Drone Physics Lesson 5

May the Force Be with You:
Drag

I can learn about the drag force on airplanes and are introduced to the concept of conservation of energy and how it relates to drag.

Vocabulary

cross-sectional area: The projected area of a three-dimensional object onto a two-dimensional plane.

drag: The phenomenon of resistance to motion through a fluid.

fluid: A continuous, amorphous substance whose molecules move freely past one another and that has the tendency to assume the shape of its container; a liquid or gas.

gas: The state of matter distinguished from the solid and liquid states by relatively low density and viscosity, relatively great expansion and contraction with changes in pressure and temperature, the ability to diffuse readily, and the spontaneous tendency to become distributed uniformly throughout any container.

kinetic energy: The energy possessed by a body because of its motion, equal to one half the mass of the body times the square of its speed.

liquid: The state of matter in which a substance exhibits a characteristic readiness to flow, little or no tendency to disperse, and relatively high incompressibility.

molecule: The smallest particle of a substance that retains the chemical and physical properties of the substance and is composed of two or more atoms; a group of like or different atoms held together by chemical forces.

surface area: The extent of a two-dimensional surface enclosed within a boundary.

​It Is Sunny.

Cherokee: A-ga-li-ha

Syllabary: ᎠᎦᎵᎭ

It Is Raining.

Cherokee: A-ga-sga

Syllabary: ᎠᎦᏍᎦ

It Is Cold.

Cherokee: U-yv-tsa

Syllabary: ᎤᏴᏣ

It Is Hot.

Cherokee: U-di-le-ga

Syllabary: ᎤᏗᎴᎦ

Open Ended

How would you define drag?

Where can you feel drag?

walk in a swimming pool.
A fisherman feels the drag on his lure as he pulls it through the water.

Drag is a force that acts in the opposite direction than an object is moving.

Open Ended

What happens to your arm/hand when you stick it out the window of a moving car?

Can you think of a situation where drag is a good thing to have occur?

An excellent example of drag being a useful force is during skydiving: skydivers rely on drag to slow down their falls so that safe landings take place — hence, the reason for parachutes.

When drag is not a good thing

Like any other object that moves through the air, airplanes also experience drag. This is not always beneficial, however, since we want airplanes to move forward very quickly and drag slows planes down. Overcoming drag has always been a primary design challenge for aerospace engineers. Airplanes overcome drag by generating thrust.

An object must be moving through some kind of fluid for drag to occur. A fluid is a substance in which the particles can move past each other freely. The most obvious fluid is water, but gases, including air, are fluids as well.

How Does Drag Slow Airplanes?

Air is the fluid that airplanes travel through. As an airplane moves through the air, it interacts with air molecules, which cause it to slow down. The moving energy of the plane is transferred to the air molecules. This means that some of the kinetic energy (which is energy an object possesses due to its motion) from the airplane is passed on to the air molecules, resulting in the airplane slowing down and the surrounding air speeding up. The total energy lost by the airplane is exactly equal to the energy transferred to the air. This illustrates the first law of thermodynamics, which implies that energy can neither be created nor destroyed. Engineers usually refer to this principle as the conservation of energy, meaning that energy is always preserved. Energy must go somewhere, so in the case of drag, the energy in the motion of an object is transferred into moving the gas in the path of the object (in the case of an airplane, moving the air around it). This transfer of energy results in three types of drag: friction drag, form drag, and induced drag.

Friction drag arises from air moving over the surface of the airplane. On a very tiny scale, the surface of an airplane is rough, like sandpaper. If you run your hand over sandpaper, it catches your skin and slows down or stops your hand movement. The same thing happens with an airplane. The "skin" of the airplane catches the air particles next to it and tries to pull the air particles along. This slows the airplane down as air particles speed up.

How Does Drag Slow Airplanes?

Form drag is caused by the airplane pushing air molecules to the side so that it can pass through them. A streamlined shape (like an airplane wing) has very little form drag (by design!). However, a non-streamlined shape, like a parachute, has a lot of form drag (by design!).

Induced drag (see Figure 1) is produced at the tips of the airplane wings. To create lift, the wings create a low-pressure area above the wings and a high-pressure area beneath the wings. At the end of the wing, high-pressure air underneath the wing tries to move around the end of the wing to the low-pressure air on top of the wing. This creates a swirling vortex of air at the wingtip. The energy required to move the air in the vortex is taken from the movement of the plane, resulting in induced drag.

What Do Engineers Do to Reduce Drag?

The challenge for engineers is to find innovative ways to minimize drag so that airplanes can fly faster and more efficiently. As the surface area of the wing and the roughness of the wing increase, friction drag increases. Similarly, as the cross-sectional area of the plane increases and the shape becomes less streamlined, form drag increases. Engineers aim to reduce form and friction drag by making the body of the plane more streamlined, the wings more narrow, or by using new materials and manufacturing processes to make the plane's skin smoother. Engineers also reduce induced drag by making the ends of the wings oval-shaped or by adding wingtips that stick up from the end of the wing. Through the efforts of engineers, airplanes are continually changing shape to improve their efficiency and performance (become more aerodynamic).

Match

Match the following

• Lift

• Drag

• Thrust

• Weight

Goes up

Pulls Back

Moves Foward

Pulls Down

• Goes up

• Pulls Back

• Moves Foward

• Pulls Down

Match

Match the following

• drag

• fluid

• surface area:

• liquid

• kinetic energy

The phenomenon of resistance to motion t

A continuous, amorphous substance whose

The extent of a two-dimensional surface

The state of matter in which a substance

The energy possessed by a body because o

• The phenomenon of resistance to motion t

• A continuous, amorphous substance whose

• The extent of a two-dimensional surface

• The state of matter in which a substance

• The energy possessed by a body because o

Match

Match the following

• It Is Sunny

• It Is Cold

• It Is Raining

• It Is Hot

A-ga-li-ha

U-yv-tsa

A-ga-sga

U-di-le-ga

• A-ga-li-ha

• U-yv-tsa

• A-ga-sga

• U-di-le-ga

Open Ended

• How does drag slow down airplanes?

Open Ended

Will the drag on an object increase if the surface area is increased or the object speeds up?

Open Ended

How does designing an airplane with less drag affect its speed and efficiency?