Modern gliders are taking full advantage of the latest technology to improve performance and efficiency. As well as enhancements for sport applications, glider designs are also being used as platforms for environmentally friendly power systems. BILL READ reports. This is a full article published in Aerospace International: July 2011

[caption id="attachment_4475" align="alignnone" width="300" caption="DG800B glider over Southern France. (DG-Flugzeugbau) "][/caption] When discussing new aerospace technology, the subject of gliders seldom, if ever, receives a mention. This is an omission that needs to be rectified, as modern glider designs are not only taking the lead in the latest technology but are pioneering new concepts of ‘green’ aircraft and space transport. The earliest form of aircraft to fly, gliders (or sailplanes as they are also known) have come a long way since the wood and canvas creations of the early aviation pioneers. The history of gliders can be traced back to Sir George Cayley’s Flyer of 1849, through the gliding experiments of Otto Lilienthal in the 1890s and the Wright brothers’ pre-powered aircraft of the early 1900s. The use of gliders for leisure and sport began after WW1 (particularly in Germany where many glider manufacturers are still based) and has continued ever since. Early gliders were constructed with fabric-covered wooden frames but, as technology improved, new designs were developed, first using covered metal frames and then complete fuselages and wings made from fibreglass and carbon fibre. Modern sports gliders are high tech creations, taking full advantage of the latest developments in construction materials, propulsion technology and avionic systems. A glider can be instantly distinguished from other types of aircraft by its distinctive shape of a narrow fuselage and long thin wings. Today’s sailplanes are built for efficiency across a wide speed range, as well as manoeuvrability and ease of handling. The efficiency of a glider is expressed by its ‘glide angle’ — a ratio of height and distance. A glider with an altitude of one mile and a glide angle of 1:60 can, in still air conditions, fly for 60 miles. For comparison, a commercial jet with all engines out has a glide ratio of around 1:17 while the Space Shuttle has 1:1. If glider pilots want to stay aloft for longer, they need to find places where the air is rising which will lift the aircraft. These can include concentrations of warm air (thermals), or areas close to the edge of hills where the wind is blown upwards (ridge lift) or flow resonances in the atmosphere (wave lift). Given the right conditions, a skilled pilot can remain in the air for long periods of time and cover long distances. The current longest glider flight in the UK stands at over 1,000km, while the world record is currently held by Klaus Ohlmann who flew a Stemme S 10-VT over the Andes for 2,463km of gliding in 14 hours in December 2000. The highest altitude reached by an unpowered aircraft is 50,722ft which was set on 30 August 2006 by Steve Fossett and Einar Enevoldson in a DG-500 carbon fibre and fibreglass research glider. To land a glider, it is necessary to slow down from normal cruise speeds. Furthermore, approach paths and margins are very difficult to judge given the very flat glide ratios noted above. All gliders are fitted with some combination of spoilers, flaps and air brakes on the wings to help the glider slow down and to adjust the glide slope. While early gliders often had skids to land on, most modern gliders are fitted with wheeled undercarriage, some of which are retractable.

