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The Four Forces of Flight
Inventors and scientists struggled for centuries to understand the basic principles of flight, and experts still debate the details of aerodynamics. Pilots need to understand a few fundamental concepts, starting with the four forces that affect flight: lift, weight, thrust, and drag.
These four forces act in pairs. Lift (the sum of all upward forces) opposes weight (the sum of all downward forces). Similarly, thrust (forward pulling force) opposes drag (rearward pulling force). The opposing forces balance one another in steady-state flight. Steady-state flight includes straight-and-level flight and constant-rate climbs or descents at steady airspeeds. You can assume that the four forces act through a single point called the center of gravity (CG).
Lift
Lift is the force that makes an airplane fly. Most of an airplane's lift comes from its wings. You control the amount of lift a wing creates by adjusting airspeed and angle of attack (AOA)—the angle at which the wing meets the oncoming air. In general, as an aircraft's airspeed or angle of attack increases, so does the amount of lift created by the wings. As an airplane's speed increases, you must reduce the angle of attack—lower the nose slightly—to maintain a constant altitude. As the airplane slows down, you must increase the angle of attack—raise the nose slightly—to generate more lift and maintain altitude.
Remember that even in a climb or descent, lift essentially equals weight. An aircraft's rate of climb or descent is primarily related to the amount of thrust generated by its engines, not by the amount of lift created by its wings.
Weight
Weight opposes lift. As a practical matter, you can assume that weight always acts along a line from the airplane's center of gravity to the center of the earth.
At first you might assume that weight changes only as fuel is consumed. In fact, as an airplane maneuvers, it experiences variations in load factor, also known as G forces, which change the load supported by the wings. For example, an airplane making a level turn in a 60-degree bank experiences a load factor of 2. If that airplane weighs 2,000 pounds (907 kg) at rest on the ground, its effective weight becomes 4,000 pounds (1,814 kg) during that turn.
To maintain the balance between lift and weight during maneuvers, you must adjust the angle of attack. During a steeply banked turn, for example, you must raise the nose slightly (increase the angle of attack) to produce more lift and thus balance the increased weight.
Thrust
Thrust provided by an aircraft's power plant propels it through the air. Thrust is opposed by drag, and in steady-state flight, thrust and drag are equal. If you increase thrust and maintain altitude, thrust momentarily exceeds drag, and the airplane accelerates. Drag increases, too, however, and soon drag once again balances thrust. The airplane stops accelerating and resumes steady-state flight at a higher, but constant airspeed.
Thrust is also the most important factor in determining your airplane's ability to climb. In fact, an airplane's maximum rate of climb is related not to the amount of lift its wings create, but to the amount of power available beyond that required to maintain level flight.
Drag
Two kinds of drag affect an airplane. Parasite drag is friction between the air and an aircraft's structure—landing gear, struts, antennas, and so forth. Parasite drag increases as the square of an aircraft's velocity. If you double airspeed, parasite drag quadruples.
Induced drag is a byproduct of lift. It is caused by air moving from the high-pressure area below a wing into the low-pressure area above the wing. This effect is most pronounced at slow airspeeds where a high angle of attack is necessary to produce enough lift to balance weight. In fact, induced drag varies inversely as the square of the airspeed. If you reduce airspeed by half, induced drag increases four times.
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Beyond the Basics: Learn from a Pro Flight Simulator's Lessons with Rod Machado covers aerodynamics and flight maneuvers in more depth. To take flying lessons with Rod, click the Lessons tab in the Learning Center. On the Lessons home page, click the name of lesson you wish to take. Begin by reading the ground school lesson, then click Fly this Lesson Now! in the upper-right corner of the Lessons pages. You can also click Fly this Lesson at the end of each ground school lesson, and Rod will take you up for a practice flight that covers what you've just learned. |
A Balancing Act
You can observe the relationship between the four forces by trying some experiments in Flight Simulator. Set up straight-and-level flight in the Cessna Skyhawk SP Model 172. Without moving the flight controls, add power. At first, airspeed increases, then the nose pitches up. Soon, however, the airplane stops accelerating and the airspeed returns to about its original value. Notice, however, that because you've added power, the airplane climbs at a steady rate. Reduce the power below the original setting, and the airspeed eventually settles near the original value, but the airplane descends at a steady rate.
