Lift

By Colin Towers

Introduction (A battle of principles)

In the blue corner we have Coanda Effect, the challenger, and in the red corner we have the reigning champion, Bernoulli Principle. Our referee, tonight, is Angle of Attack, and only benefactor is Lift.

The question is “What does give a fixed wing aircraft its lift?” Well in the discussion following I will tell you one thing about my opinion on this matter, but you will have to wait until the end of the article. There are several (commonly held at three) schools of thought. One, and the most widely held, is the Bernoulli Principle. Two, is the Coanda Effect that provides the lift, and, three, that both play a major role is providing lift. Pretty well most people, regardless of the camp they’re in will agree that Angle of Attack, when apparent, is a huge provider of lift. Ever heard the statement that if you put on a big enough engine, a barn door will fly.   Well, discounting prop hanging, a barn door will fly by virtue of Angle of Attack only. Well, I said that you’d have to wait until the end of the article to find out where I stand on this. But I can’t wait that long soooooo. I firmly believe that both Bernoulli and Coanda can take a bow for keeping a 747 airborne in level flight. But, please, don’t let me sway you. I’d like to present what I know about these two brilliant scientists, actually, their discoveries, and let you decide for yourselves. Even more. I will tell you how you, using common items around the house, can demonstrate these two incredible forces.

DING! .   And there’s the bell. Coanda Effect pounces from his corner and takes a sharp left swing from Bernoulli Principle and goes down. The ref. is counting, one, two, three….. While we wait for the ref. to do his thing, let us review a profile of the champion.

Bernoulli Principle

Bernoulli, a well know scientist of his day, discovered that in any given unconfined environment, if two masses of air were traveling at different speeds, that the mass that is traveling faster will exist at a lower pressure that the other. So how does that help us. Let us first consider what we do to a wing of a fixed wing aircraft to make it fly --- we trundle it through the air at a great rate of knots. Note: Whether we move the wing through the air, as in a real aircraft, or move the air past the wing, as in a wind tunnel is moot. So I will flip-flop between each as appropriate. Ever notice the cross section of a wing. Of course you have. The basic flying wing is one that has a curved surface from front to back across the top of the wing and a flat bottom (called a “Flat-bottomed airfoil”) or it has a curve less severe on the bottom (called a semi-symmetrical air foil).   There is a wing section that has the same curve top and bottom (called a symmetrical airfoil) but this is a special case and I will discuss this later. For simplicity, I will assume a flat bottomed airfoil but please remember that a semi-symmetrical airfoil works in the same way as far as Bernoulli is concerned. First, we do need to know the ground rules. As the wing travels though the air, the leading edge of the wing cuts into the air forcing some mass to travel over the top and some to travel beneath the wing.
Figure 1
The common wisdom asks you to picture two molecules of air that are so split by the wing’s leading edge, one going over the other going under the wing.   Assuming no turbulent air, the two molecules will connect again as the wing’s trailing edge passes by -- Figure 1. Here’s the Bernoulli kicker.   Because the upper surface of the wing is curved and the bottom is flat. The upper molecule had to travel farther than the lower molecule. To aid in this picture, consider the wind tunnel situation where the air is moving not the wing. If the upper molecule had to travel farther and did it in the same time (there’s only one leading edge and one trailing edge), then the upper molecule must have traveled faster.
Ah, ha. Two masses of air each traveling at different speeds. Bernoulli, enter left. If the air mass on top of the wing is traveling faster than the air mass below the wing, the air above the wing will exist at a lower pressure. So, Bernoulli says that lift is thus created by a virtual vacuum above the wing. That sucks…..Drum roll please. One visual indication of Bernoulli can be seen on a hot and very damp day, if you watch jet liners take off from an airport, at the point just after rotation (the wing is at maximum lift) you’ll often notice a cloud of condensing air above each wing. Since H20 will boil and condense at a lower temperature in a low pressure, this could be the main evidence that Bernoulli is alive and well in modern life. At the top of Mt. Everest, water will boil at approximately 93o Celsius, not 100o. Conversely, steam will condense at 93o, not 100o And, of course in the perfect vacuum, water will not exist at any temperature.

Lift Experiment 1

Drum roll, Maestro, if you please.
Figure 2
Take a piece of paper. A sheet from a photo copier is ideal for this. Fold it in half and stand it on a smooth table surface so that it resembles a tent with no floor. Place it so that one of the open ends is near the edge of the table – Figure 2


Figure 3
Now, place yourself so that you can blow though the tent, as if it were a tunnel. Then do just that – blow sharply into the tunnel. You might think that the sudden rush of air into the inside of the tunnel would increase the air pressure inside and lift the paper off the table. But what actually happens is that you cause all the air inside the tunnel to move.   Since the air outside the tunnel remains stationary, the air inside must be traveling faster. Remember Bernoulli says this will decrease the air pressure inside the tunnel and the tunnel will collapse. This is exactly what happens in this experiment. The tunnel collapses under the higher outside air pressure – Figure 3

DING! End Round One

DING! Round Two

Coanda Effect

Coanda Effect comes out of his corner strong and looks like he will give the Bernoulli Principle a good licking.
Coanda, likewise, was a respected scientist of his day. He developed the theory that everything is attracted to everything else, and that if left to their own devices with no external force, that two particles of matter will be attracted to each other and will eventually join. Just look at a fizzy drink and notice how the bubbles on the surface tend to collect together. They do this through a mechanism called “Surface Adhesion”.
Here, we can look at the surface of the wing and the molecules of air that are passing over and under it. If the surface is curved, as in an airfoil, the air molecules will be forced up to the peak of the curvature following the leading edge of the wing.


