Tag: SF-25

By me
All photos me too, copyrighted

With the financial situation at the airlines still worryingly precarious, I recently came to the conclusion that it’d do me good to become a bit more efficient with my GA flying (a #firstworldproblem right there). Since I usually spend my time in light aircraft joyriding around at low altitude with, at most, one other person on board, it dawned on me (belatedly!) that my usual Cessna 172 is quite a financial overkill – and that if the point is to just soak up the scenery and not actually go anywhere, I could do that for a lot less. Thus, I decided now’s a good time as any to kill two birds with one (cheap) stone, and do what I’d been wanting to do for ages: get a Touring Motor Glider (TMG) endorsement 🙂 .

Unsurprisingly, this had Achtung, Skyhawk! written all over it. And while I had initially planned to do another of my “amateur flight reports”, I soon hit upon a better idea. Since the approach to flying TMGs does differ somewhat from that of conventional aircraft, I decided to have a crack at an interesting mental exercise: try to anticipate what I’d struggle with during training – and then, having gone through the actual course, compare notes. Not only does jumping from the mighty Q400 into an aircraft that is it’s polar opposite promise to be quite an educational and humbling experience, it could also make for an interesting read – particularly on the peculiarities of the brain and the frequently amusing lack of love between the conscious and subconscious…

Tools of the trade

But, first things first: the airplane itself. The machine that has had to suffer my first attempts at “soaring with cheating” is a neat little Scheibe SF-25C Falke, registered 9A-DHD and based at Zvekovac Airfield (LDZE) to the east of Zagreb. Manufactured in 1976 with the serial 44148, it has lead a surprisingly straightforward life, having been initially known as D-KDEF and D-KLUG (likely temporary identities for delivery), before passing to the Austrian register as OE-9116 in 1977. Operated out of Scharnstein Airfield (LOLC) near Linz, it would be sold to Croatia in May 2018, to later become the founding aircraft of the SZK Dubrava flying club – and the eighth of its type to permanently reside in the country¹.

Like many other motor gliders, the Falke as a type comes with a choice of engines, with the C model’s Type Certificate Data Sheet listing no less than nine! Most of these are converted VW air-cooled four-pops from the 1.7 to 2.1 liter range (either by Limbach or Sauer), but there’s also the option of fitting the garden variety Rotax 912 – and even the fuel injected 914 of up to 115 HP (in which case the aircraft is sometimes known as the Rotax Falke). DHD itself sports the most basic fit, a 1.7 l Limbach SL 1700 EA2 unit developing 60 HP for takeoff and 53 HP continuously, spinning a fixed-pitch Hoffmann HO-11*-150-B-65-L² propeller.

At 580 kg | 1,280 lbs all-up mass, in touring mode this powerplant is good for 120-150 km/h | 65-80 kts in the cruise – while as a glider, a glide ratio of approximately 1/21 can be expected at 70 km/h | 38 kts. The type’s manual states that the usual empty weight is around 375 kg | 827 lbs; DHD however tips the scales at 409 kg | 902 lbs, leaving 171 kg | 377 lbs for the payload and fuel. The latter usually comes in the form of a 44 liter | 11.6 USG tank mounted behind the cockpit; DHD however sports the optional 55 liter | 14.5 USG unit, which will take it… some distance, depending on your use of the throttle and how much of a glide can you get out of it. On the engine alone, the performance figures say you should get up to 750 km | 405 NM at the long-range cruise regime (2,500 RPM and 130 km/h | 70 kts), with the 9.5 l/h | 2.3 GPH fuel flow giving an endurance of 5 hours 45 minutes – though I know no owners who had taken them even remotely that far (delivery flights included).

