Physics of Airplanes
At any given moment, roughly 5,000 airplanes crisscross the skies above the United States alone, amounting to an estimated 64 million commercial and private takeoffs every year (source: NATCA). Considering the rest of the world’s flight activity, the grand total is almost incalculable. In order to understand how airplanes fly, we must break down the parts of an airplane and understand the physics behind the airplanes most important components: the engine/propeller, and the wings. In the earlier days, airplanes gained thrust only by a single propeller. Nowadays huge turbofans are attached to the wings or the tail of the aircraft instead.
Of course propelled airplanes are still used, most modern planes have jet engines. I will be going over the physics of propellers, and how airplanes gain lift. Lastly, I will go over the details and aspects of jet turbofans used on commercial airlines. A propellers main function is to push the plane forward through the air. Hence it needs all the air that it can get. Since air becomes scarcer as we go higher, this is why most propelled airplanes do not travel at high altitudes like most jumbo airliners do, because these airplanes are able to fly only where there is the greatest amount of air; and that is nearer the earth’s surface.
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The physics behind propellers interestingly relates to Newton’s 3rd law. Newton’s 3rd law states “For every action, there is an equal and opposite reaction. ” Propellers are an application to this law because as the propeller spins, it sucks in air and pushes it out the back of the plane. This is the action, and reaction is the forward motion of the plane (source: dynamic science website). The more air pushed back, the more the plane is pushed forward. The way my model airplane’s propeller works is from winding a rubber band.
This model not only exhibits Newton’s 3rd law but also is an application of conservation of energy. It is an application of conservation of energy because we have some potential energy to start off once we wound up the rubber band, and then once we let it go that energy gets converted to kinetic energy, causing thrust and letting the plane fly. | | The texture of the propeller is also very important in learning how it functions. The shape of the propeller is such that it “cork screws” through the air. This motion is what causes the plane to move forward into the air. (source: dynamic science website)
Now that we have a good understanding of how airplanes gain thrust to move forward, it is also important to know how they lift off the ground. Most people make the misconception that the engines or propeller of an aircraft is what makes it fly and lift off the ground. This is partly correct. The engines and propeller create the thrust which is essential for a lift, however the wings are the main component that causes the plane to be able to lift off the ground and fly. The design and function of the cambered airfoil, or wing, follows a familiar principle we covered in class. That is Bernoulli’s Principles.
Simply put, a gas will accelerate if it is forced to pass through a constriction. There must be a drop in pressure associated with this acceleration. Early aviation designers understood and applied this relationship to the wings of aircraft. Airflow travelling above a curved wing will accelerate and travel faster than the airflow beneath the wing. The lower pressure zone created above the wing, coupled with higher pressure beneath the wing, provides lift. Sir Isaac Newton stated that for every action there is an equal and opposite reaction. The application of this law is even more important to flight.
In aviation the force involved is the movement of air against the wings and control surfaces. When air is pushed downwards, Newton’s Law correctly predicts that the aircraft must move in the opposite direction – up. If we pay attention, we will notice that many aircraft have the wing mounted at a slightly upturned angle. This built in angle ensures air is constantly pushed down by the wing. This is called the “angle of attack”, and is extremely important to both lift generation and control of all aircraft. Raising the airplanes’ nose will increase the angle of attack and lift. Airplanes are controlled by the elevator, rudder and ailerons.
By using Newton’s Law and Bernoulli’s Principle the angle of these control surfaces is changed to redirect airflow. The angle of attack (Newton) and the change in pressure (Bernoulli) both act to direct the aircraft in the desired direction. Various types of flaps are used to increase lift for landings and takeoffs. These force more air downward and increase the pressure difference on the wing. The airfoil actually can be said follows all of Newton’s laws: Newton’s first law states, “Every body remains in a state of rest or uniform motion unless it is acted upon by an external unbalanced force. This is true because if the thrust equals drag there is no change in the horizontal motion. If lift equals weight there is no change in the vertical motion. If any of these is increased or decrease so that the formula is unbalanced, a change will occur. Newton’s second law “A body of mass (m) subject to a force (F) undergoes an acceleration (a). ” F=m ?a This law defines the amount of force, produced by lift, needed to overcome the effects of gravity. This lift is achieved in part by use of the Bernoulli principle, and also Newton’s Third Law of motion.
Newton’s 3rd law of motion states “For every action there is an equal and opposite reaction. ” It can be seen in the picture on the next page that as the angle of attack is increased the resultant force from the deflection of the air both above and below the wing is also a major component to lift. As the air is deflected downward, as in the top picture, it pushes on the wing in an equal and opposite direction. Newton’s Third Law in conjunction with Bernoulli’s principle can be used to explain the physics behind lift that allows an airplane to fly.
Lift combined with drag, weight, and thrust provides the 4 forces which need to be controlled to allow an airplane to maintain flight. (source: Langley flying school website) Now that we have the basics of how airplanes fly understood, it time to move onto more modern airplanes, and the huge turbofans that provides thousands of pounds of thrust to be able to lift almost 300 thousands of tons of weight. Similarly like propelled aircrafts, turbofans apply the same idea as a propeller because essentially they consist of a propeller inside what we call its engines.
The huge fan sucks in air; the air that is sucked in goes out the back. The reaction to this is a forward movement which is what causes the plane to move forward. On the flipside, turbofans provide much more pounds of thrust than old propelled planes do. The Boeing 747 consists of 4 engines on its wings. Only one of its engines produces 60,000 pounds of thrust, producing a total of 240,000 pounds of thrust with all its 4 engines combined (source: Sanders). Turbofans are also the most efficient engines having speeds from about 310 to 620 mph. This is the speed at which most commercial aircrafts operate.
Of all the turbofans, the General Electric Company manufactured the most powerful engine on a commercial airline yet. The GE90 only found on the Boeing 777, produces 75,000 – 115,000 pounds of thrust. The GE90 series are amongst the most powerful engines ever built for commercial airlines. To be specific, the GE90-115B is the most powerful, being capable of providing 115,000 pounds of thrust. This is an absolutely enormous amount of thrust coming from only one engine! In a flight test done on a Boeing 747, one GE90-115B was attached on the pylon of a 747 wing, and three of the Boeing 747 normal engines remained on the aircraft.
After takeoff, the GE90 engine was able to keep this 747 airborne by itself, while all three smaller engines were deliberately shut down in flight. They found out that the GE90-115B was twice as powerful as the regular 747 engines. Photo of the test flight aircraft is below. In this photo we can see how the GE90-115B engine is roughly twice the size of the regular 747 engine on this Boeing. Understanding the physics behind on how airplanes fly was really interesting, and it was nice seeing how some of the concepts/principles we covered in class comes back and relates to real-life examples.