It’s a public holiday – Worker’s Day and those of us who work are restless to get some air between us and the ground. There is some discussion in the club WhatsApp group about a suitable location for breakfast – some guys want to go to Thabazimbi for the NGK Meifees (May fest) but many of us are a little twitchy about flying to a town airfield and leaving our aircraft there, being transported to the festival ground and having to rely on folk to bring us back to the planes again.
So I have a new type on my license – and some endorsements to go with it! I recently spent two days at Wonderboom Airport (FAWB) getting rated on the Sling 2.
The Sling is a locally (South African) designed and built aircraft. It’s nominally a light sport aircraft but as configured it requires a PPL to fly as the maximum takeoff weight is over 600kg. Interestingly the same aircraft, with one wing tank removed will have a MTOW < 600kg and can be flown on a recreational pilot license as a light sport airplane.
It’s also quite different from the Cirrus I have done all my training on to date. Of course, it’s much lighter, all metal and has a significantly different engine? How different? Well, it’s a Rotax 914 – a 1.2l 4 cylinder producing 115hp compared to the Continental 0-360 6 banger on the Cirrus. It’s also turbocharged and the prop is a variable pitch constant speed unit (Yes, the Cirrus had a variable speed CSU but with no direct pilot control). Importantly, it burns MUCH less fuel – 20litres per hour compared to the 50-65litres per hour on the Cirrus. And it will burn MoGas happily too.
To obtain the conversion onto the Sling was not an easy matter. Firstly, there are a lot of flight schools operating Sling 2s. There are only 2 in the country that have the Sling 2 914 Turbo on their ATO. Thus I had to travel to Wonderboom, which is a great airport. Unfortunately it is 90km from my house. Not so good. I did the conversion through FliteCare who were fantastic – they could accommodate me at short notice, and their desk staff were amazing.
I first needed to have briefings on the Sling aircraft itself – concentrating on the many differences between it and the Cirrus – this took about an hour, then it was time to dive into the nitty gritty of turbochargers and variable pitch propellers. Turbochargers are easy to understand – once you figure out how the wastegate works and the differences between the hot and cold sections. The variable pitch propeller on the other hand I found to be a lot more complicated. It’s bloody brilliant engineering but seems very complex. My biggest problem is remembering the difference between fine and coarse pitch which I find counterintuitive.
As it turns out, the intricacies of the variable pitch prop are moot in the Sling because it has a very clever system to adjust prop pitch. Firstly, instead of being controlled by oil pressure and springs, the pitch is adjusted electrically by servos in the hub. And there is no pitch lever as would be suspected, but a pitch control instrument on the panel which has a knob to select T/O, Climb, cruise, hold, and Feather. One can also bypass the selector and adjust pitch manually should the control unit fail – this is done with a rocker switch. Once a setting is selected, the CSU adjusts pitch to maintain the required RPM on the prop. Because the Rotax is a geared engine, engine RPM does not directly reflect prop RPM – so prop rpm is not set, but the CSU will indirectly control engine RPM.
Speaking about engine RPM – this is high! A regular aviation mill will turn at about 2500-2700RPM. The Rotax idles at 2000 and will go all day at 5500. An interesting feature about the 914 is that full power is actually 100hp with an additional 15hp available when you push the throttle through the gate – allows 115hp for up to 5min – would primarily be used for takeoff and initial climb to 300ft AGL and obviously for the go-around. In the gate position the engine is turning at 5800rpm.
After the briefings, it was time to fly. The conversion requires upper air work, simulated engine failure and at least 5 landings. Preflight is fairly standard with the addition of a prop control unit check – cycle from full fine to coarse with the engine off (remember those electric servos?). The other very different aspect is the dry sump oil system – oil is stored in the oil tank – so in order to check the oil one needs to ensure all the oil is in the tank and not in the crankcase. This involves turning the prop by hand (check mags off, key out master off first!) until the telltale gurgle from the tank indicates that the oil has returned.
Dragging the plane out of the hangar is a one man affair – really a pleasure. Startup is a non event – fuel pumps on, fuel on and crank. It literally starts like a car engine. Run at 2000 until oil pressure settles then warmup is at 2500 until oil temp reaches 50C. Operating temps are much lower than conventional engine due to the hybrid air/water cooling system.
Taxiing is much simpler than the Cirrus due to the direct nosewheel steering – so much nicer than the free castoring nosewheel. She’ll roll 2 up at 2500 so the engine can warm nicely en route to the run up bay. Most pre takeoff checks are done on the taxi as they have a reputation of getting quite hot while idling stationary. In the bay it’s final checks, run up at 4000RPM checking the mags (<300drop and <115between), takeoff briefing and then it’s time to fly.
The best word to describe takeoff is “sporty”. Stabilise at 5500RPM and 100hp then through the gate to 115/5800. Lots of right rudder is required and before you know it, we’re at 48kts and the rotate call. Liftoff at 55kts and best rate at 75 has her climbing away comfortably at about 1200fpm. After takeoff checklist includes setting prop to climb, coming back out through the gate and cleaning up the flaps.
For the conversion we set off to the general flying area – steep turns are a non event – very easy to keep the nose up. Stalls are reasonably benign but I did detect a slight tendency to drop the wing and not a huge amount of buffet. The (brilliant) MGL Avionics iEFIS unit has an AOA indicator too which is helpful.
Then we did some circuits at Freeway airfield – which has a massive runway which is gravel. Also a non event for the Sling. I may think twice about taking an aircraft I owned onto a gravel strip but certainly it wasn’t an issue on this plane.
Back to Wonderboom with me having no idea at all of where I was – fortunately the instructor was at home in the airspace. The plan was to do a couple of circuits, but this was foiled by a medium sized Cb cell which decided to discharge itself over the airfield. Conditions on landing were challenging to say the least – significant wind shear and gusts from about 15degrees off the front of up to 25kts. The Sling handled this with admirable aplomb.
Sadly this meant I had to make the pilgrimage to FAWB again to finish the rating. We banged off 4 circuits easily in the mid morning this week – 2 normal landings (Flap 20), 1 flaps up landing (would really rather not do those….) and 1 short field stop and go. The procedure for landing the Sling is similar i believe to the C172 – cut power just short of the threshold and glide it in. You can’t do that in a Cirrus.. The short field landing we did just to demonstrate it – in the POH it says “short field landings as per normal landing” since the landing distance is about 270m. However, we did a short field approach – flaps 30 and standard piloting, stopped on the runway, and then did max performance short field takeoff – full throttle, brakes off, through the gate and full back pressure on the stick from the moment rolling starts. The nose wheel lifted of at 20kts and we broke contact at 48kts. Lower nose to Vx (75) and then up and away. Very impressive, especially when you look at the airport diagram and see how short the distance was – we landed on 29.
It’s really nice to have excess power available. So I’m now rated on the Sling2 and have endorsements for turbocharged engines and variable pitch propellers. Onward and upward!