Smooth operators

[caption id="attachment_4477" align="alignnone" width="300" caption="Bugwiper on the leading edge can remove insects, restoring efficiency. (DG-Flugzeugbau)."][/caption] Gliders are made in different shapes and sizes and can be cheap or expensive depending on their use. In the highly competitive racing sports glider market, manufacturers are focusing on new ways to enable pilots to fly further and faster. Using computer-aided design software, designs can be tested in advance to produce the most efficient aerodynamic shape with the least possible drag. Some manufacturers had sought help from research institutions. The Diana 2 glider from Diana Sailplanes in Poland features a wing designed at the Warsaw University of Technology while German glider manufacturer Stemme used the results of aerofoil research conducted at the Aerodynamic Design facility of DLR Braunschweig to design the wings of its S 10-VT sailplane. The most noted ‘guru’ of sailplane aerodynamics is Prof Loek Boermans of Delft Universty in The Netherlands who has assisted several German manufacturers, including Lange and Schempp Hirth. The introduction of strong lightweight construction materials, such as carbon fibre and fibreglass, has enabled gliders to be built with wings that are stronger and more rigid to enable flutter-free high-speed flight. Meanwhile, designers are striving to smooth out all parts of the aircraft which could disrupt the flow of air. Wings and winglets are often made in one piece while aerodynamic losses at the wing-fuselage junction are minimised by careful design of the mid-fuselage section and through specifically designed laminar flow aerofoils. Ailerons, rudder and elevators are fitted with aerodynamic seals to prevent the flow of air through control surface gaps. To minimise drag and prevent the formation of laminar separation bubbles, some areas of the wing are fitted with zig-zag tape or multiple holes. To allow a smooth flow of air, wings and fuselages are also highly polished. Most manufacturers use polyester or acrylic gel-coat surfaces. While these can deteriorate over time when exposed to heat and light, they are not part of the primary structure of the airframe and can be refinished at some expense. However, glider manufacturer DG claims to have solved the gel-coat cracking problems while Stemme uses special paint which it claims avoids this problem.
Aerospace International Contents - July 2011 News Roundup - p4 China's airline strategy - p12 Analysis of China's civil aerospace ambitions Clearer skies ahead- p 16 Report from this year's EBACE business aviation show Silent revolution - p18 How gliders are pioneering new technology Plane speaking - p 22 Richard Deakin, ceo NATS, goes on the record Funding the future - p 26 A report on the RAeS Annual Conference AESA battle hots up - p 30 Focus on new generation fighter radars The last word - p34 Keith Hayward on the future of Heathrow Airport
Other innovations include streamlined undercarriages with minimal drag while some landing gears consist of a single wheel mounted just below the cockpit. Wing tips are also fitted with small skids or wheels to protect the wing edge from ground contact. The laminar flow of modern wing profiles is severely affected by rain droplets or crushed insects on the leading edge. Some wings are fitted with ‘bug-wipers’ which remove insects that disturb the smooth flow of air over the wing. The LS10 glider even has a ‘bug wiper garage’ to ensure that bug wipers do not cause extra drag when not in use. The heavier a glider is, the faster it will sink but also the faster it will fly. A light and a heavy version of a glider with the same glide angle can fly the same distance, the only difference being that the heavier glider will get there first. Some gliders built for speed in sporting competitions are fitted with water tanks which can be emptied as required to increase climb rates or to reduce speed for landing. Water ballast can also be used to improve the ride of sailplanes in air turbulence that might be encountered during ridge soaring. The LS10 glider water ballast system consists of 95 litre water tanks in each wing and two separate water tanks in the fin while the Diana 2 glider uses the entire internal wing volume as an integral water tank. As a result of this new technology, modern gliders are extremely efficient. Glide ratios have been improved to as much as 70:1 while cruising speeds have increased from an average of around 60kt in the early 1980s to over 100kt. Much recent development has been put into cockpit cells to improve crashworthiness. Slovenian light aircraft manufacturer Pipistrelle, for example, produces a ‘safety cockpit concept’ cabin encased with energy-absorbing structures made from Kevlar fibre, which maintains the integrity of the cabin. For extra safety, gliders can also be equipped with ballistic parachute rescue systems used on other GA aircraft.

