Basic Aerodynamics: Airplane and Airline FAQ

THRUST

Quite simply, engines produce thrust, which provides the energy to go forward. A car engine, for example, creates forward momentum by producing thrust and transferring it through an axle to wheels. The wheels are turned to provide forward movement. Unlike a car, a plane uses the thrust from its powerful engines to create what’s called a relative wind. The relative wind passes over and under the wings, creating lift.

RELATIVE WIND

If you are traveling 50 miles per hour in a car on a perfectly calm day in the desert, when there’s absolutely no wind blowing, and then stick your hand out the window, you are going to feel a wind coming from the front of the car to the back, and blowing at 50 mph. The car has generated a relative wind that’s moving at the same speed but in the opposite direction. It’s the same with an airplane: Fly at 200 knots and there is an equal and opposite relative wind.

LIFT

Lift is the upward force created by the wings to keep the plane in the air. Everyone has probably generated lift on numerous occasions. If you’ve ever stuck your arm out the window of a car and then moved your hand around to let the wind blow across it at different angles, you’ve generated lift. Tilt your hand upward slightly and it will want to rise up. Down slightly, and it will be pushed in that direction. Now cup your hand just a bit so it’s no longer flat. Feel the increase in force against your hand.

On an airplane, lift is generated in much the same way. A plane’s wing is designed so that the top has a greater curve than its relatively flat underside. This affects the way in which it passes through the air. Picture, for example, two molecules of air both getting to the front, or leading, edge of the wing at the same time. One molecule is going to go above the wing, the other below it.

The molecule traveling on top, because of the curved surface, has to cover a greater distance. Yet both molecules are going to meet at the trailing edge of the wing at the same time—a property of nature—which means they are moving at two different speeds. The one on top must travel faster. By traveling at different speeds, the molecules create a low pressure area on the top side and a high pressure area on the bottom side. The result—lift. You just learned the basics of Bernoulli’s Law and the effect of “Venturi” action. In simple terms, if you have forward movement, you have an equal and opposite relative wind; if you have relative wind, you have molecules passing over and under the wings. If you have molecules passing over and under the wings, you have lift. Since the wings are attached to the airplane, the whole thing is lifted. Interestingly, more lift is created by the low pressure on the top of the wing than the high pressure underneath.

WHAT AFFECTS LIFT?

Four factors come into play when talking about lift.

First is the angle of attack, which defines the angle between the relative wind and the wing. In a car, if you put your hand outside the window and hold it fairly level, there isn’t too much force on it. The more you tilt it upward, the greater the amount of force pushing your hand up also. By changing this angle of attack you can increase and de crease the amount of lift generated. It’s the same with an airplane’s wings. However, every wing also has a maximum angle of attack, above which the lift capability decreases rapidly. Using the car analogy again, if you held your hand blunt against the wind, you’d only feel a pushing force and no lift. Military aircraft have so much excess thrust that they can generate a relative wind even when climbing straight up. The limits for airliners are less.

Second is the wing area, which is nothing more than the size of the wings. Most people assume that a tiny Cessna flies better than a large 747. But the important factor isn’t the plane’s weight. Rather, it’s how much does each square foot of wing area have to lift.

Third is the air density. This is one of the reasons it requires a longer takeoff run on hot, humid days at high-elevation airports. The air is less dense, and as the air density decreases, the lift also decreases.

Fourth is the plane’s speed. Returning again to the car analogy, if you put your hand out the window and increase the car’s speed, you will generate more force on your hand and more potential lift. It’s the same with an airplane. The higher the speed, the more lift generated. A little more physics—the amount of lift generated is the square of the velocity. Accelerating from 200 to 400 knots quadruples the lift.

WEIGHT

The simplest of the four basic forces acting on a plane, weight is the opposite of lift. If an airplane weighs 300,000 pounds on the ground, that’s also how much it totals in the air; 300,000 pounds of lift must be generated to overcome the weight or downward gravitational pull of the plane.

DRAG

Drag is the opposite of thrust. There are two types of drag: (1) parasite drag, which is the drag of the plane itself, and (2) induced drag, which is the by-product of lift and is proportional to airspeed—increasing your speed increases your lift, but also increases the induced drag. Induced drag plus the parasite drag equals the total drag that has to be overcome to get forward velocity.

HOW DOES AN AIRPLANE FLY?

A plane needs lift to get off the ground. To generate lift, it’s necessary to have forward velocity. Forward velocity will produce an equal and opposite relative wind, and the relative wind passing over and under the wings will, in turn, create lift.

