Physics

The Physics of Airplane Flight



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Did you know Boeing wasn't one hundred percent sure the 747 could fly until it actually did? It was faith in physics that reassured the engineers (and bankers) that such a massive undertaking was worth the risk. Physics describes four basic elements involved with flying an airplane. Flight involves a constant tug of war between lift vs. gravity, and thrust vs. drag.

Lift:

The physics describing lift was established hundreds of years before such a machine would fly. Sir Isaac Newton and Mr. Bernoulli unknowingly played key roles in aviation. The design and function of the cambered airfoil, or wing, follows 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 pressure beneath the wing, provides lift. According to my FAA Handbook, the air pressure pushing the wing upwards from beneath exceeds the vacuum force lifting the top surface.

Sir Isaac Newton stated that for every action there is an equal and opposite reaction. 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. You 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 "angle of attack" is extremely important to both lift generation and control of all aircraft. Raising the airplanes's nose will increase the angle of attack and lift.

As always, there can be too much of a good thing. If the angle is too steep Bernoulli's low pressure zone will move too far back on the wings upper surface. This is known as the "burble" point. Exceeding this point creates a "stall" and is of deep concern to pilots (and passengers). The physics that caused the aircraft to fly no longer applies if the wing stalls completely.

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.

Physics describes the performance of propellers and helicopter rotors in precisely the same manner.

Gravity:

The force of gravity on the Earth is measured to be 9.8 Newtons/kg. An aircraft must overcome this force to get airborne. To be of any use it must also lift passengers, fuel and cargo. Weight reduction has always been paramount in aviation and will remain so unless anti-gravity technology is developed. Boeing is seriously exploring the possibility of using spinning super conductors to reduce the effects of gravity on aircraft.

Thrust:

The physics of lift is quite useless without thrust. A combination of piston engines and propellers took aviation from it's humble beginnings to machines with thousands of horsepower. The development of jet engines allowed enormous increases in weight and speed. Jet engines (gas turbines) can deliver tens of thousands of pounds of thrust but have voracious appetites for fuel. A wise combination of gas turbine and conventional propeller delivers large thrust in a more economical (albeit noisy) manner. South West Airlines will never equip an airplane with a rocket engine. But rockets can produce millions of pounds of thrust.

Drag:

The negative effect of drag increases dramatically with an aircraft's speed. Drag was of little concern to the Wright Flyer. As faster aircraft were developed more effort to reduce drag was needed. Abandoning the top wing and bracing wire was a big leap ahead. Howard Hughes made a substantial contribution with retractable landing gear and flush rivets. Our thick atmosphere acts much like a liquid when an aircraft speeds through it. Flying at high altitude takes advantage of the thinner atmosphere for increased performance and fuel economy. A limiting factor to supersonic speed is the high temperatures caused by atmospheric drag. An aircraft can easily suffer structural failure if temperature issues are ignored.

Without the understanding of physics the world of aviation could hardly exist. Any successes would be achieved by luck and instinct. A gamble of the magnitude of the Boeing 747 or the Airbus A380 would never be made. By understanding how physics affects the world around us, a sight as unlikely as a 747 rising off the runway can be explained without any mention of magic.

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