<< Page 6 Page 7: Page 8 >>   Pressure Drag (Separation & Wakes) Pressure drag is caused by the fact that, if the back of a body flying through a fluid is not pointy, but is instead flat, the fluid will not make the “turn” along the surface to meet itself at the very back. Instead, it will move straight back (“separate”), leaving an extremely-low-pressure “vacuum”, or wake, behind the body, which will “suck” the body backwards. This effect – separation – happens when the surface angles too steeply away from the direction of airflow: In the two diagrams above, air is flowing in from the right. Even when the surface turns away from the path of the air (as you see on the left image), the air will follow the surface, “stick” to it, as long as the turn is not too abrupt or too steep. This is known as the Coanda effect. (It was first described by Henri Coanda supposedly after he observed the path of smoke from an engine fire (!) moving over the wing of his 1910 biplane. The biplane is pictured on page 63, lower right corner). The classic demonstration involves letting just a thin flow of water out of a faucet, and holding something round (like a spoon) under the faucet with the convex side tangent to the water – the water will “stick” to it and turn away from its previous (vertical) path to follow the surface. But if the surface turns too steeply or too abruptly, the fluid will NOT follow the surface and will move away from it (diagram on the right), especially if your fluid is air, which is not as viscous (or “sticky”) as water. This causes a low-pressure “sucking” in the areas downstream from where the fluid left (separated from) the surface. Sometimes, fluid in this region will move forwards along the surface, the opposite direction as most of the rest of the fluid! This is called recirculation, and can easily be observed inside most convertibles (those that don’t have wind guards, anyways) and in the backs of pick-up trucks: Air separates once it goes over the top of the windshield of a convertible or over the cab of a pick-up truck, so a recirculation bubble forms in the space behind there, and air blows “forward” in the bubble). So the way to prevent a wake from forming is to have the body taper back in a teardrop shape. All parts of an airplane are teardropped (the fuselage, wings and tail – round at the front, thin/sharp/pointy at the back), so the low-pressure effect behind airplanes only leads to about 5% of the drag force on most airplanes. Roughening up the surface causes the boundary layer to be turbulent and messy, and to stick to the surface more. This means separation is more difficult (requires steeper angles), and thus low-pressure wakes become smaller. The two “tricks” to minimize separation and pressure-drag (teardropping, which is essentially adding a pointy “tail”, and roughening up the surface, so the air “sticks” more to the surface and separates later) increase viscous drag (because teardropping means an increase in surface area, and roughening up the surface so that the air sticks obviously will make the air stick more). Because pressure drag is usually very strong if unattended to, but can be drastically reduced fairly easily, this is usually a worthwhile tradeoff. But overdoing the anti-pressure-drag tricks can lead to raising the viscous drag by more than you’re reducing pressure drag. So engineers must figure out which combination of smooth, small surfaces and rough, long-teardropping-tailed surfaces leads to the smallest SUM of pressure drag and viscous drag. This is not so straightforward. In other words, it is important to figure out what shape is just teardropped enough to prevent separation. Any less teardropped and you get separation and a LOT more pressure drag, but any more teardropped and you have more surface area than you need, increasing your viscous drag (so it’s usually better to be just a tad too teardropped, to be on the safe side). The optimal amount of teardropping (which can be measured as the angle of the back “tip”, or as the thickness-to-length ratio) depends on the air density (and thus the altitude) and the speed at which most flying will be done. Above; Too little teardropping leads to separation and high pressure drag, but too much teardropping means you have more surface area for the same useful internal volume, leading to too much viscous drag. Wings are extremely tear-dropped in cross section (that’s because with wings you WANT more surface area, to get more lift), but everything else is teardropped just enough to prevent separation. While a SEVERE problem in all cars (especially rectangular ones like buses, vans, SUVs, etc) and THE leading cause of drag in sportsballs (baseballs, soccer balls, golf balls, etc), pressure drag is quite unimportant in aviation, responsible for 5% or less of the drag on most airplanes. Separation is only a serious factor in one situation: The stall. Remember how the stall happens when the wing tilts to a critical angle? When it does, the air separates over the crest (the “bump” on top of the wing) and a wake forms behind the wing. This is why drag goes way up during a stall. An object with a large wake behind it – like a car, or a ball, or a stalled airplane – usually has most of its drag resulting from pressure drag. So that’s something else bad about the stall. But during normal flight, pressure drag is almost negligible. Above; A stalled wing and a non-stalled wing. Normally, a wing would need a higher angle of attack to stall than to fly non-stalled, but in this case other variables have been adjusted to “undo” the stall. Pressure drag is also somewhat of an issue around lowered landing gear. Notice how airplanes with fixed landing gear (always down) have teardrop-shaped “pants” around the landing gear, unlike airplanes with retractable gear. Similarly, hydraulic pistons and rods, which move the control surfaces, are often encased in streamlined fairings, usually found under the wings: Above: Wheel pants on a general-aviation airplane. And then, some streamlined fairings concealing the rods that move the flaps under a 747's wings. The pointy, teardropped fairings often stick out the back of the underside of the wing. The rods are sometimes visible from above through deployed spoilers.