# How does an airplane fly?

1. Jun 28, 2013

### ProgressNation

I wonder how airplanes fly? I know maybe it's a tough question. Does the airplane fans produce huge force that makes the airplane boosts it's speed?The wings make it fly, and I don't know how it's tail works. But how do the airplanes fly according to it's heavy weight? Plus there is a big air resistance. I'd really love to know how the airplanes work, they're beautifully amazing!
Thank you!

2. Jun 28, 2013

### DiracPool

This is kind of what the PF mentors typically consider an overbroad question. However, I'll interject one interesting tidbit here I just stumbled across because it's interesting and relevant to your question. I've always learned that airplanes fly because the wings create lift through a differential in air pressure created by the shape of the wing. Supposedly, the curved shape of the wing at the top causes air to move more rapidly there, creating a lower air pressure over the top of the wing which causes the plane to lift. However, I just recently watched a documentary that aimed to debunk this as a myth, saying that there is no air pressure difference since the laminar flow of the wind simply wraps around the wing and meets at the other side.

The actual cause of the lift, according to this documentary, was simply the force of the wind hitting the bottom of the wing, and that lift was created because planes fly slightly tilted with their pitch upward into the wind which forces the plane upwards. They debunked the pressure differential idea by saying if that were true, then planes wouldn't be able to fly upside-down, which they obviously could do. I thought that was kid of interesting. Unfortunately, I can't remember the name of the documentary.

3. Jun 28, 2013

### ProgressNation

Thank you for sharing your answer, it's honor for me! But another thing that I wanna know is that why does the airplane fans exist? Does the airplane fans exert very huge force which causes the airplane to accelerate very fast where can air resistance go through the wings and make it fly?
Thank you!

4. Jun 28, 2013

### Staff: Mentor

5. Jun 28, 2013

### CWatters

Before long you will come across groups of people who argue endlessly over two different theories. One group say that lift is caused by the pressure difference above and below the wing. The other group say it's caused by the wing deflecting the air downwards. The real answer is both are correct....

http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html

6. Jun 28, 2013

### ProgressNation

Ok does the fan exerts a huge force to accelerate the engine?? If so, how?

7. Jun 28, 2013

### CWatters

That's one reason why it took man so long to figure out how to fly.

Put simply.. There are lots of different forces that act on an airplane. An airplane has to be designed so that if one of these forces causes the aircraft to pitch (point) up or down then other forces restore the balance. This is known as stability.

Consider a ball balanced on top of a hill. If you give it a slight knock it will roll off down the hill. The position of the ball is said to be "unstable". If you have a ball at the bottom of a valley and you give it a slight knock the ball may move but it will eventually return to it's original position. In this case the ball is said to be "stable".

The tail on a plane basically makes the plane stable. It also allows the plane to be steered but that's another story.

8. Jun 28, 2013

### CWatters

Start at the back of the engine...

Near the back there is a combustion chamber where fuel is burnt in compressed air. As it burns it expands and rushes out of the back of the engine. A turbine fan in this hot exhaust stream works like a windmill and drives a shaft running down the middle of the engine. This shaft power an air compressor in the centre and the big fan at the front.

The big fan does two jobs. It provides some air for the compressor in the core of the engine but it also sucks in a lot of air that bypasses the core and is sent straight out the back.

Pushing air backwards has the effect of pushing the plane forwards.

9. Jun 28, 2013

### Staff: Mentor

That's what I call the "Tastes great!" "No! Less filling!" debate over how airplanes fly. The real answer it's not an either/or proposition. Both explanations are correct.

Birds of a feather!

10. Jun 28, 2013

### Staff: Mentor

:rofl:

11. Jun 28, 2013

### A.T.

What do you guys make of the "New Theory of Flight" by Claes Johnson?

http://secretofflight.wordpress.com/

Is this really something completely new, or just more detail? Does his "elegant separation" apply to stalled wings only, or in general?

