Pilot report - Zlin Z-526
[Reprinted from Today's Pilot Magazine]
Zlins have always enjoyed a reputation as fine aerobatic aircraft. Clive davidson flies the classic Z-526.
A few years ago I was generously given a solo trip in a Zlin 526. I had just stepped from a Yak-52 and it struck me what a delight the Zlin was to fly. Everything seemed to happen in low 'g', the ailerons were light and precise, a bit slow in roll perhaps, but beautifully harmonised. The rear cockpit, P1 position sited over the wings' trailing edge, held a good all-round view and the cockpit workload was low. I had been keen to try a few lomcovaks but was informed before I departed that several 526s had lost their propellers whilst engaged in these manoeuvres. All these incidents had occurred in Eastern Europe with the exception of one in Germany, where the canopy was attacked and smashed by the escaping blades. The propeller fixing flange cracked between two of the six propeller bolts. The flange then disintegrated, releasing 60lbs of highly rotating propeller. "I won't even flick it" was my smiling reply and I thoroughly enjoyed my next 15 minutes airborne.
Alan Baldry, an affable and generous individual, kindly asked if I would like to fly his Zlin, G-BPNO. Of course I would. It transpired that this aircraft had originally been constructed as a Z.326 and was later upgraded to Z.526 standards by the factory. The fixed pitch propeller was replaced with an Avia V503 constant speed unit and this heavy addition altered the centre of gravity, requiring the solo pilot to sit in the rear cockpit. This position in turn was re-wired so that the pilot is able to operate the facilities the front seat, therefore, lost its switches. Alan and his partner Terry acquired the aircraft in 1988, specifically to accommodate their love of aerobatics and annually they average around 40 hours each. A typical detail consists of 15 minutes of aerobatics while the other critiques (sounds so less harsh than criticises) the performance before the roles are reversed. A PFL and then a couple of circuits conclude the flight. Fuel burn for such practical fun is an almost frugal seven Imperial gallons per hour.
During the walk-round, the impression created by the two-seat Zlin Z.526 sitting on its narrow track mainwheels and tail, is that of clean lines purposefully reflecting the advanced design philosophy of the day, from the vanes of the propeller spinner right back to the angular rudder. The wings are of all-metal construction with Duralumin sheet surfaces, including the ailerons. The fuselage is a welded tube space frame and is fabric covered, the tailplane and fin are Duralumin sheet clad but the rudder and elevators are fabric covered. The fin appears quite small in relation to the large rudder, while the forward upper section of the fin is unpainted and houses the VHF aerial.
The non-retractable tail wheel is fitted with an anti-shimming tyre and a break clutch that unlocks for short radius turns while taxying, rather like the Yak-50. The 5ft 7in main gear is relatively narrow tracked and retracts rearwards into the under surfaces of the mainplanes. In the POH the undercarriage is described as "partly tiltable", which really means it semi-retracts with about one-third of the wheel and tyre protruding into the airflow in the up position. There are no undercarriage doors but the undercarriage legs are fitted with fairings. The half-span ailerons are differential, with the aileron down throw by 3.8in and up by 4.2in. Incidentally, with the single-seat version, the Z.526A, the ailerons have a slightly greater throw of 4.4in up and 4.2in down, providing a greater response and roll rate. The ailerons sport spades under the mainplanes, aiding rapid application while providing a lighter feel. The inverted six-cylinder Walter Minor 6-III of 160hp is closely cowled with six short exhaust stubs exiting on the right. It is fitted with an electric starter, two-stage fuel and oil pumps and two impulse magnetos. For inverted flight there are fixed chokes in the two carburettors and special fuel and oil piping arrangements ensuring that supply pressures are maintained. During the model upgrade of November Oscar from Z.326 to Z.526, the oil cooler was re-sited from inboard the left wing leading edge to a more 'natural' installation within the left engine cowl. The highly-polished propeller blades are of a higher aspect ratio than normally seen on the front of our regular US training aircraft and are really quite elegant. The small blades on the spinner turn it relative to the propeller hub and these drive a hydraulic pump in the hub. The spinner also moves axially by around a third of an inch relative to the propeller hub, under dynamic air pressure. This axial movement actuates the pitch control mechanism of the blades.
From the pilot's viewpoint, there is only one 'power lever'. All that has to be done when setting rpm is to move the throttle and then, when in a dive or climb, when the rpm would normally change, the blade pitch alters to maintain the set rpm. The stable rpm also increases the efficiency of operation, both in terms of a light work load and improving aircraft performance.There are two internal wing tanks, each holding 9.9 Imp gal. Within the fuselage are two smaller tanks, a 0.65 Imp gal collector and 1.5 Imp gal header (under and above the front pilot's feet respectively), the first gives up to four minutes of inverted flight and the other, ten minutes flight should the fuel pump fail. Ferry tanks may be fitted onto the wing tips. Mechanical fuel gauges are fitted in the upper surfaces of the wings, similar to those on the Chipmunk.
