Induced Drag

[cookmg]

Can someone provide me with a thorough and clear explanation of induced drag, or perhaps point me to a source that does a good job explaining it. I'm able to understand its implications very well, but I am having trouble visualizing what is happening that results in induced drag.

Thanks.

 

[maverick_fp00]

Well.. let's start off by saying that drag is a by-product of lift, which you already knew. and the higher the angle of attack, more drag is being produced. So, if we are flying at a slower airspeed, we are flying at a greater angle of attack.So when more higher pressure air from the lower surface coes around the wingtips it forms powerful vorticies. Now, these vorticies change the relative wind for that portion of the wing creating a lower angle of attack at the wingtips. So, at higher airspeeds, a lower angle of attack is required to produce lift, which means less wingtip vorticies and therefore, less induced drag.

The faster the airspeed, the lower the induced drag.

-Nick

 

[Jason_CFI]

Induced drag is generated by the airflow circulation around the wing as it creates lift. The high pressure air beneath the wing joins the low pressure air above the wing at the trailing edge and wingtips. This causes a spiral or vortex which trails behind each wingtip whenever lift is being produced. These wingtip vortices's have the effect of deflecting the air-stream downward in the vicinity of the wing, creating an increase in down-wash. Therefore, the wing operates in an average relative wind which is inclined downward and rearward near the wing. Because the lift produced by the wing is perpendicular to the relative wind, the lift is inclined aft by the same amount. The component of lift acting in a rearward direction is induced drag.

As air pressure differential increases with an increase in angle of attack, stringer vortices's form and induced drag is increased. Since the wing usually is at a low angle of attack at high speed, and a high angle of attack at low speed, the relationship of induced drag to speed also can be plotted.

Parasite drag is caused by any aircraft surface which deflects or interferes with the smooth airflow around the airplane. Parasite drag normally is divided into three types: form drag, interference drag, and skin friction drag.

Form drag results from the turbulent wake caused by the separation of airflow from the surface of a structure. The amount of drag is related to both the size and shape of the structure which protrudes into the relative wind.

Interference drag occurs when varied currents of air over an airplane meet and interact. Placing two objects adjacent to one another may produce turbulence 50% to 200% greater than the parts tested separately. An example of interference drag is the mixing of the air over structures such as wing and tail surfaces brace struts and landing gear struts.

Skin friction drag is caused by the roughness of the airplane's surfaces. Even though these surfaces may appear smooth, under a microscope, they may be quite rough. A thin layer of air clings to these rough surfaces and creates small eddies which contribute to drag.

Each type of parasite drag varies with the speed of the airplane. The combined effect of all parasite drag varies proportionately to the square of the airspeed. For example, a particular airplane at a constant altitude has four times as much parasite frag at 160 knots as it does at 80 knots.


Check out this web site for some good information.

http://142.26.194.131/aerodynamics1/Drag/Page6.html

The greatest source for aerodynamics I believe is the AERODYNAMICS FOR NAVAL AVIATORS; ASA reprints this book and can be found at any book store, pilot shops, or on-line for less than 20 dollars.

I highly recommend this book.


Jason

 

[lucky2Bflyin]

Aerodynamics for Nasal Radiators...greatest book in my collection.

B

 

[carlos]

Here's a simple, I hope, explanation:

Induced drag increases with increased lift. Increased lift is a result of a higher angle of attack. Lift is perpendicular to the wing chord. As the angle of attack increases, the lift vector, which is perpendicular to the wing, angles to the rear of the aircraft. Since the lift vector is now pointing to the rear of the aircraft there is a portion of the lifting force that is operating to the rear of the aircraft. This rearward force is induced drag.

The other component of induced drag is wing vortices, which someone covered in a previous post.

 

[lucky2Bflyin]

Carlos-
Lift is not perpendicular to the chord line....rather it is ALWAYS perpendicular to the relative wind.
B

 

[carlos]

Lift is not perpendicular to the chord line....rather it is ALWAYS perpendicular to the relative wind

This is true and more correct than the words that I used, but needs a little further explanation. Lift is perpendicular to the local relative wind. The local relative wind is the relative wind as it flows around the wing. When the wing is in level flight, the local relative wind is parallel with the wing's movement through the air. With increasing angles of attack, though, this local relative wind is bent downward as it flows around the wing. The lift vector then is also bent backward producing a rearward component of lift, which is induced drag.

This is the way that I understand it, but I am open to corrections.

 

[VNugget]

There is a discrepancy between what I see in this thread and what I have learned -- perhaps someone could set me straight if I'm wrong.

Everyone in here says that induced drag varies with lift -- but in straight & level flight, and constant weight, lift is the same for all airspeeds, (Newtons' second law -- no vertical acceleraion means no net force in the vertical axis) no?

Rather, it increasis with the coefficient of lift (Cl), which is one of factors that go into lift (Lift = Cl * density * velocity^2 * area / 2 ). The rest are self-explanatory, but Cl is a catch-all for everything else that goes into it - AoA (duh) and other factors way beyond my understanding of aerodynamics.

Sorry, I realize it's a nitpick, but I'm a stickler for clarity.

 

[carlos]

Everyone in here says that induced drag varies with lift

Actually, I think we're all saying that induced drag varies inversely with speed, not lift. As speed decreases the angle of attack must increase to maintain level flight, which increases induced drag.

The vertical component of lift remains constant, as you say, but with increasing angles of attack, there is also a rearward component of lift, which is induced drag.

Again, I'm open to any corrections. I'm no expert, by any means, but this is what I've been reading while preparing for my PPL.

 

[VNugget]

Yeah, we're talking about the same thing now. As velocity decreases, the Cl (which angle of attack is part of) must increase (and induced drag increases along with it) in order to maintain a constant total lift.