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98

A short summary of the paper mentioned in another answer and another good site. Basically planes fly because they push enough air downwards and receive an upwards lift thanks to Newton's third law. They do so in a variety of manners, but the most significant contributions are: The angle of attack of the wings, which uses drag to push the air down. This ...


66

From Stick and Rudder by Wolfgang Langewiesche, page 9, published 1944: The main fact of all heavier-than-air flight is this: the wing keeps the airplane up by pushing the air down. It shoves the air down with its bottom surface, and it pulls the air down with its top surface; the latter action is the more important. But the really ...


46

This answer is nothing more than a variation of Sklivv's answer. I simply wish to discuss some quantitative ideas following from Sklivv's answer and discuss what I understand (from an aerospace engineering friend) to be a common conceptual mistake - that the application of "mere surface effects" and "application of Bernoulli's principle" is wrong. These ...


32

When you would enter the water, you need to "get the water out of the way". Say you need to get 50 liters of water out of the way. In a very short time you need to move this water by a few centimeters. That means the water needs to be accelerated in this short time first, and accelerating 50 kg of matter with your own body in this very short time will deform ...


29

Since you asked for an explanation appropriate to an non-specialized audience, maybe this will do: "A Physical Description of Flight; Revisited" by David Anderson & Scott Eberhardt. It is a revision of the earlier "A Physical Description of Flight" (HTML version).


25

There is a YouTube video that visualizes the air flow around a propeller for various configurations. I caught a screen shot of a moment that more or less shows what is going on: As you can see, this happens at 2:07 into the clip - this happens to be for a dual rotor configuration (two counter rotating blades) but the principle is the same. Behind the ...


25

Upside-down or right side up, flight works the same way. As you stated, the wing deflects air downward. When inverted, the pilot simply controls the the pitch of the aircraft to keep the nose up, thus giving the wings sufficient angle of attack to deflect air downwards. Most airplanes are designed with some positive angle of attack "built-in," meaning that ...


23

It's not the falling that's fatal, it's the deceleration at the end that kills you. Something like water or concrete does this on a sub-meter distance (which requires extremely high forces). On the other hand a gas is much less dense, so it cannot decelerate a falling object nearly as quick. Sometimes inflatable cushions are used as safety nets (think: ...


21

Let's look at the relationship between momentum and energy. As you know, for a mass $m$ kinetic energy is $\frac12mv^2$ and momentum is $mv$ - in other words energy is $\frac{p^2}{2m}$ Now to counter the force of gravity we need to transfer momentum to the air: $F\Delta t = \Delta(mv)$ The same momentum can be achieved with a large mass, low velocity as ...


20

The first question you need to ask is: does an irrotational, inviscid, incompressible fluid really exist? The answer is no (well, yes, sort of, if you consider super-fluids). The irrotational, inviscid, incompressible fluid is a mathematical creation to make the solution of the governing equations simpler. Lift cannot exist without viscosity! That's is ...


18

Each window represents a restriction to the air flow. The greater the pressure difference across the aperture, the greater the flow. An electrical analogy: each window is a resistor. The current through the resistor is proportional to the voltage across it - but when you have two resistors in series they must carry the same current (air that enters through ...


17

I often wondered about these things - then I came up with a simple experiment that works for me because I have a simple bike computer (thing with a magnetic pickup on the spokes that updates my speed every second). I find a flat piece of road, and ride at a certain speed (say 20 mph on my road bike, or 15 mph on my mountain bike). I then stop pedaling at a ...


17

Fundamentally, a boomerang has two arms that spin. One arm spins in the same direction of flight and the other spins away from the direction of flight. For this reason, there's a tilt force on the boomerang. Now, since the boomerang is spinning it has angular momentum. Therefore the tilt force generates precession which is pretty much what makes the ...


16

There are lots of questions here that I will try to answer, hopefully I'll get to them all... Creature Comforts It's hard to "just fly higher" when you consider passenger planes. Supersonic military aircraft like the SR-71 do fly ridiculously high. It's service ceiling is 85,000 feet! But, it has the advantage that it doesn't need to keep anybody but the ...


16

How does they move slowly in air, without falling down? One possibility is soaring using a ridge lift - typically a situation when the wind is approx. perpendicular to a mountain ridge. The air is lifted at the front side of the ridge and an eagle can soar in the lifting air stream. This can also work without the wind, Which is a situation of thermal ...


