Lift

[Editor: The following is a composite of several online websites dealing with the subject matter.]

I think we would all agree that understanding the principles of flight is a complex subject matter. None of us would feel comfortable standing in front of a group of strangers and begin a class on teaching the subject. Never-the-less, we all still have an intuitive grasp of the subject, at least enough to enable us to fly with confidence.

And probably the most important aspect of flight that we least understand is that of �Lift�. We are all too aware that without lift, all we have is a pile of sticks or foam on the ground!

But surely we must have discovered something about lift over our lifetimes. For those young enough, you probably remember discussing this during high school physics classes. And if you dig deep, you likely remember it in the context of the Bernoulli principle � as the speed of a moving fluid (air) increases, the pressure within decreases.

For a great many years, this principle has been used to educate people (lay folk and professionals alike) about how a wing creates lift due to the lower pressure on the top surface. And this fact is absolutely correct, but not for the reasons that are typically used to explain the presence of low pressure!!

The most prevalent myth that still exists in most text books to-date is that the low pressure is caused because the air travels faster across the top surface in order to meet up with its slower molecule brethren that flowed below the wing. But actual wind tunnel testing using pulses of smoke and air shows that those molecules traveling over the top actually reach the trailing edge faster than those that travel below! See Fig 1 for a depiction of this.

 

 

Fig. 1

Only a symmetrical wing at a zero angle allows for equal transit times over and under the wing.

Just to re-iterate, Bernoulli states that we get lower pressure on top because the air moves faster (in order to catch up to the air below). But it was never explained (correctly) why the air has to meet back together with its previously adjacent molecules.

What is actually happening is just the opposite of what Bernoulli stated (semantics). The air is moving faster because of lower pressure on top. Sometimes a lot faster than the air below, sometimes almost the same speed, but always covering equal to or more distance per unit of time.

It wouldn�t be fair to just leave it here, since we have to ask (don�t we?) where that low pressure zone comes from!!

Along comes Newton

For a seemingly simple set of words, Newton�s three laws of motion are a central theme in almost every aspect of our lives. It should come as no surprise that it also has a major implication on the causes of Lift.

Those three laws are (paraphrased):

  1. A body in motion remains in a constant velocity unless acted upon by an external force.
  2. Force acting on a body gives it acceleration in the direction of that force with magnitude inversely proportional to the mass of the body.
  3. A force exerted on a body is matched by an equal and opposite reactionary force.

Ok, ok, the number two law is not simple words, but you get my drift.

Starting with number one, and referring back to Fig 1, it is obvious that the air in front suddenly changes direction so a force is acting on it at or around the leading edge. Also, in line with number two and three, the air is accelerating around the wing. So far so good.

A careful examination of Fig 1 shows that the air does not just split at the leading edge. In fact it is drawn upwards momentarily before the split as shown in Fig 2 below.

 

 

Fig. 2

You will also notice that the air at the trailing edge is directed downward. The first law states that a force is being applied to direct it down. The third law tells us that an opposite force is in play, and that is what we have been calling Lift. In fact, the lift applied to the wing is equal to and opposite in direction to the air�s change in momentum while being directed down.

Momentum is a force and mathematically it is the product of velocity and mass (F = m * v). Newton�s second law also can be written mathematically as acceleration times mass (F = m * a), so rephrasing the second law explains what is happening: �Lift of a body is proportional to the amount of air diverted down times the vertical velocity of the air�.

The diagram of Fig 1 would imply that only a small amount of air is headed downward, but that is a function of reference. If you were to ignore all but the vertical component of that air mass, you can measure the size of that lift using Newton�s second law.

Without doing all the math, the aircraft in Fig 3 demonstrates that lift. It is flying along the top edge of a fog bank. You can see the size of the �dent� put into the �air� below (and the vortex turbulence). This air comes from the downwash created, with an equal force pushing the aircraft upward.

 

 

Fig 3

 

So, why does that air travel downward instead of just straight back? It is a direct cause of something known as the Coanda Effect (named after the discoverer Henri Coanda of Romania). It defines the ability of a fluid (such as air) to flow around a curved object. Fig 4 is an example of this effect.

 

 

Fig 4

The mathematics behind all of this behavior is extremely complex when it comes to airfoils, and perhaps that is why the educational systems are slow to update their out-of-date explanations. If you wish to learn some more and look at some of the math, feel free to visit some of these websites;

 

At any rate, the air does indeed reach the trailing edge of the airfoil and begin to wrap around the edge until enough turbulence occurs to break it away. Due to the nature of laminar flows, the air molecules directly above the bottom layer are also dragged around, but to a slightly lesser degree. Eventually, when far enough away from the bottom layer, the air is barely dragged downward at all.

Ok, you guys in the back row, it�s time to wake-up!!!