How do airplanes fly?

Answer 1

Here is a link to one site with an explanation.

http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html

Air traveling over and under the wing of an airplane is deflected downwards and causes it to be lifted. This is known as aerodynamics.

Answer 2 (Wrong)

Bernoulli's principle of flight. Air traveling over the curved upper surface of the wing moves fast. This air becomes low pressure. The air traveling under the wing moves slower and maintains a higher pressure. As the speed of the airplane increases, the pressure over the wing becomes far less then the pressure under the wing, and the airplane is lifted up.

Answer 3

By the way, the same is true of birds wings and helicopter blades.

Answer 4

Circulation and downwash. For every action, there is a reaction. There is a large amount of air being forced down by the wings....thus the opposite force would be the air keeping the airplane up. The theory that the air above the wing has to travel faster than the air beneath is bogus. Two particles hitting the front of the wing (one travelling on top, the other on the bottom) will NOT meet at the same time. That is called slippage. How do you think airplanes fly upsidedown?

Answer 5 (Incorrect)

Whoever wrote the above is bogus. The Bernoulli principle is the primary aerodynamic force for subsonic lifting force. Pure Deflection is a terribly inefficient method of providing lift. The variation in flow speed above and below the wing ARE the reasons for circulation and downwash, circulation is merely the physical reality of the two flows not meeting perfectly because of the different travel lengths, it is an inefficency, not a cause of lift. Airplanes fly upside down because each airfoil shape has a plotted relationship between lift force and the angle of incidence of the incoming flow stream and the airfoil. Even asymmetric chambered wings will produce a negative lift when the angle of attack is lowered enough below the zero lift line, however they are just not as efficient that way. That's why you don't fly cross country upside down. Aerobatic planes use symetrical airfoils, thus they work just as well either right up or upsidedown.

Answer 6 (Partly Right)

Money is what makes airplanes fly... However even as an airplane is flying upside down, the angle at which the air strikes the wings causes the flow on the upward side of the wing to travel farther. Also there is the downward push of the air itself, it is a combination of both. It must be noted also that an airplane flying upside down with a conventional airfoil shape, is horribly inefficient, most airplanes with that type of wing can hardly maintain level flight. In order for an aerobatic aircraft to fly upside down the wing is shaped the same on the top and the bottom.

Answer 7

Without researching the link offered in the first answer, the second answer contains the most comprehensive and accurate information on the aerodynamics of lift. Interestingly, the same principles apply to sails on boats.

Answer 8 (Weird... but not entirely wrong)

High air pressure is trapped below the plane, pushing it up, and low pressure is above the plane, pushing it down. Both cancel each other out, causing the plane to hover. The turbines pull air into it, and blow it back out behind, thus creating thrust, which propels the plane in the forward direction.

Answer 9 (Right, right, then Ka-Boom....)

There is no reason why an air particle would go faster if there is a longer distance to travel, it just would take longer. there is also no relation between pressure and the distance between particles; the distance between particles has something to do with the density of the material. The only things correct on this answer are the observations: The air will go faster over the upper side of the wing and the pressure will become lower. but this is due to the following rules:

1. rule of conservation of energy/mass:
the flow over an airfoil (wing) can be defined as a stream tube ( a tube through which a certain amount of air flows). In this stream tube the mass flow is by definition the same since mass can't be destroyed. The mass flow is defined as:

Mass flow= dm/dt = rho * A * V 9.1

where m is the mass, t is the time, rho is the density of the gas or liquid, A is the Area through wich the mass flow is flowing and V is the speed with which this goes. And since as has been said the mass flow on one moment in the stream tube is the same as on the next moment the following formula is true as well:

rho1*A1*V1=rho2*A2*V2 9.2

For speeds under 100m/s (or to be more correct for speeds under Mach =0,3) the difference between rho1 and rho2 can be neglected. therefore the formula becomes:

A1*V1=A2*V2 9.3

When one looks at the following picture of streamlines/stream tubes over an airfoil: http:/sol.sci.uop.edu~jfalwardphysics17chapter5airfoil.jpg
In this picture one can see that the streamlines flow closer to each other at the upper side of the wing, this means that the stream tubes get narrower and the Area through which the flow flows gets smaller. This means that A2 gets smaller in formula 9.3, and therefore V2 needs to get bigger. At this point Bernoulli will kick in, Bernoulli's equation is stated below:

p+0,5*rho*(V^2)= constant 9.4

In Bernoulli's equation p is the static pressure, this is the pressure which you will feel on your head when you are sitting behind your computer for example. rho is again the density and V is the speed at which the air particle travels.
since is has been proven that the speed on the upper side of the wing will go up and the rho will remain constant, the static pressure has to become lower. This means that the pressure on the upper side of the wing becomes lower. On the lower side of the wing the opposite happens and the pressure will become higher. due to this pressure difference the airplanes fly.

as stated I have talked about flying speed which are lower than M=0.3. For faster speeds the equations get a lot more complicated, though the principle stays the same.

Observations 9 of (Right on.... Somebody is awake!)

Answer 1, although very abbreviated, is correct, but does not fully answer the question asked. Also, the link is very good.

Answer 2, also VERY abbreviated and incomplete, is however the MOST ACCURATE AND BEST explanation [as observed by answer 7].

Answer 3, contains NO answer, but is an accurate observation.

Answer 4, is ALMOST totally inaccurate ["bogus" as observed by answer 5], WITH THE EXCEPTION of sentence 2 which has only a tiny bit of accuracy. It is true that a board rotated at steep angle [of attack] in relation to its movement through the air, WILL HAVE a slight vertical component of force [which imitates lift], but is so inefficient as to be considered almost negligible. Additionally, there IS also a SMALL UPWARD true lift component, due to the Bernoulli principle, BUT due to the severe effect of induced turbulence, this lift is very small. BUT, with the combinaton of these two UPWARD forces, AND enough muscle [power] a flat board can be made to fly.

Answer 5's first 3 sentences are very good contributions to this answer, but from there on, the connection to accuracy deteriorates.

Answer 6 is accurate in sentence 2 ["However even as..."], sentence 3 ["Also there is..."], and sentence 4 ["It must be..."], but the rest miss the accuracy mark.

Answer 7 as brief as it is, is totally accurate, but only observations, no answer to the question.

Answer 8 is totally inaccurate, other than the fact that "...thrust...propels the plane in the forward direction."

ANSWER 10 (Great Summary)

Most references contain misconceptions about the lifting force, and they explain it incorrectly. The first NASA link here is correct, and it also gives details about the wrong answers found in many books.

To understand how wings work, you need to learn about the many mistaken answers. That way you can remain focused on real information while avoiding picking up misconceptions. For example, it's wrong to say that air molecules must race over the top of the wing in order to meet the slower molecules moving below the wing. This mistake is called the "Equal Transit-Time Fallacy." While air does flow more rapidly above the top of the wing, the molecules on top will never meet the molecules flowing below. Instead the molecules above the wing will greatly outrace those flowing below, and the two groups remain separated after the wing has passed by.

Also see these correct explanations:

Denker: See how it flys, AIRFOILS
http://www.av8n.com/how/htm/airfoils.html

Weltner: Physics of flight, reviewed
http://user.uni-frankfurt.de/~weltner/Flight/PHYSIC4.htm

Raskin: How Wings Work
http://jef.raskincenter.org/published/coanda_effect.html

(Note: I have closed this answer to changes. Please contact me if you want to reopen it. --Eric M Jones, Physics Supervisor)

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