Jumbo Jets: How Massive Planes Float on Air
For most people, the thought of an object weighing more than 200 tons taking off, and flying
500 miles per hour at 30,000 feet is perplexing to say the least. With that being said, the
process of getting a jumbo jet from the ground to the sky and back down is rather simple. The
principle steps to a successful flight are as follows:
1. Takeoff
2. Steady Flight
3. Landing
For several examples the case of a Boeing 747 will be used as it was one of the largest and most
recognizable jumbo jets. The Boeing 747 specifications vary based on model, but average
figures include:
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Empty weight of 400,000 pounds
Maximum weight of 800,000 pounds
Wingspan of 200 feet
Length of 231 feet
Seating for 400 passengers
Takeoff:
For an airplane to get from point A to point B it has to go forward. The way it does so is by
taking advantage of Newton’s Third Law which states that for every action there is an equal and
opposite reaction. The very start of a successful flight is when the jet engines begin pushing air
behind the plane at high speeds (up to 1300 mph). The air being pushed back is the action, and
the plane being accelerated forward is the reaction. This effect is visualized in Figure 1 below.
For planes without a jet engine the same principle is applied in the form of a propeller.
Figure 1: Newton’s Third Law applied to a jet engine
To understand how a plane flies, it is important to note the Bernoulli principle, another crucial
physical principle applied in this process. The Bernoulli principle states that as a fluid moves
faster, the pressure it exerts decreases, air being the fluid in this scenario. Airplane wings are
shaped in such a way that takes advantage of this principle and as the plane moves forward the
air that goes over top of the wing moves faster than the air moving underneath it. This results
in a higher pressure acting on the bottom of the wing. The force as a result of this pressure
difference is called lift, and is visualized in Figure 2 below.
Figure 2: Bernoulli Principle Visualized
Each type of plane has its own approximate speed at which it is able to lift off the ground. Most
commercial grade planes takeoff between 120 and 200 miles per hour. For a Boeing 747, one of
the largest commercial planes in use today, takeoff speed is approximately 180 miles per hour.
After accelerating across the ground to its necessary takeoff speed the pilot of the plane will
increase the angle of attack of the airplane using the airplane’s elevators. The elevators of an
airplane are what controls its pitch. This is shown in Figure 3 below.
Figure 3: Elevator and pitch visualized
The increase in angle of attack increases the amount of lift on the wings to a force greater than
the weight of the plane. Anyone who has stuck their hand out of the car window before knows
that when you turn your palm up to face forward the force on your hand becomes much larger.
This is the exact same effect as increasing the angle of attack of an airplane. Since there is a
larger force pushing the plane up compared to its weight pulling it down the plane accelerates
upward to its cruising altitude.
Steady Flight:
As the engines continue to push the plane forward and it continues to rise it will eventually
reach its cruising altitude. The cruising altitude for each plane is the height in the atmosphere at
which its long-term flight capabilities are maximized. It is a function of the planes size, speed,
fuel capacity, etc. Larger commercial grade planes tend to cruise at altitudes of 25,000 to
30,000 feet. Smaller civilian grade planes will cruise anywhere from 2,000 to 15,000 feet. Upon
reaching its cruising altitude the plane will level out and come to an equilibrium. Being in an
equilibrium means there is zero net force acting on the plane; therefore, it is not accelerating in
any direction and it is moving with a constant speed. The four primary forces acting on a plane
while it is cruising are its weight, lift on the wings, drag from the air, and thrust from the
engines. These are shown in Figure 4 below. In an equilibrium these forces are balanced and
the plane continues to move forward at its cruising speed. Cruising speed for a Boeing 747 is
approximately 580 miles per hour.
Figure 4: Airplane in equilibrium
While for most commercial flights very little maneuvering occurs during flight it is important to
know how a plane is able to move in three dimensions. It has already been shown that the
plane uses its elevators to control its pitch so we will examine the other two dimensions. These
ranges of motion are called roll and yaw and are controlled by a plane’s ailerons and rudder.
This is visualized in Figure 5 below.
Figure 5: Roll and yaw visualized
For a plane to roll onto its left wing, the left aileron is raised and the right aileron is lowered.
This results in more lift acting on the right wing compared to the left, and the plane rolls onto
its left side. One way to visualize this is to think of a pencil held horizontally. If the tip is pushed
up towards the sky, the eraser will point towards the ground, and vice versa. The rudder works
in a similar manner to control the yaw of the airplane. If it is deflected to the right side of the
plane there will be more drag acting on that side, and the plane will twist to the right.
Landing:
After spending the necessary amount of time at its cruising altitude the plane will have to make
its descent for a landing. The first step in this process is to decrease the angle of attack of the
airplane to a point at which the lift on the wings is smaller than the weight of the plane, thus
moving the plane downward. At the same time the engines of the plane are slowly throttled
back in order to slowly bring the plane to its proper landing speed. To assist in slowing the
motion of the airplane, flaps on the wings are raised to increase drag. Figure 6 below shows the
location of the flaps and other previously discussed parts of a commercial grade airplane. The
figure also gives brief descriptions of the purpose these parts serve for the plane.
Figure 6: External parts of an airplane and their functions
As the plane makes its final approach to the runway its landing gear is deployed. When the
airplane touches down it will always touch down on its rear landing gear first, followed by the
nose landing gear. For smaller civilian airplanes this is less important because the front landing
gear bears a smaller percentage of the plane’s weight. The approximate landing speed for a
Boeing 747 is 160 miles per hour and it will take around 5,500 feet or one mile to come to a
stop.
Summary of a Successful Flight:
The process of a successful flight begins with the jet engines pushing air back to accelerate the
plane forward. Smaller planes utilize propellers for the same principle. A jumbo jet will
accelerate along the ground to speeds up to 180 miles per hour. As it does so, air is split by the
wings and moves over the top and bottom of the two wings. The wings are shaped in such a
way that air moves slower on the bottom of the wing and thus exerts a higher pressure on the
bottom of the wing. This higher pressure results in a force called lift and is how the plane is able
to overcome its weight and fly. The pilot increases the angle of attack of the airplane so that it
can rise to its cruising altitude. Once at its cruising altitude it comes to an equilibrium and
moves forward with constant velocity. After some time, the angle of attack is decreased and
the plane descends for a landing. The engines are throttled back, flaps and landing gear
deployed, and the plane touches down safely on the runway.
While the idea of all this coming together for large metal objects weighing hundreds of tons can
seem far-fetched; it is very much a reality and one that has developed into a massive modern
industry. Even more fascinating is that this process is being pushed and advanced further every
day by the smartest people in the world. A Boeing 747 class 800 is capable of carrying six
apache helicopters in its cargo bay. An F-22 fighter jet is capable of reaching speeds more than
twice the speed of sound. One can only hope that these capabilities only scratch the surface of
what man-kind is capable of in terms of flight.
References:
“Airplane Cruise.” NASA, NASA, www.grc.nasa.gov/www/k-12/airplane/cruise.html.
Anonymous. “Roll, Pitch, and Yaw.” How Things Fly, howthingsfly.si.edu/flight-dynamics/rollpitch-and-yaw.
“Boeing 747.” Wikipedia, Wikimedia Foundation, 26 Mar. 2020,
en.wikipedia.org/wiki/Boeing_747.
“Parts of Airplane.” NASA, NASA, www.grc.nasa.gov/www/k-12/airplane/airplane.html.