View Full Version : Just a Thought on the SLA

05-02-09, 12:47 PM
Best of British
By David Bremner
Once upon a time, there were myriad British microlight manufacturers, all beavering away in small units, producing machines of variable quality, from the world-beating to the distinctly dodgy. In 1984, the advent of airworthiness regulations saw off most of them. And since then the combined effects of airworthiness approval and increasingly effective competition from the rest of the world has cut the numbers drastically, so that today thereís only P&M, Thruster, Medway and Reality holding the fort.
And of these, Medway is the one with the longest continuous history. The constituent parts of P&M go back a similar time, but throughout all the ups and downs, Medwayís been quietly providing quality product for their devoted client base from their factory on the marshes on the Isle of Grain on the Thames estuary.
Proprietor Chris Draper is a big chap with a big personality. He has more experience than most in the microlight business, and strongly held views and he doesnít tailor his views to his audience. Heís made a few enemies along the way, but heís smart enough to know the effect of what heís saying, and strong enough not to take negative opinions to heart. Unlike some others, heís had not financial backing for his business, and has had more than his share of bad luck Ė arsonists burned down his office building a few years ago, and itís now in a temporary mezzanine in the Nissen Hut style factory. His team includes wife Karen, the Rev. John Kitchen, and Martin Ingleton, who helps Chris with the fancy engineering and test piloting in his spare time,
The factory has its own airstrip at Stoke, separated only by a short walk across the railway.
The strip itself is very interesting Ė sandwiched between the railway line and power cables on one side, and the marshes on the other, itís about 480 yards long, and banana-shaped. The power lines dominate ones attention on the way up and on the way down, but no-ones got hooked up yet, so it canít be too much of a problem, and I couldnít see any wreckage sticking out of the marsh eitherÖ
Medway has always been a flexwing outfit. Theyíve produced a succession of nicely-produced units featuring the Raven wing (originally produced in 1984 and very much ahead of its time) and culminating in the EclipseR series which has sold in some numbers.
I was asked to pay a visit to look at their newest model, the SLA, which is their first venture into three axis. Itís a crowded market, and punters with money to spend are presented with an astonishing array of goodies to choose from. Most of it is imported, of course, and Ė at least until the new Thruster facility is up and running, Medway is the only British manufacturer making a ready-to-fly model which should make it an attractive proposition for schools, since you should be able to expect better service from a local manufacturer.
The SLA design opts for the tried-and-tested. Itís a tube-and-Dacron construction, side-by-side high wing tractor layout, like many of its competitors. Chris freely admits using ideas from other manufacturers Ė not surprising, since thatís what all the other manufacturers do! So the engine mounts look a bit like the Sky Ranger, the wing and empennage remind one of a Rans, and the fairings and cockpit are reminiscent of the South African Cheetah. But each of them uses ideas from the others Ė itís standard industry practice, and of course itís how we improve the breed.
I was introduced to Lilac Lil, the company demonstrator. Sheís an early production model, but representative of whatís coming off the production lines.
The initial impression is quite distinctive Ė the long-stroke undercarriage, unusual cowling and zip openings on the fuselage side mean that itís unlikely to be mistaken for any other type in the UK. The cowling in particular seems to have holes everywhere Ė no less than six openings in the front, with huge vents each side and underneath at the rear. Itís not classically good-looking, but as I came to appreciate on closer acquaintance, the features are there for a purpose.
One distinctive feature of all Medway designs is the use of stainless steel fittings. The SLA is no exception and it means your pride and joy will look sparkly and pristine long after the plated fittings on others are going dull, with bits of rust starting to break through.
And once you start to probe around a bit, you can spot other interesting features too. The skins are full of zips. These make it easier to inspect the aircraft than almost any other Iíve seen; look at the pictures and you can reach pretty much anywhere inside the fuselage Ė including the tail Ė without having to unlace things. You can even get your head inside the wing for a look round. The cabin is mostly non load bearing composite panels, but there are easily removable panels to get at the innards.
Poke your head inside the sharks-mouth-shaped zip panel on the fuselage side, and there are a couple of other nice surprises. The fuel tank stretches the full width of the fuselage and holds a MASSIVE 100lt. But take a look just behind. The flaps are electrically operated (not quite a first for a UK-approved microlight, but pretty swish nonetheless), and thereís a bungee running aft from the operating arm. That is connected to the elevator circuit to provide just enough bias to remove the stick forces when the flaps are operated. Good, init? Now all you need is a reason to use the flapsÖ
But Iím jumping ahead of myself. I should be describing the whole thing logically from nose to tail, and Iím cherry-picking all the exciting bits first.
