Nothing to do with intelligent design, as such, but Sal Cordova brought up the issue of how wings produce lift in this thread.
hmmm.
[Edited to add xkcd, which my son just reminded me of….]
Like most people, I bought the airfoil myth for many years until I actually started to think about designing a kite (it was a fun first-year architecture project). And then I didn’t think much more about it until an interminable thread about a wind-powered cart that could travel downwind faster than the wind arrived at Talk Rational, in the course of which I discovered something I hadn’t appreciated about air, which is that it isn’t readily compressible unless in a confined space (I’d always imagined propellers in water behaving rather differently to propellers in air, but it turns out they don’t). Anyway, thought it might make a change from ID, and maybe Gil Dodgen might like to weigh in on Something Completely Different :). I’ll invite him.
When I first saw this thread title I thought, “WTH? Has Lizzie lost it?” But after reading the OP I get your point.
I for one would not call the airfoil explanation a myth so much as either incomplete, mischaracterized, or just plain misunderstood. Like the concept of evolution, flight and lift and actually rather complex and nuanced concepts and unfortunately they are very easily simplified to the point of being inaccurate. Along the same line, there was the canard for years that aeronautical engineers and scientists were baffled by bumblebees because based upon the principles we understood, such creatures should not be able to fly. This was false, based likely on a misunderstanding (or a quotemine) of an aeronautical engineer’s hyperbole in an interview about how fascinating the bubble bee’s ability to fly is and that we can’t quite mimic it. Here’s one explanation:
http://www.westmtnapiary.com/bee_flight.html
In any event, it’s fairly easy nowadays to test the old flat-surfaces-have-no-lift question – while in a moving car, put your arm out the wind and hold your hand parallel to the ground. Now, tilt your hand. The airflow will either force your hand and arm downward or upward depending on the direction you tilted your hand. No airfoil necessary.
This guy is well-intentioned but is also in over his head. Some of what he says is correct and some of it is flat wrong.
For example, this should elicit a double facepalm:
And this does not make any sense:
It is easy to see that pressure differences involved are fairly small. A Boeing 777 weighs about 200 tons. It’s wing span is 500 square meters. A pressure difference required to provide a lift is 200×10^3 kg × 10 m/s^2 / 500 m^2 = 4,000 Pa, or 4 percent of the atmospheric pressure. In a gas, a 4-percent pressure difference is equivalent to a 4-percent density difference (for an isothermal process; adiabatically, it would be even less). Air compression plays essentially no role.
NASA has some great education web pages on the subject, complete with Java simulators and descriptions of right and wrong theories of lift.
Incorrect lift theory #1.
Use navigation buttons at the bottom to explore the other pages.
You are making an airfoil with your arm and hand- by tilting your hand you made an airfoil.
IOW all you are saying is that one can make an airfoil in various ways.
This is the same with flying a plane/ jet upside down- there are wing adjustments that have to be made that re-create the airfoil effect.
Awesome! Thanks! I hadn’t realised there were three incorrect explanations!
I suspect I’ve subscribed to all three at various stages of my life….
The key parameter is angle of attack. Most of the shape differences relate to minimizing drag.
The big difference between living things and airplanes is in the method and means of propulsion. We have built small flying machines with flapping wings, but despite a recent YouTube hoax, we haven’t built anything that flies like a bird and can carry a man.
If there is a myth associated with airfoils it is that the relevant force is suction (lift?). It’s the same kind of loose language that causes us to call a vacuum cleaner a vacuum. We experience what the machine does as sucking, but the force is actually coming from the other direction. This is just a matter of semantics unless you are a physicist or engineer.
There is (to me) and amusing analogy between this semantic game and biological selection. There’s a lot of verbiage wasted on whether variation or selection produces change. Same with whether temperature flows from hot to cold or vice versa. Or in solid state circuits, whether electrons are flowing, or holes.
That was a YouTube hoax? This one?
