Flight Test: Sling 4
A nicely-designed new four-seat kit plane with good handling and an excellent cockpit view
Attractive and slick: the four-seat tourer that you have to build yourself (currently, at least) and which gives competitors a good run for their money
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!
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 hornbalanced, 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 onepiece 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.
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. 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.
’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. The Turbocharger Control Unit (TCU) uses the throttle position in conjunction with ambient pressure, manifold
There’s a definite surge of power as the turbo cuts in
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.
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 cameraCessna. 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 280340 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.
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. 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.
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 wingrock, 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 low-speed handling, flaps up or down, is impeccable
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. 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!
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.
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!
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.
THIS PAGE, CLOCKWISE FROM TOP: the gull-wing doors allow excellent cockpit access, even if the sills are high; the flaps go down to 30° – a bit more would help kill the landing float, we think; inspection hatches on both sides are a nice bit of design detail – with a Rotax nine-series you really need to be able to check the coolant hoses as well as oil and coolant levels; and the constant-speed propeller, which gives the best possible takeoff and climb performance on relatively limited power by four-seater standards
THIS PAGE, FROM TOP TO BOTTOM: even the test aircraft’s stand-by instruments, located centrally above the avionics stack, are ‘glass’ – note the tall sticks (a bit too tall, we would say) and the rather vulnerable row of tumbler switches below P1’s primary flight information display (PFD); it’s not like your Porsche 911 – the rear seats are proper, adult-sized ones; the throttle is gated at ‘100%’ (maximum continous power) – takeoff power (‘115%’) is selected by lifting the latch to get full travel; PFD and starboard multi-function display (MFD) in operation, showing how the navigation and engine/vital systems information can be duplicated alongside the flight instruments on the PFD; and a detail shot of the central part of the panel showing the propeller controls (blue knob with ‘T.O.’, ‘climb’, ‘cruise’, ‘hold’ and ‘feather’ selection, manual ‘fine’/‘coarse’ adjustment switch below) and the flap selector (black knob at the bottom)
BELOW: while coming in at minimum speed and arriving tail-down help kill the landing float, greater flap travel would be useful!
ABOVE: part of the appeal of the Sling 4 is its good looks, from any angle
ABOVE: there are times when it is reassuring to know you have a feathering prop to maximise glide performance
ABOVE: room for one more – this is a genuine four-seater