So here goes.
The engine (you can have it with a 80hp Rotax 912 (vanilla flavour) or a 100hp 912S (99?), though Lilac Lilís got the 100hp version) is tightly cowled to ensure maximum view over the nose. Thereís a three-bladed Woodcomp propeller on the front (though other propeller options are being investigated).
The cowlings are composite and split horizontally as usual. There are quick-access hatches for oil and water checks, and removing the top one (16 quick-release fasteners) will get you excellent access for most things Ė and the lower one will come off very quickly to get at the lower half. The nosewheel (which is a good old Nylite wheel) is mounted on a big stainless tube that runs in nylon bushes with long stroke bungee suspension. Propeller ground clearance doesnít look enormous, but the bungee suspension seems pretty solid, so it shouldnít cause problems.
The low cowling line doesnít contribute much to the overall look, but should improve the view from the cockpit enormously.
The door surrounds are all composite, which is a good idea since itís one of the major wear-and-tear areas.
The cockpit is entered through top-hinged doors with gas struts to hold them open. The welded steel door frames are reasonably solid, though it seems possible that theyíd need tweaking from time to keep them draught-free. The catches are easy to operate and secure. Inside, construction looks similar to the Sky Ranger, being of aluminium tube with a pair of them rising from back to front between the seats in the central console, and a single central stick. Personally, I like a central stick, but I know that instructors generally prefer two, and Medway will see instructors as an important market sector.
There are twin push-pull throttles in the instrument panel, and stainless steel tube rudder pedals.
All the main controls are cable operated, trim and flaps are both electric and the hydraulic brake lever is on the central stick, with a natty cam which acts as a parking brake.
The bucket seats are well padded, and fitted with four point harnesses which are easy to secure and adjust.
Behind the seats is a fabric baggage compartment good for 5kg which you can reach in flight. Thereís also a very useful little recess in the central console for sweeties, change for parking machines, etc. and you can stow maps on the top of the instrument panel.
The electric flaps are apparently lighter than mechanical equivalents, and are operated by a rocker switch in front of the left hand seat with a LED position indicator next to it. The two sides are mechanically linked via a torque tube, so itís not possible to get asymmetric deployment, and as Iíve said, access to the gubbins is fantastic.
The trim is also electrically operated, the switch and indicator being centrally located. The trim servo is also in the rear fuselage, with a Bowden wire link to the tab itself. The Bowden wire can easily be kinked if you push on the tab, and despite multi-lingual exhortations to keep off, I think it should have been possible to get the servo mounted in the tailplane with a short, strong pushrod linkage.
The main undercarriage is tall and fitted with long-stroke sprung struts. This should give good protection against heavy-handed students Ė or experienced pilots, even (did I blush a little bit here?). In fact Martin Ingleton said theyíd tried dropping one in from about 50ft and only managed a slight bend in the main crosstube, which is a pretty good endorsement!
The wings, rear fuselage and empennage are conventional Ė aluminium tubes covered with Dacron Ė apart from the generous inspection openings and stainless fittings already mentioned. The wing planform is more tapered than the competition, which should give a small benefit in efficiency, and I liked the fabric hinge covers on all the control surfaces.
The covering is conventional Dacron with Ultralam on the top surfaces, which should limit fading and weakening from UV radiation if itís left out in the open.. You can have Ultralam all over as an option, which (because itís a shiny surface) is easier to keep clean.
It was time to clamber in.
But clamberingís the wrong word. The stalky undercarriage may not look too sexy, but getting in and out is that much easier because the seatís at a more convenient height. The lift struts are attached aft of the door, and the door itself opens upwards to give you completely unobstructed access Ė and thereís no stick to fiddle your legs round either.
The seats are adjustable by means of pins through the sliding rails, but thereís plenty of room for longer legs as the instrument panelís not too deep. The seriously vertically challenged might need a cushion. The seat itself is very comfortable, and the cockpit width is generous enough even without bowed doors for your shoulders. Martin and I fitted in without any sense of being cramped.
The view out is also excellent Ė that closely cowled engine might not grab you with its classic lines, but it sure does give you a great view ahead.
In fact the view all round is terrific Ė with narrow door pillars and fully glazed roof and doors, the only direction you canít see is aft, and thatís a function of the design.
Some types have windows you can open in flight. Theyíre great for photography, and on those rare days when itís actually sunny. They werenít fitted to Lilac Lil, but it would be an easy option to produce.