Incorrect Joe. Changing the tile of your hand does not create an airfoil; it changes the angle of attack of a flat surface.
Which creates the airfoil affect.
Also I can change the angle of my hand and have my hand and arm not move up or down, even with air flowing rapidly all around it.
It’s an argument over definitions. Airfoils are generally more sophisticated than flat surfaces, but angle of attack is the key, regardless. The sophistication is in minimizing drag and providing control.
The “myth” is that lift is suction.
Correction. that’s not the hoax.
If the link works, this is the hoax.
Thanks!
Nice hoax. But is the one I posted real? I loved it.
I think yours is real, but the thing doesn’t get off the ground unaided. The hoax one was supposedly motor assisted. That would be cool, if possible.
The principle “magic” of the airfoil is that you can have an engine that generates 10 pounds of thrust lift an airplane weighing 100 pounds (I’m just using round figures to illustrate the point). Contrast this to a 100 pound rocket, which needs at least 100 pounds of thrust to fly. The airfoil essentially allows 10 pounds of force in the horizontal direction to create 100 pounds of force in the upward direction. It is a highly counterintuitive result, and because of this, even now it somewhat seems magical (but not really).
The airfoil myth is by-and-large correct, but there are some subtleties.
For upside down flight
See the illustration here (scroll down)
http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/airfoil.html
You’ll notice that by changing the orientation (or angle of attack) of the wing one is able to get differing velocity flows above and below the wing.
Regarding the shape of the airfoil, it is shaped such that at a certain preferred orientations lift is generated. The shaping of the wing is rather significant.
For about .3 times the speed of sound air can be approximated as an incompressible fluid for engineering purposes, beyond that the higher the velocities relative to the aircraft, the compressible characteristics of air have to taken with increasing account (from an old aerodynamics class text).
From the “hmmm” article
http://warp.povusers.org/grrr/airfoilmyth.html
The notion of what constitutes “over” and “under” is subtle. If we let the horizontal axis be the defined by the direction of flight, the notion of what is over and under is changed slightly by this coordinate transformation. With this in mind, by changing the orientation of the wing relative to the direction of flight, the airfoil can be seen to obey the airfoil myth since air will have to move faster over the top (with respect to of the direction of flight) than the bottom. If we let “top” and “bottom” be defined by the wing itself, then this introduces some confusion.
When you put your hand out of the window in a moving car you experience the force generated by dynamic pressure created by the wind. If you put your hand at an angle you deflect (turn the flow), and this creates some lift because of the deflection. This is a diffent mechanism than the Bernoulli lift. To illustruate consider being hit by a fire hose. If you angle yourself just right, you might sort of fly when hit by the water because you’ve turned the flow of water from the fire hose. But this is conceptually a different mechanism than lift generated by the bernoulli principle.
Aicraft lift is generated by both mechanisms. Airplanes fly right side up by both the bernouli mechanism and turning of fluid flow (and in the case of the F-22 raptor thrust vectoring is yet a third mechanism of lift). It is desirable however, to minimize lift from turning of fluid flow as this is energy expensive. Hence, for energy considerations, the basic airfoil is desirable so that most of the lift is generated by bernoulli effect rather than turning (deflecting) the flow.
By way of contrast, rocketships don’t really use bernoulli as a principle lift mechanism on take-off. It uses thrust almost exclusively.
If you watch the F-22 perform the cobra maneuver you’ll see it transition from bernoulli lift to turning lift to almost exclusively thrust generated lift (like a rocket) and it will use the thrust to hover almost like a helicopter.
Like evolution, the principles of flight are more nuanced than simplistic characterization and invocation of one mechanism to explain everything.
In practice the ratio is more like four to one. But there is no magic. Thrust provides forward motion. Leaving the ground would be even more efficient with something like maglev, and even better with maglev in a vacuum, where both friction and air resistance would be nearly zero.
stcordova,
Exactly, which is why the dismissal of evolution as a random, “get 500 bits right at a time” theory, is a flawed argument by Dembski and other ID supporters.
petrushka,
I took flying lessons once and for puddle-jumpers like a Cessna 152, 10 to 1 is about right.
