Flight test: Schleicher AS-K14
PUBLISHED: 16:35 16 April 2019 | UPDATED: 16:46 16 April 2019
A good motor glider offers the best of both worlds, and the AS-K14 is not just a good ’un but arguably one of the finest ever made | Words Dave Unwin Photos Keith Wilson
As the prop slows to a stop one of the great attractions of flying motorgliders manifests itself. The transition from powered to gliding flight is almost as shocking as it is sensuous.
The contrast is just so vivid, but before I can really revel in the near silence of soaring flight there are some cockpit chores to take care of. A brief stab of the starter button swings the prop horizontal, swap hands on the stick then a deft pull and twist of the T-handle sprouting from the starboard side of the panel feathers the prop. Swap hands again and set the throttle and choke ready to start.
Tweak the trim and settle swiftly into the core of the thermal. As the variometer needle swings confidently up towards six and the altimeter's 'hundreds' pointer starts rotating like a second hand I settle back in my seat with a smug smile.
I really like soaring. I like the cerebral challenge of defying gravity by using my intellect as the engine and the atmosphere as the fuel. I'm less enamored with waiting around to be launched, and landing out can be a right pain, hence the attraction of motor gliders.
And the subject of this month's flight test is surely one of the finest ever made. Descended from the classic Schleicher AS-K6 (known simply as the K6) the AS-K14 has long been on my 'wish list'.
Why? Well, having owned shares in a couple of K6s (the E model in particular still being one of the sweetest handling flying machines I've ever flown) and having always enjoyed motorgliding, the concept of a self-launching K6 really appealed. Consequently, I was both pleased and flattered when Peter Andrews asked if I'd be interested in flying his.
It's quite rare: Schleicher only built 62 between 1967 and 1972. There are only three in the UK and 'Golf Bravo Sierra India Yankee' is the sole example that's currently airworthy. Peter has owned it since 2005; prior to that it had spent 22 years based in Kenya, and even featured in an Anglia TV documentary about vultures!
The K¬14 was designed by Schleicher's Chief Designer Rudolf Kaiser, and was derived from the famous K¬6 series. The original K¬6 (more correctly known as the Ka¬6) first flew in 1955 and was built by Kaiser as a private venture. An interesting design feature was that the ailerons and elevator were interconnected so that as the elevator approached its 'fully up' stop, both ailerons were also raised.
This reduced the angle of attack at the tip, ensuring that the root stalled first. As it was not fitted with a wheel and had a wingspan of only fourteen metres, it was not suitable for serial production, although the German glider manufacturer Alexander Schleicher soon realised that the type had potential as a club aircraft. The span was increased to 14.4 metres, and 25 were built between 1956 and 1957.
The next model – the B – appeared in 1958, with the span increased to fifteen metres. Coincidentally, that same year the FAI gliding commission announced the introduction of the 'Standard Class', and although the aircraft had not been designed specifically for this class, Schleicher realised it only needed to be fitted with a wheel to qualify.
This aircraft became the K¬6CR (the 'R' standing for rad [wheel]), and well¬known sailplane pilot Heinz Huth flew one to third place in the 1958 World Championships. Although this was a good result for Schleicher, an even better result was that the K¬6 won the OSTIV prize for the best Standard Class design.
This version remained in continuous production until 1968, by which time more than 800 had been built. During this time, Kaiser and Schleicher were continually looking at ways to improve the aircraft, producing a new variant ¬ dubbed the K¬10 ¬ in 1963. This featured a different aerofoil, an all¬flying tail, a thicker fuselage and a redesigned cockpit. However, for a variety of reasons (primarily, it was simply too heavy) it was not a success, and only twelve were built.
The final development of the line was the classic K6¬E, on which the K¬14 is based. It retained all the best facets of the CR and incorporated the better features of the K¬10 (such as the all¬flying tail). In essence, the E had the wing mounted slightly lower on the fuselage (which had a reduced cross¬section), while the wing itself used a revised NACA 63 aerofoil.
