Flight Test: Sling 4

A nicely-de­signed new four-seat kit plane with good han­dling and an ex­cel­lent cock­pit view

Pilot - - CONTENTS - Words: Dave Un­win

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.

Pho­tos: Keith Wil­son

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

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