Bt Bernard Eckey

The pace of Schleicher’s new model releases continues unabated. After the introduction of the AS 33 and the all-electric AS 34, the world’s oldest glider manufacturer has just announced its latest development – a new Open Class ship – the AS 35 Mi.

It comes as no surprise that Schleicher’s has earmarked the ASH 31 as their next model for an upgrade. This very popular glider has already been in production for 12 years and to date almost 220 of these machines have left the German factory. This is a far greater number than the combined production of 21/23m gliders by all other manufacturers.

Also largely unknown are the inflight measurements recorded by the ‘Idaflieg’ institute, showing that the glider has a best L/D of 60:1 – a figure that comes as no surprise to most of its pilots. Not only that, but the ASH 31 ticks all the boxes for aviators seeking a self-launching, docile, uncomplicated aircraft with a reliable, smooth, powerful engine, a comfortable cockpit and very pleasant handling characteristics. Its fans will be pleased to know that, due to ongoing demand, Schleicher will keep the glider in production for quite some time to come.

Technology to the rescue

Soon after the legendary ASW 22 was discontinued in 2008, pilots of other aircraft began to occupy the podiums at Open Class competitions. In response, the young and ambitious team of Schleicher engineers has set out to make some changes. They are committed to success at the highest level of competition and believe that utilizing the very latest Finite Element Method (FEM) and Computational Fluid Dynamics (CFD) technology will be key to this success.

These modern tools have challenged current thinking and resulted in a new design path. Instead of wingspan, experience with the new AS 33 suggests wing loading to be more important. Several competition pilots supported this approach and reported that their new AS 33s could easily climb with other 18 metre gliders even with a 10% higher wing loading. Obviously, higher wing loading can lead to better gains compared to optimizing span.

The three consecutive first places of AS 33 pilots at three European National Championships speak for themselves and are quite remarkable, given that in each of the three contests, only one AS 33 pilot was competing. It is therefore only natural that the design team picked the new AS 33 as a yardstick and used the well-established lift and drag values of its individual components for comparisons with their new design.

Wing Modifications

Right from the outset, it was clear that retaining the AS 33’s superb ability to climb, even in weak conditions with a full load of water on board, would be crucial. Achieving this at an even higher wing loading would make the AS 35 very competitive in Open Class. Time consuming and highly complex computer analysis has finally resulted in a new family of wing sections, which can not only generate more lift but also shine with lower drag.

Many other design modifications, although minor, would also be included. Individually, they make small contributions, but the combined effect makes a noticeable difference. For example, reducing the size of pushrod outlets and integrating perfectly fitting and absolutely flush bug wiper garages eliminate unnecessary drag and help make the glider more slippery. A fully retractable tail wheel, new winglets, a perfectly covered tow release and flush mounted LED flashlights (embedded into the leading edge of the fin) are other examples.

Obviously, all of these refinements increase the cost of production but taking them into account during computer modeling confirmed that a wingspan of 20 metres with a maximum take off weight of 730kg was the optimum. This requires 170 litres of water ballast, which can just be accommodated in a very thin 20 metre wing.

Inflight comparisons

Well before more formal inflight performance comparisons could be arranged AS 33 pilots met up by chance with a modern Open Class glider while flying cross-country. It was spontaneously decided to find out how the performance of the latest 18 metre glider compares to the current state of the art Open Class ship with almost 10 metres of additional span.

While flying in close proximity to each other, and especially while cruising in the medium to upper speed range, the AS 33 easily kept pace and occasionally even managed to get its nose in front. This came as a slight surprise but when subsequent flights showed very similar results with both gliders ballasted to their maximum wing loading, the team was excited and delighted at the same time. Here was evidence that increasing wing loading matters most when it comes to improving cross-country speeds.

If an 18 metre glider with a wing loading of 60kg/m2 can keep up with a current generation Open Class aircraft, then an aircraft with only a slightly larger wingspan, but with improved aerodynamics and a higher aspect ratio and with increased wing loadings, should be the optimum. The direction for the design team was crystal clear – just like the AS 33, the AS 35 would be designed with as flat a polar curve as possible, and to maximise performance in the medium and upper speed range.

Another aspect of the design was to provide customers with flexibility in span and allow them to enter their AS 35 in either 18m or Open Class competitions. The outer wing panels are supplied as either 18m or 20m tips, whereby the Snap-On winglets are interchangeable and remain the same for both wingspan versions. In 20m configuration, the glider’s wing area is 11.75m2 and features an aspect ratio of 34. Its maximum wing loading of 62.1kg/m2 is reached when fully ballasted to its maximum take off weight of 730kg.

