SOKOL

30 LIGHT AVIATION | NOVEMBER 2015

There were plenty of smiles on the faces of the team that makes up the Turweston-based Sokol Flying Group. The group, headed up by Turweston’s resident

vintage aircraft restorer and boss of ATSO Engineering, Alan Turney, had just taken delivery of the very rare Sokol M1C G-AIXN.

The Mras M-1 Sokol (Falcon) is an all-wood aircraft that was designed in Czechoslovakia during the latter stages of WWII, as a development of the pre-war Benes-Mras Bibi, an ultralight two-seater powered by a 65hp Walter Micron engine. While sharing the same general concept as the Bibi, the Sokol featured a shorter wingspan and a mechanically-operated retractable undercarriage. Intended as a trainer, tourer, air taxi or mail carrier, the Sokol was

advertised rather charmingly as an asset that will ‘increase your personal prestige and the prestige of your business…’

In its M1C version, which was the first to be built in any numbers, the Sokol introduced a 105hp Walter Minor engine and a fuselage stretched to accommodate a third seat behind the existing side-by-side pair. In this form, which first flew in 1947, apparently almost 200 were built, followed by over 100 of the later M1D version. Few survive today because, as with so many wooden aircraft in the UK at that time, the majority were condemned in the early 1960s due to concerns over longevity and structural integrity. A few survivors linger on, flightless, in museums.

G-AIXN, serial number 112, was built by Automobile Zavody in 1947 and Czech-

registered as OK-BHA. It was put onto the UK register in 1955 and operated on a Certificate of Airworthiness for many years. Based at Elstree during the 1950s, the logbook records that the aircraft was flown to Prague in 1956 for a detailed check. The undercarriage apparently collapsed in 1958, but caused relatively little damage. Moving to the Blackpool area in 1959, the aircraft was little used, having only acquired 450 flying hours by 1974, when it was transferred to PFA control. In 1974 the aircraft was based at Thruxton and flown by Mike Stapp, who wrote the aircraft up, very enthusiastically, in the July-August 1974 issue of Popular Flying.

Sadly, in December 1974 ‘XN was damaged in a wheels-up landing in the Bristol area which, as it turned out, was to

THE MRAS M-1 SOKOL FALCON

QUIRKY BUT CLEVER

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Francis Donaldson delights in an early post war Czech three-seat retractable

SOKOL

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keep the aircraft out of the air for a quarter of a century. For the next two decades the aeroplane was to move numerous times – it was acquired in 1979 by John Evetts and stored at Booker, where LAA Chief Inspector Ken Craigie recalls seeing it in a fully dismantled state awaiting a full rebuild. It subsequently moved on to Tony Smith at the Real Aeroplane Co at Breighton. In the early 1990s, I corresponded with new owner Peter Knott of Bradford, who reported that while the wings had been extensively restored, the fuselage appeared to have suffered from a major oil leak from the header tank that had soaked the front bulkhead and cockpit.

Passing through the hands of Alan Witt who stored it at Crosland Moor, the stop-go rebuild appears to have gathered momentum only after being acquired in 1996 by Mike

Howells, from the Barton area, who moved it to Ken Fern’s workshop near Stoke-on-Trent for Ken to complete. This was carried out under the expert eye of PFA inspector Chris Turner. The rebuild included a great deal of assorted woodwork, replacing the corroded magnesium alloy fuel tanks with welded aluminium ones and, with PFA’s approval, replacing the rivets that held the wing mounting lugs to the wooden spars with steel bolts.

A zero-timed Walter Minor was also acquired, along with a new wooden Airsport propeller. For peace of mind, a modern gascolator and Facet type electric pump replaced the aged back-up hand pump and twin filters. Shoulder harnesses were also fitted at this time, the aircraft previously having flown with lap straps only. Now

owned by Adrian Wood of Linton-on-Ouse, the restored aircraft finally flew again in December 1999, almost exactly 25 years since its accident, and was granted a Permit to Fly through the PFA system in early 2000. Since then, in Adrian’s proud ownership, it has continued to clock up hours for 15 years, before he reluctantly passed her on to Alan’s group in late summer this year.