The canopy was significantly larger, and more non¬load¬bearing parts (such as the turtle deck) were made from fibreglass. The net result of these changes was a significantly improved glide angle (up from 29:1 to 34:1), a slightly flatter polar curve and a lower minimum sink rate. Around 390 were built between 1965 and 1972.
Peter flew over to Saltby from Husbands Bosworth on a lovely day in late June, and even as it rolled to a stop his aircraft's lineage was obvious. Designed by Rudolf Kaiser and produced by Alexander Schleicher GmbH, the AS-K14 is−in very simple terms and as you may have guessed−essentially a K6E with an engine.
The fuselage is a wooden semi-monococque structure skinned in plywood, while the cantilever wing uses a single spar of pine and plywood, covered with fabric. The wings are almost identical to the K6E's, the only differences being that the span has been reduced by 0.7m and the Schemp-Hirth type airbrakes replaced by upper surface spoilers, which seems a retrograde step.
The forward fuselage was redesigned to house the engine and retractable mainwheel, and the wing relocated lower down, but the fuselage aft of the wings is pure K6E. The wings, fin, tailplane and rudder are skinned with plywood and fabric, and all control surfaces (except for the spoilers and ailerons) are also fabric-covered.
The engine is interesting. Made by Gobler-Hirth, it is an air-cooled two-stroke flat-four which is mounted so that the thrust-line is slightly down and offset. Originally it was fitted with a recoil starter, but now has an electric starter.
As there is no alternator or generator, there are a finite number of starts before the battery needs recharging, but what makes it unusual is that each cylinder has its own carburetor and ignition coil. This idiosyncratic engine even has an uncommon firing sequence in that the front two cylinders fire simultaneously, and then the back two. It produces 25hp at 5,000rpm and drives a Hoffman two-blade fully-feathering propeller.
The fuel is contained in a single twenty-litre fuselage tank located, somewhat disconcertingly, immediately behind your head and within the cockpit. There is no fuel quantity indicator. Although typically referred to as a 'monowheel', strictly speaking this is inaccurate as the undercarriage has four wheels−a large main wheel, a solid, non-steerable tailwheel and two small solid wheels built into the wingtips.
As delivered from the factory it simply had small skids built into the tips but Peter has fitted wheels. The main undercarriage is quite stalky, presumably to provide adequate prop clearance. The main wheel retracts forwards and upwards, and as about half of it protrudes when retracted it is possible to land safely 'gear up', as long as the prop is in the horizontal position. The main wheel is fitted with a cable-actuated Tost drum brake. There is no shock-absorption or damping.
As Peter had flown in, we didn't have to rig 'India Yankee', but he said there's no mystery to it and that it's very similar to rigging a K6E. The ailerons have to be connected manually using L'Hotelier connections but the spoilers auto-connect.
The all-flying tailplane is very easy to attach and hooks up automatically−simply push the port tailplane's torque tube through the fuselage, line up the hole in the starboard tailplane with the hole in the torque tube and secure them together with a locking pin and safety-pin.
The canopy hinges to starboard but before getting in it is important to adjust the rudder pedals as this is tricky (though not impossible) to do once you're in.
The K14 is very much the kind of aircraft that you put on, as opposed to get into, and once you've strapped it on with the four-point harness you really feel as if you're 'wearing' the aircraft. The seat is very firm and, as the cockpit is quite small, it's important to be comfortable as there's not much room to wriggle.
The cockpit is completely in character with the rest of the aircraft, being very basic and uncluttered. The panel carries only a few instruments. Flight information is provided by an airspeed indicator, altimeter, compass, slip ball and two variometers (one pneumatic and fitted with a MacCready Ring, the other electric) while engine health is simply shown by a tachometer, CHT gauge and voltmeter.
All the (very few) controls are easy to see and reach. The stick is quite an unusual shape but feels very natural, while the throttle and choke levers, fuel valve and blue spoiler handle are all on the port cockpit sidewall.