The Axes of Flight
Aircraft rotate around three axes: the longitudinal axis, the vertical axis, and the lateral axis. In an airplane, movement about each axis is controlled by one of the three primary control surfaces.
Ailerons, Rudder, and Elevator
Pilots use ailerons to bank or roll about the longitudinal axis. Rudder controls yaw about the vertical axis, and the elevator controls pitch about the lateral axis. The three axes intersect at the center of gravity. Smooth, coordinated use of controls separates pilots from airplane drivers. Good pilots use all the flight controls together to produce coordinated motion about the three axes.
Straight-and-Level Flight
Flying straight and level may look simple, but it's actually one of the more difficult flight maneuvers to master. Because pilots want to control airplanes, they overdo it most of the time and interfere with the airplane's basic stability. Like a balancing act, straight-and-level flight requires that you make smooth, small corrections to keep the airplane from wobbling all over the sky.
Divide and Conquer
It's best to break down the task of establishing and maintaining straight-and-level flight into two parts: Holding a constant altitude and airspeed. This part requires that the pairs of opposing forces—lift and weight, thrust and drag—remain balanced.
Holding a Constant Heading
This part requires you to monitor the heading indicator and turn coordinator to hold the wings level, maintain coordinated flight, and correct minor deviations in heading.
Pitch + Power = Performance
Fortunately, there's a simple rule that can help you handle the first task.
The basic equation "Pitch plus power equals performance" is a pilot's golden rule. It means simply that if you establish a specific pitch attitude and set power at a constant level, the airplane will fly at a particular airspeed and either maintain level flight or climb or descend at a constant rate.
For example, to set up a typical cruise configuration at 3,000 feet (915 m) in the Skyhawk SP, set the throttle to deliver about 2,500 rpm. To maintain level flight, adjust the pitch attitude so that the miniature airplane on the attitude indicator is level with the horizon. The top of the instrument panel is below the real horizon when you look out the front window.
If you keep the nose from rising or falling and leave the power set at 2,500 RPM, the Skyhawk SP will maintain altitude and cruise at about 110 knots indicated airspeed.
If the airplane starts to gain or lose altitude, make small, smooth corrections to the pitch attitude and adjust the elevator trim so eventually the airplane flies "hands off."
Keeping It Straight
Maintaining a constant heading is a little easier than holding altitude, but you still need to keep a close eye on the flight instruments. Check the heading indicator frequently to make sure the nose stays pointed in the right direction.
Cross-check the turn coordinator: If the wings on its miniature airplane are level, the airplane isn't turning. If the wings aren't level, you need to apply smooth, slight pressure on the ailerons and rudder to level the wings and maintain coordinated flight.
Turns
An airplane turns because some of the lift that the wings produce pulls it "around the corner," not because the rudder swings the nose left or right. In theory, you could skid an airplane through a turn with the rudder, but that's an inefficient (and uncomfortable) way to change direction. That's why airplanes bank to turn.
The Horizontal Component of Lift
Banking the wings with the ailerons deflects sideways some of the lift that the wings produce. This part of the airplane's total lift is called the horizontal component of lift. It's this force that pushes an airplane around in a turn.
Adverse Yaw
Banking the wings changes the angle of attack of each wing. And the deflection of ailerons changes the drag of each wing. These two factors create a tendency for the airplane to yaw opposite the turn. That is, if you bank to the left, the airplane's nose tends to swing toward the right.
To compensate for this effect, called "adverse yaw," you must apply rudder pressure in the same direction as the turn. As you bank left, you should add a little left rudder, and vice-versa.