Unique in the UK, the scarlet and white Sokol has aroused a fair bit interest at Turweston, despite the fact that it shares hanger space with some pretty exotic hardware. Noticing my interest in the new acquisition, Alan generously offered me a chance to fly it, and having taken in Mike Stapp’s report all those years before, I was quick to accept – although as Alan warned, I was to ‘make sure I fitted in first’.

SOKOL

32 LIGHT AVIATION | NOVEMBER 2015

From the outside, to my eyes at least the Sokol is a beautiful aircraft with 32ft of wing, built in three sections with a short centre section and tapered outer panels with pleasingly swept back leading edges and rounded tips. Seen in front view, the centre section is quite thick at the root but the aerofoil sections thin rapidly toward the wing tips. Without having particularly noticeable washout, it raised questions in my mind about the likely wing-dropping tendencies at the stall. We couldn’t think why the frieze ailerons are made in two short portions rather than in one piece, and the single-skinned plywood split flaps looked flimsy in comparison with conventional flaps. I made a mental note to be particularly careful about flap limiting speeds.

The Sokol’s plywood skinned fuselage is of pleasingly rounded cross section, giving it a somewhat sophisticated look alongside the rather basic flat-sided Jodel and Robin types. The main visual feature of the fuselage is the multi-faceted cockpit enclosure, with its twin front-hinged doors, reminiscent of wartime types like the Bucker Bestmann and more ancient-looking than contemporary British designs such as the Miles Gemini and Messenger with their full-width blown windscreens. It’s no surprise to read that the later, definitive M1D version had an enlarged single-piece canopy.

105HP POWERPLANTUp front, the inverted inline Walter Minor 105hp engine sits under a very neat set of aluminium cowlings with a fixed front nosebowl, fixed bottom section and two top-hinged side pieces, each stiffened by riveted on top hat sections so that they can be adequately retained by just three quarter-turn quick-release fasteners along their bottom edges. Opening these six Czech Camlocs and raising the cowlings, where they can be secured by stays, gives ready access to the whole engine compartment in a matter of seconds – a luxury not afforded by many of today’s installations where designers seem to feel that engines are best not tinkered with, so hide them behind

out in unison as the elevators are raised and lowered. Helpfully the scuttle, which covers the mechanics of the yoke controls behind the instrument panel, is a one-piece component held down by screws around its periphery, removal of which gives ready access to the mechanism and, incidentally, to the back of the instruments, making connection and fault-finding a cinch. Not many homebuilders enjoy this level of access, and many, with a fixed scuttle, can only access the back of their panels by diving inside head-first and upside down, an agonising as well as undignified procedure not lending itself to meticulous workmanship.

At the rear, the Sokol‘s tail surfaces are configured similar to a Chipmunk’s, the fin and rudder extending only above the fuselage to accommodate a one-piece elevator arrangement. The elevator is fitted with a very neat eyelid-like central fairing which finishes off the lines of the rear fuselage with a flourish, blending in the lines, the fairing being made in two nesting sections which overlap as the elevator deflects, as the photos reveal.

Climbing up onto the wing root walkway to look inside, the Sokol’s cockpit abounds with character features. There’s a pair of fixed bucket seats side-by-side up front, and it’s almost a surprise to see a third, single seat nestling centrally in what one initially takes to be a generous baggage area. Most strikingly, and testament to its post-war origin, the Sokol’s instrument panel boasts a pair of chromed control yokes rather than joysticks. In between, on the centreline of the panel, there’s a dinky throttle pedestal. The throttle and mixture levers are alongside one another, on the same pivot axis but move vertically rather than conventionally fore and aft, full throttle being ‘up’. As with Gipsy-engined Austers and other aircraft of this era, the mixture control works in the opposite direction to what might be expected, full rich being with the mixture lever in the ‘down’ position – the arrangement being determined by an automatic feature which, as a concession to forgetful pilots, discretely returns the mixture knob to ‘full rich’ whenever ›

Chipmunkesque tail surfaces. Note the tailcone that telescopes neatly together as the elevator rises and falls

flexible GRP cowlings secured by a multitude of loose screws that so easily get lost in the grass, or by off-putting lengths of recalcitrant piano wire.