The faster planes, due to drag, are less efficient.
During the war, the Lockheed designers added 2000 HP to a P-38 for a test, and got a 15 MPH increase out of it!
stcordova,
The whole point of all of this is that we do not model our flight after that of birds.
Birds use their wings for thrust AND lift, something we just can’t seem to make work practically.
So our designers did NOT model our flight after ID’s “intelligent designer”.
As a qualitative guess, I would suppose the principle lifiting mechanism of a typical flat kite is turning (deflecting) the flow of wind, not the bernoulli effect, but this is just a guess.
I like the F-22 cobra maneuver as it illustrates 3 mechanisms (bernoulli, flow turn, thrust) in getting an aircraft to fly.
The bernoulli effect is important since it is an energy efficient means of creating lift. The most energy efficient are balloons which employ static lift — a 4th mechanism of lift. In theory one can build a device that employs all 4 means of lift, but that would probably more of toy than a serious aircraft.
I took lessons in a J-3, so I have you beat in the featherweight department.
In any case, airplanes evolved from more from kites than from birds. And WWI was over before we had a formal theory of airfoils. Early aircraft design was more cut and try than theory.
Which is how evolution works.
stcordova,
The whole point of shaping the wing like an “airfoil”, is for the purposes of preventing the air on the top of the wing from becoming turbulent and thus stalling the wing.
The standard NASA airfoil will become turbulent at about a 15 degree angle of attack.
If the top and bottom were flat, they would stall very quickly when increasing the angle of attack due to turbulent airflow over the wing.
So the airfoil “shape” is for control, efficiency and predictability.
As far as aircraft design adopting bird “design”, the Wright brothers actually warped the wings to bank their plane instead of using ailerons.
That’s a false dichotomy. It’s a bit like saying that the bouncing of a ball off the floor is caused by energy conservation rather than Newton’s equations. It’s both. See this: Bernoulli and Newton.
We get into the same kind of semantic tangles when someone describes selection as “pressure.”
It’s a rubbish argument. It’s all metaphors, except for the math.
Trying to win arguments by citing dictionaries is simply dishonest.
An honest person wants to understand the other person’s position.
Which is why I keep hoping that ID proponents will put forward a theory of design that can account for the designer’s ability to produce those highly improbable sequences, despite their being beyond the computational resources of the universe.
A theory that doesn’t invoke something like Harry Potter wand waving.
stcordova,
I too would like to know how the designer gets around his design obstacles.
For instance, if we have “free will”, we can change the future.
How does the designer know that his “object under design” will work in an unknown future we might create with our free will?
Well, your’e the physicist, so I’m reluctant to disagree.
However, I’ll point out, force can be generated parallel or even at an angle to the direction of flow even without flow existing on one side of a “wing” — i.e. solar sails. Solar sails use radiation pressure, but it is not hard to imagine a directed stream of particles (like a fire hose) accomplishing the same task.
The reason I have issues with characterzing lifting force as soley a reaction force is the idea is that 10 pounds of thrust can in pricniple lift a 100 pound aircraft.
If by reaction force one means the force in reaction to a change in momentum, then clearly the bernoulli lifting force does not appear to be the result of a change in momentum any more than the lifting force of the legs of a chair (which opposes the gravitational force) is the result of a change of momentum.
But you’re the physics professor and I’m just a trouble maker. So I’ll defer to you.
There are two ID positions that can be held without violating observed regularities in nature.
One is the anthropic principle, which most scientists find vacuous, but which can’t be disproved. You might call it fine tuning or front loading, but it has no relevance to the evolution argument. the universe looks the same whether it is true or not.
The other argument, which I think is derived from mysticism, is that the universe itself is conscious (or alternatively that we are God’s thoughts). but again, there is no way to differentiate a universe in which this is true from one in which it is not.
