Category Flying and Gliding

Ask an aerodynamicist what the most efficient wing is, and she’d say ”One that has no wingtips.“ Of course, every wing has to have a tip of some sort, since every wing has to end somewhere. So what does this curious statement mean? Let’s find out.

As we saw in Chapter 8, the curved, or ”cambered,“ upper surface of the wing creates an area of low pressure, or pressure that is at least lower than the air traveling along the bottom side of the wing. Physics tells us that the relatively high-pressure air under the wing has a natural tendency to migrate toward the area of low pressure above. For the most part, the wing’s structure gets in the way, but there’s one place on every wing where the high-pressure air below can see its way clear to the low- pressure air above: the wingtip.

Because the high-pressure air has an escape route at the wing tip, the air below the wing takes on a slightly ”spanwise flow,“ as designers would call it. In other words, instead of flowing straight from the leading edge toward the trailing edge, the highpressure air under the wing shuffles slightly toward the wingtip. And the closer to the wingtip the air moves, the more rapidly it moves.

At the very tip of the wing, the high-pressure air swirls upward, creating a little tornado of twisting air called a ”vortex.“ This vortex creates a force that pulls backward on the wing in the form of drag. To reduce this drag, aerodynamicists discovered that if air has less distance to travel over from the wing’s leading edge to its trailing edge, the spanwise flow can be reduced. So they shaped the wing as thin as possible, while lengthening them to keep the same total area.

With the spanwise flow reduced, the power of the wingtip vortex is weakened, and a weak vortex means less drag.

Plane Talk

Aircraft designers are smart folks. They figure, if the wingtip vortex is always going to be there anyway, why not make it do a little work? They answered that question by turning the very end of the wing upward to create a “winglet" A winglet looks like someone has bent a couple of feet of both wingtips straight up and down. This shape helps squeeze a little bit of upward lift from the wingtip vortex.

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For some, the beginning of summer is not signaled by the first pitch of a baseball team or by the beginning of school vacation. No, for those who love gliders and glider flying, summer truly begins the day they see the first glider circling overhead in the gentle grip of a sun – powered upward rush of air.

In most regions of the United States, glider flying is a sport that must be enjoyed during the few warm months when the sun’s warmth and steady but benign winds provide the energy gliders require in order to spend hours at a time in silent, birdlike flight.

The Glider and the Plane

Perhaps the best way to start looking into the world of gliding is by comparing the glider to the airplane.

When you look at a glider resting beside a conventional powered airplane, one of the first things you notice is how long and skinny the wings are. Compared to those of a glider, the broad wings of an airplane look like a pair of barn doors.

The glider also sits very low to the ground. In fact, it sits so low that you might not be certain it has any landing gear at all. But if you bend down, you can see the curve of the rubber tire peeping out of a little cave where it mostly hides out of the airstream. That’s your first sign that glider designers are obsessed with reducing air resistance. They never stop fussing about drag and how to reduce it.

Max Karst flies his ASW15B over the mountaintops of the Cascade Range east of Seattle.

(Vince Miller)

Sleek modern gliders are a far cry from the pioneering gliders that Otto Lilrenthal and the Wright brothers built in the very early days of flight (See Chapters 1 and 2 for more on these great aviation innovators.) Early experimental gliders were largely responsible for perfecting the shape of wings and the engineering techniques used to make aircraft strong and durable. That meant they were often unsophisticated and bulky. Lilienthal didn’t sur­vive the experimental phase of his research into flight, but the Wright brothers picked up his baton and refined his gliders into the first airplanes. All fixed-wing aviators trace their legacy to Lilienthal and his gliders.

You also notice how the glider is tipped over onto one side. It rests on one wingtip or the other because there’s only the one tire under the cockpit. There’s nothing like the conventional airplane’s two main landing gear plus a nose wheel to stabilize it on the ground.

What’s more, you see that the body and skin of the glider are amazingly ”clean“— that is, there are no parts of it that jut into the air. The skin, which is usually made of Fiberglas or some other very light material, is buffed and polished until it gleams. The light weight of the Fiberglas is a testament to the glider designers’ aversion to excess weight, which is almost as pathological as their fear of drag.

That careful attention to reducing drag and weight is due to the final difference between the glider and the conventional airplane: The glider has no engine. How is it possible, you ask, for a plane to fly without an engine? Read on!