The doors are easy to shut, so Martin and I did that and fired up the donkey. The Rotax 912S isnít as silky smooth as the 912, but the noise and vibration levels in the cockpit were on a par with similar types. While the oil was warming through, we did the usual pre-flight checks. Because you canít see out the back, youíll need to check the elevator and rudder control operation before you get in, or by leaning out of the open door. You can specify your own instrument fit, but the factory preference is for digital, and Lilac Lilís got the Skydat unit which I found easy to see and use.
Once the oilís warmed up, we line up, and open the taps. You can only adjust the throttle friction from the left hand seat but otherwise itís a good firm control, with one surprise Ė the carburettor springís been altered to fail closed, not open, something that will please many readers.
You can use flaps for take off if you wish Ė and Martinís keen to demonstrate a short takeoff, so he selects half flap.
With 100 horses up front and a healthy 10mph breeze on the nose, weíre off in less than 50 yards. With the power lines looming large out of the right window, weíre glad to be over the top of them in short order, and we power on up at an indicated 800ft/min, with the ASI reading about 60mph.
The view ahead under these circumstances is unusually good (see the picture). You canít see the full horizon ahead, but it only takes a small amount of neck cranage to check all clear ahead. You need to keep a small amount of right rudder on, but surprisingly small considering the large horsepower and low speed.
We settled down into the cruise, with 95mph showing on the clock and a leisurely 4000rpm on the tacho. A check against the GPS showed that the official calibration for the ASI was spot on, which gives a calibrated airspeed of 85mph.
Once settled in the cruise, the view is really good. The low cowling, big windscreen and fully glazed doors make for a great view out, and the ability to see through the roof into a turn Ė even a relatively gentle one Ė is a great plus. I found that my head was just brushing the fuselage tubes in the roof. Iím 6ft 3in, and even so it was a mild irritant, not a nuisance. But I understand that plans are afoot to alter the fuselage geometry to provide better clearance for giraffes like me.
We tried a few gentle turns to get a feel for the controls. The controls are generally firm, requiring stick forces that are slightly bigger than average. But their effect is crisp and positive. Turns require co-ordinated stick and rudder, but you donít (in my view) need to lead with rudder. On the other hand, if you use only aileron, youíll get some noticeable adverse yaw. Pitch stability is positive, both stick fixed and stick free. The stick free test is done by trimming, then raising or lowering the nose until you get a 10mph change in speed, and letting go to see what happens. In the case of the SLA, it takes about one and a half slow cycles until it settles back to the trim speed. That means you can be reasonably confident that you can let go of the stick to fold the map or look up your destination airfield in the book without being taken for a roller coaster ride. Stick fixed pitch stability requires that the stick forces increase steadily the more out of trim you are, and thereís no doubt about that.
The next check is yaw stability, which involves putting the aircraft into a sideslip, and seeing whether the rudder self-centres. Here itís a bit marginal, which indicates that the fin area is only just enough, and explains why you need to use rudder in turns. Roll stability (sideslip it again and check that the ailerons self-centre) is definitely positive.
Turns are pretty straightforward Ė one simply uses the same amount of rudder as aileron, and the ball stays pretty much in the middle.
One other check then Ė the effect of power on trim. So we set it up in a trimmed cruise at 80mph, let go the stick and brought the power back to idle. The nose dropped gently, and the speed remained constant at 80mph. we increased power to full Ė and the speed remained constant at 80mph. thatís a pretty unusual characteristic, and a really useful one. Clearly itís essential that the nose drops when you cut the power in order to avoid accidental stalls if the donkey quits, but most conventional aircraft will end up with an increased speed in the glide, and a reduced speed on full power Ė indeed on some types the glide speed exceeds Vne, and / or the climb speed is less than the stall speed.
But whenever the speed changes, it means you have to retrim every time you alter power; on the SLA100 you donít, which is great, and something very few other types can match.
The trim itself is not particularly effective Ė we got a range of speeds from 60-85mph IAS, and there was relatively little feedback through the stick. I would have liked a little more effect Ė perhaps a larger tab would help. And while Iím whingeing, would it be possible to have an inset trim tab please? The current one is a bit unsightly and vulnerable.
While we were tinkering with trim, it seemed a good opportunity to try out the flaps, to see if the bungee connecting the flap mechanism to the elevator would do its stuff. Reader, it did.