The ID position that I think is insupportable is that which asserts that gaps in the fossil record represent points of intervention by some agency outside nature. Not all ID advocates believe in interventions, and those that do seem to see different gaps.
Behe, for example, does not cite the fossil record as an argument for intervention. Being a molecular biologist, he sees gaps at the molecular level. On the other hand, Berlinski likes fossil gaps, such as in whale and bat evolution.
I find it amusing that the designer might have intervened just in those lineages that are badly preserved. Kind of a cosmic trickster. Perhaps Loki is the Designer.
Thought experiment —
take something like a door that has hinges that allow relatively free motion. Take a high powered hose and blast one side. The velocity of the water is much faster on the side being hit (call this the bottom) and virtually zero on the other side (call this the top). Clearly the fluid flow is faster on the bottom than the top, yet lift is generated. In fact the fluid velocity profiles are the reverse of Bernoulli flow!.
The lift is reacton to turning of the fluid, not the velocity profiles that create Bernoulli lift. The mechanisms are different as far as I can see.
Bernoulli lift force may seem counter intuitive, but so do balloons and legs of a chair which require no change of momentum (reaction forces) to create forces to oppose gravity.
I’m not a physicist, but I am certain that you are simply applying an incorrect analysis. You are not getting something for nothing. My gut feeling is that there is something equivalent to leverage.
Imagine the aircraft engine attached to a pulley system. A small engine could lift the aircraft. Not being a physicist, I don’t have the terminology to express this, but I think the relative wind on the airfoils is equivalent to a lever or pulley.
So in my thought experiment, I attach the aircraft engine to a pulley system and ask if it could lift the aircraft on the pulley system at the same rate it lifts it by use of the airfoil. If it can, there is no magic and no mystery.
scordova,
An airplane is no different than an inclined plane in physics.
I can take a wagon and with one finger tow it up an incline but I may not be strong enough to pick it directly off the ground with one finger.
That’s the same effect you feel riding your bicycle up a steep hill versus a gentle slope.
You are still the same weight but the difference depends on how quickly you want to gain “altitude”.
The quicker you want to go up, the more power you need.
Ah, so I had the wrong metaphor, but the correct analysis.
It is in fact a false dichotomy. stcordova, your intuition isn’t wrong, just incomplete. If you direct a fire hose at a tilted surface, the water will reflect off at an angle and a “lifting” force will be exerted on the surface due to the “turning” or redirecting of the water. Bernoulli’s principle really isn’t relevant in this case. Nonetheless, the lifting force on the surface due to the fire hose can still be thought of in two distinct ways, one of which is directly analogous to the “Bernoulli approach” with wings. But these two ways don’t represent different phenomena, just different ways of looking at the same phenomena.
On one hand, the turning of the water direction constitutes a change in its momentum. But a force has to be exerted to change momentum. Newton’s third law says that if the surface exerted a force on the water to change its momentum, an equal and opposite force must have been exerted on the surface. If we measure/calculate the change of momentum of the water, we can determine the force on the surface. On the other hand, as the water hits the surface it exerts a pressure on the surface. Everywhere water impinges on the surface, pressure is being exerted. Since water is only hitting one side of the surface, there is only a water pressure on one side and not the other. If we knew the value of the water pressure at the surface, then by multiplying the water pressure at the surface by the area over which it acts (or integrating if the pressure varies from point to point) we can determine the total force exerted by the water. This value should be identical to the value we get by doing the momentum calculation. They aren’t two different things. They are simply two different ways of getting at the same thing.