The four-cylinder Walter Minor engine is mounted on an elegant beam-type engine mount, fabricated from aluminium alloy and steel, rather than the common welded tubular type. By virtue of the H-section fore-and-aft beams having substantial bending strength, this type of mount gets away without needing any diagonal cross bracing to carry side loads, which leaves better access to the engine accessories at the rear of the crankcase and contributes to a beautifully clean, simple looking engine installation. A large cylindrical aluminium alloy oil tank sits transversely across under the engine at the rear, complete with a small oil cooler. The engine also has a beautifully-fashioned aluminium alloy air duct to route the cooling air from the front intake past the fins of each cylinder, and a clever sliding section of the duct’s vertical face opens to reveal access holes for servicing the four spark plugs on the ‘cold’ side of the engine.

As on the smaller Walter Micron 3 engine, which is more common on LAA types, the Minor’s magnetos hang vertically downward at the rear of the crankcase, where they are readily accessible – no bad thing as like the Micron, this installation has a reputation for engine oil leaking into the magnetos as the oil seals deteriorate, collecting in the distributor caps at the bottom and eventually compromising the ignition. Equally accessible for maintenance, there’s a substantial electric starter mounted at the rear of the engine, plus a generator on the left-hand side of the crankcase, gear driven from the accessory case.

Intruding into the engine bay are two large-diameter (approximately two-inch) foot-long metal tubes which sprout horizontally through the firewall and don’t seem to be attached to anything. It takes a moment or two to twig that these are the forward ends of the control shafts which the pilot’s and co-pilot’s yokes are attached to, using the firewall to mount the front bearings, and that the tubes move in and

Note yellow undercarriage screw jack handle between seats with flap lever adjacent. Handbrake is centre, below the throttle/mixture binnacle

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“Climbing up onto the wing root walkway to

look inside, the Sokol’s cockpit abounds with character features”

52 LIGHT AVIATION MAY201134 LIGHT AVIATION | NOVEMBER 2015

SOKOL

The multi-faceted canopy has generously size, forward hinging doors

the engine is throttled back, and so protects against a lean cut on short final.

Under the throttle and mixture controls is a handbrake control which you push down (i.e. towards the floor) to activate both wheel brakes together, the left/right mix being dependent on the position of the rudder pedals. All mechanical, and cable-operated, the workings of the system are cleverly hidden from view. A devilish locking button on the brake lever provides a park brake feature, but operates as a ‘fly-off’ handbrake, rather than the more modern arrangement, meaning that to release the handbrake, no amount of pressing on the button will free it, you have to squeeze on extra brake on the lever before the button springs back and releases the lever again. All of this takes much longer to write than to do, but is worth rehearsing carefully before taxying because the unusual arrangement of throttle and brake will present enough of a challenge initially, without the additional risk of finger troubles making the brakes lock on.

Just behind the throttle pedestal, a vertical shaft emerges by the pilot’s right wrist, topped with a crank – the sort of crank that one might expect to see for lowering a shop front sun blind, or a heftier version of a vintage car’s manual window-winder. This is the one and only control for the Sokol’s delightfully simple retractable undercarriage, which through a clever system of gears, torque tubes and over-centre links, effortlessly folds the main legs backwards through almost 90° by winding the crank half a dozen turns. A rather ambiguous

notice adjacent to the crank fails to convey which way the crank works, but red and green lights on the panel confirm when the wheels are in transit or ‘down and locked’. Again, perhaps, the designer had forgetful pilots in mind when he arranged that the retracted wheels lie exposed under the wing, rather than buried away in wheel wells, so that at the expense of a little extra drag the aircraft is saved from major damage in the event of a wheels-up landing. When it comes to this particular misdemeanour, as they say, ‘there’s them that have, and them that will’, as I was forcibly reminded by Alan during the preflight brief.

A conventional manual flap lever, like a car handbrake of the era, is mounted just behind the undercarriage crank, and offers a choice of three angles of deflection. Intriguingly, the full flap position feels ‘dead’ on the lever, as if the gate isn’t engaging to hold the flap fully deflected. This turns out to be because an over-centre action comes into play which also cleverly (I’ve used that word again) provides maximum mechanical advantage when the flap air loads are highest, to make the lever easy to work. This is a design feature that could do with being copied on one or two modern microlights where the pilot seemingly needs the arms of a gorilla to get the flaps down.

The fuel system is equally pilot-friendly, the Sokol originally having just one 12 gallon tank in one wing root. ‘XN has the optional second tank a mirror image of the first, the two being plumbed so that both feed the engine simultaneously via a simple on/off valve. A

single electrical fuel gauge sender is mounted in the left tank, calibrated such that it in theory at least, the panel-mounted fuel gauge indicates the total fuel on board. Having said that, wary of asymmetric feed from interlinked tanks and of the accuracy of ancient electrical gauges, any prudent pilot would back this up by wielding the provided wooden dipstick before flight and making mental calculations of elapsed flight time, likely consumption rate and likely fuel remaining.

As a first indicator of ergonomic issues to come with the aeroplane, the fuel on/off selector is mounted out of the pilot’s normal line of sight under his knees, where the seat joins the floor, convenient no doubt in terms of pipe runs and fuel flow but not so good if you want to quickly access it in an emergency in flight. Equally unsatisfactory, there’s no clear placard to confirm the on/off orientation of the fuel lever, and the ‘on’ position defies convention in being with the lever at right angles to the fuel pipe rather than in line with it – which just goes to emphasise the importance of a good pre-flight brief! Needless to say, a placard is now on order…

Briefing over, sliding into the non-adjustable pilot’s seat I found that like many old aeroplanes, the Sokol clearly hadn’t been designed with tall pilots in mind, or especially, long-legged ones. However much I squirmed around trying to place my feet in different ways on the rudder pedals, my knees pressed against the bottom edge of the instrument panel. A not unfamiliar problem

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The long wings, nicely cowelled in-line engine and rounded

fuselage give the Sokol a lovely vintage charm

and uncomfortable, but not normally an issue, except that in this case there’s a pair of electrical lever switches at the bottom of the panel which I could unwittingly turn off with my left knee cap, while my right knee conflicted with a rather flimsy looking push-pull carb heat control when hot air was selected.

Compounding matters, the shiny control yoke comes up against my legs well before the ailerons reach their stops, limiting the available travel. A potential flight safety issue? It’s not unusual to find control throws limited by the pilot’s frame with vintage and homebuilt aircraft, but certainly something to bear in mind. Without doubt, if operating in maximum crosswind conditions, not getting full control deflection might make the difference between success and disaster, so it’s a matter of judgement what’s acceptable. In this case the available aileron movement seemed quite adequate, and with a 10kt wind slap up and down Turweston’s Runway 09, the conditions could not have been more benign.

On closer scrutiny, the switches at risk from knee activation turned out to be marked ‘aux’ and ‘radio master’, so were not flight critical – no-one seemed to know what ‘aux’ did anyway, if anything (!) – so after a minute or two’s deliberation about the cramped cockpit situation I decided we were good to go.

Starting procedure with the Walter Minor involves priming the engine with a couple of shots on the plunger and then, with throttle ‘closed’ and switches ‘off’, turning the prop over by hand through four blades to suck in, at the same time checking that stray oil will not cause any of the inverted cylinders to hydraulic lock. The prop swinger then places the propeller carefully at an angle that puts the engine at ‘top dead centre’ of the next compression, which is meant to give the electric starter a chance to get the engine turning with momentum before coming up against the hindering force of the next compression. Wheels chocked and with brakes on, master ‘on’, throttle cracked a mite open, pressing the starter button results in a somewhat alarming ‘thump’ from up front as the starter engages and the propeller jerks into life, the engine fires up sporadically, and after a few seconds’ juggling with the throttle, settles into a steady idle.

Taxying the Sokol initially seems a bit of a challenge. The tailwheel is essentially free castering, with only a feeble detent to encourage it to track straight ahead. Taxying at low speed, with little propwash over the rudder, and without applying handbrake,

there’s no sign of any differential braking action and a distinct absence of directional control. Trying to engage the handbrake by a couple of clicks to liven up the braking action on the rudder pedals, as on a Chipmunk, doesn’t seem to work on the Sokol. Instead, after a few tentative turns you get the knack of applying rudder in the direction required with your feet and then using a squeeze of hand brake to grab the wheel you want to hold back. With a bit of practise, you realise that the layout of the throttle pedestal is quite clever (that word again!) because you can operate the brake lever with the palm of your hand while simultaneously working the throttle with an outstretched index finger. This technique learned, by the time I have reached the threshold the Sokol has been tamed and I feel reasonably on top of the quirky controls.

