Drag

Drag is the force that slows an aeroplane down, or holds it back. You can feel drag when you ride a bike - the air and the wind will hold you back and make it hard to go faster. You may have noticed that the faster you go, the more drag you feel. This is because drag increases as speed does. It is also affected by size. The bigger something is, the more drag it will have. This is why sportsman like cyclists or skiers duck down into as small a shape as possible.

Aeroplanes are very big and move very fast, so they have a lot of drag. This is why they are long and thin in shape and have such powerful engines - it reduces the amount of drag that slows them down. The reason that planes lift their wheels up when they are in the air is to reduce drag.