Drag
Drag is the force that acts rearward, counteracting thrust. It tries to slow the airplane. There are two types of drag, parasite and induced.
Parasite Drag
Parasite drag is caused by either form drag or skin friction.
Form Drag
Form drag is created by antennas, struts, and uneven edges on an airplane. Anything that can catch air as the airplane moves will create parasite drag. Airplane manufacturers shape these items so that they are streamlined and small, in an attempt to minimize the amount of form drag they will produce when the airplane flies.
Skin Friction
Skin friction is created by rough surfaces. Even a surface that appears very smooth will create some amount of skin friction, since every surface has imperfections if examined closely enough.
The form drag and skin friction together make up the parasite drag produced by the airplane. The faster an airplane travels, the more parasite drag it will produce. In fact, parasite drag increases as the square of the airplane's speed. This means if the airspeed were doubled, the parasite drag would increase by four times. If the airspeed were tripled, the parasite drag would increase by nine times.
Induced Drag
Induced drag is generated as the wings produce lift. It is commonly said that induced drag is a byproduct of lift. Induced drag increases with increased angle of attack of the wings.
To maintain level flight, the wings must produce lift equal to the airplane's weight. They must be placed at a higher angle of attack to produce this lift a slower airspeeds. At higher speeds, the same amount of lift can be made at a lower angle of attack. As a result, induced drag is reduced as the airplane's speed is increased and is greater at slow airspeeds.
Induced drag varies inversely as the square of the airspeed. This means if airspeed were doubled, induced drag would be reduced by a factor of four.
Total Drag
Parasite and induced drag act together. When the airplane is flying at slow airspeeds, parasite drag is low. However, since a high angle of attack is required to counter the airplane's weight, induced drag is quite high. When the two are added, the result is a high total drag at low airspeeds.
If the airplane is at its maximum speed, the wings can produce enough lift to counter the airplane's weight at a low angle of attack, resulting in a minimum of induced drag. However, since parasite drag is at its highest at this high speed, total drag is high at high speeds.
Total drag is at its lowest at an intermediate speed. It this speed, neither parasite or induced drag are at their minimum or maximum values. But, when added together, their total is an a minimum.
Ground Effect
When the airplane is flown very close to the ground, such as during takeoff and landing, the ground changes the way the air flows around the airplane, resulting in what is known as ground effect.
Ground effect reduces induced drag, resulting in a sort of false performance boost very close to the ground. This can cause the airplane to become airborne prior to the recommended takeoff speed. In such a situation, the airplane could be flying, but incapable of flying more than a few feet off the ground.
Another time ground effect is noticeable is during the landing, as the airplane floats down the runway just a few feet off the ground. Any excess speed when its time to land can cause a considerable amount of floating, burning up a lot of usable runway.
The pilot can't do anything about ground effect, it simply exists. Be aware of ground effect and realize the importance of executing the takeoff and landing at the proper speeds.
Ground effect has its greatest impact when within one wingspan of altitude.