Engine On The Way

Coming soon to my workshop: a Rotax 915iS engine!

The very first Sling TSi, with a Rotax 915iS engine on the front

Those familiar with the Sling lineup know that the 915iS engine normally is found on the Sling TSi, not on a Sling 4. So, my airplane is going to be somewhat unique, but that’s part of what experimental aviation is about. I won’t get quite the same top-end performance of the Sling TSi, but Mike Blyth (the creator of the Sling) tells me I should see much improved climb performance, and slightly better cruise speeds. The Sling TSi is also a wildly popular model right now, so I’m pretty happy that my plane will have the same engine.

There are many technical differences between the 914UL engine (the standard engine for a Sling 4, and the 915iS engine. The big difference is hidden in that little ‘i’, which stands for “injected,” and that’s what I really wanted. The previous generation of Rotax engines (912, 912S, 912UL, 912ULS, 914, etc.) all used Bing carburetors. Then, Rotax introduced the 912iS engine in 2012, which replaced carburetors for a fuel injection system. The 912iS is a normally-aspirated engine making 100 HP, but it didn’t take long for Rotax to bring the same technology to their turbo-powered engine, and so the 915iS was born, making up to 141HP.

Even when I started building my plane, I knew I would be putting a fuel-injected engine on my plane. I REALLY did not want carburetors on my plane. The 914UL engine is a fine engine, but it has not one, but two carburetors. Along with some of the usual challenges of carbureted aircraft motors (higher fuel consumption, tough starting necessitating a choke valve, to name a few), a dual carburetor must also be synchronized occasionally… and the engine runs rough when they go out of sync. With the 915iS engine, that all goes away. Carbs are replaced with a set of high pressure fuel injectors and a redundant pair of engine management controllers that continually adjust the fuel-air mixture and other parameters based on sensor measurements, which qualifies it as a Full-Authority Digital Engine Control (FADEC).

FADEC is something of an aviation buzzword, but even in 2020, FADEC is pretty rare in general aviation planes, even though 99% of automobiles sold today have the equivalent (the remaining 1% are battery electric vehicles which don’t have an engine!). Many general aviation engines sold today are still dominated by old-school but proven technology that hasn’t changed much in the last 50 years – carburetors, magnetos, choke valves, mixture controls. Lycoming and Continental are slowly starting to adopt some high tech – for example, you can finally buy an electronic ignition system on a Lycoming engine under their Thunderbolt brand.

FADEC allows the engine to develop more power while burning less fuel. The interface to my avionics is also simplified; since the engine ECU already has a full suite of sensors, I don’t need to add any more; instead I simply tap into a CAN bus to monitor my engine vitals on the G3X display.

I’m going an unusual route to buy my engine – I’m buying it used from a local seller. It’s a very new engine design — it only launched in 2018 — so it’s pretty incredible that I was able to find a used one, right nearby. I’ll save the full story for a future post, but the engine I’m buying has just 12 hours on it, so it’s almost new.

To put the 915iS on my airplane, I had to consult with Sling Aircraft for some technical assistance. There are a few airframe differences needed to work with this engine; most notably, I needed a new engine mount which makes room for the turbo intercooler, and a new engine cowling, which adds a large air scoop for the same intercooler. Other than those two changes, I’m ordering mostly a standard Sling TSi firewall-forward (FWF) kit, rather than a Sling 4 FWF kit.

Sling 4 engine mount
Sling TSi engine mount – note the difference in the lower left tube, which makes room for the intercooler

I put in the order for this custom FWF kit this week, but it will take a few weeks to arrive, probably not until early 2021. In the meantime, I intend to work on avionics and finishing the canopy install, there’s plenty of work for me to do between those two tasks.

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