Posted by: Alex | April 8, 2010

What is gliding?

This is a common question given the relative obscurity of the sport, and since I plan to be making posts detailing some of my flights, I’d better explain up front.

The closest corollary to gliding is sailing, where success is determined by knowledge and successful interpretation of the weather as it relates to the most efficient and strategic piloting of your vessel to achieve your goal. People go gliding for fun, of course, just as people go sailing for an afternoon with friends, but gliding enters the realm as a competitive sport when you introduce cross-country flying.

(NB. In aviation for any type of aircraft, “cross-country” is defined as any flight greater than a certain distance — say, 50km — from the home airfield. Flying away from your home airfield brings to the forefront different skills such as navigation and appropriate decision-making in the event that you have to land somewhere other than you intended, say, due to inclement weather or an emergency.)

Glider pilots are continually working to improve their cross-country technique and skills, and week-long regional, national, and world competitions are organized around flying daily cross-country tasks the fastest. With no engine, things rapidly become quite interesting. It’s a complex sport, requiring many areas of knowledge, so I’ll do my best to summarize the most important aspects. Here follows a hypothetical interview.

So, what is gliding, what’s a glider, and how does one fly without an engine?

Gliding, also referred to as soaring, is the sport surrounding the pursuit of unpowered flight through aircraft called gliders. To understand how gliders fly, here’s a quick lesson on the basic principles of flight.

There are four key elements to consider when thinking about an aircraft in flight. First, the wings of an aircraft are responsible for flight — specifically, wings generate lift as they are moved through the air. Engines, whether a propeller on a small plane or a jet engine on a military fighter jet, all produce movement in the intended forward direction of the aircraft, which we call thrust. Gravity keeps you and everything else on the surface of the Earth, and the effect it has on matter is called weight. Finally, familiar to anyone who has stuck their hand out of a moving car, or tried to run through the shallow end of a swimming pool, is a force that opposes movement called drag.

When the captain turns off the fasten seatbelt sign and you’ve reached your initial cruising altitude in a commercial airliner, it’s at this point that all forces are balanced. You’re not climbing or descending, so the lift generated by the wings must be equal to the weight of the aircraft, and because your speed is constant and not changing, the thrust generated by the engines must be exactly equal to the drag caused by the aircraft moving through the air at high speed. This is the basic model of the forces involved in flight.

What happens if the engine suddenly fails? Does the aircraft become a projectile and immediately fall out of the sky? No. But why? The reason is that gravity pulls the engineless aircraft towards Earth, and as this happens, the wing acts to convert some of this downward motion into forward motion. The ratio between the two, and hence the slope the aircraft descends along, is called the glide ratio. Your aircraft just became a glider.

For the purpose of comparison, let’s give a few examples. A single engine Cessna that carries 2-3 people might typically cruise at around 7,000ft and has a glide ratio of 1:7. In the event of an engine failure, you’d be able to glide just over 9 miles (15km). Not too bad, you think, maybe enough to get you to the nearest airfield for landing. How about a 747? Big, heavy, and needing four engines, it probably doesn’t glide too well. In fact, from a typical cruise altitude of 35,000ft, your 747 would be able to glide almost 113 miles (182km) — it beats the Cessna with a glide ratio of 1:17.

Now that we’ve established that all aircraft will glide to a greater or lesser extent, let’s come back to what a glider is. A glider is simply an aircraft without an engine that has been optimized to achieve a very high glide ratio. The latest glider designs are approaching practical glide ratios of 1:60. From 35,000ft you’d be capable of gliding an astonishing 398 miles (642km).

Since a glider will fly as far as the eye can see, you simply start high and then see how far or fast you can go?

Not at all. These glide ratios are only approximate ideals and they also assume completely still air. The atmosphere is never entirely still, which is where flying skill, weather knowledge, and assessment of constantly changing conditions come together to form the sport of gliding. Air currents, particularly in the mountains, can rise or descend in excess of 2,000ft per minute, and the best pilots will know how to read the conditions accurately to spend as much time in rising air as possible. By doing this, you can cover large distances: a new world record currently awaiting verification, just shy of 1,563 miles (2,500km), was recently set in New Zealand.

Sounds similar to hang-gliding. What’s the difference?

Hang-gliders rely on the same flight principles, but a hang glider’s performance doesn’t do much better than the 747 in my example above, putting hang gliders in a separate class. You’re also directly exposed to the elements with no cockpit. Gliders are true aircraft in terms of their construction and their performance is optimized as much as possible.

It sounds like gliders aren’t made from fabric or nylon stretched across a frame.

Not anymore, but they used to be. I learned on gliders made from a combined metal and wooden frame with fabric stretched over the top to form the wings and fuselage. Most Americans learn on metal skin gliders (similar to the construction of a Cessna). Neither have particularly high performance, but are simple and easy to fly aircraft, suitable for initial instruction.

Modern day gliders are made from composite materials, mostly fiber glass with some carbon fiber, covered with a very smooth exterior coating to encourage smooth, “laminar” airflow over the wing. The construction technique provides various benefits: extremely precise manufacturing, particularly of the airfoil (wing cross-sectional shape); absolute minimization of drag along all surfaces; and a high strength-to-weight ratio to allow both higher airspeeds and the ability to carry water ballast (held in the wings).

You mention water ballast, so lighter is not always better in a glider?

Not necessarily. The primary reason is that the same glider, when made heavier with water ballast, will have the same glide ratio when flying faster than without the extra weight. What you gain in speed, you give up in your sink rate. The glider will sink a little faster (perhaps an additional 100 feet per minute), which means that it will take you a little longer to climb in any rising air (lift) you find. Pilots will elect to take on water ballast, which can be dumped at any time, if the soaring conditions are expected to be strong (sufficiently strong lift to outweigh the negative of the slightly increased sink rate) so that they can fly faster overall.

