Flight test: Staaken Z-21A Flitzer
PUBLISHED: 09:56 23 November 2018
Twins but not identical: Bob Grimstead samples two aerobatic Flitzer homebuilts constructed in different hemispheres | Words: Bob Grimstead - Photos: Karen Grimstead
Windsong in the wires; a stiff breeze tweaking your eyelids; taut fabric controls felt through safecracker-sensitive fingers; a whiff of warm oil; the thrill of high-speed airflow; and the rising, falling thrum of a fast-revving engine−nothing charges the senses and stirs a true pilot’s heart like open-cockpit aerobatics!
I have sampled two of the dozen or more flying Flitzers. Englishman Rupert Wasey’s award-winning G-ERIW (the natural wood-coloured one in these photos) has an 80hp Aerovee 2,180cc VW conversion with a single magneto and electronic ignition with dual Hall effect coils, each firing two spark plugs. It also has the unusual airborne VW luxuries of a starter and an alternator, charging an onboard battery.
Rupert crafted a wooden airscrew specifically designed to optimise his Flitzer’s performance, and this later developed into his full-time, and very successful business, Hercules Propellers, which now supplies Spitfire airscrews.
The second Flitzer I flew belongs to Australian design and technology teacher Mark Crawford. His has a production four-cylinder Jabiru 2200 motor, also nominally developing 80hp at 3,300 rpm. This originally drove a second-hand, home-whittled propeller he bought with the engine, but was later swapped for an Australian Heliptera propeller.
Subsequently he too has fitted a Hercules propeller for improved performance and reduced vibration. Mark’s blue fuselage mimics that of German WW1 fighter ace Lt Jansen of Jasta 6.
After a decade or more of submissions, calculations, correspondence, and an independent handling assessment, Rupert Wasey’s British Flitzer has finally been approved for aerobatics. This is my second flight in it and this time I can turn it upside-down.
By virtue of being certified as ‘Experimental, Amateur Built’ under the superior, FAA-like regime, Mark Crawford’s similar Australian Flitzer was aerobatic from the outset more than ten years ago. Such are the frustrations of the presumably safer but more restrictive British system.
The Staaken Flitzer is a remarkable aeroplane−the only biplane I know of that will not only fly, but actually fly really well behind the puny power of a VW Beetle car engine. And its handling is delightful; light, nimble and responsive. The brainchild of British aviation artist and lifelong aeronautical enthusiast, Lynn Williams, his Flitzer is a modern design but with a distinctly period flavour, masquerading as a 1920s German sport biplane of Great War layout.
Williams supplies plans for several sub-types of the basic design. The original Z-1 Flitzer was his bright red prototype, powered by an 1,835cc VW. Next came the stronger Z-21, incorporating a longer-span tailplane and other small refinements, but still using a 22-inch wide fuselage. Larger pilots can build the later Z-21A featured in this article, with its roomier, 24-inch cockpit.
The slightly more compact StummelFlitzer has a shorter fuselage with raked cabane, a larger tank bay and rounded flying surfaces. This model is available in two versions, Types S and R, optimised for intermediate level aerobatics, using more powerful engines in the 60 to 110hp range (including the Australian Rotec or Czech Verner radials). The StummelFlitzer’s structure is strengthened to cope with a +6/-3g loading, and balanced slaved ailerons have been added to its upper wings to improve roll performance.
A smaller version still, the Kobold (or Goblin) Flitzer is under development, designed to use a less powerful 1,835cc VW engine bolted directly to the firewall, and has no fin for added excitement. There is a two-seat Z-2 Schwalbe (Swallow) version, plus the bigger, longer-range Z-3 Falke (Falcon). Being a compulsive doodler, Lynn is forever inventing new sub-versions, like the Walter-powered F.2 Tiger ‘Fighter Trainer’, Vale Stormcock, Rotec-powered Maksimov SK26 ‘Russian Fighter’, and ‘racing’ Meteor Speedwing. For the current status of these and his other brainchildren, you should visit the FSV website (see spec box).
Over 366 sets of Flitzer plans have been sold. They come as thirty-three, comprehensively-illustrated, three-foot by two-foot sheets, with much text and many exploded views of individual components. All metal fittings are drawn to full size, and the original plans have been updated to reduce fabrication time.
The sub-versions are all derivatives and so come as modifications to the basic plans. The fuselage is the longest component, but being less than twelve feet long means it can be built in a small workspace, like a single-car garage or even a spare bedroom.
