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How Gliders Fly, And How They're Different Than Powered Aircraft

While airplanes and gliders share many design, aerodynamic, and piloting factors, the lack of an engine fundamentally changes the way a glider flies.

GolfCharlie232 / Flickr

Streamlined Fuselage

Since there's no engine taking up space, a glider is sized around the cargo it carries; the fuselage is designed to be as small and light as possible. Most gliders have seats for two people in the small cockpit, with pilots sitting in a reclined position, vs. powered airplanes, where pilots typically sit upright. Why the difference? By sitting reclined, the cockpit and canopy can be more streamlined, creating less drag in flight.

The surface of a glider's fuselage is designed to be as smooth as possible, allowing the plane to fly through the air with little parasitic drag. The earliest of gliders were constructed from wood covered with canvas; later versions were made from riveted structural aluminum skins. Unfortunately, the seams and rivets typical of aluminum significantly reduced performance due to parasitic drag, so gliders continued to adapt. Today, many advanced gliders are constructed from seamless materials like fiber glass and carbon fiber.

GolfCharlie232 / Flickr

High Aspect Ratio Wings

Gliders have high aspect ratio wings, which means they are longer and narrower than wings on normal, powered airplanes.

Aspect ratio is calculated by dividing the square of the span of the wing by the area of the wing. As you can see in the diagram above, the Schleicher ASH 31 glider has an aspect of 33.5, while the Piper Cherokee has an aspect ratio of 5.6.

High aspect ratio wings produced less induced drag, which is what makes them so efficient on gliders. So why don't all aircraft have high aspect ratio wings? There are several different factors.

First, high aspect ratio wings bend more than shorter wings, which means they need to be designed with stronger design specs. Since gliders are light, the bending isn't as much of a problem. But with heavier aircraft, like airliners, a high-aspect ratio wing would be impractical. Next, high aspect ratio wings are more susceptible to wing warping when ailerons are used. Since gliders fly a relatively slow speeds, wing warping isn't as pronounced, but it would be a real problem in a fast aircraft.

Maneuverability is another major factor. High aspect ratio wings decrease maneuverability, because they have a higher moment of inertia. Think of it like a tightrope walker: they carry a long rod to balance themselves, preventing them from quickly falling left or right. It's great if you want to stay in one place, but not so great if you want to quickly move (or roll) left or right.

Finally, airport size limits the aspect ratio an aircraft can have. Take the Boeing 777 for example. The 777 has an aspect ratio of approximately 9. If it had an aspect ratio of 30+, it wouldn't be able to park near any other aircraft on the ramp, and its wings would be so long that they would hang over the taxiways during takeoff and landing. Obviously, that wouldn't be practical.

GolfCharlie232 / Flickr

Control Surfaces

Like most airplanes, gliders use ailerons, a rudder, and an elevator to fly. Flaps are fitted on gliders to control descent rates by producing drag and increasing lift. Many modern gliders also use airbrakes or spoilers which, when used, drastically disrupt airflow over the wing, increasing drag and reducing lift.

Another significant difference between powered airplanes and gliders is that gliders normally have only one landing gear, situated directly below the pilot. Having only one gear save a lot of weight, but what happens to the wings on takeoff and landing when you've only got one gear? The wingtips are protected by skids or small wheels, and when the glider lands, it comes to rest on the main gear and one of the wingtips.

GolfCharlie232 / Flickr

Glider Takeoff

Since they don't have engines, gliders normally use one of two methods to get off of the ground:

1) Aero-Tow: A powered airplane tows the glider into the air using a long rope. Inside the cockpit, the glider pilot uses a quick-release mechanism to release the tow rope. Once the glider is at a desired altitude, the rope is released and the glider and tow plane turn in opposite directions.

2) Winch Launching: An engine on the ground powers a winch, connected to a cable launch system. The cable is then attached to the bottom of the glider. Once the winch is activated, the glider is pulled along the ground at high speed toward the winch and takes off. In a short amount of time, the glider gains substantial altitude during this process and releases the winch line before continuing flight.

