Introduction to Drag Characteristics — Page 83, Lesson 94

Let’s start with the definition of drag, because everything else in this lesson hangs off it. Drag is the aerodynamic force that acts parallel to the relative airflow and opposite in direction to the flight path. So if the aircraft is moving forward, drag is the force pulling backward along that same line of motion. It’s the price you pay for producing lift. Now, just like lift, drag can be expressed as a coefficient — a number that is independent of dynamic pressure and surface area. That’s important because it lets us compare the drag of different wings without worrying about their size or the airspeed. The equation is D = Q × CD × S. Let me unpack that. D is the drag force itself. Q is the dynamic pressure — the pressure you feel from the air rushing past. S is the surface area of the wing. And CD is the drag coefficient. So drag is the product of dynamic pressure, drag coefficient, and surface area. CD itself is defined as the ratio of drag per unit wing area to dynamic pressure. Now, if you plot CD against angle of attack for a representative wing, you get a curve. At low angles of attack, CD is low, and small changes in angle of attack produce only small changes in CD. But as you increase the angle of attack, the rate of change of CD per degree increases — the relationship becomes exponential. And beyond the stalling angle of attack, which is where CLMAX occurs, there’s a further large increase in CD. So drag really climbs steeply once you pass the stall. Next, we look at the lift-to-drag ratio, or L/D ratio. This tells you how efficient the wing is at producing lift. A high L/D ratio means more lift for a given amount of drag — so it’s more efficient. For each angle of attack, you can calculate the proportions of CL and CD. As angle of attack increases, the L/D ratio rises until it reaches a maximum at about 4 degrees. That specific angle is called the “optimum” angle of attack. Beyond that, the L/D ratio decreases as angle of attack continues to increase, right up until CLMAX is reached. One caution here: the plots of lift, drag, and L/D ratio in the figure are all drawn at different scales. So you should not draw any conclusions from where those curves happen to intersect on the graph — the intersections are meaningless because the scales differ. The maximum lift-to-drag ratio, called L/D MAX, occurs at one specific angle of attack for a given aerofoil section. If the aircraft is flown in steady level flight at that optimum angle of attack, drag will be at its least while still generating the required lift force. Any angle of attack lower or higher than the one for L/D MAX reduces the L/D ratio, which means drag increases for the same required lift. Let me make that concrete with numbers. Suppose L/D MAX is 12.5. In steady flight at a weight of 588,600 newtons, and at an IAS that gives the required lift at 4 degrees angle of attack, the drag would be 47,088 newtons. That’s 588,600 divided by 12.5. Any higher or lower speed would require a different angle of attack to generate the required lift, and any angle of attack other than 4 degrees will produce more drag than 47,088 newtons. Now here’s a subtle point about weight. The same aircraft could be operated at a different weight, and you’d still get the same L/D MAX of 12.5 at the same angle of attack. But a change in weight requires a change in IAS to support the new weight at that same angle of attack. The lower the weight, the lower the IAS needed to stay at the L/D MAX angle of attack — and vice versa. So weight changes the speed, not the optimum angle. Finally, there’s a limitation to keep in mind. For a given configuration — meaning a fixed set of flaps, gear, spoilers, and airframe contamination — and at speeds below Mach 0.4, changes in weight will not change L/D MAX. So below that Mach number, L/D MAX stays constant for a given configuration, regardless of weight.