Wednesday, August 8th, 2012 09:54 am GMT -6 Wednesday, August 8th, 2012 09:54 am GMT -6Wednesday, August 8th, 2012 09:54 am GMT -6
Airliner Wing in Flight

These two types of airplane drag are a major pain in the neck of airplane designers.





Drag is, well, a drag. It is a force that slows down the airplane. Thrust is always working against it.

There are a lot of different types of drag. Not everybody agrees on what the major categories of drag are or even what exactly they should be called. Regardless, minimizing drag is a major goal of airplane design.

Here I will discuss the two primary types of drag in our model airplanes.

Parasitic Drag

Parasitic drag is made up out of many different specific types of drag. We need to be concerned mainly with form, skin friction, and interference drags.

Form drag is the drag caused just by the air having to work its way around the airplane. This is probably what you think of when you think of drag. It can be reduced by having a more streamlined airplane shape.

Skin friction drag is proportional to the surface area of the airplane. The surface area is also called the wetted area, meaning the area of the airplane that would get wet if the airplane were submerged in water. The smaller the airplane, the lower the skin friction drag.

Interference drag is when one part of the airplane joins another and the air has to work its way around the junction. For example, when the wing joins the fuselage. This is probably the least understood type of drag among airplane designers.

The different types of parasitic drag are proportional to the square of the air speed. This means that if you double the air speed, you quadruple the parasitic drag. This also means that, if you are trying to go fast, even a small reduction in parasitic drag can make a big different in the top end speed.

Induced Drag

Induced drag is the drag due to the generation of lift.

An airplane wing generates lift by having a lower air pressure over the wing than below it. This air pressure has to equalize at the wing tips. Normally a vortex forms at the wing tips and this represents lost energy. This lost energy is the induced drag.

The higher the aspect ratio of the wing, the weaker these vortices will be. That is a primary reason why airplanes with long wings, like gliders, are more efficient.

The more lift you try and produce from a wing, the stronger the vortices. So the slower you go, the stronger they get. The induced drag is inversely proportional to the square of the air speed. This is the exact opposite behavior of the parasitic drag.

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