The anticipated polar curve looks exciting. With a sink rate of less than 2m/s at 120 knots (220 km/h) the AS 35 will be hard to beat, especially under conditions of streeting or while flying in wave. Just as impressive for a self-launching glider is the empty weight of 448kg. It will make for very relaxing un-ballasted outings with a modest wing loading of around 44kg/m2.

Handling – on the ground and in the air

The design team continued to look for additional areas of improvement. Reports from AS 33 owners indicated that the handling and control coordination of this new design is of exceptional standard and a glider pilot’s dream. The obvious challenge was to give the AS 35 the same standard of
handling.

Traditional Open Class gliders are renowned for their lack of manoeuvrability and slow rate of roll. This means that it can take quite some time find the core of thermals or waste valuable seconds while attempting to stay in the strongest part of the lift. In contrast, a smaller and more agile glider is far more efficient when it comes to thermal centring and by comparison to an Open Class glider, can save at least half if not a full unproductive turn at every thermal.

Considering that an average glider race requires taking about 15 thermals, the savings could easily amount to 10 full turns per flight. At 20 seconds per turn this would total to around three minutes per race and an astonishing 15 to 30 minutes over the duration of a competition. That’s almost an eternity, considering that past championships have been won and lost by just a minute of difference between first and second place.

Outer Wing and Flap Design

With this in mind, it was decided to incorporate the outer wing of the new AS 33, which would most certainly add considerable agility to the aircraft. The inner wing panels are a totally new design but feature wing sections very similar to the AS 33. It also transpired that a more refined control mixer would not only boost the roll rate but also enhance the lift distribution over the wing while thermalling. This in turn would further enhance the climb rate and improve the aircraft’s manoeuvrability at the same time.

Just as important was integrating the popular Schleicher landing flap design. It has long been proven that feeding in slightly negative aileron deflections on selection of landing flaps makes landings easier and safer, and ensures aileron control until the glider comes to an almost complete stop.

Easing the ground handling was next on the agenda. The strict application of FEM technology had not only proven useful in terms of optimising strength, rigidity and stability but it has also helped to minimise the weight of individual components. As an example, each AS 35 wing is about 5kg lighter compared to the ASH 31, which not only helps make ground handling easier but also resulted in a welcome increase in the weight of the non lifting components. Winglets with Snap-On fittings and automatic connections for all controls also ease rigging and speed up assembly. For more convenient ballasting, the wings are filled on the upper side of and the air vents are now located in the tips of the winglets.

The AS 35 even fits into a standard trailer for 15m gliders – another novelty for an Open Class Glider. The total length of the inner wing panels is below 6 metres and the fuselage is just half a metre longer. Some owners might even secure a hangar spot by removing the rather lightweight outer wing panels, which reduces the remaining wingspan to just 12.2m. A single person can easily fit or remove the outer wing panels, as their weight is only 16kg each.

Proven Fuselage Design

When a thorough CFD (computational fluid dynamics) review of the ASH 31 fuselage showed no scope for aerodynamic improvements, the development team adopted its external shape for the new AS 35. It meant that the comfortable cockpit ergonomics, the inflight adjustable backrest and all safety features that customers expect from Schleicher, could also be retained.

However, the interior was modernised and the new rudder pedal design was copied from the AS 33. The most noteworthy modification is the new canopy seal, which stops cockpit ventilation air from leaking past the canopy frame and hence prevents a premature tripping of the laminar airflow around the front fuselage into a draggier turbulent one.

To ensure sufficient cockpit ventilation, two separate air inlets plus the usual air scoop in the sliding window are provided. The air vent on the right cockpit side comes with a fully adjustable nozzle. To allow such vast amounts of ventilation air to escape again, a CFD optimised air extractor is placed under the aircraft’s belly and positioned to cause a minimum of airflow disturbance.

Retractable Tail Wheel

Another welcome design change is a steerable, fully retractable tail wheel, which instead of being an optional extra, is part of the standard design. This will allow unaided take offs as well as limited taxiing in conjunction with the small removable wingtip wheels.