The secondary reason is that flight through turbulence (for example, while flying along a ridge) is going to be less violent if your aircraft weighs more. Think back to a time you were on a commercial flight and were going through some turbulence. Imagine how much worse it would be if the aircraft only weighed as much as a small car versus many tons. The atmosphere is a powerful place.

We’ve talked a lot about theoretical gliding performance in a constantly changing atmosphere with areas of sink and lift, so let’s turn to the practical. How does a glider get airborne?

There are two primary methods. The first is an aerotow, which is where the glider is attached to a powered aircraft (often a re-purposed crop duster, capable of taking off with lots of extra weight) via a towrope around 100 feet long. The glider releases from the rope at the desired altitude, ideally in an area of lift. The second is a winch launch where the glider is attached to one end of a long steel cable (around a mile in length) and the other to a drum that is rapidly wound in at around 60mph, enough to get the glider airborne to 1,500ft or higher within a minute. It’s an exhilarating ride to the top, but the disadvantage is that you’re released right above the airfield, not necessarily where the best lift is.

Once you’re up there, how do you stay in the air?

There are three primary forms of rising air: ridge lift, thermals, and wave. Ridge lift is available when wind blows perpendicular to a ridge, forcing the airflow upwards along the face of the ridge and producing lift that a glider can fly in. This will work whether a low-level ridge, or a mountain range many thousands of feet high. Ridge lift can sustain your glider’s flight, but you’ll have to stick close to the ridge, making long cross-country flights challenging using ridge lift alone.

The second type of lift, and most commonly used by gliders, is thermal lift. Thermals are rising bubbles of warm air, triggered when the sun heats the Earth’s surface and the air immediately above it. Once this pocket of air warms enough, being warmer than the surrounding air, it will rise like a hot air balloon. By circling tightly and staying in the core of the thermal, glider pilots can use thermals to gain height. Thermals form over all different types of terrain as long as the sun is able to heat the ground directly (eg. no high cloud cover). Both ground features and clouds can be used to locate thermals.

The third type is wave lift. Wave forms in a similar way to ridge lift, but only in certain atmospheric conditions and geographical regions. Wind blowing over mountain ranges can set in motion a large wave-like system potentially extending hundreds of miles beyond the original mountain range and into the stratosphere that can be used to gain many thousands of feet. In addition, by flying along waves, large distances can be covered at high speed. Most of the current world distance records were set on flights largely flown in wave. As with thermals, clouds can be used to locate the rising crests of a wave system.

In order to know what type of lift to expect and where, various weather resources are available, some specifically tailored to glider pilots. Wind speed, direction, changes with altitude, expected solar heating and cloud cover can all be accurately predicted for the day. Most gliding operations will have daily morning weather briefings.

What if you can’t find air currents to keep you aloft?

With no engine, you have no option but to land! Either rapidly changing conditions or an incorrect assessment of the weather ahead can catch you off guard, but either way you need to find a suitable landing site. This can be a nearby airfield, but since gliding flights tend to take place away from built up areas, landing in a field or a farmer’s airstrip is most typical.

Landing out is by no means the norm, but it happens fairly frequently. An aerotow retrieve may be possible (the tow plane will fly to where you’ve landed and tow you back to your home airfield), but otherwise gliders can be easily dismantled and put into a trailer once your retrieve team has figured out how to get into the field you’ve ended up in. Derigging normally requires two people: the two wings come off and sit in a long trailer either side of the glider’s fuselage.

How long does it take to learn?

If done all at once, most people will be ready to go solo within a few weeks if taking lessons every day. Usually people will learn by coming down to the airfield every weekend for a several months. Compared to learning how to fly a powered aircraft, gliders have far fewer systems to worry about with no engine and fewer instruments. The main learning curve involves learning to fly the glider accurately to minimize drag, and how to find and fly in areas of lift. One of the areas a student will spend most of their time on is developing the appropriate judgement to land the glider accurately every time. With no engine, you only ever have a single chance to get it right.

The biggest commitment is that it requires a lot of time.  Time from the point of view of spending it at the airfield waiting for the right weather before launching, but also a significant time investment in learning. Beyond achieving your basic license after going solo, there are multiple levels of proficiency, ultimately leading to longer and more demanding flights. You not only have to know how to fly (arguably the easiest part), but it requires a deep understanding of weather systems, what types of conditions and terrain are favorable to generating lift, and then a refinement of your basic flying skills to be able to take advantage of these conditions as effectively as possible.

How did you get into flying?

When I was younger I lived in the UK and had the opportunity to learn through an RAF (Royal Air Force) gliding club for about US$4 per launch. Later on, I joined a youth program at an airfield nearer my home that traded regular work for flying hours. I went solo when I was 16, gained my basic instructor’s rating at 17, and when I left the UK at 18, I had 190 hours and 350 flights under my belt. Since then, I’ve been looking for an opportunity between jobs to head down to Omarama and experience some mountain soaring first hand.

And is Omarama turning out to be everything you’ve been hoping for and more?

Absolutely. An “average” or “weak” day here has turned out to be far better for gliding than an average day in the UK! Terrain and weather conditions are completely different. Not to mention the friendly people, beautiful scenery, and the opportunity to share a few beers with some of the best glider pilots in the world. The other day I was poking around Steve Fosset’s old custom ASH-25 glider that he and Terry Delore broke multiple world records with. You can’t beat that. It’s a very unique spot from all perspectives.


Responses

  1. Ahh, so that’s how gliding works.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

Categories

%d bloggers like this: