<< Page 5 Page 6: Page 7 >>   Skin Friction (Viscous Drag) Skin friction, a.k.a. parasite drag, shear drag or viscous drag. This is perhaps the most obvious and intuitive of the components of air resistance. As the external surfaces of an airplane fly through the air, they are moving and in contact with air that is not moving. But the friction in this interface pulls the air forward, because of viscosity. (Viscosity is a fluid’s resistance to having different layers and/or surfaces slipping past each other. Honey, for example, is really viscous). Imagine some air had a grid drawn in it with smoke particles. If you look closely at the air as it moves past a surface (air going to left, i.e. surface travels to the right through the air), it would look like this: The air which is being dragged along by the surface due to viscosity is called the Boundary Layer. It has a speed that is closer to that of the surface, in effect an envelope of air tightly surrounding each of the airplane’s surfaces. Of course, each layer of air dragged along drags the layer above it, which then drags the layer above it, and so on, so as more of the airplane passes, the boundary layer gets thicker (see image below). Typically, the boundary layer of, say, an airliner is a few mm thick at the nose but about two feet thick at the tail. So, the obvious question is, how to reduce this kind of drag? Well, smooth surfaces will have less viscous drag than rough surfaces. Other than that, the only way is to decrease the surface area of your airplane. Don’t have a lot of stuff sticking out, a lot of holes, a lot of channels or creases, etc. Why are airliners round in cross-section? Two reasons. One, which is structural, is that a round (cylindrical, or ideally spherical) container distributes the pressure stress over it evenly if the inside fluid is at a very different pressure than the outside fluid. This is why submarines, fire extinguishers, aerosol bottles, and rocket fuel tanks are round. If they were square (or otherwise polygonal/polyhedral), they would crack along the edges, where stress is greatest. The second reason is because of viscous drag (I suppose this also applies to submarines). As you know, the circle has a lower perimeter-to-area ratio of any shape. This means to enclose a certain area, the circle has the smallest length when unfolded. This means that to make an airplane fuselage that encloses a certain volume, the fuselage would have a larger surface area if it were square (like a bus) than if it was round (like an airliner). A cylindrical airplane body will have less viscous drag than a rectangular one of the same size. The only other way to reduce your airplane’s surface area is, well, to make it smaller. Viscous drag is a fact of life: If you have a certain exposed area, you are going to have at least a certain amount of viscous drag. Unlike the other three components of drag, there are no tricks to reduce it. It is a major component of the drag on most airplanes, especially on efficient airplanes: 40 to 50 percent of the air resistance on an airliner, and over 60 percent of the drag on a glider, is viscous drag. Inefficient airplanes like fighter jets and aerobatic airplanes have less of a problem with viscous drag, but that’s just because their wings generate a lot of induced drag. It should also be mentioned that viscous drag causes heating. The force caused by viscous drag increases approximately with the square of the speed, and the power this force requires/delivers is force times speed, so the heating power goes up as the CUBE of the speed. Fly twice as fast, and you have eight times the heat to get rid of. This is not a problem until supersonic speeds, but becomes a very big problem at speeds greater than MACH 2 (about 1300 mph). Airplanes that fly that fast must be built with materials with really high melting points, materials that will not soften at high temperatures. Viscous drag will heat the skin of an SR-71 to hundreds of degrees, so hot that the pilots – wearing pressurized “astronaut” suits – cannot keep their gloved hands against the window for more than a few seconds at a time. And this is at 90000 feet, where air is much less dense so there is that much less viscous drag. A Blackbird flying that fast at lower altitudes would melt (but as you will soon see in the section about engines, its turbine blades would melt long before the plane got that fast in air that dense). Heating at supersonic speeds is in part caused by compression at the shockwaves, but much of it is due to friction and viscosity. One last interesting note about the boundary layers created by viscous drag regards the air intakes of fighter jet engines. While most commercial jets have their engines held by pylons (and thus their circular intakes are quite far from any other surface), many military aircraft have jet intakes flush against the body of the aircraft. This would mean that the engine “swallows” the Boundary Layer of the surface ahead of it (the BL along the sides of the fuselage). Now, remember that, due to viscous drag, this boundary-layer air right by the surface is moving more slowly (relative to the airplane, and to the engine) than the freestream air. This means this air is not as “energetic”, and does not offer as much ram-pressure in the diffuser. The solution? Fighter jet intakes are actually a few inches away from the surface right in front of them. The part of the intake closest to the surface of the airplane often has a deflector plate to keep the boundary-layer air out. This ensures that the slower boundary-layer air is not ingested by the engine, only the more energetic (and easier-to-compress) freestream air. (One exception to this is the Joint Strike Fighter intake, which has a “bump” to accelerate the boundary layer air much in the same way that the convex top of a wing accelerates air that flows over it). Below; the air intakes of an A-4, F-4, F-5, F/A-18, F-104 and F-22 – notice how they are all held a few inches from the surface ahead of them, so that they do not ingest the boundary layer. And lastly, an F-35 intake with a “bump” that accelerates the boundary layer.