Angle Of Attack
(or another way to make lift)
NASA decided to put various shaped wings inside of a wind tunnel with smoke so they could see just what is going on when air flows over a wing. Below is a rough sketch of what they saw as air flowed over a typical light airplane wing.
Notice how as the air flows across the wing towards the back, it changes direction. The air coming off the trailing edge is not going the same direction as the air meeting the leading edge. Instead, it has been bent slightly downwards because of the shape of the wing. I asked Bernoulli how this would affect us, and he was absolutely clueless. Luckily, there was another guy nearby, his name was Sir Isaac Newton. Turns out that a while back he had studied objects in motion and forces and how they all work together. One of the things that he discovered is that for every action, there is an equal but opposite reaction. When I asked Newton about the airflow's change in direction over the wing, he told me that because of that action, the reaction would be a resultant upwards force on the wing...LIFT!
The lift is created as a reaction to the air being deflected downwards (what we call downwash). Remember, it is an EQUAL but opposite reaction to the original action. Because of this, we can reason that the amount of lift created is directly proportional to the amount of air deflected downwards. As we fly faster through the air, we are deflecting more air downwards in the same amount of time, which means our resultant lift force increases. So, we can now conclude that lift will be directly proportional to airspeed. This makes sense as we all know that an airplane has to reach a certain speed before it can takeoff. But wait a minute, didn't we learn in the previous section that when an airplane is in unaccelerated flight, lift must equal weight? We know that airplanes generally go faster in cruise than they do during takeoffs and landings. How is it that we can maintain straight and level flight at a faster airspeed than takeoff? There must be another way to control lift than by changing airspeed! Don't worry, there is...
Angle of Attack
I mentioned earlier that one of the reasons NASA studied this was because a flat/symmetrical wing can also produce lift. Below is a rough sketch of what they saw in the wind tunnel when they placed a flat wing in there.
You can see that by tilting the wing slightly upwards, they were able to get the airflow to bend slightly, just as it did with the wing in the previous diagram. Below is another rough sketch of an airplane wing (cross section). This time the wing has been tilted upwards, just like the flat wing in the above diagram.
You may have noticed a few vocabulary words that we need to cover. One of them is relative wind. Have you ever been driving down the road with the window down and you notice wind moving opposite the direction of the car? Then you stopped and the wind went away. This is a perceived wind and has nothing to do with the actual wind. Since you are moving forward through the air, you can feel the air moving backwards relative to you. So you perceive a relative wind in the opposite direction of your movement. Same thing happens when a wing moves through the air, the relative wind is opposite the direction that the wing is moving. Also, in the above diagram, the wing is not pointed in the same direction that it is moving. There is a name for how much we have tilted the wing. It is called Angle of Attack. Angle of Attack is really just the angle measured between the relative wind and the chord line. Wait, what's a chord line?!? Chord line is simply the line that you see drawn through the wing. It is the line that would extend from the leading edge of the wing to the trailing edge of the wing. A simplified definition for Angle of Attack is the difference between where the wing is pointed and where it is actually going.
Below is a diagram that illustrates what happens to the air when an airfoil's angle of attack is increased.
By increasing the angle of attack, we bent the air even more, which increases downwash. We determined earlier that lift was directly proportional to the amount of downwash created. So, not only can we control lift by changing airspeed, we can also control it by changing angle of attack!
Controlling Angle of Attack
The amount of lift created is a direct result of the amount of downwash created by the wing. There are two ways that we can change the downwash of the wing, we can either change how far the air is bent, or how much air we are bending. Think of it this way: if I am on a row boat and I want to go faster, I can either use a bigger paddle (which will move more water), or I can just paddle faster. In an airplane, if we are flying at a particular airspeed and the resultant lift force is not enough to counteract gravity, we can increase angle of attack (tilt the wing upwards) until the downwash has increased enough to make up the difference. Then, if we decide to fly faster, as lift increases from the increase in airspeed, we can reduce the angle of attack to balance the forces and keep the lift the same. So how do we control angle of attack? The wing is attached to the fuselage of the airplane. So, if we tilt the airplane upwards, then the wing has no choice but to tilt upwards as well. As a result, we can use our pitch angle (angle of the nose relative to the horizon) to control angle of attack. This relationship is very important because just about everytime you make a speed change, you will also need to make a pitch change.
From your first flight you've seen that the elevator (moving the control stick or yoke fore and aft) controls pitch. So, in essence, the elevator is our Angle of Attack Control. Well, with this ability to control angle of attack, it would seem that you could fly an airplane at just about any speed, including very low speeds. But, from your first flight you've also seen that the airplane has to be traveling at a certain speed to takeoff, and also has to be going about at least that fast coming in for a landing. So why can't we fly slower?
On to the next section...Critical Angle of Attack