The pitch trim is similar to the K6E's, consisting of a large spring, one end of which is attached to the floor and the other to the elevator circuit. Moving a small lever under your left thigh forward or backward biases the spring tension in the same direction.
Immediately outboard of the trimmer is the undercarriage lever and its rather quaint wire safety catch. The wheel brake is actuated by a T-bar between and just aft of the rudder pedals. It's not ideal, and Peter is looking at putting a bicycle-type lever on the stick and possibly making the tailwheel steerable. The lack of a steerable tailwheel is a glaring omission.
The whole point of a Self-Launching Motor Glider (SLMG) is to be self-sufficient, and I just don't know what Kaiser was thinking. He and Schleicher really took their eyes off the ball here. Another thing it lacks is a CG hook (so that you could at least take a winch launch if the engine was U/S) and Peter tells me that later models did in fact have this as an option, mounted in the front of the wheel well.
Set the choke and open the throttle almost halfway, Master and ignition on, press the big red button and the engine starts readily, and urgently. The tick-over is set fast, as the plugs can foul quickly at low rpm. There's no warm-up needed, and the pre-takeoff checks are minimal−you can't even check the oil temperature and pressure, or ignition system.
Peter walks the wing out onto the runway and I'm ready to go, but first a look at the windsock. That monowheel means there is not much control authority at the start of the takeoff roll, so if there's any crosswind (and particularly from the right) it will swing if you raise the tail−so don't!
Although it's only got 25hp, its light weight means that the power-to-weight ratio is an acceptable 19kg/kW, and as the prop is pitched for the climb, the acceleration is not too bad. The ailerons come alive quite quickly, enabling me to keep the aircraft balanced on the main wheel with the tailwheel on the ground, while the delightfully low wing loading means that it waffles into the sky at about 45kt.
Just press the stick forward slightly so the 14 stays in ground effect until the ASI's needle swings past 55, then gently ease up into the climb. At around 350fpm the climb rate is respectable albeit unremarkable, but improves slightly when I dab the wheel brake, release the wire safety catch that guards the undercarriage lever and then pull the lever up and back.
The wheel thumps home in its well and I reduce power from a frantic 5,000rpm to a still-frenetic 4,500. The engine continues to buzz busily as I throttle back a bit more and head towards a freshly forming cumulus that already appears to have several large birds underneath it.
At 1,400ft I can feel the lift surging around the aircraft and almost immediately the rate of climb more than doubles. I pull the power right back, but this engine doesn't like to idle so, rather than foul the plugs, a quick check that I can easily get back if it shouldn't start, then shut it down. There isn't a prop brake so I slow to almost the stall to stop the prop windmilling, motor it to the horizontal and then feather it.
The first priority is to 'secure my position' so I spend the next few minutes working the thermal up to 4,000ft. Altitude gives me time and options while, without the frantic buzz of a four-cylinder two-stroke, I can begin to appreciate fully the true character of the 14. Flying it like the glider it is is where it's at−and it's just great.
As much as I love my Jodel D.9, at times the boisterous bark of its unsilenced VW engine is quite obtrusive.
Initial impressions are that this really does feel like a K6. The handling is feather-light and the controls crisp and precise.
It doesn't seem to climb quite as well as a 6E (as it has slightly heavier wing loading) but has somewhat better 'penetration' (for the same reason). The field of view is exceptional. Forty knots feels very comfortable, thermalling at altitude, but if ridge-running down low, fifty would probably be more prudent.
It quickly becomes apparent that−just like the 'E'−when flying between sources of lift there is little to be gained using speeds in excess of seventy knots as the glide angle gets very steep. It also has a small tendency to pitch up at high airspeeds, and as the spring trimmer cannot fully compensate the stick forces in pitch can get uncomfortably high. However, this is not as noticeable as it is in an E, possibly because the K14 possesses marginally stronger longitudinal stability due to the more forward C of G.
