Induced Drag

Sandesh

Sandesh

Well-Known Member
Dear Friends and Aviators,

I am a student pilot going for my private. I was in ground school and was little confused about teh induced drag and vortices portion, I couldn't grasp the idea of induced drag and how it is a byproduct of lift.
I would appreciate it if i got some explanation on this topic.

Thank You,
Sandesh
 
The vorticies are low pressure areas behind the airplane draging it back, think high/low pressure lift idea here. Induced drag is explained "is a byproduct of lift" because it would take a engineering aerodynamicist to explain and understand the science behind it.
 
Wingtip vortices are created from air under high pressure below the wing "escaping" to the lower pressure above the wing (see: Bernoulli's principle). Now, the higher the angle of attack of the wing, the greater the pressure differential, which increases lift. However, this also increases the magnitude of those vortices.

Think of the wing as dragging those vortices behind it. The bigger those vortices, the more drag - induced drag specifically.

THEORY: If the wing was of infinate length, then there would be no wingtip vortices and hence induced drag would not exist.
 
I explain induced drag as the component of lift that is acting in the direction of drag. When you increase the pitch the wing is now at an angle that has a rearward component of lift. When you go slower you need to increase AOA and thus you also increase this rearward component of lift..."induced drag".
 
subpilot said:
I explain induced drag as the component of lift that is acting in the direction of drag. When you increase the pitch the wing is now at an angle that has a rearward component of lift. When you go slower you need to increase AOA and thus you also increase this rearward component of lift..."induced drag".
Click to expand...

I usually explain it in a similar fashion. Although not technically correct, for a student - understanding that increasing the AoA changes the vertical and horizontal components of total lift. The horizontal component of lift is essentially the drag created.

If a student wants to get into detail further, then put a book in front og them :)
 
Our ground is based on the Jeppsen Private Pilot book and it does not go into great details on Induced. So, I was not able to grasp the idea.
 
jawright said:
Are wingtip vortices the only type of induced drag you can get?
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Wingtip vortices are not a type of induced drag, they just contribute to it. When the higher pressure air below the wing spills over the wingtip to the lower pressure air above, it reduces the amount of lift the wing is creating. Since you need a certain amount of lift to support the aircraft, the only way you can compensate for this is to increase the angle of attack. Increasing the AoA increases induced drag, since lift is perpendicular to the chord line (as others have said, increasing AoA increases the 'rearward component of lift').

Induced drag is most prevalent at low airspeeds, where you have to maintain a high AoA to stay airborne. At higher airspeeds, you need a lower AoA so induced drag is less, but parasite drag increases.

The negative effect of wingtip vortices can be minimized somewhat by increasing the wing's aspect ratio (works good on slow airplanes like gliders)or using winglets. Wing planform will also effect efficiency, with the eliptical wing being the most efficient, followed by the tapered wing. The rectangular 'hershey bar' wing is the worst, but at the speeds a fixed gear trainer flies at, it really doesn't matter.
 
How about a new perspective?

Lift, by definition, is the aerodynamic force perpendicular to relative wind. Drag, by definition, is the aerodynamic force parallel (and in the same direction as) relative wind. Drag has been further subdivided into two categories: induced and parasite. Induced drag is the result of a finite wing producing lift. When a wing produces lift, trailing vortices form as high pressure air swirls around the wingtip towards a low pressure region.

Two ways of explaining why vortex formation leads to induced drag follow:
1. The formation of a vortex requires energy. This energy comes at some expense, namely the motion of the aircraft being retarded through the air. This retarding force can be labeled “induced drag.”
Alternatively . . .
2. As a vortex forms, the relative wind in the vicinity of the vortex is modified. In fact, this modification of the local relative wind causes the local lift to angle slightly in a rearward direction. The rearward component of lift (parallel to relative wind) is considered to be a drag force, namely induced drag.

Do not consider the explanations above to be complementary, whereby Explanation #1 identifies some portion of induced drag and Explanation #2 identifies another portion. Both present two different ways of viewing the creation of induced drag.
 
