The propeller is both strong and heavy, often used to pull an aircraft forward into flight. It works to pull the vehicle in whatever direction it is facing.
The downside to the propeller is that it is extremely noisy, meaning if you used it in a level containing Angry Birds, your vehicle would catch their attention in seconds. It is interestingly shock resistant, but will come off if it hits something at a certain speed. It is first introduced in When Pigs Fly, and levels 3-14.