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Flight test: Airplane Factory Sling 4

PUBLISHED: 16:16 16 January 2019 | UPDATED: 16:40 16 January 2019

Sling 4 flight test

Sling 4 flight test

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A nicely-designed new four-seat kitplane with good handling and an excellent cockpit view | Words: Dave Unwin – Photos: Keith Wilson

Photographer Keith and I met up with The Airplane Factory’s James Pitman and UK agent Tim Hardy on a beautiful early autumn day at the delightful grass airfield of Fenland in Lincolnshire. As the Sling taxied towards us my initial impressions were that it is quite a handsome machine.

Indeed, the very smart paint job is vaguely reminiscent of a USAF Thunderbirds scheme from the 1950s.

Starting the preflight, one of the first things I noticed was that−perhaps surprisingly−it is very nicely finished. Why ‘surprisingly’? Well, the test aircraft is the same one that was built over seven days and completed on the last day of the 2016 LAA Rally. In fact, when I start examining the airframe closely I’m extremely impressed by the quality of the construction. I could’ve understood the odd rogue rivet, but if there were any less than perfect I didn’t find them.

The fuselage flows fluently from spinner to fin and looks well-proportioned, with the only aesthetic aberration being the twin blade-type aerials aft of the cockpit. These, however, were fitted at the request of the customer whose avionics specification included TCAS!

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Both the materials and methods used in the Sling’s construction are entirely conventional. The monococque fuselage is made from aluminium (except the cabin top, which is composite), as are the wings, which feature five degrees of dihedral and two degrees of washout.

The flaps, ailerons, fin, elevator and rudder are similarly constructed, whilst composites are used for the tips. The fin and tailplane seem nicely proportioned, both elevator and rudder are horn-balanced, and I note with interest that the trim tab carried by the port elevator has been extended, presumably to enhance its effectiveness by increasing its area.

Ailerons and elevator are actuated by pushrods while the rudder uses cables. The elevator trim and large-span slotted flaps are electric, the flaps having four settings: ‘up’, ‘1’ (10°), ‘2’ (20°) and ‘down’ (30°). Position and strobe lights are located below the elegant up-swept winglets, with dual LED landing and taxi lights built into each wing’s leading edge, near the tip.

The undercarriage offers reasonable ground clearance and is of quite wide track, with a relatively short wheelbase. The mainwheels are carried by a one-piece composite spring bow and fitted with hydraulic disc brakes, while the nosewheel is sprung by self-damping polyurethane blocks and steers through the rudder pedals. All three wheels feature snug-fitting spats.

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Power is provided by a 115hp Rotax 914 UL turbocharged air/liquid-cooled flat-four, which turns an electrically-actuated, constant speed Airmaster prop with three ‘scimitar’ blades, and is fed from an 84-litre fuel tank in each wing’s leading edge.

An interesting aspect of the cowling is that there are access hatches on both sides, even though the oil dipstick and coolant bottle are on the starboard side. While having two hatches does allow a more comprehensive inspection of the engine, to do any real maintenance you would need to take the top cowling off.

An excellent feature is that as well as being able to access the oil and coolant through the starboard hatch you can also see and reach the hydraulic reservoir, which is mounted on the firewall next to the cooling system’s overflow bottle.

Access to the cabin is excellent. Well-positioned, streamlined footsteps just aft of the trailing edge lead up generously proportioned non-slip wing-root walkways. Unlike the two-seat Sling 2 (which has a sliding cockpit canopy) the 4 has two large upward-opening ‘gull-wing’ doors, supported by gas struts.

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The door sills could be a little lower, to make it easier to step down into the cabin, but it’s certainly not difficult. However, you do have to step onto the seat first and I was loath to do this, as the interior is luxuriously outfitted in grey and red leather and looks very smart.

Stepping on the seat just didn’t seem right! I found the adjustable pilot’s seat very comfortable and even the rudder pedals can be adjusted, although not that easily. While the test aircraft was fitted with optional toe brakes, a single handbrake next to the throttle is the standard fit.

Before going flying I looked at the back seats and was pleasantly surprised at the amount of room available. This ‘4’ is more than a 2+2! There is even a baggage bay aft of the rear seats (accessible via a door on the port side), although admittedly it is not large. I suspect its small size is intentional, the better to prevent a thoughtless pilot from attempting to wedge too great a mass of stuff back there.