Pilot Hopeful said:
the relative wind in the vicinity of the vortex is modified.
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Your explanation is correct. Let me see if I can make it clear in the absence of a white board.

In an ideal world,

1) Lift is perpendicular to the relative wind (not the wing chordline).
2) The relative wind is produced by the direction of flight of the aircraft.
3) Therefore, lift is perpendicular to the direction of flight of the aircraft and has no force component that opposes the motion of the aircraft.

In the real world, wingtip vortices change all that. They distort the airflow in the vicinity of the wing so that the local relative wind comes from slightly above the direction of flight of the aircraft. The lift generated by the wing will be perpendicular to this local relative wind, not the relative wind that exists ahead of the aircraft. If you draw a perpendicular line to this local relative wind, you will see that it points slightly against the direction of movement of the aircraft. The component of this force that points backwards opposes the forward motion of the aircraft and so we label it "drag". Since the strength of vortices increases with increased angle of attack (synonymous with low airspeed), induced drag increases as well.

If you could eliminate wingtip vortices, you would eliminate induced drag. This can be done in wind tunnels, because the wing sections have the tunnel walls as endplates and no vortices can form.

I don't really care for the analogy of a wing "dragging" a vortex behind it. The forces on a wing are due to two factors and two factors only: pressure differences around the airfoil and skin friction. A vortex is only of significance to the extent that it affects those two forces. There simply is no string that attaches a vortex to an aircraft and pulls it backwards.
 
tgrayson said:
In an ideal world,

1) Lift is perpendicular to the relative wind (not the wing chordline).
2) The relative wind is produced by the direction of flight of the aircraft.
3) Therefore, lift is perpendicular to the direction of flight of the aircraft and has no force component that opposes the motion of the aircraft.
Click to expand...
I agree with the above, with one slight codicil: when you refer to lift in these statements, you're really talking about the sum of all lift produced--aka total lift--which is indeed perpendicular to relative wind. However, the individual components of total lift are perpendicular not to the relative wind, but to the upper surface of the airfoil, and can be divided into forward-, upward- and rearward-acting components of lift. The rearward-acting components produce what is called induced drag (making the forward-acting components "induced thrust", perhaps? :D); it is a byproduct of the production of lift.

Given this (which may be entirely wrong, it's just my understanding of the subject; I welcome all well-reasoned corrections), I don't see how elimination of wingtip vortices can even theoretically eliminate all induced drag, as they are but one source.
 
Alright guys, we've got this all kinds of befuddled now....

Let's start from the beginning. As the air flow passes over the wing it produces a pressure differential, and a turning of the air which causes an aerodynamic force on the wing. I am going to refer to this as a resultant force, because it can be broken down into the result (or addition) of two components.

This resultant force is neither parallel to, nor perpindicular to the relative wind. This resultant force is, rather, at some angle from the relative wind. This force can be broken down into 2 components. One component of this force acts perpindicular to the relative wind and is called Lift. The other force acts parallel to (and in the same direction as) the relative wind and is called induced drag. It is called induced drag, because it is induced (created) every time we create lift. In essence, it is a negative by-product of trying to create lift.

Now, to correct some errors about induced drag from above. Induced drag is NOT the result of wing tip vortices, although it is affected by wing tip vortices. But even if there were no wing tip vortices, there would still be some induced drag. There would just be less.

The other major category of drag that we look at is parasite drag. This is sometimes also refered to as form drag. Parasite drag is the result of viscous effects of the air. Basically that means it is the result of friction between the air and the aircraft.

So here are some properties of induced and parasite drag. First, since induced drag is the result of producing lift, it would be present even if there were NO friction. Friction causes parasite drag, not induced drag. Induced drag is a by product of producing lift, and it is greatest at slower speeds, because of the high angles of attack required to produce lift at those slow speeds. Parasite drag is the result of friction, and is greatest at high speeds, because that is when there is the most friction between the air and the airplane. Induced drag is NOT the result of wing tip vortices, but induced drag is affected by wing tip vortices. If there were no wing tip vortices, there would be less induced drag, but it still wouldn't be 0. So even with an airfoil in a wind tunnel, which has no wingtip vortices, there is still some induced drag.