Lift is generated when there is excess thrust. More thrust than drag is required in order to accelerate the airplane. If there is more thrust than drag, then there can be more lift than weight, and this makes it possible to climb.

When the plane is at cruise, it’s neither accelerating nor decelerating, climbing nor descending. At cruise, the amount of thrust is equal to the amount of drag, and the amount of lift is equal to the amount of weight. It’s in a steady state.

If less thrust than drag is generated, then the plane will slow down. When it slows, there is less air passing over and under the wings; the weight becomes greater than the lift. The result: the airplane will descend.

WHY THE PLANE WOULDN’T FALL IF THE ENGINES STOPPED

As long as there is relative wind moving over and under the wings, lift is generated. When the plane is either in level flight or climbing, the relative wind is maintained using engine thrust that provides the power to move us through the air straight and level, or uphill. But what if all the engines were to quit?

First, the aircraft’s weight hasn’t changed just because the engines quit. The plane still needs only to generate enough lift to overcome the weight. How is this done?

A car that losses its engines might try to coast downhill. A plane only has to lower its nose. If all the engines were to fail at once, the pilots would point the airplane downhill. Enough lift will be generated to keep the plane safely airborne until an engine is restarted. At 35,000 feet even a jumbo jet can glide for about 70 miles, simply by coasting. Remember, the glide ratio is a function of how much lift has to be generated by each square foot of wing, not the total size or weight.

In fact, on almost every flight there comes a point on descent when the pilots will pull the power back to idle and let the plane glide. Gliding is a normal part of almost every flight.

FLAPS AND SLATS

Every wing is designed to have a perfect speed— remember that speed affects lift—at which it’s most efficient. Air transport wings, for instance, are designed to fly at extremely high speeds, upward of 600 mph. But to achieve this type of efficiency, its low speed efficiency is sacrificed. This is where flaps and slats become important. If a commercial airliner didn’t have flaps and slats, it would have to accelerate to about 170 knots in order to generate enough lift to fly. With flaps and slats, minimum flying speed is nearer 100 knots, though greater takeoff and landing speeds are used for safety.

What flaps and slats do is increase the size of the wings about 20 percent. They increase the curvature, which also increases the lift, and utilizing the various combinations, it’s possible to increase the lift of the wing some 80 percent. Why not leave flaps and slats out all the time? Because they’d create drag, which would reduce cruising speeds and sacrifice efficiency. Most planes have a flap and slat structural speed limit of 250 to 280 knots, so they are used for takeoff and landing, allowing shorter takeoff rolls, slower approach speeds, and less required landing distance; less wear and tear on the landing gear, tires, and brakes; and better slow speed in-flight stability.

STALL

When a car engine quits, it’s said to have stalled. In terms of airplanes, a stall is totally unrelated. It has nothing at all to do with the engines, but the aerodynamics of the wing itself. A stall means very little or no lift is being generated. Not enough relative wind is available. Either the speed is too low or the maximum angle of attack has been exceeded. If the plane is on the ground, the wing is essentially stalled. There’s no lift.

If the flight crew attempted to climb too steeply while airborne, the wing could stall from an excessive angle of attack. It’s as if you were holding your hand out a car window, palms blunt to the air. You still have forward speed, but there’s not enough relative wind passing over and under the wing.

Airline pilots never, never, fly any slower than 30 percent above the minimum speed that the wing can fly, whether during takeoff, landing, or cruise. So there is a big margin for error. Let’s examine the worst-case scenario. If by some chance the plane did get too slow, a warning system would alert the pilots they were within 5 percent of minimum speed for this flap and slat configuration. If you continued to slow and actually stalled the wing, a stall recovery would involve lowering your angle of attack. Either lower the nose of the aircraft or add power—remember, there’s nothing wrong with the engines—or do both simultaneously. A full recovery can be made in seconds.

FLYING SLOW

Planes are designed so that when cruising near maximum airspeed, the aircraft will be flying level. Normally we cruise slightly slower than maximum speed for fuel economy. If the plane is flown slower, there’s less air passing over and under the wings, which means less lift. To compensate, the nose is raised. Normal economy cruise requires a nose-up pitch of 1 to 3 degrees. Slowing down further means another increase in pitch if we wish to keep from descending. Eventually we will slow to the point where flaps and slats are needed. Extending them increases lift so the degree of nose-up pitch required is less and the nose of the aircraft can be lowered. As we maneuver for our approach to land, we will slow further. Again the pitch is increased to compensate. It is common to see aircraft landing with their nose pitched up 5 to 7 degrees—this is also a design feature, so on landing, the main landing gear in the back touch down on the runway first.

