Drag

Drag is the enemy of both speed and lift in aviation.  It will slow down your aircraft, and could adversely affect the ability of the aircraft to lift up off the ground.

There are two types of drag, Parasitic and Induced. 

Parasitic Drag

That drag which is produced by the aircraft which is not associated with lift.  Example of parasitic drag include:

• Landing Gear
• Antennas
• The Fuselage of the aircraft
• Skin Friction

There's not much you can do about parasitic drag, however some air carriers have tried and claim that they save fuel dollars by doing so.  The simple task of washing the plane will reduce skin friction (a form of parasitic drag) since there will be less dirt particles attached to the aircraft causing the aircraft to slow down.

Induced Drag

That drag which is directly associated with the creation of lift.  The wings themselves create drag as they create lift. 

In the illustration above (thanks FAA), this wing is creating lift because of its angle of attack.  The higher the angle of attack (up to a point), the more lift is created.  But, the relative wind seems to be impacting on the bottom-rear of the wing causing drag to be directed towards the back of the aircraft the wing is attached to.  This is a form of induced drag and a necessary evil but one of the reasons why your aircraft will slow down during a climb. 

You can try this experiment when driving in your car:

1. When there isn't too much traffic around and your car is moving quickly down the highway, open up your window and stick your hand out flat with your palm facing down to the ground.  Relax your arm and let your hand slice through the air.
2. Slowly, angle the front portion of your hand up while keeping your hand flat -  notice the results
   • Your hand will begin to ascend (Newton's third law of motion causes your hand to lift).
   • Your hand will also begin to move backward if you relax your arm enough - this is the drag caused by the wind hitting the lower portion of your hand as you raise the angle of attack of your hand.

Another way to look at induced drag is that difference in the production of lift as illustrated below:

Lift is always produced perpendicular to the actual wing.  The light-blue arrow above shows this.  However, the resulting lift is shown with the pink arrow since the aircraft isn't actually moving backwards as it lifts because of the thrust of the propeller.  The Induced Drag arrow (black arrow) thus reflects the difference in the angles of these arrows and can be thought of as the induced drag produced by the wings lifting.

Another form of induced drag is that caused by the wingtip vortices caused by the aircraft being lifted.

Again, thanks to the FAA for the diagram above which effectively illustrates the production of wingtip vortices.  Wingtip vortices are caused by the low-pressure air above the wing trying to get down to the higher pressure area to equalize the overall air pressure in the wing area.  At the tips of the wings, however, high pressure air spills up reducing the effectiveness of the lifting of the wing near the wingtips.  This reduced the lift but keeps the drag at its normal level, thus increasing overall induced drag.