Cockpit upgrades

[caption id="attachment_4478" align="alignnone" width="300" caption="Moving map display, GPS - glider cockpits are now becoming more advanced. (Solar Flight). "][/caption] Pilots have been able to take advantage of new technology in glider cockpits. Since a glider is a relatively simple machine, innovations such as fly-by-wire are not necessary and the controls are still manually operated. Due to the narrow dimensions of a glider fuselage, the cockpit area is limited in size and pilots recline with their legs stretched out in front of them. Ailerons and elevator are controlled conventionally, using a single control stick between the pilot’s legs while the rudder is controlled using foot pedals. Many sailplanes use near full-span, narrow-chord flaps which are continuously adjusted in flight to maximise glide ratio over a wider speed range. Instrumentation in gliders used to be relatively simple. Basic equipment will include an airspeed indicator, altimeter, variometer, yaw string, magnetic compass and radio. Of these, one of the most useful to a glider pilot is the variometer which measures the rate of climb or descent — a very useful instrument when detecting the presence of rising air. Today, variometers can be either mechanical or electronic, the latter produces a modulated sound which rises when detecting rising air or lowers with sinking air. Most variometers are fitted with a ‘MacCready Ring’ (named after a mathematical theory attributed to Paul MacCready) which indicates the optimal speed that a pilot should fly between thermals, given the average expected lift from the thermal and the amount of lift or sink encountered in cruise mode. Electronic variometers can also calculate the optimal speed after allowing for such factors as the glider’s theoretical performance, water ballast, prevailing winds and insects on wing leading edges. They can also be linked to portable GPS systems to enable a very wide range of navigation and tactical function to be incorporated in real time in the cockpit. Many pilots take advantage of low-cost or open-source soaring flight software loaded on to palmtop computers which, when linked with GPS and altitude data from external instruments, can provide moving-map displays. Some software can also sense wind direction and speeds, recommend the best speed to fly, display airports within theoretical gliding distance and alert pilots to local airspace restrictions. After the flight the data can be replayed for analysis.

Transponders for gliders?

[caption id="attachment_4479" align="alignnone" width="300" caption="Arcus glider about to land."][/caption] The overwhelming proportion of sport and sailplane flying is carried out in good visibility VFR in uncontrolled airspace where the ‘see-and-be-seen’ concept has functioned for decades. An item of equipment common to most commercial aircraft but not usually fitted to gliders and sport aircraft is a transponder. There has been much debate in the flying community as to whether transponders should be mandatory for these classes. The argument for transponders is that it would make gliders more easily detectable by both air traffic controllers and TCAS (terrain collision avoidance systems) fitted to other aircraft. Collisions between sailplanes and other airspace users are extremely rare. One of the few incidents quoted in the debate was in the USA in August 2006 when a Hawker 800XP business jet had a mid-air collision with a Schleicher ASW27-18 glider. Amazingly, no one was killed, the damaged aircraft was able to land safely while the glider pilot parachuted to safety. Had the glider been fitted with a transponder the accident may have been avoided. The installation of transponders into gliders poses a number of problems. Not only would the equipment constitute additional cost, both first cost and periodic qualification for owners but transponders would need to be continually switched on which would require additional electric power to keep them operating, meaning larger or more batteries aboard the aircraft. A typical powered aircraft Mode S transponder has a large size in relation to the small area of a glider cockpit. In addition, gliders made of carbon composite would have to have the transponders mounted outside the aircraft as the fuselage would shield its transmissions — which would effect gliding performance. However, the greatest issue is that sailplane flight involves continuously changing height and developing routing using detailed weather assessment by the pilot, the essence of the sport. In the sports preferred approach, gliders can be fitted with transmitting GPS-based equipment such as the Swiss FLARM system. In this concept of operation the collision warning system shows the relative position of nearby traffic and provides visual and acoustic warnings of approaching aircraft which are displayed in the cockpit, without the need for ‘third party’ intervention. The debate continues and it has been predicted that transponders may eventually become mandatory for gliders by regulatory authorities once devices with lower power requirements become available.

Battery debate

Another current ‘hot topic’ is that of batteries. Because most gliders do not have engines, there is no way of generating power on the aircraft to operate equipment which needs electricity, such as radios, lights, undercarriage — or transponders as mentioned above. Gliders therefore need to carry on-board batteries of a design which has been approved by regulatory authorities. Up until recently, the most commonly used batteries were sealed lead acid batteries, also known as gel cells. However, new lithium batteries have since been developed which offer more power for the same weight but there have been safety concerns over the risk of them overheating or catching fire in flight.