Other stuff? Well, the same manual notes that the engine can conceivably take it up to an absolute ceiling of 6,000 m | 19,700 ft, though the service ceiling – the level at which rate of climb drops to 300 FPM – is significantly lower at 4,000 m | 13,100 ft… which begs the question of the length of time needed to climb all the way! Speed-wise, the C Falke is structurally limited to 190 km/h | 103 kts, though in level flight DHD’s SL 1700 will run out of ideas already by 175 km/h | 94 kts. However, the recommended limit in actual operations is 150 km/h | 81 kt, which corresponds to the Falke’s maneuvering speed – that is, the maximum speed at which a full and abrupt control deflection will not overstress the airframe (particularly important in thermals and mountain waves, where things are not always silky smooth).

¹ of course, this being Achtung, Skyhawk!, we have to mention the rest, at least in passing. In addition to DHD, as of May 2021 there are six other SF-25Cs on the Croatian register (9A-DBV/DDB/DDT/DLK/DVB/GDK) – and one SF-25B (9A-DGZ) that had sadly been written off due to storm damage. There was also a rare and fully airworthy early model SF-25A Motorfalke (9A-DAG) from 1965 – but that one was sold on to Serbia back in April 2016

² as with many bits of an airplane that do not require sexy marketing names, the designations of both the engine and propeller represent pretty much their entire ID card. The engine is thus a Limbach unit (SL, now just L) of 1,700 cm³ (more precisely 1,680), with a single ignition system (E), and intended for use in a tractor configuration (A) with a fixed pitch propeller (2). The prop itself is a bit more complicated, being a Hoffmann unit with a Type 11 hub connection without any later modifications (*), made of hardwood, 150 cm in span with narrow blades (B) and designed as left turning (L); the 65 is a measure of the geometric shape of the blade and refers to the forward distance in centimetres the propeller would cover in one revolution – that is, its pitch – measured at some reference point along the blade (for this prop at 75% of the blade span)

Great expectations

So, that’s the machine taken care of – now time for the guy flying it. To explain why I think I may struggle with the Falke, we first need to have a quick look at the scale of the challenge. On the face of it, short haul turboprop operations are often a curious mix of button pushing and stick & rudder flying, and tend to develop a very valuable – but also very specific – set of skills, which is then hammered into the brain by sheer force of repetition. On one hand, you don’t really need to fly manually except on take off and for landing; but on the other, the latter requires quite a bit of skill and finesse to get right, and on the Q400 even a slight lapse in that department can have some pretty uncomfortable consequences. Add to that the fact that four/five/six legs a day are the norm, and, like it or not, you’ll have ample opportunity to get your technique down pat; indeed, I myself had already crossed into the 2,000 range, far in excess of anything I ever did in GA (and then on multiple types to boot). Since all of these landings were, by their very nature, highly structured and regulated, under their sheer numbers my perception and reactions have inevitably become biased towards the speed, power, inertia and control response Q400 – albeit experienced only in a very limited set of circumstances.

At the same time, after thousands of hours of looking at the same gauges and reaching for the same switches in the same positions, the brain inevitably develops a “blind map” of the cockpit, and begins working to a well-rehearsed procedural routine that relies extensively on muscle memory and requires very little conscious effort. In essence, if left unchecked, after awhile the basic business of pulling yokes, pushing levers, turning knobs and poking buttons becomes almost automatic, and starts to depend heavily on the familiarity of the surroundings and the lack of change in them.

Thus, when change does occur – as will happen in a new cockpit – there’s always bound to be some negative transfer, despite all conscious effort to prevent it. While the brain is well aware that the situation has changed and that it needs to adapt accordingly, it will initially struggle to operate without that intrinsic, instinctive knowledge of how the aircraft behaves that it had previously always taken for granted. To compensate for this lack of data, it’ll start filling in the gaps with acquired muscle memory, various preconceptions and all manner of past experiences – all stuff that rarely (if ever) works. The magnitude of this effect depends on the person; some will be alright almost immediately, others may suffer for quite some time. In my experience, I’ve noticed that many of my “automated Q400 responses” tend to go away within not many minutes (when the brain builds up a first, crude mental image of the aircraft), and it seems that my penchant for GA and frequent manual flying the Q does help in shortening that period. However, whatever the duration and the manners of the new airplane, this is still the sort of thing that would be poor airmanship to ignore – no matter the thickness and content of your logbook.