At the slow side of the speed envelope the stick forces are so low as to be almost unnoticeable, while the stall itself is very subtle and gentle. There is practically no pre-stall buffet, the best indications of an imminent stall being the stick position and the change in the sound of the air flow. If the speed is reduced sufficiently slowly it never really stalls, but instead simply mushes with a high sink rate.
While it is not cleared for aerobatics, the 14 enters an incipient spin well and recovers promptly as soon as the correct control inputs are applied. Overall, it's a delightful aircraft to fly, although if the conditions were lively I reckon you'd be bounced around a bit, just like in a K6.
On those occasions it would require a firm hand, but on a day like today with big fat thermals I let it fly itself. I'm a 'low gain' pilot (some might also rather unkindly describe me as lazy) and prefer not to do too much unless it's unavoidable. I tend to guide it rather than fly it.
Ventilation is adequate as the canopy incorporates a large DV panel, and although I expected to smell petrol, due to sharing the cockpit with the fuel tank, I didn't. I really enjoyed soaring in the 14, and could cheerfully have stayed up for several hours probably burning less than a litre of fuel, but we must shoot the air-to-air photos, so I turn back towards Saltby and reluctantly decide to use the spoilers to speed my descent from 4,000ft.
Extending the spoilers automatically sets off the undercarriage warning horn (it has its own 9v battery and works even when the master switch is off). So, to shut it up and also expedite my descent, I retract the spoilers, extend the undercarriage and then operate the spoilers, which work quite well. For simple spoilers they're quite powerful. They open almost 90° and are probably the best I've flown with.
Having easily got back without the engine I leave it shut down. Peter warned that landing with it running is extremely difficult, if not almost impossible, as the combination of the high tick-over and ground effect really prolongs the float, and anyway I'm more than happy to land engine off.
Back in the circuit I wait until close abeam the threshold at around 600 feet, then turn onto base and final while controlling speed with pitch and the rate of descent with the spoilers. Trim for about 50-55kt, start with about 'half spoilers' and see what happens. There's a slight nose down pitch trim change with full spoiler extension.
Landing is as undemanding as every other aspect of the aircraft. The spoilers supply adequate approach control, and I just ease them in slightly in the flare to reduce the sink rate, and sweeten the landing. This ploy also allows me to extend the spoilers fully at touchdown, pinning the aircraft on the ground.
The unsprung monowheel means a rough ride if the ground is bumpy, although the ground run is pretty short. The stalky undercarriage ensures that if you achieve a perfect 'two-pointer' you've got a lot of alpha at touchdown, and a commensurately slow airspeed.
After a quick brief with Al and Keith, who'll be in the EuroFox cameraship, we launch as a flight of two and head north to Belvoir Castle, and the brief transit gives me the opportunity to examine cruise performance. A sensible cruising speed seems to be around 65kt IAS, and as the engine uses about ten litres an hour this equates to an endurance of around two hours and a still air range (no reserve) of about 120nm. If the cloudbase is high enough a 'saw-tooth' glide profile is much more efficient and would undoubtedly be the way to go.
Keith's door opens, which is the signal for me to 'close up, echelon port'. Once nicely tucked in I ease the power back a bit more and instantly sense that the engine isn't happy. It clearly doesn't like low rpm and the plugs are beginning to foul, as the motor sounds a little bit rough.
A healthy handful of throttle soon clears it, followed by prompt application of spoiler to avoid sailing past the cameraship! Like a lot of two-strokes this one has quite a narrow power band and is happiest at high rpm. Once we've got a shed-load of air-to-airs, I shut the engine down and move back into formation to shoot another sequence with the prop feathered, before floating down to another feather-light landing at Saltby.
What a delightful flying machine this thing is! It's just lovely, and I can't help but imagine how cool it would be with an electric motor and say ninety minutes of battery.
It would be possible to scratch and scrape down to just a couple of hundred feet above a decent field before resorting to the electric motor, certainly much lower than I'd ever trust a petrol engine.
In my privileged position as Pilot magazine's flight test editor I get to fly a lot of aeroplanes. Most of them I like, some of them I love and a few of them I want−but this one I need.