Thin Is In

Because gliders don’t carry their own source of power, they are designed to waste as little energy as possible. One of the glider design’s hallmarks is its long, skinny wings, which have a far higher aspect ratio than the wings of most powered airplanes. This special shape helps reduce induced drag, the kind of drag that is created as a by-product of lift. (For a review of the forces of flight, including lift and drag, turn back to Chapters 7, ”How Airplanes Fly, Part 1: The Parts of a Plane,“ and 8, ”How Airplaines Fly, Part 2: The Aerodynamics of Flight.“)

Another thing a pilot learns early on is how to turn. Turning an airplane seems simple—you just bank the airplane while staying at the same altitude. But in truth, every turn requires a smooth coordination of the elevators, the ailerons, and the rudder.

To begin a turn, a pilot first looks for traffic, other airplanes, in the direction she is going to turn.

Once the pilot is sure there are no planes in the direction she’s going to turn, she makes a gentle bank to the right by moving the aileron controls. The movement of the control column is almost identical to the turning of a steering wheel in making a right turn in a car.

When the control column is turned toward the right, the aileron on the right wing deflects upward; at the same time the aileron on the left wing deflects downward. That’s because the ailerons are rigged to always move in opposite directions to each other.

The relative wind strikes the turned-up aileron on the right wing, pushing the wing slowly downward. By the same token, the rushing

over the left wing creates even more lift than usual because the down-turned aileron gives the wing an exaggerated curve. (You’ll recall that the curved upper surface of the wing is what gives it lift, and greater curve means greater lift.)

Now the rudders come into the picture. When the ailerons are being used, each wing experiences different amounts of drag. The wing that is swinging upward has more lift, but with extra lift comes extra drag.

While the left wing is rising and feeling more drag, the right wing is dropping. Because it’s losing lift, it is also feeling less drag. So even though the pilot wants to turn right, she’s noticing the drag of the left wing is pulling the nose slightly to the left.

The solution is a small foot pressure on the right rudder pedal. Even a small amount of rudder pressure will move the rudder surface enough to keep the nose turning right, counteracting the left-turning tendency created by the differences in aileron drag. Of course, the same principles apply to left turns, which require a slight pressure on the left rudder pedal.

The elevator controls must also be used during a turn—even if the pilot doesn’t want to climb or descend. Imagine the bank angle has been established at, say, 30 degrees. Because lift acts directly perpendicular to the wings, the lift force is acting at a 30- degree angle from straight up.

But weight always acts straight down. Let’s say the airplane weighs 2,500 pounds. No matter what the bank angle, the weight remains constant. But when the wings bank and the lift force is deflected, the lift is no longer opposite to the direction of weight. Now, with the lift pointed sideways, the weight exceeds lift. Unless the pilot does something, the plane will start a slow descent.

In order to restore the lost lift, the pilot pulls back gently on the control column, increasing the wings’ angle of attack and restoring lift.

And, finally, because the pilot is creating extra lift, she’s also creating more drag. That means she has to increase the throttle to provide the extra thrust to balance the increased drag.

Here we have one of the simplest of maneuvers, the basic turn. But it involves all the flight controls the pilot has at her disposal. The ailerons are deflected, the rudder is pressed, the elevators come into play, and the throttle balances the increase in drag.

With practice, the turn, like most of the maneuvers of flight, becomes second nature.

—Ф Centrifugal force

Load

Factor

A turning airplane creates centrifugal force that
makes the airplane behave as if it has gained weight.
(FAA Flight Training Handbook)

The Least You Need to Know

Lift combines simple "action-reaction’ force with the gentle but effective force of low-pressure air.

^ Drag takes a stem toll on an airplane, and engineers work hard to reduce it

► Thrust which opposes drag, is caused when an engine accelerates air using a propeller or a jet engine.

^ The airplane’s control surfaces are used together to create the basic maneuvers of flight

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The forces of flight come alive when a pilot or flight student puts one hand on the control, the other hand on the engine throttle control, and presses her feet to the rudder pedals. Once the airplane is in flight, at a safe altitude for some practice flying and in an area mostly free of other airplanes, she can experiment with lift, weight, thrust, and drag.

In Control

One of the things a student pilot learns during a first flight is the function of the control column. The control column moves forward and backward as well as left and right. The control column is directly related to lift. Remember, when lift exceeds weight, the airplane climbs, and when lift is less than weight, the plane descends.