The electric flaps are only reachable from the left hand seat, and (like the trim) thereís a rocker switch and a LED indicator. They donít have Ďlatchí positions Ė full flap is only 22į, and for intermediate positions you simply check the indicator. But the bungee did the biz, and the trim speed hardly altered as the flaps were applied Ė one less thing to have to think about on finals.
It was time now to try the extremes of speed. We chopped the throttle again, and held the nose up to find the stall. With flaps up, thereís the normal buffet just before the stall, and then a high descent rate mush but no stall break, responding to small control movements normally. The first time we tried it, there was a weird wobble, but even so it remained in control, and it was the only time it happened.
Normally, the stall is more interesting with the flaps down, but Lil was just as docile as before. In fact it took an accelerated stall Ė zooming up at maybe 30į - to get a stall break of any sort, and even then there wasnít a hint of a wing drop. So we tried a power-on accelerated stall, and it still wasnít possible to detect any misbehaviour. Sheís a perfect lady, Lil.
Thatís the low end of the spectrum; how fast will she go, mister? We opened the throttle and let her settle down, and she settled out at an indicated 110mph (around 100mph calibrated) and near enough 5200rpm. Sheís absolutely settled at that; the controls are firm but not too firm, and the airframe feels solid and comfortable. You have to keep a hand on the stick, as she wonít quite trim out, but thatís probably no bad thing as it acts as a reminder that youíre exceeding Va, the manoeuvring speed, and getting pretty close to the Vne at 119mph indicated.
It only takes a shallow dive to get to Vne, and there is no change in the handling. Sheís as solid as a rock.
And so, duty done, itís time to head reluctantly back to the strip. No it isnít. Martinís an entertaining companion, and Iím treated to a whistle-stop tour of the island, including the site of Short Brothersí original aircraft factory and the Thames, before we head back to Stoke for a go at landing.
Martin does the first one, since heís used to those power lines looming at you, and makes it look a doddle. Approach speed is 60, reducing to 55 over the numbers, and the firmish touchdown in the gusty conditions is absorbed easily by the gas struts. He manages to keep the nosewheel off the deck until weíre pretty much down to a walking pace. Itís a good party trick, but has practical benefits, in reducing wear on the part of an aircraft which in my experience needs more maintenance / repair than any other.
Now itís my turn, and we whiz off for a tight circuit over the marshes. No flaps for this takeoff and the distance, while longer than before, is still admirably short.
Flap limiting speed is 69mph, which gives one plenty of latitude in the circuit. Applying full flaps on finals, the nose dips giving a panoramic view of the field, but the trim speed remains as previously set Ė itís a real plus not to have to retrim when thereís so much else going on.
The approach is reasonably steep with full flaps, making it easy to judge oneís height, and we arrive over the numbers with the speed satisfyingly on the button. Weíve made an angled approach to keep away from those power lines, and itís necessary to make a low level turn a few feet off the deck to line up with the strip. The controls are still nice and crisp, and thereís plenty of pitch control left to make the flare, and weíre down and rolling out in satisfactory style. The brakes are powerful and a real help in bringing us to a stop in short order.
This is the first opportunity Iíve had to check out the taxiing, and thereís no problem here either. In fact the turning circle is spectacularly tight Ė with full lock on the inside wingtip is going backwards. I reckon the centre of the turning circle is about level with the end of the lift strut. If you canít park it properly, donít blame Lil!
Summing up
The SLA joins a very crowded market place. Assuming youíre not interested in building your own, you can choose from the Ikarus C42, the Kosmik Eurostar, the P&M CTSW, the Jabiru UL-D, and maybe soon the Thruster (though they arenít back in production yet). The Jabiru is the cheapest of these at £42,750, C42 and Eurostar both sell for around £45,000, and the CTSW is the most expensive at £50,500.
So the SLA, at £36,000 (for the 80hp version Ė the 100hp comes in £800-£1000 more, depending on exchange rates) is excellent value. And if youíre a qualified pilot, the SLA is also a very practical choice. Itís easy to maintain Ė the stainless fittings are very practical, and the structure is sufficiently simple that most parts can be fitted by the most cack-handed of owners. Itís not quite as fast as the C42 or Eurostar, but the difference is unlikely to make much difference except when youíre bragging at the bar. And although itís factory-built, you can specify your own instrument fit. Thereís loads of room, the view is exceptionally good, and it should withstand normal wear and tear well. The controls arenít as sophisticated as some; stick forces are higher, and thereís more adverse yaw. But this is offset by the minimal changes of trim with power and flaps. And the huge tank capacity means you can fly for up to 8 hours at a stretch Ė in other words, once per weekend. The downsides? The looks are somewhat idiosyncratic, and the lack of opening windows, the limited range of trim speeds and the slightly clunky-looking trim tab.
