<< Page 4 Page 5: Page 6 >>   Drag Drag too is a consequence of Newton’s Third Law: As an airplane flies through the air, it drags some of the air forward with it, so that air pulls the airplane back. Drag minimization is crucial for airplane design, as it improves efficiency/mileage, speed, range, endurance, climbing abilities, and takeoff and landing distances. Now, there are several reasons why a plane drags air forward, and they are all quite independent of each other. Or, from the point of view of the airplane, air is slowed down as it passes the airplane, for different reasons. 1) Skin Friction (viscous drag) 2) Aerodynamic wakes (pressure drag) 3) Lift, and Wingtip Vortices (induced drag) 4) Shockwave/Supersonic drag (compressibility drag)   - - - - - - - - - - - - - - - - - -   Summary (of next 4 pages): Drag Caused by four kinds of phenomena: -Skin Friction / Viscous Drag: friction between air and moving airplane. Causes heating. The only way to minimize it (other than having a small surface area, cylindrical fuselage, etc) is to have smooth surfaces. A little less than ˝ of the total drag force on most planes. -Pressure drag: Separation causes a “vacuum” to form behind a bluff (not teardropped) body, sucking it back. But airplanes are teardropped and so suffer from little pressure drag, unless they stall. Parts of airplanes that are not teardropped, like wheels and control rods and antennae and any other “round” thing that sticks out, can be covered by teardropped fairings to reduce pressure drag. -Induced drag: The high-pressure air under the wing spills into the low-pressure air at the top, forming a vortex. This is more severe at slow speeds, high alpha, and high loads. It can be minimized by the use of winglets, by increasing the wingspan, by twisted and/or tapered wings, and by reduced weight. A little less than ˝ of the total drag force on most planes. -Supersonic drag: caused by shockwaves as air is shoved out of the way, then sucked back in, very violently. Can be minimized by pointy noses, swept wings, and area-ruled fuselages. Only a small part of the total drag force if subsonic, but a majority of the drag force if supersonic (which is why supersonic planes look different than subsonic ones – must be optimized for eliminating this kind of drag, not so much others). Associated with intense heating, especially over MACH 2.5.