To apply this “woke-ness” to my TMG training, I racked my head for tales of experienced Falke drivers, my own observations from watching them fly – and impressions from the 30 minutes I’d spent in control of one 15 years ago – and quickly came up with a list of things I feel I should keep an eye on:

• overestimating the Falke’s mass and underestimating its control response
• being too apprehensive about throwing it forcefully into a maneuver should the need arise
• misjudging its drag and coming in too high and fast, without the benefit of big props, flaps and retractable landing gear to control deceleration and rate of descent
• using the spoilers – not something I’ve had in this form on any airplane in the past – in a ham-fisted “Hulk smash” fashion
• and getting caught out by the ground effect (low wing + large area) and a) floating for far too long, b) landing too hard for comfort, and/or c) moving the stick with a force and displacement appropriate to the Q and thus setting myself up for an unstable approach

The precedents for this caution and introspection are multiple. In my previous encounters with wholly new airplane types – the UTVA U-75, the SOCATA Rallye and latterly the Diamond Katana – I had noted an initial tendency to flare high and with a sudden movement of the controls, a well-rehearsed Q400 reflex that tackles its quirky combination of inertia, high approach speeds and low tailstrike pitch limits. Unsurprisingly, on light aircraft this usually results in a long float some way off the deck, followed (more of than not) by a firm and inelegant touchdown as the speed bleeds off – hardly the proper arrival into the type of soft and uneven strips that I normally operate out of. Given that the Falke is lighter, more agile and aerodynamically far more efficient than any of them, it is reasonable to be on alert for more of the same – particularly given its eagerness in pitch³ and the fact that a far smaller change in Angle of Attack (AoA) is needed to produce the same results.

³ then there’s the control response with power off. With the engine running, the propwash provides additional airflow over the tail, increasing its effectiveness at all speeds. Remove the wash and that bonus is gone, resulting in a slight (and probably measurable) degradation in both stick feel and aircraft response

A similar point can also be made for use of the rudder. As on conventional gliders, the Falke’s long wings and large ailerons make for significant adverse yaw in the turns, which has to be countered by a lot of footwork – more so than on a “normal” touring aircraft. On the face of it, this should not be a problem, since I’m used to constantly keeping my tail in check; on the Q400, the P-factor of those huge props is such that you need to use rudder/rudder trim for any change in speed or power (down to as low as 3 knots or 2% torque), let alone in a turn. What does require awareness however is the magnitude of the input; the Q has notoriously heavy pedals and a very powerful rudder (part of which deflects up to 36°), and its application requires a firm, but still measured and comparatively short push action – which isn’t always compatible with the rudders and rudder pedals of light aircraft. This too was brought to my attention on the tail-happy Citabria, when the owner inquired as to why I was gingerly pussyfooting with the pedals, unaware that my muscle memory was trying not to yaw the airplane clean out of the sky and make an unholy mess back in the cabin.

Other predictions? Well, the Falke’s track record of docile handling and gentle behavior in the air suggests it has few (if any) naughty gremlins. That first 30 minute experience had hinted that it is very pleasant and relaxing to fly in many regimes (including the stall, which was a complete non-event), with the only oddity being a slow roll response due to the strong damping effect of its wing span. Another thing I imagine will feel weird at first is its low sink rate with power idle/off (me being used to getting 1/10 at best), which will initially make for an uncomfortably low traffic pattern and more than one overshoot in the descent. A further thing to keep in mind in glider mode are the temperature limits of the engine; with a minimum oil temperature of 50 °C required before you can fully open the taps, any extended soaring will have to factor in a potential warm-up period, and consequently an increase in the minimum altitude at which an air start is practical.