Loss of Lift
Because some of the lift is deflected sideways in a turn, to maintain altitude you must increase the total lift that the wings produce. To increase lift, you must increase the angle of attack, so add a little up-elevator pressure (by pulling back on the stick) as you roll into a turn. The steeper the turn, the more up-elevator pressure you must add. In steeply banked turns of 45 degrees or more, you must add considerable up-elevator pressure (and probably add power, as well) to maintain altitude. Just remember to relax that back pressure on the stick as you roll out of the turn.
Turn Coordinator
The turn coordinator is really two instruments. The gyro portion shows the aircraft's rate of turn—how fast it's changing direction. A ball in a tube called the "inclinometer" or "slip/skid indicator" shows the quality of the turn—whether the turn is "coordinated."
How It Works
The gyro in the turn coordinator is usually mounted at
a 30-degree angle. When the airplane turns, forces
cause the gyro to precess. The rate of precession makes
a miniature airplane on the face of the instrument bank
left or right. The faster the turn, the greater the
precession, and the steeper the bank of the miniature
airplane.
Standard Rate Turn
When the wings of the miniature airplane align with the
small lines next to the "L" and "R," the aircraft is
making a standard rate turn. For example, an aircraft
with a standard rate turn of three degrees per second
will complete a 360-degree turn in two minutes.
Balancing Act
The black ball in the slip/skid indicator stays between
the two vertical reference lines when the forces in a
turn are balanced and the airplane is in coordinated
flight. If the ball drops toward the inside of the
turn, the airplane is slipping. If the ball moves
toward the outside of the turn, the airplane is
skidding.
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To correct a skid
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To correct a slip
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The auto-coordination feature in Flight Simulator automatically moves the rudder to maintain coordinated flight.
Climbs
An airplane climbs when its engine or engines produce more power (thrust) than is required to maintain level flight at a particular weight and angle of attack. Airplanes do not climb because the wings generate more lift. This point may seem confusing, but it makes sense if you remember that whenever an airplane is in steady-state flight—for example, a climb at a constant airspeed and rate—lift equals weight. If lift exceeded weight during a climb, an airplane would accelerate upward.
A Steady Pull
During a steady-state climb, the component of lift acting vertically toward the ground is actually slightly less than weight, because when the airplane is in a climb attitude, some of the lift vector is directed rearward, not upward. So a climb is caused by the thrust vector pulling the airplane up at an angle. Imagine someone tugging a sled up a hill, and you'll get the general idea.
More Power
If power determines rate of climb, then it's apparent that the throttle, not the control yoke, is the primary "up-down" control in an airplane. Pulling back on the yoke to increase an airplane's pitch attitude usually does start a climb. But an increase in induced drag quickly counteracts the boost in lift, and the airplane, having gained a little altitude, settles into level flight at a lower airspeed or into a slow, constant-rate climb. To establish and maintain a steady rate of climb, excess thrust must be available, and you must add power.
Descents
Many people assume that to descend you simply push forward on the control yoke or stick to point the airplane's nose down. In fact, the pilot must adjust both pitch and power to establish a stable descent at a constant airspeed.
You can descend with the airplane in a level or even nose-up attitude. Remember that if you hold an airplane's pitch attitude constant, thrust—power—determines whether the airplane maintains altitude, climbs, or descends. If the engine produces more thrust than is required to maintain level flight, the airplane climbs. It descends if you reduce power.
As a rule of thumb, limit descents in unpressurized airplanes to about 500 feet per minute (152 m/min). This rate allows passenger's ears to adjust to pressure changes during the descent.
Spend some time with the airplanes in Flight Simulator to familiarize yourself with the performance that you can expect at different power settings and airspeeds. Remember, the lower the power, the greater the rate of descent. Practice stopping a descent by smoothly adding power.
How Wings Work
Wings—not engines—are what make an airplane fly. Although wings come in many shapes, they all produce lift by splitting the oncoming air, called the relative wind. Air flowing under the wing maintains its ambient pressure. Air flowing over the curved upper surface accelerates, and due to several factors, including Bernoulli's principle, drops in pressure. The difference between the relatively high pressure below a wing and the relatively low pressure above creates a force, called lift. Deflection of the air downward from the bottom of the surface of the wing also contributes to the total lift that a wing produces. Pilots change a wing's lift by using the elevator to adjust the airplane's pitch attitude, and thus the wing's angle of attack.