The tail wheel door design Schleicher uses is an improvement over those that open outwardly. As these doors are always only within a few centimetres of the ground (and more often than not scraping dense vegetation) they are vulnerable to damage or even being completely ripped off. The solution is a single-part door cover, which retracts entirely into the tail boom. Very smart indeed! A welcome byproduct is an improved forward visibility as the wheel extends further and hence lifts the tail higher off the ground.

Rotary Engine

Total independence and reliable self-launch capabilities are always top of the priority list for Open Class owners, and AS 35 customers are unlikely to be an exception. Therefore Schleicher will install their well-proven, liquid and air-cooled rotary engine from Austro Engine.

This power plant has been continuously improved over the years and now comes with a dual ignition system and a fully electronic fuel-injection system with automatic altitude compensation. Almost 700 of these engines are in worldwide operation already.

The engine has an excellent power to weight ratio and the folding drive belt allows it to remain stationary in the fuselage where it is permanently connected to a large, efficient muffler. Only the propeller lifts up and tensions the drive belt in the process. This not only keeps the air resistance to an absolute minimum but also prevents stretching of the belt over time and hence avoids regular re-tensioning.

For inspection or maintenance purposes, the entire power plant can be removed easily. Another welcome advantage is the very straightforward engine management and the lack of service life limitations or overhaul requirements after reaching a certain number of running hours or years of service.

Quiet, Smooth, Efficient

The engine’s surprisingly quiet, smooth, vibration-free operation at all power settings has also greatly contributed to its worldwide popularity. Maintaining an assigned altitude (for example, while flying through controlled airspace) no longer presents a problem and is best conducted at approximately 140 km/h (70 to 75kt). The previously applied ‘saw tooth’ climb/cruise method used with conventional 2-stroke engines is no longer necessary or recommended.

To convert the power output of the rotary engine into climb performance, the entire system consisting of engine, propeller and aircraft is perfectly fine-tuned. On paper, the Wankel engine has a slightly lower power output than comparable systems but its efficiency is second to none. Customer experience clearly shows that figures on paper are meaningless and that only climb rates and good aircraft acceleration count. As the propeller plays a pivotal role, Schleicher not only designs but also builds their propellers in-house. Just like the wing profiles, they are constantly fine-tuned in order to boost overall power plant efficiency.

Another advantage of this power plant is the modest fuel consumption, which is significantly lower compared to conventional two-stroke engines. It runs on high-octane fuel or Avgas and there is no need to mix in 2-stroke oil on refuelling. The removable 16-litre fuselage tank fits into the wheel well and allows approximately one (1) hour of engine running time.

Standard Issue or Optional?

Schleicher is also breaking new ground in terms of including items previously only available as optional extras. The standard package comes with a two-pack polyurethane finish, antennas for radio, transponder and Flarm (located in the fin) as well as tail dolly, fin tank, a 20 Ah (LiFePo4) avionics battery, a cockpit air extractor and the fully retractable and steerable tail wheel. An automatic refuelling system, which turns itself off on a full tank, has also been integrated. Other goodies such as solar cells on engine bay doors, leather interior, bug wipers with garages, CofG optimisation for pilot weight, oxygen system, LED flashlights and so on remain on the list of optional extras.

Coming Soon

In summary, the days when Open Class gliders with impressive best glide ratios (and wingspans of 25m to 28m) were dominating the competition scene seem to be nearing their end. At medium to high speeds, longer wings translate into more drag and as competition flying predominantly involves fast to very fast cruising, the long big wings become more of a hindrance.

On the other hand, long and flexible wings can provide sheer delight for pilots who prefer relaxing, enjoyable cross-country flying to racing. Nothing can beat sharing a flight with another gliding addict and handing over the controls from time to time. It is often said that “Sharing enjoyment equals doubling the enjoyment” and I gladly confess to be a firm subscriber to this theory.

However, contest pilots have different priorities. They are always looking at competition results and have noticed that – at least over recent years - gliders with shorter wingspans are just as successful as so called ‘bigwings’. Aircraft like Antares, Quintus or JS1 have finished ahead of traditional Open Class ships on many occasions, although their wing loadings are well below the 60 kg/m2 mark.

But gliders are evolving constantly. The very latest generations of wing profiles are capable of producing significantly more lift and hence tolerate much higher wing loadings. They also cause less drag at high-speed and, in combination with additional aerodynamic refinements, we can expect smaller wingspan gliders to give the ‘bigwings’ a run for their money. Time will tell but all indications point to the AS 35 becoming a truly formidable weapon in Open Class championships.