rotax 80hp vs 100hp auto conversion

[av8rps]

Over the years there has been most every kind of engine tried on this general aircraft design. In addition to the Rotax 912/914 series, I've seen Subaru's, 0-200's, IO-240's, C-75 and 85's, VW's, Lycoming 0-235's, Jabiru 4 and 6 cyls, a few Cam-100's, Viking 110's, a Mazda Rotary, a few Rotec Radials, and even a BMW and a Honda Goldwing motorcycle engine. But the lesson I have generally learned from all that experimentation was that for overall best performance, in this airframe, the 912 series rules.

However, in defense of some of those other engine choices, some may make the aircraft climb better, or cruise better than some of the 912's. But for overall best performance, and useful load, I feel strongly the 912 has the advantage.

And I think the 912 has the advantage for 2 basic reasons:

1st, except for a two stroke, it is one of the most lightweight firewall forward options for our aircraft . Many dispute those weights, but on average the other choices are typically 100+ pounds heavier. That extra hundred pounds has a severe negative impact on wing loading as well as power loading numbers, which is the primary reason the 912 excels in my opinion.

2nd, the 912 uses a gear reduction that allows the prop to run ultra-efficiently. I've always wondered how many of the other engine choices would work better if they just accepted a little more weight and used a prop reduction unit? Jabiru comes to mind, as the engine has bigger displacement and comparable HP to the 912, but on our type of plane the shorter prop on the Jabiru turning higher rpms just doesn't perform like the 912. For what it's worth, I have seen a few Vw's on other planes perform very well when equipped with a belt drive prop reduction unit. So again, maybe some of these other engines like the Jab and the VW would work better if they were to add a PSRU?

I'm a seaplane guy, and if there is a good test of aircraft performance it is flying from water, as to get best performance from a heavy and draggy seaplane you need lots of power and a light wing loading. Case in point, I just came back from a seaplane gathering where we did takeoff contests. The top winning aircraft were all Rotax 912 powered, and all but one was on amphib (read as HEAVY) floats. Competitors were 180 hp Aviat Husky's amphibs, 160 hp straight float Supercubs, a 225 hp Cessna 170 on straight floats, and the usual gaggle of 300 hp Cessnas. None stood a chance against the Rotaxes. (There was a super lightweight C90 straight float J3 that was close, but that particular pilot is beyond phenomenal...). And for the record, my 80 hp 912 Kitfox 4-1200 on amphib floats (with full 28 gallons of fuel - I didn't know we were doing contest...I think they planned it that way ) placed right behind the 100 hp 912 airplanes, but still ahead of the other planes.

I said all that to say this, I've seen a lot of Kitfoxes and Avids over the years show up at the Oshkosh Seaplane Base (I run it) with a multitude of powerplants. But the ones that perform best are always Rotax powered. Even the Avid Magnum (which is a great airplane on floats) with either 160 or 180 Lycoming on straight floats will not perform as well as the 100 hp 912 powered amphibs. And the magnum is no slouch. But weight is weight. Maybe if the Kitfox had a large wing like a Cub or a Cessna, the weight of the bigger engine would be less of a factor?

Oh, and for the record, years ago (early 90's?) Zoom Cambell's magazine US AVIATOR did a fly off between a Kitfox with a 125 hp Continental IO-240 and a Kitfox with a 115 hp 914 Rotax. I recall the 914 was in a Model 4-1200 and the Continental in a Model 5, but they felt that was a fair comparison since both share the same general platform and wings, and both had similar payload. The general conclusion was that the 914 airplane outperformed the Continental airplane in takeoff, climb, and cruise. But that was probably to be expected as the Rotax Kitfox 4-1200 was probably 200+ lbs lighter, and only 10 hp less. Of course that was a 914 turbo, so it really excelled when they took it to altitude. At the time I felt the test would have been more valid if they had used the same model Kitfox, but after thinking about it, since payload is comparable and the model 5 is actually "cleaner" aerodynamically than the 4, maybe it was a fair test...

Worth mentioning to newcomers contemplating a Kitfox, also bear in mind that if you go for a higher horsepower but heavier engine, the fuel burn will be higher than the fuel miser-like 912, so for a fair comparison you need to also add the additional fuel weight burned each hour over the Rotax to empty weight. Might sound petty, but it all adds up on a little airplane with only 132 sq ft of wing. If you fly for fun like me, with my little 80 hp 912 burning on average in the 3.5 gph range, I can fly around safely on 1/2 tanks while having 3 to 4 hours of range. The higher hp engines can double that fuel requirement easily. So you will probably find yourself leaving home with full fuel most often. 15 more gallons of fuel weighs 90 more pounds...and you'll only get half as far as the Rotax on it. Just one more thing to consider.

I really hope guys like John can get his Corvair powered Kitfox working well, as even though I have really been happy with my 912's I fly, I'd love to see us all have a less expensive option. And even if the performance isn't as good as the 912's, if it's even close it would make a great option. So John, don't let any of this Rotax talk slow you down. There's a bunch of us rooting for you