The baggage compartment houses the BRS (ballistic parachute recovery system), although customers can choose not to fit this, freeing up additional baggage space and reducing the empty weight by fifteen kilograms. The POH claims up to 35kg can be carried back here, but as the C of G shifts aft as the fuel burns, you would be well advised to watch the loading carefully if the back seats are occupied.

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’Sierra Alpha’s panel is loaded with advanced avionics−there’s even a blue ‘LVL’ (wing-leveller) button, more typically found in aircraft like the Cirrus SR22 or Piper M600. The avionics are mostly Garmin G3X and there are no analogue gauges as even the standby instruments (a Midcontinent SAM AI with integral slip ball and a combined ASI/altimeter) are electronic.

There’s a large EFIS in the left-hand P1 position, a slightly smaller one in front of P2, and a neat stack consisting of a transceiver, transponder, autopilot and audio panel in the centre. The panel also carries lots of toggle switches for the electrics, several warning lights, two different controls for the prop, identical silver-topped plungers for choke and cabin heat, a rotary mags/starter switch and a T-handle for the BRS. It’s quite busy.

A centre console flows seamlessly down from the centre of the panel and extends aft between the seats. It carries the single−and rather chunky−throttle lever, with an Andair fuel selector set in front and the parking brake further back. That throttle and its operation are worthy of further comment. The lever consists of a big T-handle with a spring-loaded catch (like the afterburner selector on a jet) which must be raised to push the throttle to maximum power.

There is a detent at the ‘100%’ position which gives 100hp (maximum continuous), but full power (115hp) is only achieved at the wide-open 115% throttle position. Confused? Don’t be! There is a throttle arm position sensor mounted on one carburettor, which measures throttle position linearly from 0% (idle) to 115% (full power) respectively.

Sling 4 flight testSling 4 flight test

The Turbocharger Control Unit (TCU) uses the throttle position in conjunction with ambient pressure, manifold pressure and temperature, and engine rpm to actuate an electronically-controlled waste gate, which regulates the speed of the turbocharger (and thus boost pressure). The TCU adds boost from the 108% throttle position onward.

The controls for the electric constant-speed propeller are also interesting. Not only can the prop be simply left in automatic or controlled manually, it had a feature I’d not seen in a single piston engine type before: it can also be feathered. In the event of an engine failure at altitude this could be the difference between making it to an airfield or not, as a feathered stationary prop produces a lot less drag than an unfeathered windmilling one, greatly extending the glide.

P1’s stick top carries buttons for elevator trim, autopilot disconnect and PTT, while the flap switch is on the panel to the right of the P1’s primary flight display. Overall, the cockpit is well laid out albeit perhaps slightly overloaded by ordinary day-to-day standards.

(The owner, it turns out, is a passionate aviator and IT guru who has a particular interest in the most recent and innovative tech available.) All the controls and instruments are easy to see and, more importantly, reach. Cabin heating and ventilation are good and the seats comfortable.

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The competition

While often odious, sometimes comparisons are necessary. There aren’t that many four-seat kitplanes on the market, and I have tested two of the main contenders: the all-metal Vans RV-10 and the Alpi Pioneer 400, which has a wooden airfame.

Even if you didn’t know their countries of origin, if all three were lined up side-by-side on the apron the perceptive aviator could possibly guess the nationality of two of them! The American RV-10 is the biggest, most powerful and by extension most capable of the three. It is the fastest and has the best rate of climb, greatest MAUW and most useful load, but it achieves these things not by clever design, advanced aerodynamics or the use of modern materials, but simply by having the largest (and thirstiest) engine. Indeed, when you consider its useful load as a fraction of its MAUW, it is the least impressive!

The Pioneer is – and looks – Italian. The styling is strongly reminiscent of the great Stelio Frati’s designs, and by virtue of the materials used in its construction and its retractable undercarriage it is almost as fast as the RV-10 but on fifty per cent of the horsepower. It has the smallest useful load, but the most impressive when viewed as a fraction of the MAUW.

The Sling 4 sits between the two. It has a greater useful load than the 400 but isn’t as fast, and although it can’t lift as much as the RV-10 it does what it does with much better fuel economy.

All three handle and perform well, so ultimately it comes down to personal preference and prejudices. Would you rather have another 20-25kt, but with the complexity of a retractable undercarriage - or more speed and a greater load-carrying ability, but with the expense of a large air-cooled engine? I suspect there’s also a big difference in the build, it terms of time and ease.