Self-launching

[caption id="attachment_4480" align="alignnone" width="97" caption="Stemme retractable propeller concealed in glider nosecone. (Stemme)."][/caption] The most common method of launching a glider is either to tow it into the air from the ground using a winch or a moving vehicle or to tow it behind a powered aircraft. In the past, elastic bungee ropes were also sometimes used to launch gliders from slopes. Self launching motorgliders (SLMG) are fitted with a propeller engine that can be used to take-off without assistance and to sustain flight once airborne. As an example, Austrian GA and trainer manufacturer Diamond produces the HK36 Super Dimona Motorglider which can climb at 1,000ft per minute using a Rotax 100hp engine. While glider engines have their advantages in certain situations, they also have drawbacks in that they add additional weight, require power to operate and impede air flow. To counter these problems, manufacturers have come up with a number of ingenious solutions. Engines are made as lightweight and simple as possible, particularly for self-sustainers which often have set power levels (no throttle) and air flow driven starting. The engine, cooling liquid, propeller and exhaust system on a DG-808C motor glider weighs less than 50kg (110lb). Propellers can also be retracted in flight. Stemme has developed a patented propulsion system, consisting of an engine mounted near the glider’s centre of gravity behind the cockpit which uses a lightweight carbon fibre driveshaft to operate a nose-mounted propeller. The company’s 50:1 glide ratio S10 motor glider has a retractable propeller which can fold up inside the movable nose cone. Not all motor gliders use propellers. [caption id="attachment_4481" align="alignnone" width="300" caption="Side view of Lange's Antares 18P pulse-jet powered glider. (Lange.)"][/caption] US company Desert Aerospace has developed a two-seat, self-launching sailplane called the TST-14J BonusJet powered by a retractable jet engine while Lange Aviation’s self-launching Antares 18P is fitted with a pulse jet.