Now, in the case of the wing, rather than water hitting one side of the surface, the force is caused by air that is in contact with both sides of the wing surface. The wing acts to turn or redirect the flow of air in a downward direction. As with the previous example, lift on a wing can be determined, in principle, by measuring/calculating the change in momentum of the air. Knowing what force was required to effect that change in momentum, we also know that an equal and opposite force acts on the wing. But, on the other hand, the air at any point around the surface of the wing exists at some value of pressure. The pressure on the lower side pushes up, while the pressure on the top side pushes down. Again, if we multiply pressure by the area on which it acts we get force. In reality we have to do an integration since the pressure is not the same everywhere. For a wing producing lift, the pressure on the bottom is higher than the pressure on the top and therefore we get a net upward lift force. This force will be identical to the force obtained through a momentum calculation based on the turning of the flow. Using the pressure approach versus using the momentum approach doesn’t mean we are calculating two different things. They are two different ways of calculating the same thing.
Finally, Bernoulli comes into play in understanding why the pressure on the top surface of the wing is lower than that on the bottom. The air flow over the top surface is accelerated relative to that over the bottom surface and, as Bernoulli’s principle requires, the higher velocity corresponds to lower pressure. (The reason the air flows faster over the top is not as simple as the oft repeated reason that it has farther to go in the same amount of time.) In the fire hose example, the water is only impinging on one side of the surface, so we wouldn’t use Bernoulli’s principle to determine differential pressures on the two sides of the surface in the way we would for the wing. But nonetheless the physics is the same in both cases. You have a pressure differential and you have a momentum change. Both are the natural effects of the fluid-structure interaction and both can be used to understand how lift is generated.
Toronto:
Indeed – the issue here is one of scale, as well as feasibility. This is a familiar theme in misplaced biological analogies – “organisms are just little machines!”.
To get forward motion and generate lift, you need to overcome inertia. Muscle-based systems of flapping are limited in the amount of mass they can get moving forwards, as well as the limitation on downthrust to elevate vertically. This places an upper limit on bird size. The energetic efficiency of muscle is pretty impressive – local application of ATP in filament sliding allows a decent mass to be elevated, one phosphor bond at a time. Each individual energetic molecule is directly associated with the business part of the generator.
The kind of machine that we want, however – the kind that can lift a payload of at least one human, plus fuel and mechanism – does not appear to be available via an actin-style ‘design’. We burn the fuel in bulk and convert as much as we can of that into co-ordinated forward thrust. But to divert a portion of that into flapping would be wasteful. We would need some hydraulic mechanism, not particularly responsive, and the strains on the materials for the size of wing we would need would be likely prohibitive. We sacrifice precision take-off and landing for getting there.
A while back I sorted out this out in my mind by imagining a razorblade kite being pulled through butter:
I’ve learned something. An unusual experience in evo debates.
Elizabeth,
You’ve shown in your kitchen lab, that there is no reason IDists cannot perform real scientific research!
In the spirit of the scientific method, here is an experiment you can do at home.
Take two sheets of paper and let them hang side by side. Separate them by about an inch. Blow air between in as straight a path as you can and you’ll see the two piece of paper move toward each other. This is a classic pedagogical example of the Bernoulli effect.
Now blow air at an extreme angle and the effect goes away and one piece of paper can be forced to move away from the other. This is force from change in momentum (reaction force), or flow turning.
stcordova,
Yes, but this still is not the prime reason that an aircraft wing works.
Watch a Youtube video of an aircraft taking off.
If the lift of the wing was mainly due to the Bernoulli effect, the plane would take off at a certain speed when it was level with the runway.
We don’t see that though.
The pilot has to pull back on the yoke or stick and change the attitude of the aircraft.
At this point, called “rotation”, the wings are at an angle to the “relative wind” and only then does the aircraft take off.
The lift generated by the wings “Bernoulli effect” even at take-off speed, is not enough to lift the aircraft until the nose is pitched up by the action of the elevators causing the wings to “plane”.
In light of the above experiment, I’d argue kites are lifted by flow turning, not the bernoulli effect.
What you can do with a single sheet of paper is let it hang and blow are parallel to the face (the flat part of the paper). When the paper starts to move toward the fast moving air that is the bernoulli effect. A sufficient change in angle and the piece of paper will be blown away by the air, this is flow turning. Kite’s imho, use flow turning. Airfoils by and large use the bernoulli effect. Both effects can be present, and sometimes one will work at the expense of the other.
