As an airline pilot who commutes to work by plane, it seem sometimes I spend nearly as much time riding in planes as I do flying them. Often when I ride in back, I get asked a lot of questions about airplanes from curious passengers ranging from what those fins on the ends of the wings (winglets) do or how much fuel a jet burns, to the simple question of why I’m riding in the back. The question I get most often however is: “How the heck does this big thing get into the air?”
I have always felt that people are generally curious about aviation and want to know more about it – I know this was the case for me anyway. Although many possess this inherent curiosity, aviation seems to be an elusive topic. This is why I wrote this post, to attempt to shed some light on aviation for those looking to learn a little more about those amazing aluminum machines that rocket us across the world.
There you are, sitting on the end of the runway after just being cleared for takeoff. Soon you hear the familiar sound of spoiling turbine engines and you begin to rocket down the runway. You can feel the acceleration of the jet as it hurtles itself along on the the takeoff roll. The nose rises and you launch into the air. A quick glance out the window shows buildings getting smaller – You are flying. The question is: How?
Takeoff is truly the most magical part of flight. A few different principles come together to make it happen, so I divided things up a bit:
Developing lift and the operation of the wing:
Airplanes may seem complicated and the proposition of understanding how a wing works might seem daunting.
In reality, the method in which a wing produces lift is really quite simple.
To put it in its simplest form, an airplane’s wing is essentially a long board that grabs the incoming air and pushes or deflects it downwards. Because air has mass; a small mass, but a mass no less, that deflection of air downwards causes a force which pushes the airplane upwards.
To give a real life example, imagine you are driving down the highway and put your hand out of the car window while squeezing your fingers together making a wing-like shape. If you align your thumb and pinky with the incoming air, your hand will remain relatively stationary. If you then tilt your thumb upward, you will begin to deflect the incoming air downward which will cause your hand to be pushed upward.
Although this upward force can feel fairly significant, hands are inefficient at producing lift. Airplane wings have been optimized over the past hundred years to produce as much lift as possible while minimizing the drag produced while doing so. This is why airplanes have wings and not hands 😑.
How airplanes maximize lift with airfoils:
If you have gotten a window seat with a view of the wing, you will immediately notice that an airplane’s wing is fairly flat on top and has a lot of surface area… much like a plank of wood. If you are a little more scrupulous however, you will also notice that the wing has a slight curvature. The profile of this curve is called an airfoil. Here is a profile view of an airplane wing showing the airfoil:
An airfoil helps maximize lift and reduce drag in two ways:
First, because an airfoil has a streamlined shape, it has little form drag (or drag due to its shape). You might have heard this trait referred to as being “aerodynamic”. This is why Pick-up trucks get worse gas mileage than streamlined economy cars.
Second, the curved upper surface of the airfoil allows it to take advantage of the Coanda Effect. This effect refers to a fluid’s (air in this case) tendency to hug a curved surface. An airfoil’s specially curved upper surface causes the air flowing over the top of the wing to be deflected downward. This means a lot more lift and a lot less drag in a smaller package.
Here’s an illustration:
In theory, a plane could fly with boards for wings, but its wings would be horribly inefficient. This is clear in the illustrating above. The board does produce some lift by deflecting the air impacting its lower surface downward. Nearly all of the air that flows over the top of the board however is sucked into the vacuum produced by the board’s wake and creates a bunch of drag. The airfoil on the other hand is able to use the Coanda Effect to help deflect and control the air flowing over the top if the wing as well.
Below is an excellent video of the Coanda effect in action, notice how the profile of the spoon closely resembles that of an airfoil!
Even with well designed airfoil-shaped wings, an airplane would not be able to lift off of the ground without airflow over the wing. In order to become airborne, airplanes need a certain amount of airflow or “airspeed” over the wings.
Besides being necessary to get you from point A to B, gaining speed is an essential part of how planes fly. So important in fact that all planes have a minimum speed that they can be flown at called stall speed. On takeoff, an airplane must be accelerated past this speed to become airborne.
Airliners are massive machines. Average sized passenger jets like the Boeing 737 or Airbus A320 weigh well over 100,000 pounds. The largest passenger plane in the world, the Airbus A380 has a maximum takeoff weight of over one-million pounds (equivalent to 340 passenger cars)! In order to get that much weight moving, airliners have engines which produce thousands of pounds of thrust. Unlike cars or most other forms of terrestrial transportation which use piston engines to provide motivation, modern airliners employ the aptly named jet engine. Jet, or turbine engines are best suited to aviation because of their high power to weight ratio and their ability to operate efficiently at high altitudes. Aviation as we know it today would not be possible without jet engines.
As an aircraft accelerates down a runway, airflow over its wings increases. Eventually an airspeed is reached where the wings can safely produce enough lift to support the entire plane. When this occurs, the pilot pulls back on the yoke and begins to rotate the airplane.
Rotation is the term used to describe the period of time when an aircraft begins the transition from its level attitude during the takeoff roll to the nose-high attitude found on climb out.
In order to rotate an airplane, the pilot pulls back on the yoke which is connected to a flight control surface on the tail called an elevator. This input causes the elevator to rise into the air stream and grab air, generating a downward force on the back of the plane. The main wheels act as a pivot point and the nose begins to point skyward.
Rotation raises the wings relative to the incoming airflow. This allows them to grab and deflect more air downward. This is exactly the same principle from the car window example earlier in the post. As you begin to rotate your thumb upward, the force upwards begins to increase. Eventually a point is reached where the upward force generated by the wings exceeds the downward force of gravity on the plane and the aircraft becomes airborne.
Just after takeoff
Clunk, Clunk…CLUNK. If you listen carefully, you can hear a series of clunks and whines just after takeoff. After an aircraft becomes airborne, the pilots quickly begin the process of setting it up in its lowest drag configuration or “cleaning it up” in pilot talk. This starts by retracting the landing gear into special bays and reshaping the wing for high speed flight by moving flaps and slats. Once the process is complete, the aircraft can accelerate to climb speed.
It’s not that complicated…
But a lot of things have to come together to get an aircraft into the air. Hopefully next time you ride in an airplane, the events that occur during and after takeoff will seem a little more familiar.