Flitzers use conventional construction with plank spars and built-up ribs forming the USA 35B wing section, squared (or rounded) off with laminated wood wing-tip bows. All ribs are identical for ease of building, although the lower wing’s structure is a little more complex, and the upper wing has a slightly wider chord because of its thicker spar. The interplane struts are complex works of art, utilising spruce blocks, plywood cheeks and spruce fairings, making them look like authentic single-piece wooden components.
All flying surfaces are wire-braced, the empennage with single-strand piano wire, and the wings using beefy multi-strand wound cable. This is not flexible aircraft control cable, but proper 3/16-inch and 1/8-inch structural cable, fastened over thimbles with Nicopress swages and adjusted by turnbuckles in the time-honoured way.
The short cabane struts are of welded steel tube, and the undercarriage is also a steel tube structure: a pair of triangular frames supporting two fixed cross-axles. These support a third, floating axle, held in place with tightly wound half-inch bungee cord.
The legs are wire-braced and faired with balsa wood, then fabric wrapped for streamlining. Like other components, there can be some variation in the wheels. The plans call for re-spoked Honda C-90 moped wheels with new hubs and no brakes, but both the examples I flew had cable-operated heel brakes and fat motorcycle tyres.
The Flitzer’s fuselage is the usual built-up wooden box with scarfed ply exterior skins and fabric covering, and beautifully scarfed pressings of 1.0mm and 0.8mm ply for the rear decking and cable fairleads. The tail and control surfaces have similar construction to the wings but with either built-up or solid spruce or fir ribs, drilled out for lightness. A baggage locker behind the cockpit has enough room for a tent, sleeping bag, mattress and a change of clothes or two.
Both examples are beautifully built, with exquisite cockpit finishes. There is a small black rectangle of non-slip walkway on the left wing root, from which you lift your right leg over the Tiger Moth-style fold-down hatch and step on to the seat and down into the cockpit. This is perhaps a little snug despite the widened fuselage, with a fairly upright seat and fixed four-point harnesses.
In both cases, my shoulders touched the cutout sides, but there was enough room for comfort, although my head did stick well out into the breeze because I am six foot two inches tall. Lynn tells me there is now a modification to lower the basic Z-21 seat by two inches, while the Z-21B has a deeper decking, and later sub-variants have bigger, deeper fuselages.
The small instrument panels each hold eight miniature instruments plus a radio. Both Rupert and mark use Microair M760 radios, which fit into a standard two-and-half-inch instrument hole. Rupert’s has a throttle and mixture quadrant on the left sidewall, fuel on/off selector on the right and a sprung carburettor heat knob in the centre. Mark’s layout is slightly different, with a single sidewall throttle (the Jabiru’s Bing carburettor is self-compensating and so needs no mixture control) beside which is the choke.
The carb heat plunger is under the left of the panel, with the fuel cock directly below the tank, a long stretch under the panel’s centre. Both use cork-and-wire fuel gauges and are otherwise similar.
Rupert’s Flitzer’s empty weight includes five pounds of lead ballast to compensate for its longer engine mount, while Mark’s is built from denser Douglas fir, so they both tip the scales at around 540 pounds. Adding forty pounds of fuel and 170 pounds of Bob took our total to the 750 pound official maximum. The temperature was around 22°C (70°F) for all my flights, and often with a ten-knot breeze slightly across the runway.
The Flitzer is small and short, with a tall, narrow-track undercarriage, so building it with the original fixed tailskid would only be a good idea if all your flying was to be off grass (most unlikely nowadays). Because of this, both Rupert and Mark have fitted theirs with differential footbrakes and a lockable swivelling tailwheel to improve ground handling on hard surfaces.
Your eyes look straight along the forward fuselage past a small windshield and through a low triangular tunnel of cabane struts. The nose sticks up high ahead and pretty much in the way of a direct forward view, but it is quite narrow, so your taxying weaves need only be fifteen to twenty degrees either side of track.
The main gear is nicely sprung, and both aeroplanes’ brakes were exactly right, with no play and enough effect to turn precisely. For a tighter turn, you can ease the stick forward a little and give a blast of power without too much tendency to lift the tail and stand the ironmongery on its nose.
When I opened Rupert’s throttle, that big VW emitted a most un-Volkswagen-like snarl and the little aeroplane surged forward with steady acceleration. Only a tiny bit of left rudder pressure was needed until I lifted the tail at around thirty mph.
Then it gave a wayward dart to the right, easily and immediately caught by a prompt dab of increased left rudder. At 45mph, in the tail-high attitude, she flew off cleanly with a gentle backpressure on the stick. I accelerated to sixty mph in ground effect in the time-honoured manner, after which we were soon climbing like an interceptor at an eager 700 feet per minute. By then you can no longer hear the enthusiastically blattering engine over the torrential roar of slipstream in the wires.