The undercarriage position is indicated by annunciators in both P1 and P2 positions and has wing-mounted mechanical indicators, which project above the wings when the undercarriage is down. These retract, becoming flush with the wing top surfaces when the undercarriage is raised. The undercarriage is operated by an electric strut (under the P1 seat) and has an independent mechanical unlock in case of strut or battery failure. It also has a 'dead-man' switch on the right leg.
Access to the cockpit is via either wing, avoiding the flaps as you step up. The solo cockpit (for P1) is the rear of the tandem seats and places the pilot squarely in the fuselage with a good all-round view, in line with the wing trailing edges. The front cockpit is similar to the rear but lacks the electric service switches, artificial horizon, rudder trim, primers and fire extinguisher lever. All levers and trims (elevator and rudder) fall easily to hand (and foot) and the five-point harness makes you feel secure, as if part of the machine.
In the rear cockpit a large red plastic warning light is centrally placed and illuminates should more than 6g be pulled. There is also a canny main spar pressure gauge fitted in the front cockpit and this illuminates should the sealed, gas tight, metal spar crack from being over stressed.
The front (P2) cockpit view is marred only by the position of the compass, located on top of the instrument panel and directly in line with the nose. Logically, the cockpit is easy to interpret. I particularly liked the flap lever marked with aircraft symbols with the flaps showing up, intermediate and down positions; the near friction-less controls with no play; the Eastern European fuel and oil coding (yellow and brown) on the instrument panel; and the feel of the throttle. All appear familiar with no real oddities. In fact, it is well conceived and constructed.
Oddities are saved for the Pilots Operating Handbook, The retractable undercarriage is "tiltable", graphs are termed "tablets" and when opening the throttles to full power the pilot is actually "admitting fully fuel to the engine". When discussing the 'g' overload indicator, it warns that during "acrobatics, overpass of limited permissible values is signalled"! Mixture control is referred to as "altitude balance", idle cut-off is "gas arrestment" and the electric bus bar is "sectional switches", close enough when you know the answer, but slightly removed from our idiom. Values of weight, speed and pressures are, of course, in metric with a simple and easy-to-use 'Weights and Loads' chapter.
START UP AND TAXI
For engine starting, the fuel cock is selected to main tanks (or the header) prime the hand pumps and after checking that the magnetos are off in both cockpits, have the prop' pulled through at the same time. Switch on the dynamo, instruments, undercarriage indicators, magnetos and starter with the hand brake applied. Engage the starter and once the engine is running, set 800 rpm and ensure that the stick is held back. The oil pressure shows within a few seconds of starting. As the oil temperature rises to 35°C or 45° in winter, the revs should be increased to 1,400 rpm. The magneto and spark plug check uses 1,750 rpm and idle check 600 rpm.
Taxying is straightforward for a taildragger weave and clear the area ahead. It has a good turning circle when the tailwheel castoring lock disengages; and solid brakes. The tail doesn't even think of unsticking, unlike a Piston Provost I flew but in its defence they were pneumatically operated and I was doing more than 2mph! Wing walkers are recommended for the Zlin with winds above 13 knots. The book figure for take-off, to clear a 50ft obstacle, separates a grassy surface from concrete by only 33ft 1,350ft versus 1,319ft. The still air, flat ground roll quoted is a short 755ft from a hard surface.
Taking off from RAF Wyton's Runway 16, the wind was an undemanding 140º/8 knots, well within the crosswind limitations of 13kts. With a good clearing scan, I lined up and completed the brief take-off checks. Sitting behind the prop' turning in an anti-clockwise direction, I anticipated the swing to the right. With 'T's and 'P's within the green arcs, full power coming on positively and the ASI showing life, I eased the control column forward. Swing was easily countered by a slight deflection of left rudder. She rode straight and smooth and flew herself off at 47kts. I held the aircraft down with no real force against the trim to 65kts, which came quickly, before easing the stick gently back and climbed out at 70kts. I delayed raising the undercarriage until there was no longer any useable runway ahead.
The Zlin settled into a comfortable climb, the ASI showed 75kts 3kts higher than the best rate of climb speed. I adjusted attitude and re-trimmed, then tried a few varying gentle pressures on the rudder and rocked the wings in anticipation of what was to come. She promised good handling and a lot of fun.The haze layer rested on the horizon at 3,000ft QFE (QFE set for aerobatics) and had a pink and purple hue away from the sun. Blue sky was above so we continued the climb to 4,000ft. Once there, with 2,350 rpm set and 97kts indicated, I had a superb view, particularly with the low nose attitude. Trimmed out, she flew hands and feet off with the wings level and the ball in the middle. A good start.
The Bedford Levels appeared beyond and beneath our leading edges. We were clear of clouds, crowds, cities and controlled airspace, with fields below should we need to put down in a hurry. I had memorised the best glide speed of 65kts and completed the ingrained Hasell checks. First, I examined the aircraft's stability. With the stick free, control column induced phugoids died within two short cycles. The release of cross controls from banked and level attitudes demonstrated both longitudinal and directional stability.
