I am doing a science project for school. I am planning to test how the shape of the wing effects the flight time. I also have to do a written research report. I was wondering if you could tell me the basics on how a sailplane works.?

From : Don Stackhouse

Wow! Such a small question, such a BIG answer! Well, I'll give it a try!

Most folks will tell you that a glider (which in some cases can also be called a "sailplane") is an airplane without a motor. Actually, a glider has a motor, a very large and powerful one, and probably the most reliable one around. No, it's not the wind, although that's a common misconception. It's also not skyhooks, witchcraft, or smoke and mirrors.

You're most likely sitting on it right now. No not the chair. The Earth, or more specifically Gravity. The most reliable motor in the world! If it ever quits, we all have much more important things to worry about!

A glider coasts downhill through the air like a sled, or like a bicycle coasting down a hill. Because the flight path is slightly downhill, the lift force (which is always perpendicular to the flight path) is tilted forward slightly. This means that part of the lift is now counteracting the drag force, acting like thrust. A bicycle coasting down a hill works the same way, except that lift is replaced by the force between the tires and the road. In both cases the force is tilted forward by the same angle as the slope of the "hill", which allows a part of that force to pull the vehicle forward against the forces of aerodynamic drag (glider) or rolling friction (bicycle).

In the case of a glider, the slope of that "hill" is called the glide ratio. If the glide ratio is 10:1, it means that the glider moves 10 feet forward for every foot of altitude it loses. Another term you will frequently hear around glider fliers is L/D ("ell-over-dee"), which stands for Lift divided by Drag, or lift to drag ratio. We usually use this term interchangeably with glide ratio, since the two are almost equal if the glide ratio is bigger than about five to one.

The slope of that imaginary hill, and the flying speed of the glider determine its "sink rate", the amount of altitude it loses in a given time. This is frequently given in feet-per-second or feet-per-minute. When you're coasting down a hill of a given height, you will get to the bottom sooner if the hill is steeper, or your speed is higher, or both. The same is true for a glider, a steep glide ratio or a high airspeed will make it lose altitude faster. An example of the second one is a very heavy but efficient glider, such as an airliner with the engines stopped. For example, a Boeing 727 with the landing gear and flaps retracted and the engines at idle has a best glide ratio of about 17:1, which is quite good, but it occurs at a very high airspeed, almost 200 miles per hour if I remember correctly. This gives it a sink rate of about 1000 feet per minute. A Schweitzer 2-22 sailplane has a similar glide ratio, but because of its lower speed its sink rate is only about 250 feet per minute.

So let's summarize: for a glider, the motor is gravity, and it uses altitude for fuel. The rate at which it "burns" this fuel to cover a given distance is called "glide ratio", which in most cases is about equal to the ratio of lift to drag. The glide ratio and the speed of the glider determine its sink rate, the amount of altitude it "burns" in a given time.

So what is the difference between a "glider" and a "sailplane"? Well, all sailplanes are gliders, but not all gliders are sailplanes. A sailplane is a glider that has the ability for a sort of "aerial refueling". The atmosphere is always moving, both horizontally and vertically. There are many causes of rising air currents. Two of the most common are "thermals", where air above a warm place, such as a plowed field warmed by the sun, gets hotter than the surrounding air and begins to rise, and "slope lift" where the wind blowing against a hill, building, or other obstruction is deflected upwards. If the sink rate of the glider is sufficiently low, the glider may be able to find some air that is rising faster than the glider is sinking. The glider can then fly within this mass of rising air, and be carried upward with it, replenishing the glider's "fuel supply" of altitude. It's sort of like walking down an up escalator; if you walk slowly enough, you will still be carried up. This is called "soaring" for things that fly, whether it's a bird or a glider. Gliders with sufficiently high glide ratios and sufficiently low sink rates that they can frequently soar on typical air currents are called "sailplanes".

To be consistently successful at this, the sailplane must have a good glide ratio at a fairly high airspeed to cover lots of distance both upwind and downwind while searching for rising air. It must have a very low minimum sink rate so that it can gain altitude successfully once it finds that rising air. It must also be sufficiently maneuverable and have a small enough turning radius at minimum sink speed that it can stay inside a typical "bubble" of rising air. Balancing these rather contradictory requirements to get sufficient amounts of each is one of the most difficult but fundamental requirements a sailplane designer faces.

Don Stackhouse
DJ Aerotech