There are two ways for the pilot to increase lift: by pulling the control column gently backward or adding engine power by pushing the throttle control forward. In practice, a pilot would probably do a little of both, but we’ll see what each does separately.

When the pilot pulls the control column backward, the elevator controls tilt slightly upward on the horizontal stabilizer. The relative wind strikes the up-tilted surface and produces a Newtonian reaction downward. When the tail is forced down, the nose is tilted upward. The wings also tilt up, increasing the angle of attack and creating lift.

Another way to add lift is to increase the thrust. When the pilot adds engine power by pushing the throttle forward, the airplane begins to accelerate.

When the relative wind speeds up, the pressure of the air flowing over the wings drops further, and Bernoulli’s lift takes over. The airplane climbs.

Experienced pilots know that if they carefully coordinate the change in both pitch and power, the airplane flies more smoothly. If a pilot pitches the airplane up, for example, without adding power, it will take just a few seconds for the plane to slow down and begin to descend again. It would be like starting up a hill in your car without continually pressing the accelerator, pretty soon you wouldn’t go any higher, and the steeper the hill, the faster you would lose speed.

Airplanes need thrust to provide the forward speed that the wings transform into lift.

Thrust is what we get when an engine takes in air and accelerates it. When the air gains velocity, it causes thrust. When thrust and drag are in balance, an airplane’s speed stays constant. When thrust is greater than drag, speed increases, and when thrust is less than drag, the plane slows down.

A propeller generates a thrust force by taking a relatively large amount of air and accelerating it by a small amount. A jet engine takes a relatively small amount of air and accelerates it a lot. Either way, the result is thrust.

Jet-Powered Thrust

Jet engines, or to call them by their full name, turbojet engines, rely on the principle that high-pressure air shot out of one end of an engine creates a force in the opposite direction.

In a jet engine, normal atmospheric air is allowed in at one end of the engine, is compressed and mixed with fuel, and then ignited. The explosion causes the burning mixture of fuel and air to expand and shoot out of an exhaust pipe. Whichever direction the exhaust shoots out, a force is created in the opposite direction that can be harnessed to accelerate an airplane—or a speed boat or anything else that you attach the engine to, for that matter.

Since jets were first invented in the 1930s, they’ve grown to be a lot more powerful, a lot more reliable, and a lot quieter. They now produce so much power thrust that engineers are building larger airplanes that carry more than 500 people in luxurious comfort.

In addition to parasite drag, which is caused by the physical structure of the airplane, airplanes must also overcome “induced drag,” which is an unavoidable byproduct of lift. A wing’s lift doesn’t actually produce lift that is directed straight up. In fact, the lift is directed slightly backward.

Lift is generated in a perpendicular direction to the “chord line,” which is an imaginary straight line connecting the wing’s leading edge to its trailing edge. In flight, the wing is inclined slightly upward in the front, meaning the chord line is inclined at an angle, too. The lift force, therefore, is tilted backward slightly.

To return to high school physics for a moment, a force that is acting at an angle can be mathematically divided into its vertical and horizontal components. Most of an airplane’s lift is directed upward, but it has a backward component as well. That component is the induced drag.

The higher the airspeed, the lower the amount of induced drag. The higher speed increases the wing’s Bernoulli-type lift. The pilot can decrease the angle of attack by pitching the nose slightly downward. When the angle of attack is decreased, the chord line doesn’t tilt upward as much as it does at slow speed, and a flatter angle of attack—a negative angle of attack is possible even at very high speed—shortens the backward component of the total lift, or the induced drag.

Some large planes, particularly jets, feature angle of attack meters that display the precise angle between the relative wind and the wing’s chord line, but in smaller planes, pilots use airspeed as a rough measure of angle of attack—low speed means high angle of attack and a potential danger of reaching stall speed.

That’s not to say that total drag decreases at high speed. Total drag is very high at low speed, when induced drag accounts for most of it, and decreases as speed increases. But at some speed, which is different for each type of wing, the increase in parasite drag overtakes the decrease in induced drag, and drag increases with speed.

Parasite drag is easy to visualize: Think of the force you feel on your hand when you stick it out the car window at highway speed. The same force acts on the airplane as it flies. The structure of the airplane is sleek and aerodynamic, but it still creates a lot of wind resistance.

We call this component of parasite drag “flat-plate drag.” To arrive at an airplane’s flat-plate drag ratio, airplane designers look at all the surface area on an airplane and make some allowances for the drag-reducing qualities of its design. By arriving at a flat-plate drag ratio, designers are saying that the surface area of an airplane is equal to the drag of an imaginary flat plate of a certain size.

