Physics

How Airplanes Fly



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"How Airplanes Fly"
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Airplanes fly because of a proportional combination of thrust (or power) and lift. A huge power source requires less lift, as we would find in some of the high performance jet fighters while the reverse may apply for light aircraft where the wings are designed for high lift and a single engine-driven propeller enables a relatively low flight speed. From the early days of experimental flight these two key components eluded the best minds and technology was such that engine power was insufficient to produce results.

The Wright Brothers flier was a cumbersome bi-wing with primitive controls that succeeded in achieving the first powered heavier-than-air controlled flight on December 17 1903 where the longest flight lasted 59 seconds over a distance of 850 feet. The series of Wright experiments proved and began the continuing process of refining and improving lift, propulsion and of course the most important element control!

The Wright Brothers used a 12 hp gasoline engine to propel their 605lb machine capable of carrying one pilot laying prone and facing forward. Elevator control was by way of a stick operated by the pilot's left hand while rudder (directional) control was affected by the shifting of hips from side to side. Primitive aileron control to maintain a wings-level attitude was achieved using an ingenious system of wires that literally warped the trailing wing edges in opposite directions. Clearly the Wright Brothers understood the fundamentals of control in a three dimensional environment.

As technology improved, engine power increased and more experiments all over the world began displaying sleek new aircraft the principles of flight began to take on a more scientific approach. It was well understood that thrust was achieved using a propeller where the more power delivered produced higher performance however without commensurate refinements in control surfaces and wing design many early pilots lost their lives.

As the principles were learned wing shape became one of the critical design issues enabling more lift for less thrust. The basic principle of how a wing produces lift is taught in high-school physics where the famous teardrop profile with a flat underside and curved topside is almost a universal construct. As thrust pushes the leading (fat) edge faster and faster air movement over the top is faster than the underside, which creates low pressure. The underside then generates high pressure because the air moves over less distance and this effect literally pushes the underside of the wing thus lift is generated.

With a combination of thrust and lift an aircraft will ascend however early experiments sometimes ended in disaster because the third key element of control was yet to be refined. To make the aircraft rise at a controlled rate movable flaps on the tail changed the pressures and the effect as the aircraft moves through air is to push the tail up or down and effect control over the first of three physical dimensions pitch.

This worked for very short experimental hops in a straight line yet for practical flight direction was the next important element to master. A vertical movable flap pushed the tail in a left or right direction and the second dimension of control enabled airplanes to turn in the air yaw.

The third control required significantly more effort before the physics was understood. Wing roll was the anathema of early flying machines where loss of control caused by wind gusts rolled airplanes over and they speared unceremoniously into the ground. This was eventually overcome by use of ailerons, or movable flaps on the wing tips that move in opposite directions and the dimension of roll was controlled.

The most advanced aircraft of the 21st century still require the fundamentals of thrust Vs lift, and control over the three dimensions of pitch, yaw and roll where the same basic control flaps on wings, tail and rudder remain unchanged. With phenomenal speeds and high performance numerous additional controls added to modern aircraft are computer controlled because human responses and reaction are now far too slow. In the 100 or so years since the first 850 feet of flight today's aircraft now carry more than 600 passengers at near supersonic speeds, fighter and high performance aircraft have achieved velocities more than three times the speed of sound and who knows what the next 100 years of flight will produce.

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