12. Jun 28, 2013

This is incorrect. If there is a lift force that is generated by the air, it must be accompanied by a pressure difference. Further, the concept of laminar flow has essentially nothing directly to do with the generation of lift. It can affect the values of lift and drag, but it is not part of the fundamental reason lift exists.

Well, they are mostly right. You can really look at lift two ways: either through the deflection of the air downward or through the pressure difference created on either side of the airfoil. Both of these can accurately describe the lift generated by a wing and both allow for a plane's ability to fly upside down. The thing that makes many of the pressure different explanations wrong is that they don't explain (or incorrectly explain) why that pressure difference exists (hint: it has nothing to do with "equal transit time"). If done correctly, you could solve for the flow around an airfoil and use the pressure differences to calculate lift and drag or draw a control volume around it and look at hte net momentum change due to action of the body and you would get the same answer for lift and drag. Both explanations work equally well. The pressure explanation is often a lot easier to use in practice, however.

Which brings me to...

As CWatters alludes to, this causes a downright comical debate between two warring factions who, in essence, are both correct anyway. It is sort of a chicken vs. egg debate.

This sounds like a bit of over-promoting their own work. They really didn't present anything new in that presentation, and I would even argue that some if it is wrong. For example, saying Kutta-Joukowski does not describe real physics is true by definition, as that theory makes the assumption that the flow is inviscid and incompressible, which obviously doesn't describe the real world. Further, pretty much everyone already knew that you had three major factors affecting lift and drag: vertical pressure differences, horizontal pressure differences and separation (viscosity would be another major contributor as well). I really see nothing new here. Further, I disagree with the idea that a large rounded-edge airfoil will necessarily produce lift. At low angle of attack with no separation, it would not. It requires that separation to prevent the flow from simply remaining symmetric.

13. Jun 28, 2013

### sophiecentaur

The thing that the 'pressure difference' brigade seem to ignore is the fact that there needs to be a net downwards motion of the surrounding air in order to produce a net lift force on the plane (reaction / rate of momentum change of the air). How that downward flow is achieved, and how efficiently, is what everyone is really arguing about, I think. The downdraught from a hovering helicopter is more noticeably there because all the displaced air is under the same spot in the sky. For a fixed wing craft, the effect is much more subtle because it's spread out over the whole of the flight path and any coherent motion will quickly be damped out and the kinetic energy dissipated.
Why do Scientists always want to say "this is what really happens" instead of "this model works"?

I did like the animations on that youtube sequence,

14. Jun 28, 2013

### nst.john

Well if the air is being split by the winds at two different forces, wouldn't they be split into two component pressures and add together to the original pressure in the end?

15. Jun 28, 2013

The other side of that coin is that a lot of the downward deflection group like to ignore the fact that there also must exist a pressure difference and that the two are really part of the same phenomenon.

Ego.

16. Jun 28, 2013

### OmCheeto

This model always worked for me:

There was also a version with a high power elastomer based engine, as I recall.

ah ha!

That model worked too.

17. Jun 28, 2013

### sophiecentaur

Ego can often result in Egg on Face!

Om Cheeto - the wheels came off your idea!

18. Jun 28, 2013

### sophiecentaur

That pressure difference idea really sucks.

19. Jun 28, 2013

Come now, science never sucks!

It blows.

Last edited: Jun 28, 2013
20. Jun 28, 2013

### Filip Larsen

If by "simply" they mean "only", then how did they explain loss of lift when flow separates from the upper surface (e.g. "stall"), or when a wing deploy spoilers. How would they explain that blowing air between two sheets of paper will make the papers move closer to each other rather that away from each other?

In general you have to look at the complete flow to understand and model what is going on, and from that you can often give equivalent explanations using either forces, pressure differences, conservation of momentum, etc. As others also have noted, it may not make much sense to try assign one explanation as being more fundamental. However, what I think you can consider fundamental in this context, is that an air flow give rise to a lower static pressure (as the example with the two sheets of paper illustrate).