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.
Try this . . . imagine facing an airplane in flight.
Airplane at high speed (level flight) presents very little frontal area to the air, so induced drag is low. You don’t see very much airplane, just the frontal profile. (Form drag.)
Airplane at low speed (again, level flight, not climbing or descending) has a much more nose-up attitude, and thus presents a lot more frontal area to the air, so induced drag is high. You’ll see a lot more airplane, you are looking at the bottom as well as the front. All that area has to be dragged through the air, and the air doesn’t like it.
The various other flavors of drag (skin, parasite, form, interference, etc.) all increase as speed goes up. Induced drag goes down (but never completely disappears).
The “sweet spot” is the speed at which the sum of the induced drag and all the other drag(s) is the least. This is at about 80 knots on the graph at the beginning of this article.
Best Regards,
M/M, AGI
This is part of the total explanation problem. NO Common terminology!. A multitude of different names for the same thing.
Every time you generate lift you pay the price of getting drag. Called INDUCED drag. Airframe parts constitute PARASITE drag.
To remain at the same altitude the slower you go the more Angle of Attack is needed and consequently more PARASITE drag is produced. This can be seen in the original graph.
The faster you go at the same altitude the Angle of Attack is reduced, and consequently the INDUCED drag is reduced. But now as speed is increased your faced with PARASITE drag will increase to limit the top speed.
Best Lift to Drag is where the two lines of Parasite and Induced meet on the graph.
This represents the best speed to fly to get the maximum range! Where the airplane is at it’s most SLIPPERY.speed.
Every aircraft has a drag coefficient (parasite and induced) when “leveled with all flight controls in neutral.” Any deflection away from “aircraft level with controls in neutral” in order to maintain a straight and level flight configuration will change the values for parasite and induced drag from originally designed coefficients. Informally, parasite drag varies from design expectations by how the airplane is presented to the relative wind. Induced drag is changed by what the pilot must do with the flight controls to maintain straight and level flight at any altitude and at any speed.
Often in texts on aerodynamics, induced drag is referred to as Vortex drag. Strong tip vortices yield high induced (vortex) drag. Recall the wake turbulence concerns: heavy & slow ( I.e., high angle of attack) yields a strong vortex flow field with high rotational kinetic energy and considerable energy to oppose.
Well the wing is grabbing air and throwing it downward to generate lift. That doesn’t come for free. The air is being thrown downward, but not straight downward. It’s also being dragged forward by the passing wing. And that, in turn, means the wing feels a force pulling it backward. Just like your hand feels a force pulling it backward when you stick it out the car window because your hand is dragging air forward.
No wonder students are confused when articles like this are an example of the explaination. Mr Dubois you can and should do better.
Lift is produced by pushing air down. Moving air down takes energy. Pushing a lot of air makes a lot of force. The more air, the more force.
Accelerating air downward takes power. F (lift) = ma (mass times acceleration). If you move a lot of air (mass) you do not need a lot of acceleration (a). You can move a lot of air by making a bigger wing or by going faster. Either one will allow for less acceleration and therefore less power (drag).
Is that clear?
If it is not clear nexf time you wish to go someplace get out and push the car. If that is not fast enough for you just push the car much faster.
That is a lot of work isn’t it? That is how induced drag works and the reason why wake turbulence from a slow airplane is much worse than wake turbulence from the same airplane at cruise speed.
The slower the airplane the faster the downwash required to hold it up. (Assuming the same density altitude, mass.)
Induced drag, compare the strain on a ski two rope at 25 mph to 5 mph…if you are strong enough to hang on. Your weight does not change but the stress on the tow rope does.
Wind, “Wind is the air in motion relative to the surface of the Earth”.
Page 10.1 Aviation Weather Handbook FAA-H-8083-28A
Once airborne, wind does not effect drag or lift, it may effect a downwind turn sic.
There has to be a better way of explaining this, and a much simpler way so that everybody can understand it and make sense of it, the way it is being stated now is not so simple, it is complicating, because we understand it, doesn’t mean everyone else does, so lets see how smart those of us that understand it can come up with a foolproof way of explaining it without causing confusion, ok ?? start now !!
Been flying for 52 years, and this is all still as clear as mud!
The Center of Lift still has to hold up the weight of the airplane which I assume would be perpendicular to the direction of motion. As you speed up more lift comes from the impact of the relative wind on the bottom of the wing, due to angle of incidence, reducing the amount of aerodynamic lift needed from the curvature of the top of wing to hold the weight. So that might account for the induced drag curve reducing in your first chart. Parasite drag is in a fixed relationship to the velocity since it is not changing its relationship to the relative wind. So is the lift vector changing or is the drag vector changing? I used to fly Navion and the folks that fly these birds are always looking for speed mods. One of which was changing the angle of incidence on the wing. I’m not a mathematician so I don’t know the equations that would impact this point of view.
Doesn’t matter, just see how the plane handles in different realms and be aware of adverse conditions.
“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.”
Wait. What? Lift =Weight in unaccelerated flight. You don’t need more lift at slower airspeed, but you are at a higher angle of attack with a higher lift coefficient. The rearward angle of the lift is greater at slower speeds, but lift vector opposing weight still equals weight.
I’m probably overthinking it!
You’re right – was going to question the same point. Going into slow flight, the weight doesn’t change – therefore lift doesn’t need to change. The sole purpose for increasing the angle of attack is to maintain the same lift at a slower speed. However, what does change is the induced drag vector – at the slower speed, it points further backwards increasing drag. So a slightly higher amount of thrust will be needed to keep thrust and drag equal if the objective is to maintain the same airspeed.
Induced drag isn’t really a function of lift. Imagine a 10,000 pound aircraft with no flaps or other lift devices. If it flies straight and level at its max speed it is developing 10,000 pounds of lift with very little induced drag. If it is flying straight and level just above its 1g stall speed it is also developed 10,000 pounds of lift but with much more induced drag. A better way to think of induced drag is drag from increased angle of attack. More of the bottom of the aircraft is hitting the relative wind.
BTW, when you side slip an aircraft, you’re not increasing parasite drag. You’re increasing induced drag sideways.😎