Rule One: Nothing in aviation is free, especially lift. If you do anything to increase lift, drag will increase as well. This includes, but is not limited to, increases in angle of attack, deflecting a control surface, and lowering the flaps. The bottom line is "more lift means more drag" and there is no way around it.
The first diagram you see in every pilot training handbook depicts the lift/drag/ thrust/weight relationship (below). If you don't have that relationship drilled into your aeronautical soul, you definitely don't understand your airplane. Rule Two: Drag slows things down. There are two major forms of drag. Induced drag is caused by lift. This one we can control through control surface deflection and angle of attack. Parasite drag is inherent in the airplane's design. There's nothing we can do about this -- and the faster we go, the worse it gets. Rule Three: Camber changes affect lift. Ignoring design subtleties (and oversimplifying), the lift of a given airfoil at a given angle of attack is largely determined by its camber, which is another way of saying how much a line drawn through its center curves (see page 35). The more curve, the more lift with the same angle of attack. That's why airplanes designed for slow flight have fat, flat-bottom wings and big flaps. That's also why airplanes with high-speed, symmetrical airfoils (flat camber lines) fly at a higher angle of attack on final, land faster, and often have all sorts of lift-enhancing devices (flaps, slats, etc.). Rule Four: All control surfaces are essentially flaps and change the wing's (or tail's) camber line. As flaps go down, they make the camber line through the center of the wing curve more. That generates more lift. For this discussion, we're ignoring wing area added by Fowler flaps. Beyond 15 degrees of deflection, the amount of drag generated by most flaps begins to build faster than the lift until the point at which additional deflection generates 100 percent drag. The increased drag caused by both the increased lift and drag-producing surface means that to maintain a given speed with flaps down requires either a lower nose attitude, more power, or both. The ailerons do the same thing as the flaps: They increase the camber line (curve of the wing), which increases lift and drag. The difference is that an aileron can go up too, which decreases the lift (flattens the camber line) on that wing and lowers the drag. The rudder changes the camber of the vertical tail and causes its lift forces to be exerted left or right. The elevator does the same to the stabilizer but in an up-and-down direction. Rule Five: As an airplane accelerates, both lift and drag increase much more quickly than the speed does. As the airplane goes faster, you get more lift, but increased drag dogs you every step of the way. Nothing is free, remember? Rule Six: Projected wing and tail area changes with bank angle. As you roll into a bank, the amount of wing that is lifting vertically against gravity (area projected vertically) becomes smaller and smaller until there isn't enough to keep the airplane aloft (right). The only fix is to increase the angle of attack, which generates more lift, but more drag too, right? So it slows down. This is aerodynamics brought down to the lowest possible level. If you already have a good understanding of the subject, skip to the next article. If, however, you had to reread even one of our "rules" and ponder its meaning, you may need to study it further.
The first diagram you see in every pilot training handbook depicts the lift/drag/ thrust/weight relationship (below). If you don't have that relationship drilled into your aeronautical soul, you definitely don't understand your airplane. Rule Two: Drag slows things down. There are two major forms of drag. Induced drag is caused by lift. This one we can control through control surface deflection and angle of attack. Parasite drag is inherent in the airplane's design. There's nothing we can do about this -- and the faster we go, the worse it gets. Rule Three: Camber changes affect lift. Ignoring design subtleties (and oversimplifying), the lift of a given airfoil at a given angle of attack is largely determined by its camber, which is another way of saying how much a line drawn through its center curves (see page 35). The more curve, the more lift with the same angle of attack. That's why airplanes designed for slow flight have fat, flat-bottom wings and big flaps. That's also why airplanes with high-speed, symmetrical airfoils (flat camber lines) fly at a higher angle of attack on final, land faster, and often have all sorts of lift-enhancing devices (flaps, slats, etc.). Rule Four: All control surfaces are essentially flaps and change the wing's (or tail's) camber line. As flaps go down, they make the camber line through the center of the wing curve more. That generates more lift. For this discussion, we're ignoring wing area added by Fowler flaps. Beyond 15 degrees of deflection, the amount of drag generated by most flaps begins to build faster than the lift until the point at which additional deflection generates 100 percent drag. The increased drag caused by both the increased lift and drag-producing surface means that to maintain a given speed with flaps down requires either a lower nose attitude, more power, or both. The ailerons do the same thing as the flaps: They increase the camber line (curve of the wing), which increases lift and drag. The difference is that an aileron can go up too, which decreases the lift (flattens the camber line) on that wing and lowers the drag. The rudder changes the camber of the vertical tail and causes its lift forces to be exerted left or right. The elevator does the same to the stabilizer but in an up-and-down direction. Rule Five: As an airplane accelerates, both lift and drag increase much more quickly than the speed does. As the airplane goes faster, you get more lift, but increased drag dogs you every step of the way. Nothing is free, remember? Rule Six: Projected wing and tail area changes with bank angle. As you roll into a bank, the amount of wing that is lifting vertically against gravity (area projected vertically) becomes smaller and smaller until there isn't enough to keep the airplane aloft (right). The only fix is to increase the angle of attack, which generates more lift, but more drag too, right? So it slows down. This is aerodynamics brought down to the lowest possible level. If you already have a good understanding of the subject, skip to the next article. If, however, you had to reread even one of our "rules" and ponder its meaning, you may need to study it further.