Joseph, a student pilot in Georgia, writes: I’m having a hard time wrapping my head around induced drag. Can you help me out?
Absolutely. And just so you know, a lot of people have a hard time wrapping their heads around induced drag, so don’t feel badly on that account.
I think the reason is not so much the aerodynamics of it, but its confusing name, and the company it keeps — and by that I mean the other topics that are usually taught at the same time.
Most ground schools, lesson plans, books, and YouTube videos have a real drag of a lesson that goes something like this: There are two kinds of drag: Parasite and induced. Parasite comes from all the nubbly $#@%& on your airplane catching the wind and slowing it down. There are three flavors, and they all get worse the faster you go. Induced drag, on the other hand, is a byproduct of lift, and is created when lift is created and gets worse the slower you go. Moving on…
No wonder no one’s head makes it to the destination.
First off, it really needs to be two lessons, preferably at least a week apart, as the two types of “drag” have little to do with each other. That said, I’ll invoke my hypocrite privileges (you get those as part of your Master Ground Instructor Accreditation) and talk about both today.
Parasite drag we can keep calling drag. It’s created by the relatively simple laws of physics around the motion of fluids. P-drag is a force — meaning it has both a measurable strength and a direction of movement — created when there is relative motion between a fluid and a solid, or between two fluid layers. Actually, you can get the same thing between two solids, but let’s not go there today.
If it’s a fluid-to-solid reaction with a fixed object, the drag force will slow the fluid. If the solid object is moving through the fluid, then the drag force will slow the object. Bearing in mind that air is a fluid, I think you can see where this is going.
Using Newton’s formula that says that the strength of a force is the mathematical product of weight and speed, the faster the liquid or the object is moving, the more drag force is generated. I’m sure at some point in your life you planned a nice relaxing bath, but got distracted and the water cooled a bit. The solution was to add a bit more water from the hot tap and stir it around. You might have noticed that when your hand moved slowly through the bathwater it moved freely, but when you sped your hand up, the water resisted your motion. That was a parasitic drag encounter, and you demonstrated its increased force with speed.
The plain English synonym for parasitic drag is air resistance, and I’m sure we all remember “flying” our hands out the windows of our parent’s cars when we were children, marveling at how speed through the air could push our hands back.
As to the three flavors of P-drag, they are logical enough. The first two are called form drag and skin drag. They’re both fluid-to-solid forms of drag force.
Form is air resistance to the shape — or form — of the airplane and all the little odd bits attached to it: Antennas, fasteners, rivets, etc.
Skin drag is friction between the fluid air and the skin of the airplane, which from an air molecule’s perspective isn’t as smooth as you think it is.
The third type of parasitic drag is interference drag, and it’s different from the other two in that it is an example of a fluid-to-fluid motion generating a drag force. It’s created by the collision of various streams of air moving and twisting around your airplane.
Picture your airplane submerged in a shallow river. Uhh…sorry, I guess that’s a bad image.
Let me try again: Picture the airplane of someone you don’t like submerged in a shallow river. There are eddies, and little whirlpools, and bubbly turbulence as the flowing water works its way around this new obstacle. The same thing is happening with air as you circle overhead to check out the river-submerged airplane.
But, lift-induced drag is a whole ‘nother kettle of fish altogether, and you’d do well not to bring any of your parasites to the discussion. Because it really isn’t drag at all, at least not in the sense of the collision of fluid physics. Instead, it deals with another aerodynamic force: Lift.
In addition to drag, when a fluid hits a solid object, it creates a force called lift that acts perpendicular to the direction of the moving fluid. In flight, the moving fluid is called the relative wind, and when that wind hits the airplane, lift is generated from every surface.
In some perfect, alternate, textbook universe, the lift from an airplane would act straight upward, opposing gravity. And it would in our universe, too, if the relative wind was flat. But it’s not. It’s curved thanks to the downwash off the back of the wings.
Now wait a second, you say, how can lift be perpendicular to the relative wind, if the relative wind is a curve?
Well, that gets into tangents and geometry and some pretty deep math, so let’s just say it acts perpendicularly to the average shape of the relative wind, which effectively means the lift vector is pointing somewhat rearward rather than straight up. You can compare it to what happens to lift in a turn, where you have horizontal and vertical components of lift. It’s basically the same thing.
And because lift-induced drag is basically a “component” of total lift, the more lift there is, the more of this “drag” you get. And as we require more lift at lower airspeeds, induced drag is greatest when flying slow. Flying fast doesn’t require as much lift, so induced drag — opposite of parasite drag — goes down with speed.
How is this even “drag” at all, being as it seems to be all about lift?
It all comes down to the relationship between speed and lift. With a lot of lift lost — well, diverted from vertical — we need to compensate with speed. So the net cockpit effect of induced drag at low airspeed is akin to the net cockpit effect of parasite drag at high speed: They are both, for all practical purposes, speed inhibitors.
And it’s good for pilots to understand that the speed inhibitors act criss-cross applesauce to each other on the speed range, with the most efficient flying in the middle speed range.
As a side bar, and probably where induced drag should be taught, these are the same forces that are in play with ground effect. Ground effect isn’t a cushion of air under the wings as some pilots believe, instead close to the ground, the effect of downwash is minimized, somewhat “straightening” the relative wind, and shifting the lift vector forward, which provides more vertically-acting lift. All other things being equal, the total volume of lift hasn’t changed, it’s just that a greater percentage of that lift is now directly opposing weight.
It’s really not that hard to wrap your head around, but I think that calling this redirected lift a “drag force” confuses students and inhibits learning. I think it needs to be rebranded.
I wonder if people would get less confused if, instead of calling this phenomenon “induced drag,” we called it “lift reallocation.” Or lift-shift. Or even induced lift shift. I’m open for ideas, but we need to jettison drag here. Induced drag needs to be rebranded to better reflect what it really is.
Oh, and speaking of rebranding, parasite drag should probably find a new ad agency, too.
The Pilots Handbook of Aeronautical Knowledge tells us that it gets its name because it “in no way functions to aid flight,” apparently making it a blood-sucking parasite.
I disagree, as should you if you’ve ever deployed a slip, used “aerodynamic braking” to slow down, or watched the speed brakes pop out of the wing as your airliner landed at your holiday destination — all of which are examples of using “parasite” drag to aid flight, in defiance of its name.