But overall, itís a great product, and we should be proud that Britain can produce such an excellent product.
Instructors have been buying the Ikarus C42 and the Eurostar in big numbers which has undoubtedly helped their sales, but one school has invested in the SLA, and itís to be hoped that more follow suit.
As a school machine, there are a number of factors that make this an interesting proposition. The local manufacture should make repairs or modifications quicker and easier. The stability and firm controls make it easier to demonstrate the effects of controls to heavy-handed students. The lack of changes in pitch trim with throttle will mean that the trim control can be more or less ignored in the early stages. The long-stroke suspension and ease of entry are also in its favour, and the price is certainly tempting.

Medway Sla 80 or 100 Executive
Medway Microlights
Burrows Lane
Middle Stoke
Me3 9rn
N/A; tel +44 (1634) 270780; mob ; fax +44 (1634) 270648;
Side-by-side two seat high wing monoplane with conventional three-axis control. Wings have unswept leading edges, swept forward trailing edges and tapering
chord; conventional tail. Pitch control by elevator on tail; yaw control by fin-mounted rudder; roll control by ailerons. Wing braced by struts from below; wing
profile Clark Y-type; 100% double-surface. Undercarriage has three wheels in tricycle formation; Compression spring main and bungee nosewheel suspension, on
all wheels. Push-right go-right nosewheel steering connected to aerodynamic controls. Hydraulic Disc brakes on main wheels. Tube and fabric construction with
composite panels. Engine mounted below wing, driving tractor propeller.
Length overall 5.72m, 18.76ft. Height overall 2.26m, 7.41ft. Wing span 9.69, 31.78ft. Taperingchord 1.567m, 5.14ft at root, 1.343m, 4.40ft at tip. Dihedral 1.2į.
Sweepback N/Aį. Main wing area 14.1m2, 151.68ft2. Aileron area N/Am2, 0.00ft2. Aspect ratio 6.66/1. Fin area .4m2, 4.30ft2. Rudder area .46m2, 4.94ft2. Elevator
area .92m2, 9.89ft2. Tailplane area 1.08m2, 11.61ft2. Wheel track 1.6m, 5.25ft. Wheelbase 1.4m, 4.59ft. Main wheels dia overall N/Am, 0.00ft. Other wheels dia
overall N/Am, 0.00ft.
Rotax 912S engine, liquid-cooled. Max power 100hp at 5600rpm. Woodcomp Classic 3-blade propeller, 1575m, 5166ft diameter x 18į pitch,. Gear drive reduction
, ratio 2.26/1. Max static thrust N/Akg. Power per unit area 7.09hp/m2, 2.19hp/ft2. Fuel capacity 98litre, 21.59Imp gal, 25.89US gal.
Empty weight 268kg. Max take-off weight 450kg. Payload 182.00kg. Max wing loading 31.91/m2. Max power loading 4.50kg/hp. Load factors +4, -2
recommended, +6, -3 ultimate.
Max level speed 110mph, 95knots, 176km/hr. Never exceed speed 119mph, 103knots, 191km/hr. Economic cruising speed 85mph, 73knots, 136km/hr. Stall
speed 38mph, 33knots, 61km/hr. Max climb rate at sea level 860ft/min. Min sink rate 600ft/min at 68mph. Best glide ratio with power off N/A/1 at 60mph. Takeoff
distance to clear 15m obstacle 66m** on grass. Landing distance to clear 15m obstacle 202m on grass. Service ceiling 10,000ft. Range at average cruising
speed N/A miles. Noise level N/AdB(A) LEL.
* Under the following test conditions
Airfield altitude 10ft. Ground temperature 20įC. Ground pressure 995mB. Ground windspeed 10kt. Test payload 450kg.
£36000as tested, with specification as above
NA = Not available
Figures above are manufacturerís/importerís data
Figures in text are testerís experience.
Many Thanks to Dave Bremner
AND MF magazine where this was first Published

Tony Smith
12-03-09, 09:36 AM
Takeoff distance to clear 15m obstacle 66m** on grass.
Is that really correct? "Take off distance required", not "ground roll". That seems very short, even on best-case short grass on firm ground and without any safety factor.

Tony S

17-05-09, 07:06 AM
Yes that the best we've acheved