My understanding of how an airplane actually stays up has always been the basic standard normal explanation, and not much more. That is: one side curved, one side flat. Air moves over the two surfaces at different speeds, creating a pressure differential, which is lift (drastically simplified, I know, but I always thought this was the essential mechanism of lift).
The May 2005 Discover magazine claims this is a myth.
Now, I’m having a really hard time just buying this. I know it says “soley,” leaving the possibility that the lift I described before is just a secondary mechanism, but still… can someone help me out here?
Discovery has it right. (Former museum education program coordinator here, and I had to research the literature on this very topic).
Many have been taught that airplane flight is due to the Bernoulli effect. The idea is that the larger curve on the top of the wing causes the air to travel faster, creating a pressure differential. Unfortunately, this assumes that there is some physical reason why the two streams of air (one above, one below) need to arrive at the trailing edge at the same time. There is no such phenomenon. The Bernoulli effect may play some role in flight, but it is a minute one at best. A cross-section of what a wing would have to be shaped like for the Bernoulli effect to be more or less solely responsible for flight in air can be found here. Would you ride on a plane with wings like that?
What really happens is that the wing meets the oncoming air at angle, deflecting it downward. Newton’s Laws dictate that the plane must then go upward. The downward deflection of air is due on the bottom side to simple deflection, and on the top side due to (oh, crap, what was it?), I believe its called the Coanda effect, the way fluids have of adhering to a surface along which they are travelling. So the curved shape serves a purpose, just not Bernoulli’s.
This can quickly get moved into Great Debates.
The Bernouli vs Newton argument has a long history.
My understanding is they are both right, but they look at the same phenomena from different perspectives.
Not disagreeing with the importance of angle-of-attack, or the insignificance of the Bernouilli effect, but if the streams of air don’t arrive at the back of the wing at the same time, lift is still generated, because there will still be a region of decreased air pressure i.e. because the air isn’t getting there in time to occupy the necessary space.
An interesting point is that for most airfoils most of the time, the air flowing over the top gets to the trailing edge sooner.
N91WP is right: both explanations are correct (provided you omit the silly notion that the air molecules want to get back together at the trailing edge). But the “Newtonian” perspective is a far better explanation: people have everyday experience of action-reaction, so saying that a wing pushes the plane up by pushing air down is both correct and intuitive. Appealing to Bernoulli amounts to explaining the unknown in terms of the unknown, and by itself comes across as mumbo-jumbo.
This part of your otherwise good post is not correct. Bernoulli can completely explain wing lift, if it’s applied correctly. Deflection of air downwards can also completely explain lift.
The problem of how lift has been taught historically (like when I learned about it in flight school in 1979) is that it was taught with two incorrect parts:
the air stream, as its split above and below the wing, must arrive at the trailing edge at the same time.
wings do not fly by deflecting air downward.
These are both incorrect - first, the bits of air do not arrive at the trailing edge at the same time - as someone else mentioned, the air on top gets there first. The second is also not true, because wings do fly by deflecting air downward - a wing that does not deflect air downward would necessarily be producing zero lift. You could apply Bernoulli to calculate this lift, but you’d have to know exactly how fast the air on top is moving, and without the equal transit time assumption, it’s hard to say how fast it must be moving. The explanation of deflecting air downward is much better to explain lift to laymen because it depends on concepts that everyone understands.
My point is that both are correct, just like it’s correct to calculate how fast a ball falls by either of two methods: conservation of energy, or forces and accelerations. The problem, and I came up with this principle myself, is that you can’t use conservation of energy to explain anything to a layman. Call that CurtC’s law. And Bernoulli is a conservation of energy equation.
I thought I’d clarify the icing role a bit, and the wing shape role…
The cross sectional “Bernoulli” shape plays a minor role in lift in normal circumstances, other posts and links here show that well. However, it plays a critical role in affecting when the critical angle of attack of the wing will change from laminar to turbulent flow at a given speed (i.e. stall speed).