Electric revolution

One of the most interesting developments in glider technology in recent years has been the development of electric powerplants. To give some examples, Pipistrel has developed a two-seat self-launching motorised glider called the Taurus Electro G2 fitted with a 40kW retractable electric power train powered by lithium batteries. Schempp’s Arcus E two-seat self-launch glider features a retractable propeller powered by a Lange Aviation lightweight electric motor. The motor is powered by rechargeable Li-Ion electric batteries mounted in the wings which can supply power for up to 13 minutes at maximum power and maximum climb speed. Stemme’s S6 and S8 gliders are equipped with electrical Mulhbauer constant-speed propellers powerful enough to tow other gliders. Meanwhile, Stuttgart-based Flight Design is developing electric engines which can be used to replace existing internal combustion-powered models. More electrically-powered gliders are on the way. Chinese company Yuneec has launched a lithium battery-powered E430 two-seater light sport aircraft while German company PC-Aero has developed the battery-powered Elektra One. Electric motors also have the advantage that their batteries can be recharged — a point that has not been lost on glider manufacturers. DG’s LS10 glider has solar panels fitted on to the fuselage to provide continual battery charging while Stemme’s two-seat S10 sailplane features a rack of solar panels fitted to a removable fuselage cover behind the cockpit capable of generating up to 60kW of solar energy. While not powerful enough to recharge an electric motor, the cells can recharge batteries powering the aircraft’s avionics and engine starter. [caption id="attachment_4482" align="alignnone" width="300" caption="Sunseeker solar-powered glider (Solar Flight). "][/caption] The ultimate ‘holy grail’ is, of course, to be able to harness enough solar power to achieve self-sustainable flight and a successive number of designs are moving towards this goal. Slovenia-based Solar Flight, has developed the Sunseeker and Sunseeker II solar-powered gliders, the latter of which became the first solar-powered aircraft to cross the Alps in 2009. A third aircraft, Sunseeker Duo is under development. A particular challenge being tackled is that the top surface of the glider’s wings needs to be covered in solar cells which need to be smooth to avoid causing more drag. Another problem is that solar cells are still very expensive and generate heat which could have an adverse effect on a composite wing. Alternative ideas that have been considered include using the propeller as a wind turbine but the propeller is not efficient in this mode and would inhibit the smooth running of the glider. However, as technology progresses, these problems (including the possible fire risk posed by lithium batteries) may yet be overcome. In the meantime, manufacturers are looking at innovative ways to recharge electric motor gliders while on the ground. Electric aero engine company Bye Energy in Colorado is researching the recharging possibilities of solar panels fitted to a hangar roof or from power provided from wind turbines. Pipistrel also produces a special stowage and transport vehicle, the Solar Trailer, fitted with a solar panel which can recharge its electrically-powered glider both when it is inside the trailer and also when the trailer is empty. Electric propulsion also has the advantage of zero carbon emissions and has thus attracted the interest of aircraft designers seeking to develop more environmentally-friendly ‘green’ aircraft. Pipistrel has developed the four-seat Taurus G4 proof-of-concept electric aircraft, constructed by attaching two Taurus G2s side-by-side, which will compete in the 2011 CAFE/NASA Green Flight Challenge later this year. Research using glider airframes is also being carried out into the possibilities for powering aircraft using hydrogen fuel cells. In March 2008, Boeing flew a fuel cell-powered Dimona motor-glider for 20min at its Spanish test site which was followed in 2009 by the Antares DLR-H2 aircraft (based on the Lange Aviation Antares 20E motor glider) to test a fuel cell system developed by the German DLR Institute of Technical Thermodynamics. Lange is also working on an optionally manned fuel cell-powered UAV technology demonstrator called the H3, to be followed by the H4, a long-endurance fuel cell-powered UAV. A follow-up fuel-cell powered aircraft, the Antares H3, is scheduled to fly across the Atlantic in 2012. Fuel cells also have their problems to be overcome, not the least of which is the weight of the cells themselves. They also have the problem that the hydrogen in the cells needs to combine with oxygen to operate. The Antares DLR-H2 has currently only been tested at altitudes of 3,000ft because of the risk of power losses in reduced air pressures. The contribution of gliders is also clear in current research into long-endurance UAVs, many of which share the aircraft’s distinct long wing shape. QinetiQ’s solar-powered Zephyr UAV currently holds the world UAV endurance record after remaining aloft for over 14 days in July 2010.

To infinity and beyond

[caption id="attachment_4485" align="alignnone" width="300" caption="The Perlan 2 glider aims to reach 90,000ft. (Perlan Project)."][/caption] In the meantime, manned gliders continue to soar to new heights. A team in Oregon, USA, is working on an attempt to use a sailplane to reach 90,000ft without an engine. Fitted with a pressurised cabin, the two-pilot Perlan 2 sailplane will attempt to use an extreme weather condition called the Polar Vortex to reach the required height. Meanwhile, Virgin Galactic’s SpaceShipTwo sub-orbital spacecraft, currently under flight test, will be capable of gliding back to Earth from an altitude of 80,000ft — although some glider pilots might concur with the famous quote from Toy Story 1 that Sheriff Woody gives to Buzz Lightyear: “That’s not flying — that’s falling with style.” Looking to the future, US company PlanetSpace is looking at plans to develop a space glider called the Silver Dart, based on a 1960’s US Air Force Flight Dynamics Laboratory project for the FDL-7 hypersonic glider. The 45ft craft could carry eight people into orbit using ten rocket engines and bring them back down by gliding to a runway landing. The age of the glider may only be just beginning. This is a full article published in Aerospace International: June 2011. As a member, you recieve two new Royal Aeronautical Society publications each month - find out more about membership.

Royal Aeronautical Society
1 July 2011