The tear drop shape is a pleasantly aerodynamic shape. It is "streamlined" and causes very little disturbance and drag as the wind smoothly passes around it. It and variations of it are the only shapes you need to be interested in from now on.
NOTE: Drag cannot ever be totally eliminated as long as the moving air is caused to change its flow path even the slightest amount. We can only hope to reduce drag to a minimum.
ANGLE OF ATTACK Picture a plate, or surface as we call it from now on that has a streamlined cross section and is standing on one end like a sail on a sailboat. (See "Developing Thrust - 1"). It is set so that the wind attacks it directly head on and flows smoothly around it on both sides causing little drag.
Developing Thrust - Image 1
In the second view (See "Developing Thrust - 2") you are looking directly down on the same streamlined surface. It is twisted and set so that it is attacked by the wind at an angle. (This angle is called the "angle of attack"). In this view it can be noticed that the surface is no longer the docile little thing that it was. It is now generating a little more drag (notice the drag arrow), but it is also generating a powerful thrust in the direction shown by the arrow marked "thrust". If it were mounted on a boat, it would nicely drive the boat toward the top of the page.