So if you have a Bernoulli wing versus a symmetric wing, the former can sustain a laminar flow at much higher angles of attack than a flat wing would. Ice doesn’t usually induce turbulence when it forms in flight, that is something of a myth. It usually forms very smoothly. What it does is changes the shape of the wing, particularly at the leading edges. This doesn’t become a danger until the takeoff and landing regions of flight, when the aircraft is slow and the wing is at a steep angle. For example, all of a sudden a plane that normally flies quite well at 40 knots (like my little Cessna) will drop out of the sky like a rock at 60 knots if it is iced up. But over that speed I’d never notice the difference and the plane would fly quite normally.
So the shape isn’t really about lift, it is all about holding the laminar flow for as long as possible.
The explanations make some sense to me for a wing on an airplane, in level flight.
What bothers me is the boomerang. I used to have a nice Australian boomerang, made out of a very light wood. If you looked at it closely, it had the classic airfoil shapes on each “arm”. The pamphlet provided with the boomerang said that the spinning arms behaved like aircraft wings, generating lift in the direction predicted by the Bernoulli explanation of how a wing works. That seemed to match the observed behavior of the boomerang when it was thrown as instructed in the pamphlet.
Why does the boomerang bother you? Because it has an airfoil shape? No one here is arguing that the classic airfoil shape doesn’t work well for generating lift. It’s good at taking the airflow on the top side of the wing, and giving it a nice laminar flow so that it (the topside airflow) gets thrown downwards as the wing passes. If you could carefully observe the boomerang in flight, you’d see that it’s not perfectly flat with respect to the airflow, it is tilted, for a positive angle of attack, so that it can efficiently throw the air down as it passes.
Here’s my layman’s explanation of how a plane flies.
Almost anything can stay aloft by throwing something with mass downward. A “flying machine” (for example) could stay aloft by shooting baseballs at a very high velocity through a downward-pointing nozzle. But there’s an obvious problem: the machine will soon run out of baseballs. A practical flying machine must therefore have a cheap and inexhaustible supply of mass to throw toward the ground. There’s only one cheap and inexhaustible supply of mass available to a flying machine: air. While not ideal, there’s lots of it.
The wing of an airplane is basically a barn door angled in such a way that it throws air downward when the plane moves forward (i.e. the wing is an inclined plane). Because air has mass, throwing air downward creates lift.
But there’s a problem with a barn door wing: the shape creates a lot of turbulence in it wake. Furthermore, a barn door wing has a partial vacuum along the topside of the wing. This creates drag and instability.
To optimize efficiency and stability, it would be very desirable for the air to “hug” the top and bottom surfaces of the wing, i.e. if the air behaved as if it was “magnetically attracted” to the wing. By creating a specially curved surface on the top of the wing, it was found that the air would hug the wing surface. And this is where I believe Bernoulli’s Principle comes into play… Bernoulli’s Principle helps explain how the specially curved shape of the wing makes the air “hug” the wing surface, thus improving efficiency and stability.
The practical problem with asserting that deflection explains how wings create lift ignores that fact that in a stall (when the airflow over the TOP of the wings separates) a conventional airplane pretty much falls right out of the sky. The “deflection” of the airflow against bottom of the wing is uninterrupted, but obviously that’s not much help.
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Actually, it’s the Coanda effect that makes the air hug the wing. Bernoulli just lets you calculate the air velocity from the known pressure differentials, or vice versa.
I’m in the middle of writing up an article on this very question. If all goes well, the Teeming Millions can see it soon.
In very approximate terms, the air flowing over the top of the wing contributes 2/3 of the lift. In a stall, there is a rather abrupt loss of this, but the remaining 1/3 is still there. Yet the loss is enough to explain the plane’s reaction to a stall.
And (as I know that you know) it doesn’t “fall right out of the sky” - a typical plane drops its nose (perhaps substantially) resulting in an increase in airspeed and a decrease in angle of attack. In a small airplane, it’s normal to recover from a mild straight-ahead stall with the loss of a couple hundred feet - sometimes less.
Of course, if you stall just one wing, you may find yourself in a spin, the recovery from which is likely to require a bit more altitude.