Anyone out there with a 912s (100hp) in a Mk3?

[Monocock]

I’ve got a 912UL 80hp in my Mk3. I love the way it performs, but with used 912ULS engines being sold at seriously low prices now that the 912iS version is available, I’m slightly tempted.

However, is it simply too much power for a Mk3? I know I’d need to change the prop, but would it cruise notably more than the 85-90 mph that is does now, and would it climb that much better than the 1,000 fpm it does now?

Thanks.

[av8rps]

I'm somewhat surprised no one responded to your question yet, so I'll give you my opinion based on being around these airplanes since their creation. I still own the 43hp Cuyuna powered Avid Flyer prototype, another early model Avid Flyer with a 532 Rotax, a 912ul Kitfox Model 4, and a 912ULS Highlander. Plus, kind of "growing up" in that community allowed me the privilege to experience most of the new designs as they evolved. All in all I have a couple thousand hours in a multitude of these airplanes.

The Model 3 Kitfox having the highly under-cambered airfoil like the original Avid Flyers enjoy some benefits from that. But also some detriments.

The positive: Lower stall speed and a better climb rate (my 532 powered, 390 lb Avid stalls at 22 mph, and climbs at 2,500 fpm. Early, light Kitfoxes had numbers similar). And if you learned to use that drag from the under-cambered wing effectively, you were able to do some spectacularly short landings. It is very significant when compared to the later Kitfox Riblett designed wing, which is much harder to land as short.

The negative: When you try to push that high lift / high drag wing through the air faster, it creates more drag than a typical wing. Avid was able to get their Magnum to cruise at 130 mph, but not until using a 160 hp Lycoming. And when you were in that airplane at 130 mph you could tell it was "against a wall", unable to go much faster. The Riblett airfoil Kitfox went to starting with the Model 4 is much "cleaner" at higher speeds, therefore goes faster on less horsepower.

I had the opportunity once to fly a 95 hp 912 ULS powered Avid Flyer having a similar airfoil to your Model 3. It weighed around 600 pounds empty with the 912 ULS. I would expect that to be somewhat similar to your Model 3. The Avid was built with the 912 ULS so the owner didn't have any way to compare previous cruise speed, but overall felt it was about 5 mph faster than another friends 912 UL Avid. But the climb rate went up about 20% compared to the other UL powered Avid. Solo climb rates were in the 1800 fpm range. It was a good performer.

Ironically, I have a similar question to yours. I have a 80 hp 912ul Kitfox 4 that I fly on Aerocet amphib floats. It performs very well, but one can never have enough horsepower, especially on floats. I happen to have a used 912ULS (I bought for another Highlander project) that I thought about putting on it just to see how it would do. But the motor mounts would have to be changed so I've decided if or when I ever make a change I will probably just install the 105 hp Zipper kit on my original engine. My friend Larry did that on his Model 4 and has had awesome experience with it.

My two cents...

[Monocock]

Thanks for that. Great advice.

So, out of interest, apart from a prop change, I’m assuming the ‘S’ would bolt straight onto my mount/exhaust etc?

[Av8r3400]

Depends on the age of the engine. The newer engines have a different bolt spacing for the Kitfox style mounts.

I love my Zipper big bore and would second the recommendation of this option compared to a complete engine swap, simply for economics.

[avidflyer]

As I understand it, the difference in motor mounts is with the arms and where they bolt to the gearbox. They are about 1/8" longer or shorter between older and newer engines. I have a 1993 912 that has the older dimension arms, and the 1999 912 I have uses the newer dimension arms. If a guy is at all handy with a welder it would be easy to rework a set of arms to make then fit the other dimension engine. It's been about 1 1/2 yrs since I messed with those mounts, so that's why I'm not clear on the exact details. JImChuk

[av8rps]

The 105 hp Zipper kit installed on your old engine will provide you with a 25 hp increase. A new 912 ULS without the Rotax airbox only makes 95 hp. So another 10 hp from the Zipper kit over a new 912 ULS makes the most sense for me. Especially if you like your old 912 UL like I do, and it has some good hours left in it.

And then there is always something about knowing what you have.