FLIGHT CONTROLS

There are three basic flight controls: the elevator, the ailerons, and the rudder. The elevator, as the name implies, controls the pitch or the climbs and the descents. The ailerons control the angle of bank, which determines the rate of turn. The rudder controls the yaw, which keeps the tail of the fuselage following the nose. The basic flight controls share the same aerodynamic principles as the wing itself. All of the simple explanations about relative wind and lift still apply.

The elevator, located horizontally on the tail of the air craft, generates lift just like the wing. By moving the control column in the cockpit back and forward, the elevator moves up and down. When the tail climbs, the nose of the airplane must go down an equal and opposite direction, since they are securely bolted together. Pushing the control column forward makes the airplane descend. Climbing is exactly the opposite. Pull back on the control column and the tail loses lift and goes down. The loss of lift causes the tail to descend and the nose to rise.

A stabilizer just forward of the elevator is used to reduce the amount of work the elevator has to do. During a long climb, you would get tired if you had to maintain back pressure on the control column all the way to cruise altitude. With an actuator switch on the control column, the stabilizer can be moved up or down to infinite fixed positions. This is called trimming an airplane. Once the stabilizer is moved to the new position, the force required by the elevator is removed and the plane holds the new pitch attitude. The new pitch attitude will be maintained until another change is needed.

The ailerons are mounted on the trailing edge of the wing near the tips. If there’s also a second set, they will be located about halfway between the wing root and the first set. An airplane is banked when it’s turned, it leans left or right. Bicycles and boats are similar; they both lean when they turn. How steeply does an airplane bank? Pilots limit the bank angles to 25 to 30 degrees, even though the planes are capable of more than double that amount. In fact, in recurrent training, pilots regularly practice steep turns to keep those skills sharp.

The mechanics of a turn are simple. When the control column is turned, one set of ailerons on one side goes up, while the set on the other side goes down. The wings then develop a slightly different amount of lift. The side developing more goes up, the other side goes down. Moving the control column back the other way rolls the plane out of the bank. The speed brake panels located on the trailing edge of the top portion of the wing are used to augment the bank capability. When turning left, the speed brakes on the left side raise up slightly and automatically assist the ailerons. This is the only time the left and right speed brakes can be used independently.

The rudder is located on the vertical portion of the tail, and is controlled by foot pedals. Both pilots have a set of rudder pedals. The rudder controls the yaw of the airplane, which is the level left and right movement of the nose. The rudder is used only slightly in a turn—to offset the asymmetric drag on the wing created by a bank, not to actually turn the plane.

A fundamental reason for the rudder is to keep the airplane flying straight were an engine failure to occur. If the right engine failed and the left engine was flying at full power, there would be a tendency for the airplane to want to turn (yaw) right because of the asymmetric thrust. Stepping on the left rudder pedal would stop this motion. The rudder also enables landings in a crosswind. Let’s say there was a strong wind from the right side during the landing. If the plane lined up straight for the runway, the wind would have a tendency to push the plane left, or “downstream,” away from the runway centerline. Banking the airplane slightly into the wind will stop this drift, just like when crossing a flowing river you aim upstream to end up on the opposite bank. But by banking into the wind, the plane’s nose will not be straight down the runway. Using the opposite rudder, the nose can be accurately aligned.

INHERENT STABILITY

When looking at an airplane from the terminal building, you can see that the wings are higher at the tip than the root. This upward angle of the wings is called dihedral. Dihedral gives an airplane a great deal of inherent stability. Lift is always generated exactly perpendicular to the wing surface. If the two wing surfaces are pointed slightly toward each other, the lift vectors (lines) will cross somewhere above the fuselage. The resulting stability is similar to the stability in your car. If you turn the steering wheel in your car and let go, the tendency is for the steering wheel to return to center, and the car will eventually straighten out. A plane has the same built-in stability.

G FORCES

There are occasional times in an airplane when you feel heavier than you really are. The normal g (gravitational force) is greater and presses you into your seat a little harder. This force is felt the most during a turn or a quick level-off or bumpy turbulence. For example, in a 30-degree bank turn, the g force is 1.15, 15 percent greater than the normal 1 g felt on earth. Pilot techniques can eliminate some of the g forces felt in flight, but not all of them.

The basic aerodynamic principles of flight are straight forward. No matter how advanced and complex an airliner becomes, the four basic forces—lift to overcome weight and thrust to overcome drag—remain unchanged.

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