Having a silencer as standard, Mark’s Jabiru engine is much quieter, and it rotates the other way (like a Continental or Lycoming) so right rudder is required, but otherwise its handling was much the same, although its performance was a little less sprightly with the original Heliptera prop.
Maximum level speed at full throttle for both aeroplanes was around 95mph IAS, but setting 3,000rpm on the VW gave a comfortable 85mph cruise, burning perhaps sixteen litres per hour, giving an effective safe endurance of two hours. Rupert usually cruises his at 75mph at about 2,750rpm.
Being a biplane, visibility in flight is inevitably less than brilliant forwards past all those wings and wires, but it is generally more than adequate, and especially behind the trailing edges. And of course it is the work of but a moment to waggle the stick to whip any impinging bit of the airframe out of your line of sight.
Both windscreens were a little small for me; I could feel the wind biting between my goggles and the helmet’s rim. Nevertheless, it was great to wheel and turn over the countryside, framing golden villages and green fields between those silver wings.
Turns each way need just a little squeeze of rudder to coordinate; overdo it with a bigger, Tiger Moth-sized push and the nose will slew across the sky. As expected, Flitzers have low aileron forces, with instant response and a high roll rate. The ailerons are very powerful and the elevator is light, but with plenty of feel.
Indeed, all the controls are light, and yes, actually very nicely harmonised, something writers often claim without it really being true. Believe me, this biplane has classically harmonised controls, with light ailerons, slightly heavier elevators, and a rudder that is just slightly firmer in feel−perfect!
I made tight turns left and right with the cabane struts parallel to and just above the horizon. This gave a neat and easily judged sixty or so degrees of bank, and I was satisfyingly able to hit my own slipstream virtually every time. A properly coordinated wing-waggle was a bit harder because at first I used too much rudder. Like Fairy Liquid, just a squeeze is all you need.
I know the Flitzer’s supposed to be a mythical inter-war sportplane, but this thing’s an eagle in dove’s plumage−a Great War dogfighter in civilian colours. Although, while it’s a pussycat in the air, it can be rather more of a tiger on the ground!
The slow handling is fine. Throttled back, the airspeed reduces more gradually than the low inertia and apparent high drag had led me to expect−even despite the fine-pitched propellers and all those unstreamlined wires.
However hard you raise the nose, a Flitzer won’t really stall, but just mushes downwards at between 41 and 45mph (depending on example, weight and configuration) despite pulling back quite hard on the stick. With no elevator trim, eight to ten pounds of stick backpressure is needed to achieve a stall, which is good. Even with some power on, these aeroplanes just nod their noses, with no warning buffet, but no wing drop either. But they do sink power-off. Doing just a couple of stalls lost me 500 feet!
Lynn told me that the Z-21 is significantly stronger than the prototype Z-1, which was calculated to a strength of +8.5g ultimate at top weight. That was designed to the old British Semi-Aerobatic category. All reserve factors on the Z-21 were subsequently increased for those components with less than a 1.5 reserve (so the Z-21 tailplane forward spar is 87% stronger than that of the Z-1 – which Lynn says was already adequate).
This makes Z-21 Flitzers fully aerobatic, with a manoeuvring speed (Va, the speed at which full control travel may be used) of 100mph, and a 120mph Vne. Nevertheless, Lynn recommends against flying snap manoeuvres, sustained inverted flight or outside manoeuvres.
Rupert manages Immelmanns with about a +4g pull-up, and everything else you might want to do can be accomplished at or below that figure. Entry speed for a loop is typically 110mph, although during testing Lynn has dived to 125mph IAS. ‘Design diving speed’ is 160mph.
In each aeroplane, I first tried a few wingovers each way. These were very easy and tremendous fun, so I went further, flying gentle aerobatics from 3,000 to 2,000 feet. Rupert had warned me that his could flick if you pulled too hard over the top of a loop, which surprised me, considering the Flitzer’s light weight and low wing-loading, but I didn’t encounter this problem with either example.
Perhaps he didn’t then have the sensitive fingertips needed to ease her over the top of gentle aerobatics at below the 1g indicated stall speed.
You need some rudder each way in the loop (right as you dive, left as you slow with the VW motor, and the opposite for the Jabiru) but not much. I started my first at 110mph and saw forty on the dial over the top. None of my loops would have been properly circular, but they were quite tight, and it was very satisfying while floating across the top to look up over the centre-section to admire the inverted world above, immediate and undistorted through any canopy.