EAGER LIFT OFFChecks complete, and with first stage of flaps set, the Sokol runs commendably straight on take off and lifts off eagerly from the level attitude at 45mph after a run of hardly a hundred yards into the gentle breeze, climbing out indicating 2,100rpm at full throttle with 60mph on the ASI. Clearing away the flaps was straightforward and, passing through 500ft agl with all indications looking good, it was time to try out the retraction gear for the ‘chassis’, as early retractions were sometimes termed. Finding that the handle wouldn’t go anti-clockwise but turned freely clockwise, and knowing how easy it can be to get in a muddle in an unfamiliar aeroplane, I immediately made a note on my kneepad that I needed anti-clockwise for wheels down.

After half a dozen turns on the handwheel, the system came up against a positive resistance which clearly signified that the wheels were fully up, which was confirmed by the aircraft’s more sprightly feel and a nose-up pitch trim change with the dangling ‘chassis’ now lying flush under the wing. Cleaned up, and without taking any particular care to keep the ball centred, the rate of climb at this solo weight seems to be a respectable 700fpm. The aeroplane felt nicely harmonised with light and effective ailerons (albeit restricted in their travel), a precise and not unduly light elevator and a rudder that did its job without fuss. The aeroplane was also blessed with a very effective fin and could be held in balance without conscious effort.

Levelled off, the Sokol felt pleasingly taut and responsive as it accelerated quickly through 100mph, steadying at 130mph indicated at 2,300rpm. Flat out, apparently, she will do

150mph on the level but conscious that this was just a familiarisation sortie rather than a formal test, out of deference to the age of the engine I didn’t choose to investigate that test point. She certainly felt as if she would cruise comfortably at 130, not bad for a 105hp three-seater that can operate off a short strip.

Holding her level in cruise at increasing speeds needed increasing amounts of nose down trim, indicating positive longitudinal stability at this forward cg. The elevator trim tab, which is hinged centrally on the elevator trailing edge, is operated by a small lever at floor level buried in between the front seats, but easily located by feel, and entirely intuitive in use.

While easy to fly generally, the photoshoot element of the mission showed up one or two minor challenges. The exercise of holding position in close formation with another aircraft, and to make precise changes in station compared to the camera ship, is good for bringing to light issues with stability and control about all three axes. I always find photoshoots more difficult with a control yoke than with a stick – perhaps this is partly through being less practised with this arrangement, but it does feel more difficult to make precise movements of a yoke control. Yoke systems also tend to use sliding pitch bearings and to have more components in them than stick systems, bringing more friction, backlash and stretch into the system, all of which add to the pilot’s difficulties. In the Sokol, the control system itself seemed nicely engineered and to be more direct-feeling than some aircraft blessed with stick systems, but I couldn’t help feeling I’d have preferred it with a stick.

The photoshoot also showed that the vision from the aircraft was quite restricted by its glazing arrangements and, as anticipated, I found myself flying in some unusual postures to keep the photoship in sight – it’s not so easy to fly accurately when you’re leaning forward as far as possible while craning your head back to try to look upward under the top lip of the windscreen, or when leaning hard to the right to look upward through the upper part of the passenger’s door! I was also conscious of the fact that with me flying solo, sitting in the left seat, it needed a significant amount of right aileron to fly wings level (you can see the deflected ailerons in the pictures) and this brought the right-hand bottom edge of the control yoke close to my right thigh, meaning that there was not much more right aileron movement available for manoeuvring. The fact that my achievable roll rate to the left was a lot greater than that to the right was

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“She certainly felt as if she would cruise

comfortably at 130, not bad for a 105hp three-seater that can operate

off a short strip"t”

52 LIGHT AVIATION MAY2011

36 LIGHT AVIATION | NOVEMBER 2015

SOKOLcertainly welcome when it came to the break-away manoeuvres, at the end of the session, which fortunately involved departing stage left.

