It is easy to take the physics of flight for granted, as well as the ways in which we exploit them to achieve flight. We often glimpse a plane in the sky with no greater understanding of the principles involved than a caveman.
How do these heavy machines take to the air? To answer that question, we have to enter the world of fluid mechanics.
Physicists classify both liquids and gases as fluids, based on how they flow. Even though air, water and pancake syrup may seem like very different substances, they all conform to the same set of mathematical relationships. In fact, basic aerodynamic tests are sometimes performed underwater. To put it simply, a salmon essentially flies through the sea, and a pelican swims through the air.
The core of the matter is this: Even a clear sky isn't empty. Our atmosphere is a massive fluid layer, and the right application of physics makes it possible for humans to traverse it. We'll walk through the basic principles of aviation and the various forces at work in any given flight.
First, let's examine thrust and drag. Thrust, whether caused by a propeller or a jet engine, is the aerodynamic force that pushes or pulls the airplane forward through space. The opposing aerodynamic force is drag, or the friction that resists the motion of an object moving through a fluid (or immobile in a moving fluid, as occurs when you fly a kite).
If you stick your hand out of a car window while moving, you'll experience a very simple demonstration of drag at work. The amount of drag that your hand creates depends on a few factors, such as the size of your hand, the speed of the car and the density of the air. If you were to slow down, you would notice that the drag on your hand would decrease.
We see another example of drag reduction when we watch downhill skiers in the Olympics. Whenever they get the chance, they'll squeeze down into a tight crouch. By making themselves "smaller," they decrease the drag they create, which allows them to zip faster down the hill.
A passenger jet always retracts its landing gear after takeoff for a similar reason: to reduce drag. Just like the downhill skier, the pilot wants to make the aircraft as small as possible. The amount of drag produced by the landing gear of a jet is so great that, at cruising speeds, the gear would be ripped right off the plane.
For flight to take place, thrust must be equal to or greater than the drag. If, for any reason, the amount of drag becomes larger than the amount of thrust, the plane will slow down. If the thrust is increased so that it's greater than the drag, the plane will speed up.
Weight's opposing force is lift, which holds an airplane in the air. This feat is accomplished through the use of a wing, also known as an airfoil. Like drag, lift can exist only in the presence of a moving fluid. It doesn't matter if the object is stationary and the fluid is moving (as with a kite on a windy day), or if the fluid is still and the object is moving through it (as with a soaring jet on a windless day). What really matters is the relative difference in speeds between the object and the fluid.
As for the actual mechanics of lift, the force occurs when a moving fluid is deflected by a solid object. The wing splits the airflow in two directions: up and over the wing and down along the underside of the wing.
The wing is shaped and tilted so that the air moving over it travels faster than the air moving underneath. When moving air flows over an object and encounters an obstacle (such as a bump or a sudden increase in wing angle), its path narrows and the flow speeds up as all the molecules rush though. Once past the obstacle, the path widens and the flow slows down again. If you've ever pinched a water hose, you've observed this very principle in action. By pinching the hose, you narrow the path of the fluid flow, which speeds up the molecules. Remove the pressure and the water flow returns to its previous state.
As air speeds up, its pressure drops. So the faster-moving air moving over the wing exerts less pressure on it than the slower air moving underneath the wing. The result is an upward push of lift. In the field of fluid dynamics, this is known as Bernoulli's principle.
First, let's consider the angle of attack, the angle that a wing (or airfoil) presents to oncoming air. The greater the angle of attack, the greater the lift. The smaller the angle, the less lift. Interestingly enough, it's actually easier for an airplane to climb than it is to travel at a fixed altitude. A typical wing has to present a negative angle of attack (slanted forward) in order to achieve zero lift. This wing positioning also generates more drag, which requires greater thrust.
In general, the wings on most planes are designed to provide an appropriate amount of lift (along with minimal drag) while the plane is operating in its cruising mode. However, when these airplanes are taking off or landing, their speeds can be reduced to less than 200 miles per hour (322 kilometers per hour). This dramatic change in the wing's working conditions means that a different airfoil shape would probably better serve the aircraft. Airfoil shapes vary depending on the aircraft, but pilots further alter the shape of the airfoil in real time via flaps and slats.
During takeoff and landing, the flaps (on the back of the wing) extend downward from the trailing edge of the wings. This effectively alters the shape of the wing, allowing it to divert more air, and thus create more lift. The alteration also increases drag, which helps a landing airplane slow down (but necessitates more thrust during takeoff).
Slats perform the same function as flaps (that is, they temporarily alter the shape of the wing to increase lift), but they're attached to the front of the wing instead of the rear. Pilots also deploy them on takeoff and landing.
Pilots have to do more than guide a plane through takeoff and landing though. They have to steer it through the skies, and airfoils and their flaps can help with that, too.
THE LIFT COEFFICIENT
In determining the lift of a given airfoil, engineers refer to its lift coefficient. This number depends on air speed, air density, wing area and angle of attack.
The tail of the airplane has two types of small wings, called the horizontal and vertical stabilizers. A pilot uses these surfaces to control the direction of the plane. Both types of stabilizer are symmetrical airfoils, and both have large flaps to alter airflow.
On the horizontal tail wing, these flaps are called elevators as they enable the plane to go up and down through the air. The flaps change the horizontal stabilizer's angle of attack, and the resulting lift either raises the rear of the aircraft (pointing the nose down) or lowers it (pointing the nose skyward).
