Drag and best glide

Drag comes in two main types.

Parasite drag

Parasite drag is caused by anything that disrupts the airflow and doesn't help make lift. It has three parts: form drag (the shape of the airplane pushing through the air), interference drag (where airflows around different parts meet, like the wing-fuselage joint) and skin friction drag (air rubbing on the surface). Parasite drag increases with the square of airspeed: double the speed and parasite drag roughly quadruples.

Induced drag

Induced drag is the price of making lift. High-pressure air under the wing curls around the wingtips toward the low pressure on top, forming wingtip vortices and tilting the lift vector rearward. Induced drag is greatest at high angles of attack and low airspeed, and it decreases as airspeed increases.

Total drag and L/D max

Add the two together and you get a U-shaped total drag curve. At low speed, induced drag dominates; at high speed, parasite drag does. At the bottom of the curve, total drag is lowest. That point is where the airplane gets the most lift for the least drag, called L/D max (the maximum lift-to-drag ratio).

Best glide speed

If the engine quits, flying at the airspeed for L/D max gets you the farthest distance for the altitude you have. That's your best glide speed, published in the POH. Gliding faster or slower than best glide gives you less distance. Best glide speed is lower when the airplane is lighter, but most POHs publish one number for max gross weight; use it and memorize it.