# Tag Info

63

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 ...

38

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 ...

26

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).

18

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 ...

16

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 ...

13

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 ...

12

Nowadays, rockets use 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 swiveled from side to side. As the nozzle is moved, ...

11

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, ...

10

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 ...

9

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 ...

8

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 ...

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 ...

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

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 ...

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

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. ...

4

Lift is roughly proportional to angle of attack, and to speed squared. As a pilot, you instinctively balance these two. ADDED: Like if you suddenly drop a heavy weight, making the plane lighter, its lift isn't any less, so it starts to accelerate upward (climb). You notice this and either push the nose down with the trim wheel (lessen the angle of attack, ...

4

One thing in your argument is that more lift, means a higher speed. This may not be what airliners do. Airplanes (at long flights) choosse their cruising altitude based on their weight. Higher weight means lower altitude. I think this should be included in the incremental cost calculation of additional piece of luggage. First, simple Google hit: ...

4

More fundamental than the gimballed thrust system or verniers is the relationship between the "center of gravity" and "center of pressure" on a rocket (or any kind of projectile (e.g., bullet). For the rocket to fly nose-forward and not flip around, the center of gravity must be ahead of the center of pressure. In building small amateur or model rockets, ...

4

I would guess you've heard that an airplane in a spin or some other critical state can dive to build up speed, then when it pulls out of the dive the increased speed increases the lift and can allow the pilot to regain control. You are presumably asking if the same idea can be used for a falling person. The problem is that an aircraft wing is carefully ...

3

I am not sure that "more lift needed ⟹ more speed needed", as another way to increase lift is to increase the angle of attack (http://www.centennialofflight.gov/essay/Dictionary/four_forces/DI24.htm ). But I guess in both cases (if either speed or the angle of attack is increased to increase lift) drag is increased, so fuel consumption is increased. I don't ...

3

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 ...

3

At the start of the launch, the rocket has the largest mass of its entire flight. Any rocket that can make it to orbit necessarily is fairly big, making its fully loaded mass enormous. The combination of large size and large mass makes its relative air drag smaller than compared to a smaller and less massive rocket. The rocket's speed is also a ...

3

Leaving alone the feathers and everything, I would look at the power law. As I am unaware of the powerlaw concerning fluids (e.g. the interaction with air in this case), I would even ignore it and look at the drivetrain. As most birds take off more like choppers and less like planes (landing on spot) they need most of their muscle for liftof and can glide ...

3

I don't think this can work, even with your clarification that the balloon is tethered to the ground. Let's identify all the forces. The balloon has a buoyancy, pulling it upwards. I will call that $F_b$ (b for balloon or buoyancy). The balloon pulls the pulley up. I will call the tension in the rope tethering the pulley to the ground $F_s$ (for stake). That ...

3

There's two important differences between air and water: Air is compressible, and the densities are about a factor of ~1000 apart - 1 kg/m³ vs 1 t/m³! For most concerns where you use propellers, compression plays no role because the pressure diferences are very low. The densities, however play a large role. The thrust can be described as \$F = \dot m * ...

3

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 and by Newton's 3rd law an equal and opposite force (lift) is applied to the wing. Bound ...

2

I haven't done the calculations, but I doubt that this scheme would generate net energy. As was pointed out, electrolysis uses a lot of energy. However, after the H2 and O2 rises up the water column, you could get some of the energy back with a fuel cell that would convert the hydrogen and oxygen back to water and supply additional electric power, but ...

2

Lift is a function of the speed of the air from the leading edge to the trailing edge. In a flat turn, the inner wing is moving slower than the outer wing therefore there will be a difference in the amount of lift produced. But in fact, an airplane can not change direction by flat turning this way. Rolling into the turn by the use of the ailerons is the ...

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