Physics

How Airplanes Fly



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"How Airplanes Fly"
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It is a testament of engineering that a machine weighing hundreds of thousands of pounds can fly just as gracefully as a single engined Cessna. Such an accomplishment is the result of countless discoveries going right back to Sir Isaac Newton and Mr. Bernoulli. These two scientists unknowingly described the laws and principles that would make this feat possible hundreds of years into the future.

Several fundamental problems had to be conquered before an airplane could fly, painfully and slowly. The tug of war between lift and gravity was solved, at least in theory, by Newton and Bernoulli. The relationship between thrust and drag was more difficult. Even after a modest internal combustion engine appeared, an efficient way to convert this power into forward thrust had to be developed. The Wright brothers deserve much credit for their work on a modern propeller. Still, flight was impossible with no means of controlling such a machine. Again, the Wright brothers came through with a functional flight control system.

Lift vs. Gravity:

To make a heavy machine fly required a way to generate enough lift to overcome the force of gravity. An effective airfoil (wing) was crafted with Bernoulli's Principles in mind. Shaping the wing's upper surface with a cambered curve caused the airflow to accelerate faster over the top than below the wing. A low pressure zone occurs above the wing, generating lift. A lesser known fact is that a high pressure zone beneath the wing provides an upward push. According to my FAA Handbook this contributes more lift than the low pressure zone does. Even so, this does not provide enough lift to get an aircraft off the ground.

By tilting the wing at an angle, with the leading edge higher than the trailing edge, Newton's Laws were even more useful. His Laws stated that for every action there must be an equal and opposite reaction. This explains why a wing is forced upwards at the same time that it pushes air downwards. Lift augmentation devices, called flaps, were developed to get even more lift in this manner. Sigificantly more air can be forced down in order to perform slower landings and takeoffs on shorter runways.

Thrust vs. Drag:

A great deal of power must be produced and transferred into forward motion in order to achieve lift. Piston engines and jet engines are the two main sources of power. Both engines can utilize the propeller to convert horsepower or thrust into forward motion. The propeller/gas turbine combination is known as a turboprop and is widely used in very small single engine airplanes up to very large military transports. The efficiency of the propeller, when combined with the power and reliability of the gas turbine, ensure the turboprop will be around for a very long time. If tens of thousands of pounds of thrust are needed, a pure jet engine is ideal. They are incredibly reliable, although not terribly fuel efficient. With no propeller, a combination of turbine power and kinetic energy from the exit of very hot gasses provides the thrust.

Earth's atmosphere acts like a liquid to a fast moving airplane. Engineers go to extreme lengths to reduce drag as much as possible, especially on fast aircraft. Even reducing "parasitic" drag caused by mushroom rivets can make a substantial improvement in performance and efficiency. Flying at a high altitude to take advantage of the thinner atmosphere will increase speed and fuel efficiency.

Control:

Three primary control surfaces enable an airplane to maneuver in three dimensions. Imagine a line running through the fuselage from nose to tail. The ailerons, mounted on the outer trailing edges of the wings, control this "longitudinal" axis. When a pilot moves the control stick, the lift on one wing increases while the other decreases. This assymetrical lift causes the aircraft to roll left or right.

A vertical line running through the middle of the fuselage from top to bottom is the "vertical" axis. Movement of the rudder pedals will move the rudder (on the vertical fin of the tail) to deflect air and push the tail in the opposite direction, causing a change in "yaw". Combining the rudder and ailerons will achieve a nice roll and bank maneuver. Using just the rudder will unduce a type of skid which is ideal for bleeding off excess speed for landing.

A horizontal line running through the middle of the fuselage from left to right is the "lateral" axis. Moving the control stick forward or backward will move the elevator (at the rear of the tail) to deflect air and push the tail up or down.

I enjoy working on aircraft and understand how and why they can fly. Still, no matter how many times I watch the "heavies" accelerate down the runway, it almost doesn't seem possible for them to just rise into the air and climb out of sight. Such an amazing invention they are.

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