One other feature that had particularly piqued my interest is the landing gear. Though the outriggers mean there’s no chance of pulling a U-2 and tipping over onto one wing, the central main wheel nevertheless looks like it requires extra attention – since it, and not conventional main gear legs, is now the point around which the aircraft pivots during ground maneuvering (which contributes to its somewhat large 13 m turn radius). This I imagine requires a specific technique when operating out of rough or rutted strips, since countering the motion of the main wheel as it goes wherever the terrain wants it to go requires quick and energetic work with both the tail wheel steering and brake. This too is not particularly kosher on the Q400, since its large main wheel span (8.8 m), carbon brakes (which take their time to warm up and lock without regrets if you ride them too hard) and large nose wheel steering arc all require limited, well timed and patient inputs – just like the rudder. However, having put this thought to paper/screen, I then had a chat with a captain of mine who also has a TMG endorsement – and had, more so, done it on DHD itself. After I’d articulated my assumptions, he dispelled many of my ground maneuvering concerns – but did draw my attention to the need to actively keep the wings level during taxi, take-off and landing by using the ailerons, something that had not occurred to me at all in my initial analysis.

And last, but definitely not least, user-friendliness. Here I don’t anticipate any major issues; I know I fit… I’d used the metric system in the air before… and with my fair share of 60s Cessnas behind me, no panel setup – no matter how convoluted – is able to faze me anymore. The only thing that jumps out really are the spoiler levers: two big, handbrake-type affairs located on either side of the pilot seat. The catch is that they also operate the main wheel brake, which is activated by pulling the levers beyond the spoilers’ fully extended position; there are no pedal brakes, which is definitely something to keep in mind in the heat of the moment. However, given the Falke’s sedate touchdown speed – just 72 km/h | 39 kts – and its draggy, tailwheel-first touchdown attitude, there’s little conceivable reason for all-out braking on any GA runway in Croatia… particularly on the 630 m of it available at Zvekovac.

The levers do, however, raise an important question of ergonomics. Personal experience so far has shown that I appear to be ambidextrous as far as flying is concerned (and ONLY then!), and can operate the controls pretty much equally well with both hands. That said, being right handed, I prefer and feel physically more comfortable flying with my right hand, as I do at work (there’s muscle memory for you). When soaring with the engine off, this is a non issue: right hand on the stick and left hand on the spoiler handle, located almost exactly in the place I’d expect to find the up/down controls on the Q400 (the power levers). However, since DHD has only one central throttle lever, when flying under power, I’d have to switch hands and fly with my left (like on the C172), with the right reserved for the throttle, carb heat, the other spoiler handle and ignition/starter. Echoing the dilemma I’d faced on the U-75, the question now is whether to a) fly solely with the left in all regimes, b) switch between left and right as necessary or c) switch to the right only for extended periods of soaring. I guess trial & error will tell!

Keep the pointy end forward

So, how did it all work out in the real world? Unsurprisingly, I got some things right – but also missed the mark by quite a lot elsewhere. To make sense of the results, I felt it best to break the experience down into segments, roughly corresponding in theme (if not sequence) with the paragraphs above. Starting then from the top, we kick off with:

GROUND HANDLING

As foretold by my cap’n, this turned out to be quite easy, despite the bumpy runway at Zvekovac and a persistent (and annoying) crosswind. Quick footwork is definitely required, but the Falke’s response turned out to be very predictable, and I managed to get a hold of it already on my second time out. The aforementioned 13 meter turn radius does take a while to get used to, and on narrow runways one definitely needs to keep the outriggers in mind (located just outboard of the spoilers), lest he/she snag a runway marker or park them in a drainage canal. I was also surprised by how little braking was necessary, even during faster taxiing; with comparatively little mass, a low pressure main tire and some assistance of high grass, stopping was usually just a mater of closing the throttle and giving it a few seconds to run out of steam – meaning that I did not really miss classic foot brakes. Indeed, on my very first taxi, I felt distinctly unnerved by the mismatch between engine note (60 HP, so you have to rev it to get going) and the sedate pace of movement (rarely above 10 knots), half expecting it to suddenly accelerate and roar off like all hell broke loose (like the Q400 likes to do).