Figure 4
Now, if no other force (such as that suggested in the Coanda Effect) were to come into play the air molecules would simply travel straight back, leaving the surface of the wing, also causing a vacuum to exist between the air molecules and the wing’s surface – Figure 4. Coanda suggests this does not happen. Instead, the molecules, as they pass over the peak of the curve stay in tight contact with the wing surface as it fall away due to surface adhesion


Figure 5
Since the air would prefer to travel straight back, there must be a downward force applied to it (surface adhesion) to keep it in contact with the wing.   Newton says that any force must have an equal and opposite force, and surface adhesion is happy to comply. In this case, the equal and opposite force is “lift” – Figure 5.
The Coanda Effect, of course, only works on a curved surface. So this force will provide high lift to a flat bottomed airfoil. It will provide equal and opposite lifts (on up from the top surface and one down from the bottom surface) to a symmetrical wing, thus providing no wing lift at all.


Lift Experiment 2

Take a plastic teaspoon and hold it by the tip of the handle so that the spoon part hangs down. Hold it loosely so that the spoon will swing when rocked or pushed.
Figure 6
Now, hold the bottom of the spoon part near the running water from a faucet – Figure 6.   Gradually move the spoon towards the water. You may expect that when the convex curved surface comes in contact with the water, the pressure of the water will force the spoon away.

Figure 7
What actually happens is the spoon is drawn into the gush of water – Figure 7. This is due to the surface adhesion of the water to the spoon.


Angle Of Attack

It looks as if both the Bernoulli Principle and the Coanda Effect are just about tied in this contest which leads many folks to contemplate the possibility, nay, the probability, the both contenders add to the lift of a flying wing.   So where does Angle of Attack (AOA) come in to play?

Well, if we agree that either the Bernoulli Principle or the Coanda Effect, or both, cause the lift on a flying wing, then AOA flies because of those two forces as well. For this discussion, we’ll consider the proverbial “Barn door”.   You know the one that will fly if you put a big enough engine onto it. Well actually we’ll consider a wing with a flat airfoil both top and bottom. If you move this wing through the air edge on, it will create zero lift.   (By the way, this is also true for a fully symmetrical wing too). If you now elevate the leading edge slightly it is said that we have increased the angle of attack of the airfoil. There then forms a cushion of relatively still air beneath the wing. This cushion of air exists at a relatively high pressure and would, alone, give the appearance of lift by “Pushing” the wing upwards (remember, true lift is caused by a low pressure forming above the wing rather than a high pressure forming below it). However. If this alone would cause the aircraft to fly, then by all means call it lift.

Figure 8
But something else happens. The cushion of air acts as if the wing is no longer symmetrical and the air forced over the top of the wing is presented with a curved airflow – Figure 8.   Notice how the effective airfoil shape is similar to a semi-symmetrical airfoil……..
Voila! Enter Bernoulli and Coanda, yet again.

Of course, a flat or symmetrical airfoil in a high angle of attack attitude creates huge amounts of drag. Thus the reference to a barn door needing a big engine to make it fly.

When is AOA used for giving or adding to lift??
Always for a symmetrical airfoil
High rates of climb
Takeoff
Slow flying
Landing approach

An experiment?? Well, OK then.

Lift Experiment 3

Actually, you’ve probably done this one a thousand times. When you are next traveling by car, and when it is absolutely safe to do so, open the window and place your arm outside keeping it straight and horizontal. Now outstretch you hand so that it is as flat as you can get it, horizontal and with the index finger acting as the leading edge of this, now, flat wing.   Slowly rotate you hand so that the leading edge (index finger) rises. This will increase the angle of attack. Your arm will instantly feel the additional list your hand is providing.

That’s it, them? We’re done, right?

Well not exactly. There is another force that acts on a wing to provide lift. This is called “Ground Effect”

Ground Effect

Ground Effect and AOA work somewhat together. As the term “Ground Effect” implies, it is a lifting effect on the wing that occurs near the ground, such as on takeoffs and landings.

Essentially, it is that cushion of air that is semi trapped between a moving wing and the ground when both are in close proximity and the former is at a high angle of attack.
Figure 9
When the wing is below the wing span distance of the ground and is flying at a high angle of attack, the cushion of air below the wing increased dramatically – Figure 9. This effect is much more noticeable when the height is at ½ the wing span.
This is a great help on takeoffs if you remember that ground effect increases lift near the ground and GOES AWAY as you climb out. All aircraft, models and full size, can have the tendency to take off too early because of ground effect only to suddenly loose sufficient lift and stall back onto the runway with sometimes disastrous consequences.
On landings, it can easily make the aircraft “Float” and cause an extended flare.

This really is the end of the article. I hope it will help you understand flight better, or, at least, I hope you enjoyed reading about this stuff.