Pictures taken, I was free to have more of a look at the Sokol’s flying characteristics. As I’d expected from the wing’s rather thin tip sections, the wing drop at the stall was pronounced, vicious and came without aerodynamic warning – Mike Stapp had commented 30 years before, one moment you are flying, the next you aren’t! Interestingly, Mike’s take on this, as a flying instructor, was that these characteristics would make the Sokol an excellent trainer, because unlike more benign alternatives, the Sokol would demonstrate quite unequivocally how an aeroplane can bite if it is mistreated. At the same time, the stall speeds are so slow (around 40mph indicated, depending on flap position) and the angle-of-attack so extreme, that a half competent pilot should have no excuse for an inadvertent stall.

Looking at the lateral/directional characteristics, there was no point in trying aileron releases in a sideslip because the roll response due to dihedral effect would be masked by the lateral out-of-trim. But the Sokol banks smartly in response to rudder inputs, in the correct direction, indicating positive lateral stability. Rudder releases in sideslips showed quite strongly positive directional stability, but without overshoots. Inputting rudder doublets, the dutch roll mode is well damped, so all in all the lat/dir behaviour is hard to fault.

Time to look at the approach configuration. Slowing to 60mph, winding the wheels down (two greens – great) and using full flap results in a moderate nose-down pitch trim change, which even full up trim couldn’t quite counter. Roll control at this speed was somewhat

diminished but still adequate, and the rate of descent – power off – impressive. When it came to the landing, I found that pulling the carb heat control on the approach meant my right knee having to come further back, further limiting right aileron travel and making me feel even more uncomfortable. The pre-flight brief with the photographer had included giving him several landings to shoot, from a vantage point alongside Turweston’s grass runway, and for the first of these I had had it in mind to make a full-flap, power-off approach culminating in a touch-and-go off a three pointer. But recognising that the ergonomic difficulties were starting to stack up, I decided to make things easier on myself by doing a power on approach with two stages of flap, ending with a wheeler landing. I could then sort myself out, backtrack and do another circuit finished in the same way. The Sokol turned out to be an absolute natural for wheeler landings, with no tendency to bounce, and in the benign conditions of the day it hardly even seemed necessary to pin it on the ground with a prod of forward yoke once the wheels had touched. Leaving the tail up until it could no longer be held up with the elevator, there was no perceptible swing as the tailwheel eventually dropped, and even without the benefit of full flap, she rolled to a halt in a very short distance.

WHAT DO WE THINK?So what do we make of the Sokol? For its day, it seems to have been a very successful rendition of a low-powered three-seater. Carrying three people at 130mph on 105hp, it would hold its head high (except perhaps in terms of manufacturing man hours) against its western European competitors such as the Jodel 100/1050 series that followed. Perhaps in some ways, coming in the years of immediate

SOKOL SPECIFICATIONS

DIMENSIONSCapacity: 2 passengersLength: 7.35m (24ft 1in)

Wingspan: 10.0m (32ft 10in)Height: 2.20m (7ft 3in)

Wing area: 13.8 m2 (149sq ft)Empty weight: 415kg (915lb)

Gross weight: 770kg (1,700lb)Powerplant: 1 × Walter Minor inline engine

78kW (105hp)

PERFORMANCE Maximum speed: 240km/h (149mph; 130kt)Cruising speed: 212km/h (132mph; 114kt)

Range: 1,000km (621m; 540nm)Service ceiling: 4,800m (15,748ft)

Rate of climb: 3.0m/s (590fpm) Fuel consumption at cruise

(2,200rpm): 24.4lph

post-war austerity, the Sokol of 1947 came too early to find a market. On reflection it’s the ancient-looking cockpit transparency, plus the inverted inline engine installation, which most gives the M1C version away as a vintage type, and with a modernised airframe, a Perspex bubble on top, a cockpit with another inch or two here and there, a rakish fin and rudder, and a Rotax 912 iS up front, it would not look out of place coming out of the Czech Republic today as another ‘plastic fantastic’. And it would be a good one too! ■

Although the throttle/differential brake arrangement initially appeared a little quirky, Francis found it easy to master as he taxied down to the hold at Turweston

Beautifully engineered beam mounts and hinge-up cowlings allow

excellent access to the engine

Third seat tucked in the back limits baggage capacity

somewhat if occupied