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Hot answers tagged lift

115

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 ...

71

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 ...

67

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 "...

28

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).

26

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 ...

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 ...

19

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

Work is calculated as force times distance. $$W = Fd$$ The purpose of a simple machine like a screw jack is to lessen the force required. However, the work needed is still the same, so the distance over which you exert the force has to increase. Halving the force requires doubling the distance. In this problem, you want to lift 2000 lbs a distance of 1 inch....

14

Nowadays, rockets use a Gimbaled Thrust System. The rocket nozzles are gimbaled (An appliance that allows an object such as a ship's compass, to remain horizontal even as its support tips) so they can vector the thrust to direct the rocket. In a gimbaled thrust system, the exhaust nozzle of the rocket can be swivelled from side to side. As the nozzle is ...

13

In your own question you recognize that the Bernoulli equation is the wrong thing to apply to this situation, because obviously there are dissipative losses involved. My preferred way of looking at this is recognizing there is a lift to drag ratio that exists as a metric for aircraft. This can be 4:1 or 25:1 depending on the plane. Regardless, provided ...

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, ...

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 ...

9

As with any other rocket, ejecting the propellant out the nozzle generates an equal and opposite thrust. The difficulty here is that the propellant is in the body of water the device flys above. If you read the FAQ at the jetlev website you will see that power and pumping is provided by a separate floating unit. The buoyancy of the boat unit supports the ...

9

Without going into the excellent and detailed mechanics explaining reaction lift that others have provided for this answer, I just want to say that contrary to popular belief/high school physics textbooks, airplanes do not fly solely on account of Bernoulli's principle. According to Walter Lewin's excellent "For the Love of Physics": "Bernoulli's principle ...

9

Because where they come close together the air in between circulates in such a way as to join them in a single path. Floris is right, but maybe this picture helps.

8

This "air glider" works as a hovercraft, using air pressure to lift itself and its load. From a commercial model we get the following specifications: 8-1/2 in. x 36 in. (22 cm x 91 cm) dual pads (0.4 m2 total area). 750 pounds (~3500 N) of lifting capacity. 1.75 HP blower. The required (relative) pressure to lift 3500 N of weight using a 0.4 m2 platform ...

7

Well I ought to be studying for a physics exam, but I'll consider answering this to be my studying. Newton's third law states that for every action, there is an equal an opposite reaction. In this case, the jetpack is ejecting water at high velocity toward the ground. This is generating a significant force downward. The resulting opposite force pushes ...

7

There's an interesting book by H. Tennekes on the subject of scaling in flying. If you want to go fast and far then the size of your plane scales up, while the speed of sound gives a limit, approached by a Boeing 747. But if you simply want to get off the ground with little effort (what was meant by "easy" in my book), then it is worth while to be small (I ...

7

Viscosity is necessary in order for the wing to generate lift. Without the change in circulation caused by flow separation from the trailing edge, there will be no lift. In an inviscid fluid there will be no separation, and hence no lift. A similar flow pattern can be observed in viscous fluids when the Reynolds number is extremely low (Re<<1), and you ...

6

Most of the lift comes from the main wing, and in fact the tail lifts down, so the main wing also has to support that. (That's for a stability reason.) The lift of a wing is roughly proportional to two things: angle of attack, and airspeed squared so, the slower an airplane is flying, the more it raises the nose. You will notice this the next time you ...

6

First to Address the Equivalence of Bernoulli's Equation to Conservation of Momentum: There are (at least) three popular explanations for lift on an airfoil: Faster air on the top has lower static pressure than slow moving air on the bottom. The resulting pressure difference multiplied by the area is equal to the lift. The airfoil deflects air downward ...

6

The key point is that wings allow you to "tilt the engine" much more efficiently than actually tilting it. Tilting an engine converts the power only at 1-to-1 ratio, but wings do it better - a Boeing 747 has a lift/drag ratio of 17 at cruise speed, the wing is generating 17 times more lift than the applied engine power.

6

The principle of conservation of energy is your friend. The wind turbine generates electricity. That energy has to come from somewhere. The other energy in the system is the kinetic energy of the wind. Therefore, the wind turbines convert the kinetic energy of the wind into electricity, and the kinetic energy of the air downwind of the rotor is less than ...

6

From the geometry, you can state that in order to move the screw by 1 unit of distance, you have to move the end of the handle by $10\times 20\times 2\pi$ units of distance. Let's call the unit of distance $[L]$ - in this case, an inch might be a good unit but we don't have to be explicit about that. Conservation of energy says that work done on the system ...

5

Seems to me your question contains two physics questions which depend on the definition of "easier". Certainly in an atmosphere it is easier to balance gravity the larger the ratio of surface to weight due to the viscosity of the medium. On the other hand this does not make "easier" the maneuverability of the system in energy demands. So you are asking ...

5

The mass difference of the air it would have contained and the Helium it does = the volume of the balloon multiplied by the difference in density of the helium and air. Suppose the balloon is spherical and 12" in diameter (physicists can only do the arithmetic for spherical objects, and preferably in a vacuum) . That gives it a volume of $\frac43 \pi r^3$ ...

5

Rather than try to debug a Wikipedia page, I suggest two things: Take your time and read this delightful on-line book. Also, why not get a copy of Stick and Rudder? It's been a classic for 70 years. Take an introductory flight lesson. It's a lot of fun and totally safe. They let you take off and fly around, and then you will understand all the basics. Don'...

5

It is merely a long comment but hopefully it gets your intuition on the right track. I try to detail the physical part of the reason why having fixed wings is a good thing: You basically ask, why are airplanes more efficient (hence people still produce them despite their less nice maneuverability) well you probably noticed helicopter rotors work pretty ...

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