Figure 5.7 illustrated L/D ratio plotted against angle of attack. An… — Page 131, Lesson 153

Let’s pick this up right where the drag story gets its most useful picture. We’ve already seen L/D plotted against angle of attack in Figure 5.7. Now I want to show you the alternative way we display that same ratio — the polar diagram. Instead of putting angle of attack on the horizontal axis, we plot CL against CD. CL is the lift coefficient, CD is the drag coefficient. So on the vertical axis we have CL, on the horizontal axis we have CD, and each point on the curve corresponds to a particular angle of attack. This is Figure 6.17, the whole-aeroplane polar diagram. Here’s what the curve tells us. As we start from low angles of attack, CL increases initially much more rapidly than CD. That means early on, we’re getting a lot of lift for very little drag penalty. But ultimately, as we keep increasing angle of attack, CD increases more rapidly than CL. The curve bends over, and the lift-to-drag ratio stops improving. Now, the key trick: the condition for maximum Lift/Drag ratio is found from the drag polar by drawing the tangent to the curve from the origin. The origin is the zero-zero point, where both CL and CD are zero. You draw a straight line from that origin so that it just touches the curve at one point — that tangent point gives you the maximum L/D. That’s the angle of attack where you get the best lift for the drag you’re paying. I want to stress this: this polar diagram is a very common method of displaying L/D ratio. You will see this again and again, so make this display well known to yourself. Now let’s move to what changes that total drag curve. First, the effect of aircraft gross weight on total drag, shown in Figure 6.18. Think about a flight where fuel is being consumed. As fuel burns, gross weight decreases. With less weight, less lift is required — that means a lower CL. And a lower CL reduces induced drag. So total drag will be less, and VMD — that’s the speed for minimum drag — will occur at a lower IAS, a lower indicated airspeed. Now flip it: if the aircraft is operated at a higher gross weight, more lift is required. More lift means more induced drag. So total drag is greater, and VMD occurs at a higher IAS. And here’s an important parallel: if the aircraft is manoeuvred so that the load factor is increased, the result is similar to an increase in gross weight — induced drag increases. So a tight turn, which raises load factor, behaves like adding weight. Next, the effect of altitude on total drag. Aircraft usually operate within limits of Indicated Airspeed, IAS, so we care about drag as a function of IAS. If you fly at a constant IAS, dynamic pressure is constant. As density decreases with increasing altitude, TAS — true airspeed — must be increased to maintain that constant IAS. The relationship is Q = ½ ρ V², where Q is dynamic pressure, ρ is air density, and V is true airspeed. So if density drops, V must rise to keep Q the same. And the key result: if the aircraft is flown at a constant IAS, drag will not vary with altitude. The drag stays the same even though you’re higher and flying faster in true terms. Finally, the effect of configuration on total drag, Figure 6.19. When you extend the landing gear, air brakes, or flaps, you increase parasite drag. But these do not substantially affect induced drag. So the effect of increasing parasite drag is to increase total drag at any IAS, but to decrease the speed VMD compared to the clean aircraft. In other words, dirty configuration — gear down, flaps out — shifts the minimum-drag speed down and raises the whole drag curve. So to tie it together: the polar diagram gives us the best L/D point via the tangent from the origin. Weight changes shift induced drag and VMD. Altitude at constant IAS doesn’t change drag at all. And configuration changes parasite drag, which raises total drag and lowers VMD. That’s the full picture of what drives total drag.