That’s my experience when I flew aircraft.
Also it is dang hard to get it to land if you’re going too fast in the horizontal direction.
A classic maneuver if your airplane is stalling is to go into a dive, pick up speed and then fly horizontally, and then you’ll fly.
stcordova,
I don’t follow.
Are you saying your plane would take off before you pulled back on the yoke?
As a matter of completeness, with the F-22 raptor, part of its ability to go up are not just the lift mechanism, but simple momentum like a baseball going up. When the plane has some vertical velocity, it can’t help but go up. This isn’t lift in the conventional sense (which is probably define with respect to the aircraft centerline), but it does “lift” the aircraft to higher altitudes.
stcordova,
High performance fighters are a good example of why the Bernoulli effect does not really contribute to lift.
If you look at the cross-section of the wing of a fighter and that of a WW2 transport like a DC3, you’ll see the slower plane has a very thick wing.
By being thick, there is a larger difference between the top surface of the wing and the bottom.
This increases the “Bernoulli effect” in the fat wing since there is a larger difference in distance that the air has to take over the top as opposed to air over the bottom.
On the fighter, the thinner wings have a smaller difference in distance for air to travel over the top as opposed to the bottom.
For some reason, this diminished “Bernoulli effect” doesn’t cause a problem when it comes to generating lift at take-off speeds.
I’m not addressing the pilots who do an “Edwards” take-off, (vertical), as this is just one of those fun things you can do in a high-performance fighter as opposed to puddle jumpers!
If the Bernoulli effect was so important for lift, it would have been almost impossible for an F104 Starfighter to take off with tiny stubby almost flat wings.
It’s because what you describe is not the Bernoulli effect. It’s a bastardized version of the real thing. One of those wrong theories of lift described at the NASA web page for high-schoolers.
Yeah, that’s what I learned from the kite-designing episode. I was convinced that the airfoil was the key, so I designed a quite cool kite with a sort of curved back. Then I realised any old thing would fly as long as you got the bridle angle right.
Yes, provided you had enough elevator trim to begin with. Part of the initial lift is due to yet another lift mechanism called “ground effect”. But these are all moot points. If one wanted to build a plane that flew primarily by bernoilli lift one can. It is generally accepted as true that for the case of gliders, the lift is primarily bernoulli when the lift is not caused by thermal lifting (yet another lifting mechanism).
Consider this pilot’s account of a soft field takeoff
Just for grins sometimes on long runways I just kept going horizontal, and you can’t help but go up. This is partly the bernoulli effect and yet another mechanism called “ground effect”. The reason you want to put the nose up before too long is so you won’t run into the trees or a building and you don’t want to be buzzing the people near the airport. You want to get up high as possible, and you’re usually gunning the engine, but in terms of physics, you’ll pop start flying eventually if you get enough horizontal speed.
But once you are at sufficient altitude, you want to cruise with as little fuel consumed as possible. The bernoulli effect makes this possible. Notice that gliders can get up to 50,000 feet with no engine if they catch thermal lift (yet another lifting mechanism).
The problem of unwanted lift is very noticeable when you practice emergency landings and simulate engine out. If you have too much horizontal speed, short of crashing, you can’t put the airplane down. What you are taught to do is to go in cricles and bleed off speed and altitude.
Coming in for a landing, with the nose pointed down, if you push the thottle without even pulling on the yoke, the aricraft will start to pitch up partly because of the bernoulli effect. If you get enough airspeed speed, you won’t be able to land the aircraft (at least not safely).
olegt,
What I described was not the Bernoulli effect, what I described was, Sal’s “Bernoulli effect”.
Notice the scare quotes! 🙂
I cannot formally describe what the real Bournelli effect is I just know wings don’t work like Sal says they do.
They plane, hence the term, airplane.
Thanks for the link.