With height held and the area cleared, the stall characteristics were explored and proved what a mild-mannered machine the '526 really is. No wing drop is present when the airframe is clean at 61kts and there was only a slight tendency for the right wing to drop with full flap (40º) and undercarriage down at 59kts. Even then, just as one might expect, the wing could be held with opposite rudder. There is no aural or light warning of the impending stall, other than the undercarriage warning bell when the gear is up. You just feel it through the stick, upright or upside-down. It was in the stall we had identified one of the great benefits of such a low wing loading and wing section. With its docility we could promptly fly away from the clean stalled condition with complete control and no appreciable height loss.
A later marque of the Zlin (the Z.526AFS Akrobat Special) had 5ft 9in truncated from its overall wingspan. This undoubtedly increased its rate of roll but it lost its 'forgiving' ability at the slow end of the envelope. Apparently, the shortened wing was stressed for a higher 'g', but with the increased wing loading and now lower aspect ratio wing, the induced drag increased dramatically when 'g' was pulled. So much so that the whole feel and performance of the marque was degraded.
Time to enjoy a different perspective of the Zlin. Aileron rolls left and right from cruise (2,350 rpm) at 97kts are completed without height loss; slow rolls from 118kts, both left and right, so as not to become handed. The break-out force in roll is very light with the underwing spades. The wing can be rolled to the angle of bank and promptly stopped and held there. Their feel is marvellous but the rate of roll is just 60º per second, similar to a Bücker Jungmann or Beagle Pup. The wing does not have a great deal of inertia and it is possible to arrest the roll at a quarter, half, three-quarters and wings level for a precise four-point or hesitation roll. Aileron drag is not pronounced and easily countered with the correct rudder footwork.
And herein lies the real character of the Zlin Z.526; through its controls, light wing loading, crisp response and power. The roll rate is slow enough to be familiar and not too far removed from the everyday ordinary A-to-B machines, but it is perhaps partly the precision of control that endears you to this machine. G-loads are not inordinately different from a Chipmunk, (which has less power and weighs almost the same). Its upright seating position shows its vintage. Today, some 30 years later, a dedicated aerobatic aeroplane's seat would be inclined for the pilot to accept maximum 'g'. However, the Zlin is restricted to 3g and manages all of its manoeuvres within that envelope, so perhaps, the seating is fine for such low 'g' values.
The stall turn to the left against the engine, left us hanging on a knife-edge having used the upper POH figure of 65kts. I felt sure a reduction towards the lower figure of 59kts would be better. With a small amount of vertical sideslip she pivoted around cleanly. Next, with a stall turn to the right, holding her vertical and feeling as if we are slightly on our backs, the right rudder introduced positively at 37kts lets the right wing drop and the left wing slide over 180º so that we were heading down hill, opposite rudder required with lead to prevent the pendulum effect. We had the vertical down line and with in excess of 108kts up for a loop, then a roll off the top. The Zlin is a clean machine and picks up speed downhill noticeably quickly. A couple of Cubans, all completed with a minimum of control forces and low poundage (stick forces), a cleanly and well flown slow roll off the top continued to show her nice, well mannered character in classic, gentle aerobatics.
APPROACH AND LAND
For type conversions, the circuit is the most obvious place where all the exercises of flight are condensed into a short period and high relative workload. Again, similarities with the Chipmunk spring readily to mind when considering the circuit manners of the Zlin. Remember to lower the gear (check for two green lights and the external undercarriage stalks projecting above the upper wing surfaces) although the low-speed warning bell will sound upon reducing power on finals if you forget. PUFA checks on finals; propeller pitch, undercarriage, flaps and altimeter set, the normal constant speed prop lever would be forward and fully-fine but once again, treat this system as fixed-pitch and just throttle back.
Slipping speed is between 70-75kts. The flaps are effective and the slight change in attitude further adds to the view forward. Engine revs are kept a little lower than expected, at 1000rpm, from base leg. The recommended flare height commences at 23ft, somewhat higher than my 'double-decker front seat' standard flaring height, and then level at 1.6ft. Either way it didn't bounce (much). There was a small tendency to float but this would have been considerably more had I used the initial figures determined from the stall information and multiplied by 1.3. Landing roll on Wyton's long 9,800ft runway was a short 820ft with little or no braking, although a short field landing should be accommodated within 650ft on grass. A gentle, weaving progress to the hangar was followed by a short cooling down pause. Shut down involves turning off the electrics, fuel and magnetos, in that order.
The Zlin has so much pure capability with such a long wing and little horse power, but things have moved on. The Z.526 was capable of flying all known manoeuvres of the day with acknowledged ease in a 3g airframe without the loss of height, plus its pilots introduced the lomcovak to the world at large. For the observer on the ground watching the aerobatics, the size and shape of the aircraft help its viewing, having good visual density. I hadn't realised the length of the wings until I later saw some of Keith's air-to-air photographs of November Oscar. This, coupled with its engine's low, rather melodic note, echoing and sounding not too far removed from a much larger V-12, adds well to the flavour and its acceptance locally. Competition aircraft now have higher wing loadings and greater power-to-weight ratios, higher rates of roll and climb performance, more grunt, more zaz! The Z.526 has been eclipsed, it is true, and it certainly had its day holding centre stage. Yet it is still an aircraft to fly and enjoy the sky in marvellous, curving, light, dancing flight.
Thank you Alan.
[END OF REPORT]
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