: Since the fluid goes faster in the narrow part of the tube, the pressure is lower there. Since the pressure is lower, there's a pressure gradient, and that's what causes the fluid to speed up.
Neither of these explanations make sense, because there is no cause and effect in the problem, it's just that one state globally obeys all constraints on the fluid and that's the state with higher speed and lower pressure in the narrow part.
Once I realize something like this it makes me want to go back and do an integration / cleaning pass on my web of knowledge. Not sure how to do that in practice, though.
Some of the popular explanations of aerodynamic lift are outright wrong, but most of them are correct about part of the story. Our CFD equations are obviously correct, otherwise our simulation software would give bad results. Maybe it's just hard to explain the results of these equations in an unambiguous way.
It is true that "air goes down, wing goes up", but this explanation is only one "why" closer to the heart of the matter.
http://www.amazon.com/What-Makes-Airplanes-Peter-Wegener/dp/...
This book was part of a course he taught for non aerospace engineers. His explanations of tough topics like boundary layer theory and airfoils are clear
http://opa.yale.edu/news/article.aspx?id=6066
Apparently he worked on the V2 rocket. But there are lots of scientists that can write technical books targeted to experts. This book is worth it because this guy is devoted a lot of his teaching career to explaining difficult topics to non-experts. I think he did a marvelous job, of mixing technical material and history. I think this book was part of a course he taught at yale univ.
It is an expensive book, but I'll go in and say it's worth it. Check your univ library.
http://www.betterworldbooks.com/What-Makes-Airplanes-Fly-id-...
The discussion in there is the best I've seen, it centers on the concept or circulation, the vorticity of the flow field around the wing. One example that's used is the fact that if you just take a wing by itself (try it with a long, flat piece of cardboard) it will "fly" while rotating in the air. It doesn't fly very well, admittedly, but it certainly exhibits a glide ratio a lot better than something with the same cross-sectional area that's not a wing. In that case, there's no top or bottom of the wing at all.
Windmill blades and airplane wings have a lot in common.
I wrote some python software for it to model the curvature and get an idea of how to get the maximum effect out of a blade cut from a given blank.
The neat thing was that without the software being interactive we'd have never clued in to some of the possibilities.
In case anybody is interested here is a snapshot of the python program:
http://pics.ww.com/v/jacques/renewables/windmill/snapshot3.p...
That documents the whole building from start to finish, including the 3d computer controlled router/plasmacutter we built in order to fabricate all the parts.
The whole thing including making the tools took about a year and half.
A single picture of the completed machine is here:
http://pics.ww.com/v/jacques/renewables/windmill/firstrun.jp...
It's a variable pitch 3 blader with a 'drum' type rotor that holds 18 2x1x.5" neos. Total power about 2000 Watts, design power was 2.5 KW so not perfect but still pretty good.
Thank you to the OP for pointing out the flaw in the popular explanation !
Bernoulli lift only really comes into effect once you have level flight. Then the curvature alone can provide enough lift to support the weight of the plane without inducing drag. The Koanda effect is where ailerons and flaps come into play. The produce additional lift but also contribute more drag. As the angle of attack increases laminar flow drops. Using the authors example, placing a glass in a stream of water redirects the flow, but you'll notice there is a bit that sort of fans out. That is the equivalent of spoiled air. Too much of it and you've lost all laminar airflow, bye bye lift.
Anyways, I'm getting out of my area of expertise. It's been ages since I studied aerospace and my AIAA bible is in my parents' garage.
A bit dangerous, I know.
> Ask yourself why planes can hang tons of massive crap (engines, bombs, etc.) off of the bottom of their wings if the bottom of the wing is so important for flight
I guess that since the top of the wing generates the fast flowing air which then generates the low pressure that generates the lift, by slowing the air on the bottom of the wing with bombs and engines is only going to increase this pressure differential and hence increase the lift (at the expense of drag).
The definitely don't seem to hang anything off the top of the wing.
http://podcast.geekcruises.com/index.php?search_string=airpl...