Flight Path vs. Pitch Attitude
It's important to remember that the relative wind does not necessarily come from the direction in which the airplane's nose is pointed. To put it another way, angle of attack is not measured relative to the horizon. It's the angle between an airplane's flight path and its wings.
Stalls
A stall occurs when a wing reaches its critical angle of attack. Regardless of load factor, airspeed, bank angle, or atmospheric conditions, a wing always stalls at the same critical angle of attack. Pilots control angle of attack with the elevator.
A stall is an aerodynamic phenomenon—it has nothing to do with an airplane's engine. Gliders, airliners, jet fighters, and prop-driven trainers all stall when their wings reach a specific angle of attack—not because their engines falter.
Anatomy of a Stall
Up to a point, increasing the angle of attack increases the amount of lift a wing produces. Eventually, however, air flowing over the top of the wing can no longer follow the wing's contour and it begins to swirl like water flowing over rocks in a stream. At this point, called the critical angle of attack, total lift drops suddenly, and the wing stalls.
Every wing has a specific critical angle of attack, and it always stalls at that angle. Most general aviation aircraft have wings with a critical angle of attack of about 15 degrees. Inexperienced pilots often mistake pitch attitude for angle of attack. Remember that the airplane's flight path (and therefore the relative wind) may not be in the direction that the nose of the airplane is pointing.
Warning Signs
A slight shaking or buffeting often precedes a stall. This vibration begins as the air flowing over the top of the wing becomes turbulent. When this air hits the horizontal stabilizer and elevator you may feel a slight vibration in the stick. Most airplanes have a stall warning horn to alert you as the airplane approaches a stall.
Recovering from a Stall
There is only one way to recover from a stall—reduce the angle of attack. Apply forward pressure on the stick to reduce the angle of attack, and add power to minimize loss of altitude.
Center of Gravity
The center of gravity (CG) is the point at which an airplane would hang in perfect balance if it were suspended by a cable. The CG is also the point at which the longitudinal, vertical, and lateral axes intersect and the point at which the four fundamental forces of flight—lift, weight, thrust, and drag—are assumed to act. . To ensure that an airplane is stable in flight and responds properly to control inputs, you must load your airplane carefully to keep the CG within its design range.
The CG Seesaw
An empty airplane is like a seesaw: It balances on its center of gravity. Each item added to the airplane shifts the CG slightly. Objects placed forward of the original CG tend to tip the airplane forward. Objects placed behind the CG tend to tip it backward. The amount of tipping force, or "moment," depends on the weight of the object and its "arm"—the distance between the object and an arbitrary reference line called the datum. In many airplanes the datum is the firewall that separates the engine compartment from the cockpit.
Managing the CG
Pilots manage the CG by controlling how weight is distributed in the aircraft cabin. In most small airplanes, the fuel tanks and seats are located close to the optimum CG, so the CG doesn't move much as fuel, people, and luggage are added. Nevertheless, before every flight a pilot must ensure that the CG of the loaded airplane falls between the forward and aft limits specified by the manufacturer.
The CG and Stability
Keeping the CG within its design limits is critical because the position of the CG affects an airplane's stability, just as the position of a child on a seesaw changes the board's balance point.
As the CG moves aft (toward the tail), an airplane becomes less stable in pitch. If the CG is too far aft, it may be impossible to lower the nose to recover from a stall.
If the CG is too far forward, the airplane is "nose heavy," making it difficult or impossible to flare during the final phase of landing.
Landings
For most pilots, landing is the most challenging part of flying. The secret to a soft, smooth landing, odd as it sounds, is to try to keep the airplane from touching down too quickly. You can learn more about landings in the Lessons with Rod Machado. To take flying lessons with Rod, click the Lessons tab in the Learning Center.