Another thing that had taken me by surprise was the poor ride comfort. The RWY 22 end at Zvekovac is a bit bumpier than the rest of the strip, and maneuvering there (particularly in a 180 degree turn to line up for take off) quickly showed the limits of the C model’s simple rubber shock absorber. On take off and landing, it was not so bad; but during taxi, when the full weight of the aircraft is on the wheel, it was quite uncomfortable and borderline physically tiring (mind you, I’ve been told this is Rolls-Royce smooth compared to the earlier B model, which had no shock absorber at all!). What’s worse, that this could be an issue had not even crossed my mind previously, being used to light aircraft that had been designed for rough(er) field operation right from the outset.

Adding to the workload was also the need to keep the wings level, as cap’n had also warned me. When rolling for take off or on landing, this wasn’t much of a job, since the ailerons become/remain effective at very low speeds, down to as little as 30 km/h | 16 kts. But, at taxi speeds – and particularly over the rougher bits of the runway – the Falke was quite eager to dip onto its outriggers, making for an even less comfortable ride. However, some experimentation with the opposite rudder – using inertia to tip the entire aircraft back into a wings level position – usually did the trick, though experience will undoubtedly reveal better ways to do this.

The major problem with this tipping tendency that I can see so far is that during touch and goes, the torque and gyroscopic effects of the rapid increase in propeller RPM will swing the nose significantly to the side, making it all but certain that you’ll end up on the outrigger. Its additional rolling drag – as well as becoming a pivot around which the aircraft can turn – could then swing the nose even further out and make for a lively departure. The key, it seems, is not to be aggressive with the power; I found that, even with a long landing, I had more than enough runway to slowly advance the throttle and get into the air with plenty of space to spare. This then allowed me to parry any swing more precisely and with less rudder deflection – though the downside is that the aircraft could become airborne before full throttle is reached (as had happened to me once after touching down with something like 60 km/h | 32 kts – and 70 km/h | 38 kts being at that point enough to get me airborne).

GENERAL HANDLING

On this front, I’m happy (?) to report that my assessment of both the Falke’s handling characteristics and my own response to them was pretty much on the ball! The old Q400 muscle memory had inevitably gotten the better of me in the beginning, and the first few circuits were as elegant as a brick falling down a flight of stairs (and I can’t even scapegoat the day’s thermal turbulence). Thankfully, progress improved dramatically after half a dozen landings, and by the end of the first session, I at least had general handling down pat. Things were, surprisingly, best in pitch; as noted previously, the Falke’s aerodynamic setup means that it is very willing to maneuver around its lateral axis, and that even a small change in pitch/AoA produces a significant change in lift. This I was prepared for; what did surprise me after the initial “shock” was how much its response reminded me to that of the Q400 (except in control feel, which is quite heavy on the airliner). Once I’d gotten used to the sight picture from the cockpit, I was outright shocked at how easy I could read what the nose was doing and how much input was needed to keep things in check. The only thing I really had to concentrate on was the need to correlate extension of the spoilers with a backwards pull on the stick; this took a bit of practice (since I had the impression that the spoilers had a pronounced non-linear effect), but by the sixth or seventh landing, I was pulling tailwheel-first greasers despite the turbulence.

Handling the spoilers had also solved another dilemma: which hand to fly with. As I’d been instructed during the course, for take-off and during normal engine operation, I should keep my left hand on the stick and the right on the throttle – and then, for soaring, approach and landing, switch to right on stick, left on spoiler lever. Though I myself had also considered the option of switching as needed, this particular method seemed flawed, since such a switch can lead to a momentary disruption of the flight path (something I’d experienced already on the U-75), particularly on final approach if you’re short and need to shuffle hands to add a bit of power. However, after the first few approaches, the rationale became clear: the spoilers are incredibly powerful, and you can come in close to the runway – keeping well within safe gliding range – and still make it just by modulating spoiler extension and leaving the throttle alone. To make it even better, the spoiler lever is spring-loaded to the retracted position (full forward), so operating it is simply down to varying hand back pressure; and with moderate and quite informative resistance in the handle, this means you can be incredibly precise in metering out drag.

