How do airplanes really fly

phenom01

Because the faster you go, the more air flows between the wing; thus effecting pressure differences. You follow?

russ_watters

More thrust doesn't necessarily mean more speed. But anyway, if you hold the angle of attack constant, lift DOES increase as a square function of speed, as Bernoulli's equation would predict: http://www.grc.nasa.gov/WWW/k-12/airplane/lifteq.html

phenom01

constant in a straight line? How does that increase lift?

DaveC426913

How does increasing speed increase lift??

Just as in a car, wind resistance increases as the square of speed, so, in a plane, does lift.

boneh3ad

Holy cow there are some huge misconceptions here.

In a most basic sense, lift can be explained through Newton's laws. The flow is "pushed down" do the plane must be "pushed up". Of course in this sense, it doesn't matter how this downwash is generated or how efficiently, only that it is generated.

Bernoulli's principle is one way to calculate the lift on a wing in certain situations (the wing cannot be separated, for example). Given a velocity distribution over a wing, you can use Bernoulli's equation to deduce the pressures on the wing and hence the lift. It says nothing about how you find sai velocities or the best shape of a wing. Bernoulli's equation is merely a tool; it cannot explain lift completely.

Now, any wing that generates lift must, by definition, deflect the air downward, and the same wing will also have a higher pressure below than above. You can connect the two using what is called the Kutta condition. This states that for an object with a sharp trailing edge, the rear stagnation point must be at that trailing edge rather than the location predicted by inviscid methods, which results in a net circulation around the airfoil and therefore a velocity difference and pressure difference on the surfaces as well as deflected flow coming off the back. This works for any shape with a sharp trailing edge (as all wings have) and does not require any assumptions about how the plane is flying.

A plane with a symmetric airfoil can fly because it has angle I attack and a sharp trailing edge. This allows the airfoil to deflect the flow downward. The same applies for a traditional airfoil flying upside down. In this case, flying inverted is her inefficient, but with enough angle of attack it can be done.

Flying vertically doesn't have lift in the traditional sense. The lift is provided solely by the thrust.

russ_watters

I'm curious as to why you would say that about Bernoulli's equation but not Newton's laws, since they have the same limitation. Predicting what the flow around a wing looks like is extraordinarily difficult.

russ_watters

Moving faster means more air is flowing over the wing. More air thrown downward means more force pushing the wing upward.

boneh3ad

That's fair enough, but the distinction I hope to make is that Bernoulli's equation is a useful tool but in no way explains where lift comes from. Newton's laws can, even if they don't explain necessarily how that downwash is generated, only that it's generation signifies lift.

rcgldr

I'm not sure how accurate this would be. Part of the lift is due to a non-Bernoulli interaction between air and a wing that increases the mechanical energy of the air (wrt ambient air).

russ_watters

What interaction is that?

boneh3ad

Explain that because either I just am not understanding what you mean or else I completely disagree. The lift is exacty the opposite reaction to the deflection of the air downward. That isn't a practical calculation, but the pressure distribution, which in many cases gives the same answer, is practical.

The wing only adds energy to the air through the action of viscosity since the wing therefore drags some air along with it. That creates drag, though, not lift.

russ_watters

So again, same limitation, isn't it? You can find the lift by using the velocity profile to find pressure or momentum change, but neither tell you what that velocity profile will look like.

CWatters

I'm afraid this subject (is it Bernoulli or Newton) comes up all the time on forums around the world. People argue strongly. For every person who favours one theory there is another that has the opposite view.

It's a false dichotomy. Neither is right or wrong.

http://www.pprune.org/professional-p...li-newton.html

DrewD

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

This site was posted on PF and I read it a while back and felt that it was informative. It has a simple explanation on the first page and some deeper stuff on the second. My recollection is that it says that either a freebody force explanation or bernoulli fluid laws can work. But it helps to clarify the common mistakes that are made. I haven't the time to reread it now, so I hope that it is as I remember (if its not lets pretend that they changed it since I read it)

boneh3ad

Yes. For that you can apply the Kutta condition and get your answer for the case of no separation. Otherwise you need the full Navier-Stokes equations.

rcgldr

After a wing passes through a volume of air, the affected air ends up with a non-zero exit velocity (the velocity of the affected air when it's pressure returns to ambient). Using the unaffected air as a frame of reference, the affected air's mechanical energy is increased by a wing, which means work is done, which violates the rule for Bernoulli.

A similar process occurs with a propeller or rotor, except that the pressure differential is greater than a typical wing, and there is a greater amount of induced flow (the inwash ahead of the propeller or rotor). From a NASA article:

... at the exit, the velocity is greater than free stream because the propeller does work on the airflow. We can apply Bernoulli's equation to the air in front of the propeller and to the air behind the propeller. But we cannot apply Bernoulli's equation across the propeller disk because the work performed ...

propanl.htm

boneh3ad

Yes, you refer to the wake. However, this really relates to drag, not so much to lift.