In Flight

Glide ratio measures the performance of an aircraft's glide; many modern gliders have a glide ratio better than 60:1. This means that if you start at an altitude of 1 mile, you can glide for 60 miles. In comparison, a Boeing 747 has a 15:1 glide ratio.

But if glide ratio was the only thing keeping gliders in the air, they wouldn't flying for very long. So how do they stay airborne? There are 3 main types of rising air that glider pilots use:

1) Thermals are columns of rising air created by the heating of the Earth's surface. Air near the ground expands and rises as the surface of the Earth is heated. Certain types of terrain absorb the sun more rapidly than others, like: asphalt parking lots, dark fields, rocky terrain, etc. These spots absorb heat, and heat the air above them, producing thermal air currents.

Newly forming cumulus clouds, or birds soaring without flapping their wings, are typically signs of thermal activity. When a glider pilot is "thermalling," they are finding and riding those thermal columns. And since the thermals can often cover only a small area, thermalling often involves a tight turn to stay inside the pocket of rising air.

2) Ridge Lift is created by winds blowing against mountains, hills or other ridges. Along the windward side of the mountain, a band of lift is formed where air is redirected upward by the terrain. Typically, ridge lift extends only a few hundred feet higher than the terrain which produces it. Pilots have been known to go "ridge soaring" for thousands of miles along mountain chains.

3) Wave Lift is similar to ridge lift in that it is created when wind meets a mountain. However, wave lift is created on the leeward (downwind) side of the peaks by winds passing over top of the mountain. Wave lift can be identified by lenticular cloud formations - they look like flying saucers. Wave lift can reach thousands of feet high, and gliders riding on wave lift can reach altitudes of 35,000+ feet.

Detecting Lift And Yaw

The vertical speed indicator in your cockpit will tell you if you're climbing or descending. If you're flying a glider and suddenly see the vertical speed indicator jump up, you've probably hit a thermal column and should try to stay inside of the rising air for as long as possible.

GolfCharlie232 / Flickr

The glider is slipping or skidding through the air when it is not pointing directly in the direction it is flying, relative to the air mass around it. A string on the windshield indicates to a glider pilot whether or not the glider is flying straight (the string is straight) or if it is yawing (the string is to the right or left). In general, glider pilots try to keep the string straight since the least drag is produced when flying straight through the air.

Jumbero / Flickr

Ballast

Some gliders carry ballast tanks filled with water. Heavier gliders sink faster than lighter gliders. Glide ratio isn't affected by weight because while a heavier glider may sink faster, it will do so at a higher speed. The glider comes down faster with more weight, covering the same amount of distance; this is ideal for cross-country flying. A heavier glider, full of ballast, has a reduced climb rate and shorter flight endurance while in a lifting environment. Water ballast can be jettisoned at any time through dump valves to minimize these flight characteristics, and to slow down before landing.

GolfCharlie232 / Flickr

Landing

Compared to landing in a powered airplane, there are a few key differences when flying a glider. First, gliders cannot add power if they won't make the touchdown zone. It may seem like a simple concept, but glider pilots are trained to judge their approach so they don't land short, and always wait until they're confident that they have the field made before introducing drag through flaps or spoilers.

The landing itself isn't too different from that in any airplane, you flare until lift reduces, and try to touch down lightly. Since gliders have one wheel, it's a bit of a balancing act to keep the wings off the ground as long as possible.

Gliders are incredible aircraft, and with the right atmospheric conditions, can stay aloft for hours or days at a time. The careful aerodynamic design that goes into building one makes these birds swift and unique.

And if you've never had the chance to fly a glider, we'd recommend you give it a try it.



Swayne Martin

Swayne is an editor at Boldmethod, pilot, and commercial aviation student at the University of North Dakota. He's the author of the articles, quizzes and lists you love to read every week. Swayne's flight experience ranges from King Airs and Carbon Cubs, to flying international cross-country flights in a 1943 Grumman Widgeon. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures at http://www.swaynemartin.com.

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