And if you are keeping track, since induced drag is high at slow speeds, and parasite drag is high at fast speeds, all aircraft have a speed somewhere in the middle where these two types of drag equal each other. This is also the point of the least TOTAL drag, and is refered to as L/Dmax. Lot's of your best performance numbers occur at that point, including your best angle of climb (Vx), Best power off glide distance, and maximum endurance (at least in jet type aircraft- I'm not sure about prop-types for this one).

Hope this helps.
 
fish314 said:
Now, to correct some errors about induced drag from above. Induced drag is NOT the result of wing tip vortices, although it is affected by wing tip vortices. But even if there were no wing tip vortices, there would still be some induced drag. There would just be less.
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Well, you've just contradicted every aerodynamics book that I own. I will provide you with innumerable references when I get home.:)

Interesting that wing section data provided by NACA does not include induced drag. Why? No wing tips, no vortices. Check out "Theory of Wing Sections."

The induced drag equation is Di = Cl^2/(Pi*e*AR). Note that as AR (Aspect ratio) goes to infinity, meaning no wing tips, then induced drag goes towards zero.

One thing you might be confusing is "Induced drag" vs. "Drag due to lift". There is one other component thrown into "Drag due to lift" that is not induced drag, and that is the extra parasite drag due to a larger angle of attack.
 
fish314 said:
And of course, the wikipedia link:
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And your own link contradicts what you say:

---------<snip>-----------
In aerodynamics, lift-induced drag, or induced drag, is a drag force which occurs whenever a lifting body or a wing of finite span generates lift.
---------<snip>-----------

Note the words "finite span". Finite span means wingtips, which means vortices.

---------<snip>-----------
On a wing of finite span some air 'leaks' around the wingtip from the lower surface towards the upper surface producing a wingtip vortex. The vortices then create a down flow or 'downwash' behind the wing. This modifies the airflow around the wing, relative to that on a wing of infinite span, tilting the total reaction force rearwards. The angular deflection is small and has little effect on the lift as defined above. However, there is an increase in the drag equal to the product of the lift force and the angle through which it is deflected. Since the deflection is itself a function of the lift the additional drag is proportional to the square of the lift. Unlike parasitic drag, induced drag is inversely proportional to the square of the airspeed.
---------<snip>-----------

Pretty good descriptions and matches what I said.
 
aloft said:
you're really talking about the sum of all lift produced--aka total lift--which is indeed perpendicular to relative wind.
Click to expand...

You're right, of course, but all physical models of reality are fictitious to some degree. Whenever we represent forces using vector diagrams, we're lying. In truth, the forces are spread out, but we're pretending that they act at one point. For many purposes, the difference is irrelevant.


aloft said:
elimination of wingtip vortices can even theoretically eliminate all induced drag, as they are but one source.
Click to expand...

Because that's how induced drag is defined. There is some increase in lift induced drag with increasing AOA even in the absence of wingtip vortices, but this isn't called induced drag. It's called "drag due to lift", of which induced drag is one component, though the largest. It isn't affected by aspect ratio.
 
Oops. Sorry about that. I re-read what I posted. You're right. For a 2d, body, there is no induced drag. only parasite. I was looking at "drag due to lift" and "induced drag" as the same thing. Disregard what I wrote.

I should know better than to ever contradict you Tgrayson! I'm definitely going to be looking you up as I start my masters.


Now I need to find one of those "bowing down in worship" smileys. Just imagine one here!
 
Induced drag is caused by wingtip vortices. That's what I've learned. That's what the FAA preaches. That's what I will teach.
 
fish314 said:
I'm definitely going to be looking you up as I start my masters.
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That's very kind of you, but you will be disappointed. :) I know the basics real well, because I've read the same material over and over again in many different books. Unfortunately, I run out of steam quickly with the more advanced stuff. I have several books that are almost incomprehensible to me. Due to lack of time, they will probably remain so.:mad:

BTW, we are sortof neighbors, if you're in Columbus, MS. I used to be based in Olive Branch, MS, and still fly out of there on occasion. The examiner I'm using for my upcoming student's checkride is based in Columbus.
 
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