All of this had three important effects: a) it alleviated my fears of coming in high, fast and without enough drag to slow down, b) it meant that floating would not be an issue and that I could flare late and low without worries, and c) the same time-honored method of controlling speed with the stick and rate of descent with power/drag works beautifully on the Q400 as well (especially since it has a big, four-meter air brake on each wing). Hence, I ended up on an aircraft whose pitch response and performance in the flare are reassuringly similar to that of my daily driver, with the flight path controls all in the same place and operated in exactly the same way – resulting in shocking ease and speed with which the Falke and I had managed to work together!⁴

⁴ this similarity/familiarity should not be overlooked. In my case, it enabled my muscle memory to work WITH the aircraft as opposed to AGAINST it. One student, who only ever flew Cessnas and Pipers beforehand (left hand for yoke/speed, right for throttle/vertical rate) ended up doing a nasty hard landing in a Falke when he pulled back on the spoiler lever instead of the stick, and dropped the airplane right onto the runway from an altitude of about 2-3 meters (this had occurred after a long circuit-bashing session, likely a reversion to “previous instincts”)

However, if at this point I had any illusions about being one with the Falke, they were quickly dispelled by events in the other two axes. As expected, the long wing span makes it quite lazy in the roll; but just how lazy I discovered after entering my first thermal on the climb out, when the aircraft rolled violently to one side – and my full opposite aileron input had no effect for at least a second or two. Though this is perhaps an extreme example (open plains at 3 PM on a cloudless day make for pretty strong thermals), it is nevertheless a welcome one, since such behavior is uncommon on the types of airplanes I normally fly – which definitely warrants keeping it in mind!

But, what let the whole side down at first was my use of the rudder. Here, the Citabria Experience came back in full: countering the adverse yaw of both intended and unintended rolling required quick and occasionally significant inputs, something which my brain was reluctant to do (despite objective evidence that it should) for fear of over-yawing the airplane and sliding it about like something out of Fast & Furious. Compounding the problem was that in a steeper turn, the Falke, like all gliders, wants to continue banking in the same direction. The cause lies once again in its large wing span; in a tight enough turn, the outer wing moves significantly faster than the inner one, thus producing significantly more lift – so much so that it can overpower the glider’s natural rolling stability and effectively tip it over itself. This is easily sorted out with a bit of opposite aileron⁵ – but this again requires opposite rudder to cancel out the adverse yaw, which results in having to constantly jiggle the rudder from side to side (and even on occasion fly with crossed controls). All of this is perfectly doable – generations upon generations of glider drivers will attest to that – but for a high-performance airplane driver, all of this is subjectively new ground… despite logically being a clear as day.

A new yaw-related problem then appeared on landing – but this time had nothing to do with aerodynamics as such. While I’d quickly managed to nail down my vertical and horizontal profiles, for awhile I persisted in landing with a slight crab, something I was not really aware of until my instructor pointed it out. He said this was common for people transferring from touring airplanes, since the Falke’s smaller engine allows the cowling to taper off significantly in order to eek out a bit of extra streamlining. If you’re used to using a conventional “straight cowl” as one of your visual references on landing (as I am on the C172), you’ll subconsciously try to get the same sight picture on the Falke, and invariably land at an angle (which the grass runway at Zvekovac duly forgave, many thanks!). Rather embarrassingly, this is the same trap I fell into on the Diamond Katana not two months ago; and while in both cases my brain eventually got the message, it was definitely unpleasant to have to add rudder in the flare by conscious force and without a “visible” need to do so. The mind boggles!