Meanwhile, the vertical tail wing features a flap known as a rudder. Just like its nautical counterpart on a boat, this key part enables the plane to turn left or right and works along the same principle.
Finally, we come to the ailerons, horizontal flaps located near the end of an airplane's wings. These flaps allow one wing to generate more lift than the other, resulting in a rolling motion that allows the plane to bank left or right. Ailerons usually work in opposition. As the right aileron deflects upward, the left deflects downward, and vice versa. Some larger aircraft, such as airliners, also achieve this maneuver via deployable plates called spoilers that raise up from the top center of the wing.
By manipulating these varied wing flaps, a pilot maneuvers the aircraft through the sky. They represent the basics behind everything from a new pilot's first flight to high-speed dogfights and supersonic, hemisphere-spanning jaunts.
As we explored on the last two pages, flaps and slats enable a pilot to
move an aircraft through three-dimensional space. In other words, the
pilot alters the plane's orientation around its own center of gravity,
producing torque. Imagine this center of gravity as a fixed point in the
middle of the fuselage. Next, imagine an invisible horizontal line that
travels straight through the plane's nose, center of gravity and tail.
We call this the roll axis.
By adjusting the plane's ailerons (or spoiler) a pilot can cause the lift to increase in one wing and decrease in the other. One wing rises, the other descends. This causes the body of the plane to rotate along its roll axis, which results in a maneuver known as a roll. When a plane makes a complete rotation of its roll axis, the maneuver is called a barrel roll. However, when a pilot merely rolls enough to tilt the angle of the airfoil, the aircraft banks or turns.
Now imagine an invisible vertical line intersecting the center of gravity, shooting down through the top of the aircraft and out through the belly. This is called the yaw axis, and it comes into play when a pilot manipulates the aircraft's rudder. The rudder's deflection results in a side force, rotating the tail in one direction and the nose in the other. This is called a yaw motion, which helps the pilot to maintain course.
Finally, imagine an invisible horizontal line moving through the sides of the aircraft's center of gravity, roughly parallel to the wings. This is the pitch axis, which necessitates the pitch motion due to changes in the airplane's elevator. When the tail tilts down, the nose rises and the plane ascends -- and vice versa. Some aircraft can actually perform complete loops in this manner.
As we covered earlier, an aircraft's flight is a careful balance of
thrust, drag, weight and lift. Should lift decrease and drag increase
suddenly, such as when an aircraft's angle of attack surpasses that for
maximum lift, a stall occurs. The airframe shakes and the plane falls,
at least for a few feet. In most cases the pilot merely corrects for the
stall by lowering the plane's angle of attack. However, an improperly
corrected stall can result in a secondary stall, or degrade into a spin.
If you've ever attended an air show, you've probably witnessed stunt pilots intentionally entering into spins as part of an aerial acrobatics show. Typically, you'll see the prop-driven plane soar upward in a steep ascension, only to stall out and fall into a dramatic spin. The principles of an accidental spin are much the same.
A spin has three basic phases. The initial phase is called an incipient spin, in which the dropping aircraft starts to enter the spin. This phase lasts only a few seconds in light aircraft. If uncorrected, an incipient spin degrades into a fully developed spin composed of a near-vertical helical flight path -- as if the plane is descending an invisible spiral stair. Such a spin can cost an aircraft hundreds of feet with every turn.
In a flat spin, the pitch and roll axes remain steady, with the spin occurring around the plane's center of gravity. In other words, the plane is mostly level as it falls in an extremely dangerous spin.
Spin recovery techniques vary depending on the design of a given aircraft and where its center of gravity is situated. Generally speaking, a plane with its center of gravity more toward the nose is less likely to enter a spin than one with the center of gravity located closer to its tail. As such, some aircraft have specific spin recovery procedures, but the idea is to disrupt spin equilibrium and force the craft to stall and from there correct back into controlled flight. Most pilots aren't looking to take their passengers for a spin though. They're too busy manning the flight instruments we'll talk about next.
Airspeed indicator: Essentially, this gauge tells the pilot how fast the aircraft is traveling in relation to the ground. The indicator depends on a differential pressure gauge, not unlike a tire gauge.
Altimeter: As the name implies an altimeter measures altitude. The indicator in this case is a barometer, which measures air pressure.
Attitude indicator: Remember the three primary principle axes we mentioned before (pitch, yaw and roll)? Well, an attitude indicator illustrates the aircraft's orientation along all three. By use of a gyroscope, the indicator provides spatial clarity even in disorienting flight conditions.
Heading indicator: The heading indicator simply tells the pilot in which direction the plane is heading. The device depends on both a gyroscope and a magnetic compass, however, as both are susceptible to different errors during flight.
Turn coordinator: A typical turn coordinator indicates the plane's yaw or roll rate while also indicating the rate of coordination between the plane's bank angle and the rate of yaw. This device depends on a gyroscope, as well as an inclinometer ball in a glass cylinder to indicate when the aircraft is skidding or slipping.
Variometer: Also known as a vertical speed indicator, this device indicates the rate of a plane's rate of climb or descent. Working along similar lines as the altimeter, the variometer depends on atmospheric pressure readings to determine how swiftly altitude changes are occurring.
The total number of flight instruments has increased over the years with the speed, altitude, range and overall sophistication of the aircraft.
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