15

I doubt if anyone has come up with a complete explanation, but some laboratory simulations have created similar patterns. They happen if the central and surrounding areas in a flat, circular disk of fluid have different velocities. Emily Lakdawalla at The Planetary Society covers it at this site. She also explains how other patterns (triangles & ...


14

Sonic boom refers to the explosive sound caused by the shock wave from an object traveling faster than the velocity of sound. Yes, It's actually spoken out as breaking the sound barrier. Felix jumped from an altitude of 39,044 km (which is 128,097 ft.) and reached a peak speed of 833 mph. Yes, He did produce the Sonic boom. Most likely, we use the term ...


13

Colin's answer is right. Let me see if I can clarify a little bit. First, forget that old Bernoulli explanation. It's not wrong, but it confuses everybody. If you create a simple symmetrical teardrop-shaped airfoil, and place it in a wind stream, then the air will flow past it, and it you turn it at an angle to the wind, it will deflect the wind stream, ...


13

No it's an urban myth. It's impossible for them to fly using a very simple and inappropriate model of wing behaviour - possibly closer to say that bumble bees can't glide like albatrosses


13

The ocean surface is not as hard as the ground but if you drop from a plane, you would hit it with such a high velocity that the pressure would most likely kill you or cause very serious damage. Considering air resistance, the terminal velocity of a human, right before reaching the water, would be at most some $150\text{ m/s}$. If you weigh $70\text{ kg}$, ...


13

Consider jumping into a swimming pool. Do a barrel-roll (sorry I mean cannon ball, that just kind of slipped out). It's fun, you enter the water nicely and make a huge splash, probably soaking your sister in the process (that'll learn her). Now do a belly flop. Not as fun. You displace exactly the same amount of water in the same time, but this time there is ...


12

Let's look at this another way: you're just moving from one fluid to another. Sounds harmless, right? By specification of the problem, we're at terminal velocity when we hit the water. The force of drag (in both mediums) is roughly: $$ F_D\, =\, \tfrac12\, \rho\, v^2\, C_D\, A = \rho \left( \frac{1}{2} v^2 C_D A \right) $$ You can imagine that ...


12

The most important thing to take away is that you don't want to approach physics in terms of a long list of equations, where the goal is to choose the right one for the question you have at hand. (That's why I edited the title to your question, so it no longer simply asks for an equation.) Physics is a method, and the equations are a tool for answering the ...


12

The air in an elevator does tend to move with the elevator, because it has relatively little inertia. However, thinking about the problem in these terms seems, to me, misleading. The simplest way to think about this is to consider the acceleration of the elevator as and addition to the normal acceleration due to gravity. In this light, it would be as if the ...


12

I think the reason is that when you are blowing on an object, you are making lots of air particles collide with it perpendicularly in one direction thus transferring a lot of momentum to the object. When you are sucking air in, the only force that's acting on the object is by the air particles that rush in to fill up the gap that you just created. These ...


12

The car is behaving like a closed pipe, so you get a resonance set up. There's a Wikipedia article here, but for once the Wikipedia article isn't that great, so there's another better article here. I imagine you (like most of us) will at some point have discovered you can make a sound by blowing across the top of an opened bottle, and it's the same thing ...


11

The primary factor that determines the ability of an aircraft to takeoff is having a speed exceeding that of the liftoff speed: that is the minimum (air) speed of the aircraft to generate sufficient lift by its wings to counteract the gravitational pull from the earth. Large passenger planes at takeoff often change the wing configuration (lowers flaps etc) ...


11

Apparently that is possible. From http://en.wikipedia.org/wiki/Formula_One_car: Indycars, for example, produce downforce equal to their weight (that is, a downforce:weight ratio of 1:1) at 190 km/h (118 mph), while an F1 car achieves the same at 125 to 130 km/h (78 to 81 mph), and at 190 km/h (118 mph) the ratio is roughly 2:1. From ...


11

Intuitive start of an answer: If you have counter rotating vortices they have zero net angular momentum (to first order). If they merged they would have to have no motion -> where did the energy go. In between the two axes of rotation the fluid moves in the same direction and has no mechanism for dissipation. By contrast for two vortices with the same ...


10

The Kutta condition is completely artificial. The potential equations are completely artificial. The potential equations are a mathematical construct we use because it's much simpler than the full Navier-Stokes set of equations. We know the Kutta condition is never actually upheld in any real flow ever. However, when we perform all of our mathematical ...



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