For example, a small, two-seat Cessna 152 has a flat-plate area of slightly over 6 square feet. That’s pretty small considering how much total surface area the airplane has. But by comparison to other planes, the Cessna 152 is a “dirty,” or drag-intensive, airplane. The Beechcraft Bonanza sports a flat-plate area of only 3.5 square feet, and the sleekest of all mass-produced general aviation airplanes, the Mooneys, have a flat-plate area of around 2.8 square feet.

Some flight instructors dust part of the airplane’s skin with talcum powder to demonstrate the fact that the jagged surface of the airplane’s skin holds air still at a microscopic level. Because talc is so light, is should blow off the airplane at high speed. But because the powder is so fine that it settles into the micro­scopic nooks in the wing, it is sheltered from the main flow of air, just as air molecules are. At the end of the flight no matter how fast the plane moved, the talc will still be where it was before the flight.

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Another component of parasite drag is the wind resistance caused by skin friction. If you examine the skin of an airplane at a microscopic level, you’ll see a jagged surface with lots of nooks and cavities that are too small for us to feel, but more than large enough for air molecules to hide in. Like small eddies along a riverbank that hold water stationary while the stream nearby flows rapidly, the jagged irregularities on a plane’s surface hold a thin layer of air perfectly still, even though the airplane might be moving at a very high speed. As you move a little farther from the plane, but still at a microscopic distance, the air moves a little faster, and at a few millimeters from the skin, the wind is moving at full speed. The viscosity of the air, or its resistance to flowing smoothly, is to blame. That viscosity adds to parasite drag.

Weight, or acceleration caused by gravity, is the most familiar of the four forces of flight, because it’s something we encounter each day. In straight and level flight, weight gradually decreases as fuel is burned during flight. Other than that, weight is a constant, at least whenever the airplane is flying straight and not climbing.

But during turning flight, the centrifugal force created during the turn adds to the weight of the plane. In very steep turns, the centrifugal force can double the apparent weight of an airplane. Of course, no mass has been added to the plane, but the centrifugal force caused by the turn makes everything feel heavier.

Manufacturers limit the maximum weight that a plane can weigh at takeoff because of the strength that must be built into the plane’s structure to withstand the punishment of turbulence and harsh handling of controls by pilots.

What a Drag

Lift is a marvel, but it’s not free. For every ounce of lift created by an airplane’s wings and other control surfaces, we pay a price in drag, a force that works to slow the plane down. It is because of drag that we have to equip airplanes with engines to produce thrust (which we’ll get to in a minute). There are two primary forms of drag:parasite drag and induced drag.

Every wing has a limit to how slowly it can fly, and that limit is based on its angle of attack. Whenever a plane flies too slowly for the wing’s angle of attack to produce lift, the wing “stalls,” and the plane quickly starts descending.

Sometimes you hear news reports about an airplane “stalling.” Those reports usually don’t have anything to do with the engine stalling as can happen sometimes to a car on a hot day. They usually are referring to a wing stall. If a wing stall happens at an altitude high enough for the pilot to recover, the only result is a shaken-up pilot. But if it happens at a low altitude, such as during an approach to landing, the plane often hits the ground before it can recover enough speed to keep flying. Those kinds of accidents make up most of the accidents on approach to landing. Flight instructors spend a lot of time drilling their students on maintaining safe speed during the approach to landing.

Engineers have created a large variety of airfoil designs based on whether they want their planes to make use primarily of Newton’s action-reaction lift or Bernoulli’s low-pressure lift. The type of lift depends on the type of plane and the type of flying it will be expected to do.

If you slice a wing from leading edge to trailing edge, you see its cross section. Some cross sections have a stout, thick shape. These generally produce a lot of Bernoulli-style lift thanks to the highly curved upper surface, and enable the plane to fly at relatively low speeds of 50 to 60 m. p.h. You find these wings on general aviation airplanes and any others that need to fly slowly.

Other wing cross sections are nearly as slender as the cross section of a knife blade. These wings produce very little lift on the basis of their shape, so they rely on high-powered engines, usually jets, that can produce lots of speed. You generally find knifelike wings on high-speed jet fighters and civilian planes like the Concorde, which create tremendous engine power, or thrust. These wings rely less on Bernoulli’s low-pressure lift and more on Newton’s action-reaction lift.