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I’m no expert but the RV-10 is constructed with solid rivets, and these require assistance. The Sling on the other hand is blind riveted, consequently it is quicker to build - and if you want one in the UK you’ll have to build it. In the UK it is only available as either a Standard or Quick build kit. The ready-to-fly option is available in South Africa only, as there is no requirement in that country to meet the ‘51%’ (i.e. more than half the construction work done by the owner) rule.

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Fine view over the nose

With Tim in the back we set off towards the runway behind Fenland Aero Club CFI, Steve Brown and Keith in the camera-Cessna. Taxying is easy−there’s a fine view over, and either side of the nose as the seats are set quite high, while the nosewheel steering is effective and differential braking can be used to tighten the turn.

Ambient conditions are slightly above ISA (Fenland is essentially at sea level but the OAT is 18˚C) with a very fickle breeze vacillating languidly between 280-340 and the grass short and dry. In horse racing parlance ‘the going is good to firm’. It isn’t quite tarmac, but after weeks of drought it isn’t far off. With three POB, about 100 litres of fuel and no baggage we are still some 190kg below the maximum all-up weight of 920.

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With pre-takeoff checks complete, I line up on Runway 36, open the throttle smoothly and slowly to the detent, pause for a second, then raise the catch and push the lever fully forward. There’s a definite surge of extra power as the turbo cuts in, the controls come alive almost immediately, and as the rudder has plenty of authority there’s no difficulty keeping straight.

We’re soon airborne and climbing away strongly at the Vy (best climb rate speed) of 75kt so I quickly retract the flaps and then set the prop to ‘climb’. The change in pitch as the flaps retract is very subtle. Full power can only be used for up to five minutes at a time, but in reality this is no hardship−once you’ve cleared the highest obstacle you might as well pull the lever back to the detent anyway.

As mentioned, the throttle’s design is reminiscent of a jet’s afterburner selector, and although you don’t get the same increase in performance (or fuel consumption) as with an afterburner it’s best to use it in the same way i.e. sparingly.

The initial plan is to fly out over the Wash for the air-to-airs but the Holbeach range is active so we head further south. The light is glorious, the air smooth and the mudflats provide an almost surreal backdrop as I manoeuvre around the cameraship.

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For a tourer, the 4 is easy to fly in formation. The controls are crisp and authoritative, there’s plenty of power (I even briefly ‘tap the turbo’ once or twice when on the outside of the turn or to speed up a re-join) and the field of view excellent.

Sometimes holding formation while Keith shoots can be quite hard work but today it’s great fun, and whereas at times I’ve been secretly relieved when Keith eventually says, “it’s a wrap”, on this occasion I could’ve cheerfully kept going, I was having so much fun. With the shoot complete I set the prop to ‘cruise’ (I’d left it in climb while in formation, as having the prop in fine pitch aids deceleration), checked the fuel and headed back towards Fenland while beginning my initial examination of the general handling.

As I’d found during the close formation work, the ailerons are light, the elevator powerful and the rudder well-weighted, with low break-out forces and minimal ‘stiction’. Strangely, I feel as if the stick is slightly taller than optimum, and find myself holding it about ten centimetres down from the top (which isn’t ideal when you want to trim and need to reach the button) but this observation is entirely subjective. The electric trim itself is nicely geared.

A logical progression from holding the stick is not holding the stick, and an investigation of the stick-free stability proves interesting. The C of G has moved further aft by now, and although the stability is still positive longitudinally it isn’t that strong, while directionally it is rather ‘weak’, and laterally it’s just barely neutral.

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As Tim is sitting on the starboard side I leave the fuel selector on ‘right’, as any significant lateral imbalance could adversely influence the spiral stability. As it transpires, the spiral stability proves to be neutral to the left and just slightly divergent to the right, which is pretty much what I was anticipating.

Slow flight is impressive. The low-speed handling, flaps up or down, is impeccable and, although the pre-stall buffet is quite subtle and followed by a gentle wing-rock, the ailerons remain effective all the way to the stall. In fact, when the airspeed is reduced slowly the aircraft never actually drops its nose at the stall−instead, the sink rate steadily increases and the 4 simply mushes earthwards with the nose gently hunting in pitch while remaining controllable laterally and directionally.