⁵ as always, there’s a catch: if too slow and in too tight a turn, yanking the stick to the other side will cause more problems than it will solve. If conditions are right, the upgoing aileron on the inner wing will raise AoA sufficiently to stall the entire wingtip, converting the turn into a full blown spin

SOARING

Being “just” flying without the engine, soaring as such does not present any new problems handling-wise (elegance, however, is another matter entirely!). Indeed, the only things that I had pegged in my analysis were the control response without propwash – and the need to warm the oil to 50°C or above before flooring it. The former turned out to be a non-issue; and while there was a slight drop in responsiveness in yaw and pitch, it was not nearly as significant as I thought it would be – and had I not been specifically looking for it, I might have chalked it up to the effects of thermals or turbulence. Additionally, the Falke’s soild glide performance meant that flying downwind just 600 ft above ground quickly became normal – an impression later reinforced by a simulated engine failure on upwind, during which I managed to make a 270° procedure turn to line up with the opposite RWY 04 and almost overshot the threshold with the spoilers extended (OK, I had 15 knots of tailwind to help, but still).

Unfortunately, the short time I’d spent so far on flying with the engine off – some 10 minutes – meant I could not get a meaningful impression of the latter; with 28°C on the ground and 25° in the pattern, the oil temp went from 92°C to around 75°C in that time, which doesn’t really tell me much. However, I did note that even in the heat, the oil does tend to cool quickly and warm slowly, so the real acid test will be prolonged soaring (later in the course) and/or lower temperatures (later in the year).

One thing that did particularly intrigue me was the whole process of shutting down the engine in flight. Procedurally, it’s a piece of cake: throttle gently to idle, leave it there for a spell so the engine temperatures and pressures stabilize – and then just flick the ignition switch to off. What I did not anticipate was the need to actively stop the propeller from windmilling, accomplished by raising the nose and letting the speed bleed off (and briefly punching the starter if the blades had stopped in a vertical position). The reason for this quickly became obvious, and with hindsight should have been obvious from the outset: like the Rotax line, converted VW units are shut down by cutting the ignition, and not by starving it of fuel as is the case with conventional Lycomings and Continentals. If the prop is allowed to windmill, it will not only create additional drag, but also suck fuel into the cylinders (since it is readily available); and if the cylinder walls and heads are sufficiently hot – which they will be after a prolonged low speed climb – the fuel will spontaneously ignite on contact and combust roughly in the same manner as if the engine was running, adding to the rotation of the prop. Normally this happens only once or twice after an in-flight shutdown (as had happened to me), since the cylinders tend to cool pretty quickly and it doesn’t take long to stop the prop; however, it is nevertheless a potent reminder to be very wary around a VW nose even if the airplane is shut down. Indeed, I’ve been told stories of hot Limbachs and Sauers coughing into life after nothing more than a quick yank of the prop…

Update 19 MAY 2021:

Having gotten a bit of additional soaring time in, I thought I’d report back with a couple of fresh observations; nothing “revolutionary”, but enough to add some extra substance to this section. As I’d noticed previously, the oil does tend to cool quite quickly even at moderate ambient temperatures. Upon reaching 3,000 ft with 17°C outside, the oil was pegged at its usual “power on” 90°C; but after gliding down to 2,000 ft (some four minutes at an average 300 FPM), lighting the engine, climbing back to 3,000 and repeating the process, the oil was down to 60°C, by which time I decided to start the engine again and leave it running to keep it warm despite ample altitude left to go. Having then gently settled into a cruise at 1,500 ft with 20°C outside, at 2,100 RPM it took a good 10 minutes for the oil to go past 70°C. It is worth noting that I had kept the cowl flaps open the entire time (on advice of my instructors, given the hot nature of training ops), and I’ve been told that for prolonged soaring closing them measurably reduces cooling rates for both the oil and cylinders…

USER FRIENDLIESS

Among the many things I did not expect while writing this piece was that this section would end up being the one with the most interesting revelations! The spoiler levers I had tagged as a potential issue turned out to be anything but; instead, what did require additional brainpower was something as basic as reading the Airspeed Indicator. No, not the fact that it’s metric – but that the Pitot tube that feeds it is mounted on the vertical stabilizer (as it is on a “purebred” glider) and thus sits square in the middle of all the propwash. Hence, it over-reads by default whenever the engine is running – and does so by a different amount depending on the throttle position. Thus, despite the Falke’s aerodynamic efficiency and pussycat stall behavior, I found myself adding slightly to my climb speed “for the wife and kids”, ending up at 110 km/h | 59 kts – well above the 85 km/h | 46 kts called for by the manuals and the 90-95 km/h | 49-52 kts briefed by the instructor due to the day’s thermal turbulence.

