Effect of Lift on Parasite Drag — Page 125, Lesson 150

Let’s start with the name itself. Parasite drag is the sum of three things: form drag, friction drag, and interference drag. I want you to hold onto that definition, because it’s the backbone of everything we’re about to do. Form drag comes from the shape of the aircraft pushing through the air. Friction drag comes from the air rubbing along the skin. Interference drag comes from the airflow disturbing itself where parts meet, like where the wing joins the fuselage. Add those three together, and you have parasite drag. Now, the key phrase in the book is that parasite drag is “not directly associated with the development of lift.” That’s why it’s called parasite — it’s drag that’s always there, even when the wing isn’t producing lift. But here’s the twist, and I want you to really get this: even though parasite drag isn’t directly tied to lift production, in reality it does vary with lift. The book says it plainly — parasite drag does vary with lift, even though it’s not directly associated with producing that lift. Let me show you what that looks like. Look at Figure 6.14. On the horizontal axis we have the lift coefficient, CL. On the vertical axis we have the drag coefficient, CD. There are two curves on that figure. One is labelled CDp, which is the parasite drag coefficient. The other is labelled CDi, which is the induced drag coefficient. Now, the parasite drag coefficient curve — CDp — has a minimum value, and the book labels that minimum as CDpmin. That minimum occurs at zero lift. So at zero lift, parasite drag is at its lowest. As lift increases, the parasite drag coefficient rises above that minimum. Now here’s the important part about how we handle that in practice. Look at Figure 6.15. The book says that the part of parasite drag that sits above the minimum at zero lift is included with the induced drag coefficient. So we don’t count that extra parasite drag as parasite drag anymore — we lump it into the induced drag. The induced drag coefficient, CDi, is given by the formula CDi = CL² / (π × AR). Let me unpack that. CL is the lift coefficient, squared. AR is the aspect ratio — that’s the ratio of wingspan to average chord, a measure of how long and slender the wing is. And π is just the mathematical constant, about 3.1416. So induced drag grows with the square of lift, and it shrinks as aspect ratio increases. A high aspect ratio wing gives you less induced drag at any given lift coefficient. So the total drag coefficient, CD, is the sum of the minimum parasite drag coefficient, CDpmin, plus the induced drag coefficient, CDi. That’s the equation you see in Figure 6.15: CD = CDpmin + CDi. The minimum parasite drag is the baseline, and everything above that baseline gets folded into induced drag. Now let’s move to the effect of configuration. The book makes a very clear statement here: parasite drag, Dp, is unaffected by lift, but it is variable with dynamic pressure and area. Dynamic pressure is the pressure you feel from the air rushing past — it depends on air density and the square of the true airspeed. And area here means frontal area — the cross-section the aircraft presents to the oncoming air. If all other factors are held constant, parasite drag varies significantly with frontal area. Here’s the concrete example the book gives: lowering the landing gear and flaps might increase the parasite area by as much as 80%. At any given indicated airspeed, that aeroplane would experience an 80% increase in parasite drag. So configuration changes — gear down, flaps out — are a huge deal for parasite drag. Next, the effect of altitude. In most phases of flight, the aircraft is flown at a constant indicated airspeed, or IAS. If IAS is constant, then dynamic pressure is constant, and therefore parasite drag does not vary. But here’s the altitude twist: to maintain that same IAS at higher altitude, the true airspeed, TAS, must be higher. That’s because the air is thinner up high — lower density — so you need more true speed to push the same dynamic pressure onto the pitot tube. So at altitude, TAS goes up, but parasite drag stays the same because IAS is what matters for dynamic pressure. Finally, the effect of speed — and the book calls this the most important one. If all other factors are held constant, doubling the speed gives four times the dynamic pressure, and hence four times the parasite drag. Conversely, half the speed gives one quarter of the parasite drag. That’s the square law at work: drag goes with the square of speed. This tells you that parasite drag is of greatest importance at high indicated airspeed, and of much lower significance at low dynamic pressures. Let me give you the illustration the book uses, because it really drives this home. An aeroplane flying just above the stall speed could have a parasite drag that is only 25% of the total drag. So at low speed, most of the drag is induced drag — the drag from making lift. But that same aeroplane at maximum level flight speed would have a parasite drag that is very nearly 100% of the total drag. At high speed, induced drag has fallen away, and parasite drag dominates almost completely. And that’s why the book ends with this point: the predominance of parasite drag at high flight speeds emphasizes the necessity for great aerodynamic cleanliness — what we call streamlining — to obtain high speed performance. If you want to go fast, you have to keep the aircraft clean, because at high speed, parasite drag is nearly everything you’re fighting against. So let me pull it all together. Parasite drag is form plus friction plus interference drag. It has a minimum at zero lift, and anything above that minimum gets counted as induced drag. It varies with configuration — gear and flaps can add 80% to parasite area. It doesn’t change with altitude as long as you hold IAS constant, even though TAS rises. And it scales with the square of speed, making it the dominant drag at high speed, which is why streamlining matters so much.