You may have heard the words lift and drag, but do you know what they mean? Lift and drag are aerodynamic forces that act on an object moving through a fluid (a liquid or a gas). Understanding which way lift and drag point can be important for an aerodynamics science project. This page will try to clear up some common misconceptions about the directions of lift and drag.

Figure 1 shows a diagram frequently used to represent the aerodynamic forces on an airplane.

• The plane’s weight pulls it down toward the Earth.
• Thrust generated by the engines pushes the plane forward.
• Lift generated by the wings pushes the plane up and keeps it in the air.
• Drag (also called air resistance) pushes back on the plane as it moves through the air.

The diagram of forces in Figure 1 is accurate for the plane shown, but you have to be careful because Figure 1 can create the misconception that lift always points “up” and drag always points “sideways.” In fact, lift and drag are defined based on the object’s motion:

• Drag points opposite (and parallel to) the direction of motion.
• Lift points perpendicular to the direction of motion.

Figure 1 can also create the misconception that thrust always points “sideways,” directly opposite the direction of drag. Since thrust is generated by the airplane’s engines, which are attached to its body, the direction of thrust will depend on which way the airplane is pointing.

The motion that determines the directions of lift and drag is defined relative to the fluid. This means that the resulting drag force points in the same direction for both a plane flying forward into still air and a plane sitting still in a wind tunnel (Figure 2). In both cases, the air is moving relative to the plane, even though the plane is not moving relative to the ground in the wind tunnel.

What about when a plane is taking off or landing at an angle? Its direction of motion is no longer horizontal, so the directions of the forces relative to the ground or an outside observer change (Figure 3):

• Weight still points down toward the ground.
• Thrust points forward relative to the airplane’s body, but is no longer horizontal relative to the ground.
• Drag points opposite the direction of motion, so it is no longer horizontal.
• Lift points perpendicular to the direction of motion, so it is no longer straight up.

Note that not all flying objects always have thrust. For example, when you throw a paper airplane, you apply some initial thrust, but there is no ongoing thrust as the plane glides through the air (Figure 4). Check out our paper airplane drag project to learn more about how the drag force affects paper airplane flight.

Now that you know drag always points opposite the direction of motion, and lift always points perpendicular to it, you can apply this knowledge to other situations. For example, for a person falling with a parachute, the upward force exerted by the air on the parachute is drag, not lift (Figure 5). Since the direction of motion is downward, the drag force acts upward. You can learn more about this in our parachute size project.

It is still possible for a parachute to generate lift if the person is falling sideways. As always, drag will be opposite the direction of motion, and lift will be perpendicular to it (Figure 6).

For a model rocket moving vertically, thrust generated by the rocket’s engine pushes the rocket up. Weight pulls it down, and the drag force acts opposite the direction of motion, in the same direction as the weight (Figure 7).

If the rocket tips so that it is not flying straight, the fins will generate a sideways lift force that helps stabilize the rocket and get it pointed back in the right direction (Figure 8). You can learn more about this in our model rocket stability and vertical landing model rocket projects.

More generally, you can apply this concept to any situation where an object changes direction, like when you throw or hit a ball (Figure 9). Drag will always point opposite the direction of motion. When the ball is first moving up and to the right, the drag force is down and to the left. When the ball is coming back down and to the right, the drag force is up and to the left. The drag force is only horizontal when the ball is at the peak of its trajectory and temporarily has no vertical velocity.

Hopefully, by now, you understand that lift is not always “up” and drag is not always “sideways,” and how this applies to different scenarios you might encounter in a science project.

Image credits:

• Airplane: https://pixabay.com/vectors/airliner-airplane-flying-plane-8886817/
• Parachute: https://pixabay.com/vectors/paraglider-sign-parachute-310288/
• Rocket: https://pixabay.com/illustrations/rocket-leadership-up-upward-space-4231305/