With a dribble of power you could probably walk it right down to the ground at a sink rate of about 600-700fpm. Of course, if you did touch down like this there probably wouldn’t be any air left in the tyres, but I doubt you’d break anything.

We are a good way below MAUW (maximum all-up weight) but nevertheless, with a combination of full flap, a bit of power and very gradual deceleration, the stall speed fluctuated around forty knots, which really is impressively low.

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With full flap and throttle at idle the 4 quit flying at around 43-44 knots indicated, and flaps ‘up’ this increased by about six knots. A more vigorous stall did produce a nose drop, but recovery is effortless and immediate: simply release the back pressure. When stalled in a turn it rolls wings level and the overall impression is of a good, honest aeroplane with no hidden vices or unpleasant traits. I like it!

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What would I change?

There’s always room for improvement, and one of the fun things when writing a Pilot test report on a kitplane is a little conjecture on how I might do things differently. Here are some observations.

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- The front seats only have three-point harnesses, and regular readers will know that I prefer a four- or even five-point restraint system

- The back seats only have lap straps, which is acceptable but not ideal

- Because gull-wing doors can suck open, I’d recommend a ‘Door Unlatched’ annunciator light, and also ‘Master Warn’ and ‘Master Caution’ lights above the P1’s primary flight display, even though the G3X does provide aural warnings

- Toggle switches operate most of the electric systems, but for any aircraft which you must climb in and out of I always recommend rockers – they are just less prone to damage. Alternatively, if I were building a 4, I’d seriously consider moving most – if not all – of the switches to the cabin roof

- The choke and cabin heat knobs are identical in shape, colour and movement which is poor ergonomics; they really should be different shapes and colours, as you might intend to turn the cabin heat on and instead get a ‘rich cut’!

- The parking brake unit needs to be turned through 180° (strictly speaking, it should be ‘forward to fly’) and the flap control should be aerofoil shaped

- There should be at least one DV panel in the P1’s door

- Finally, I’d turn the fuel valve about 45° clockwise, so that the selector points at the selected tank.

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Good performance, smooth engine

Cruising back at a typical UK GA operating altitude of 2,500ft with the prop in ‘cruise’ (5,000rpm) and the throttle at the ‘max continuous’ detent produces an IAS of 108kt and a true airspeed (TAS) of 113 for a fuel flow of 23 lph. James tells me that in South Africa he typically cruises about 8,000ft higher (his base airfield is 5,250ft AMSL), and at 10,000ft the TAS is around 125kt−not bad for a Rotax-powered four-seater!

Incidentally, the 914 seemed smoother than the 100hp 912iS I often fly, and I wondered if perhaps this was down to the lower compression ratio of the 914 or a better engine mount.

By now I am getting a bit warm, but the car-type air vents located on the panel and fed by large NACA ducts on both sides of the fuselage work well.

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Back in the circuit the Sling 4 continues to impress, although it is important not to arrive with an embarrassment of energy (either speed or height) as the limiting speed for full flap is only 85kt. We have the circuit to ourselves, and although the slight wind favours R36, the light is better on R26−so you can guess where we see Keith lurking.

For the first approach James recommends 65kt, but the warm air and lack of headwind cause a prolonged float, so for my second attempt I use sixty. This is better, but even with a brief side-slip on short final we still float past Keith, so for the third I tell James I’ll nail the speed to 55.

This probably looks disconcertingly slow to him (his ‘hot ’n’ high’ base gives a fast TAS at touchdown) but the 4 is nicely speed-stable even with the slightly aft C of G, and the main wheels finally kiss the grass right in front of Keith.

Nevertheless, I couldn’t quite shake the feeling that a bit more flap would be nice; maybe change the flap settings to 10, 25 and 40°? I decide to make the final landing onto R36 and into wind but−would you believe it−on very short final a thermal wafts the windsock back around to 26!

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I liked the Sling 4. In fact, I liked it a lot−and if you built it a bit lighter than the test aircraft (which was quite heavy with all the avionics and autopilot) and didn’t fit a BRS you’d have a useful load that could be described as very useful indeed.

Of course, when operating near MAUW it would be wise to calculate where the C of G will be at the end of the flight, and not just the beginning, but then the same can be said of most four–seaters.

It really is a very impressive machine, and I’d love to own one.

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