Then there was a Grade A rookie mistake, one I’m still trying to wrap my head around (when I manage to stop laughing at it): like many light aircraft, DHD has a friction control for the throttle lever, which was set quite high when I first sat inside. I though that a bit too much for my taste (mistake #1) and on subsequent circuits loosened it slightly. However, when I decided to see how far a thermal would take me – keeping the engine at idle since I was just 500 ft above ground at this point – I assumed (mistake #2) that the throttle would stay put, and thus failed to note the exact idle RPM (mistake #3). Having topped out and decided to head back down into the circuit, I was somewhat puzzled to find that I was struggling to descend at nearly 140 km/h | 76 kts and with the spoilers fully extended, barely making 3 m/s | 600 FPM. Normally, this speed with brakes out should be seen with at least 15° of nose down pitch, so my first thought was that I had hit another of the many thermals that had been lingering along the edge of the circuit. However, having covered some distance, my rate of descent actually started decreasing towards 2 m/s | 400 FPM, at which point I started to suspect something else was up. Belatedly, my ears then informed me that the engine note was far too loud for idle; and lo and behold, I noticed that the throttle lever had moved in slightly in all the commotion and increased RPM by 300 revs… quite a lot on the Falke, as I then discovered! Needless to say, I kept the friction pretty much all the way up from then on…

Another prediction that ended up being a bit off is “I know I fit”. While I most certainly do and can get comfortable even with my headphones on (more so than on the similarly tight Katana), problems began to arise when I needed to deflect the stick fully. To enable it to be moved to its extremes in such a small cockpit, the one on DHD is, by necessity, a bit short; and when you place your hand around it, taller people like me find that your legs get in the way left-right – and your “gentleman’s area” to the back. While I successfully managed to hit myself in all three on the first few flights, the frequency of… hmmm… “interfering with myself” soon began to decrease, though I’m still not entirely sure whether due to the reduced deflection of my control inputs (having began anticipating the aircraft better), subconsciously altering my sitting position to compensate, or a combination of both.

One other flight control to get used to is the elevator trim, a small lever located between the seats (and in roughly the same place I’d expect to find the electric trim switch on the Katana). On DHD it is a bit tightly set, which actually suits me just fine, since this too approximates the heavy controls of the Q400. What is an issue however is that to operate it in soaring flight or when at idle, I have to switch hands – again, not something I feel comfy doing at low altitude. However, the good thing is that the control forces are light whatever the trim setting, so you can easily “overpower” the trimmer and just keep additional forward or back pressure until you have time to adjust it.

And though it may not exactly fall under the category of “user friendliness”, all of the Falke’s controls have excellent feedback, so once you get a hang of the airplane, you can almost tell what each bolt is doing just by judging the feel in the pedals and stick. This makes it very relaxing and easy to fly in an old school manner – solely by visual reference to terrain – with just an occasional glance at the oil and cylinder temperature gauges. Indeed, the club is planning to stick on a yaw string, which will then make for a proper “soaring with cheating” experience – and hopefully another Achtung, Skyhawk! article!

Sources:

• EASA – SF-25 Type Certificate Data Sheet (TCDS) & Type Certificate Data Sheet for Noise (TCDSN) (PDF)
• Hoffmann Propeller – designation decode (PDF)
• Limbach – designation decode (PDF)
• SF-25C Pilot Operating Handbook (for both Rotax 912 and L1700 versions)

Changelog:

• 23 MAY 21: added photo as OE-9116
• 19 MAY 21: added soaring update