Barrel rolling to the left in Rupert’s Flitzer was a bit difficult because of interference between the stick, my knee and his throttle. It was easier rolling to the right despite that being against the engine’s rotation, but either way you do have to raise the nose very high before applying aileron, or else it falls a long way during the manoeuvre, even if you roll with full aileron.
The same was true to a lesser extent with ballistic aileron rolls in either direction in both aeroplanes. Neither aeroplane’s fuel cap had been modified for aerobatics and Rupert said he got a face-full of fuel when he tried a slow roll, but I did make one each way without any problems other than preventing a slight roll-rate increase in the last quarter – a common issue, easily cured with practice.
Half-Cubans, with their momentary inverted period were also fine, rolling right in Rupert’s and remembering that tiny kick of left rudder immediately after the check forward and as I hit the aileron to coordinate the initial roll back to upright. That helped in keeping straight, and I actually hit my slipstream as I pulled out of the second one, which was very satisfying.
Stall turns of course took increasing rudder on the way up (again, left with the VW, right with the Jabiru). Holding each aeroplane in the vertical for just a moment (as anticipated, they slow quickly), I hit the appropriate rudder at forty mph and round they both went, pirouetting precisely on the spot, sweet as a chocolate-coated nut, and with very little need for either forward elevator or opposite aileron.
Throttling back halfway around kept the radius tight and gave me plenty of time to accelerate in the downward vertical before pulling out. Even a stall turn against the engine went okay! Of course the trick was to use a little and increasing left aileron against increasing right rudder on the way up, to give plenty of left rudder availability for the kick at the top.
This aeroplane is just instinctive to fly, a marvellous dog-fighting aircraft−ike a biplane Turbulent or Corby Starlet, but much stronger. It was great fun, I learned a lot, and I can’t wait to do more. Oh, and I never exceeded 3.5g. It just wasn’t necessary.
Back in the circuit, I flew downwind with 2,500rpm and around 65mph, and then throttled back to 2,000rpm for the approach at sixty. Initially Rupert briefed me “don’t fully close the throttle”, because she sinks rather rapidly in this configuration although later, when I was more used to the type, I did try several glide approaches. Some stick backpressure is needed at this speed, but there is still plenty of control authority for countering gusts.
Aeroplanes of the notional period would normally have flown glide approaches because of their engines’ unreliability, and that is still not a bad idea when using a converted automotive powerplant today. But with the throttle closed, Flitzers glide like a Chubb safe. Add full rudder with aileron for a sideslip and they come down like a safe with the door open! Helicopter pilots would be familiar with the forced landing glide angle. Sailplane pilots need not apply.
A curving approach is preferred, both for decent forward visibility and to keep the threshold in sight. With the nose well down to hold seventy mph, I found I actually got the best view of the runway during the turn by stretching my neck to look over the top wing.
There is plenty of elevator effectiveness, with or without power, and on one occasion, when the Jabiru engine stopped on final, I found there was still enough elevator control for a proper flare. Despite a fairly prolonged hold-off, these little aeroplanes don’t float far, thanks to that vast amount of drag, especially with the nose raised and the undersurfaces of all four wings presented to the airflow.
You have to rotate the nose through a very big angle to get it up to the correct three-point attitude, and this takes time. However, if you are not patient, and your stick is not fully against the back-stop by the time the wheels brush the turf, the Flitzer will bounce on its stiff main gear legs.
Trying to correct this by pumping the stick back and forth merely causes pilot-induced oscillations and divergent bounces. The only solution is opening the throttle to go around. Mark’s Jabiru Flitzer was better in this regard; its slightly softer bungees had a little more give and generally allowed a softer, bounce-free landing, despite my repeated inability to get his stick all the way back to its rear stop.
When you do get the flare right, and everything comes together nicely, it can be truly satisfying. For a proper three-pointer you hold the stick right back into your gut as she touches down, and then ride out the little bounces and waddles as she gradually loses momentum, dancing your feet lightly on those rudder pedals (and, just occasionally, the brakes) to keep straight.
Nearly every time both aeroplanes slowed, and despite their engines rotating in opposite directions, they both showed a distinct attraction towards the trees along the right runway margin. Sometimes an early rudder input would stop this before it got started, but on other occasions a good jab of left brake was needed to rein them in. I did not land either of them on a hard runway, but I suspect that would be a little more challenging.
Partly because of their ground manners, Flitzers are the kind of aeroplane you would never tire of − endlessly challenging to fly accurately, but immensely satisfying when you get things just right. Nimble, manoeuvrable and sprightly, their handling is perfect for the connoisseur.
Above all, they’re just so much fun!