rotax 80hp vs 100hp auto conversion

[jtpitkin06]

Don,

This is a great subject that I make sure my student pilots understand before solo.

Most normally aspirated engines are rated at maximum RPM, sea level, standard day. It’s the standard by which we can compare one engine to another on “rated” output. There may be some engines out there rated differently but I’ve not run across one in 46 years of hangar flying.

Normal cruise by definition is something less than max cruise. Let’s use the O-200 in a Cessna 150 for an example.

The O-200A engine is rated for 100 hp at 2750 RPM. It isn’t possible to get 2750 during normal takeoff and normal climb because there is too much load on the prop when the airspeed is low. In cruise at 2500 feet the manual shows max power at 2750 RPM resulting in 92% bhp.(or 92 hp if you like). Because cruise airspeed is faster than climb and the loading on the prop is less, the RPM could exceed maximum allowable with full throttle..In this case the engine is not power limited, but RPM limited. The throttle must be reduced to keep the engine from over speed. This is pretty much true for all fixed pitch prop engines. But it is possible to get nearly the rated power at 2500 feet.

If we had a cruise prop installed we might be able to get full throttle without exceeding 2750 but the altitude and induction losses will rob about 3 inches of manifold pressure. The maximum power available might be 95% or so.

As altitude increases the throttle cannot compensate for the drop in
atmospheric pressure and the power starts to fall off rapidly above 5000 feet. By 7500 feet the power is down to 74% with full throttle and 2700 RPM.

Also note that just because the engine is spinning at 2750 RPM it does not mean it is producing 100%. Pull the power back to idle and shove the nose over. The RPM increases but there is no actual increase in power.

Both Continental and Lycoming small engines are rated for 100% power continuous. That means you may run it at full power for climb and cruise without a time limit. It’s not very economical on fuel and won’t give the best range unless you are bucking a strong headwind. But, if the manufacturer says continuous, then you can run it that way all day if other parameters are not exceeded. As always you must not exceed the RPM redline.

In the case of a conversion engine, anything goes. On my Corvair engine I have put a redline of 3200 RPM continuous because that is where it produces 100 HP. (3200rpm*165 lb-ft / 5252= 100.53 hp.) It is also loafing at that speed whereas the original redline from GM was set at 5500 RPM.

John Pitkin
Greenville, TX

[Av8r3400]

Look at the reality. How many Rotax powered Kitfoxes are flying (regularly or even occasionally) versus how many auto conversions. The ratio is very, very lopsided.

I could give example after example of auto conversions that didn't live up to the hype, but I can not give any magazine articles or published engineering papers as evidence. The circumstantial evidence is overwhelming, though.

One of my chapter mates has recently given up on trying to make an Aerovee VW work in a Just Escapade and installed a Rotax 912 UL 80 hp. He spent thousands of dollars on props, engine mounts, modifications, rebuilds, not to mention hundreds and hundreds of hours and never got the performance that the engine was guaranteed to produce. Now with the 912 he can sustain climb at 1200 fpm, instead of 300, and cruise at near 100 mph instead of 85.

Auto conversions work, and work well in airframes designed for them. Sonex is a great example. Our planes were designed around the Rotax engines. Other engines have been made to work, anything is possible, but compromises will need to be made in order to do it. Constant tinkering, balasting of the tail, forward sweep of the wings, etc. will all be part of the plan.

So many seem to love to hate Rotax engines. They are light powerful and economical to own and operate. Most of all, they are extremely reliable and safe.

[kmach]

From my experience of dreaming, following, searching for, buying a Kitfox, there does seem to be quite a few auto conversion powered kitfoxes that end up switching engines to a Rotax 912 80hp or 100 . It seems the 100hp 912 uls is the most common and desired.

[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

[TahoeTim]

The gyro guys have a couple of suzuki sled options, 3 and 4 cylinder. They have made adapters to fit the rotax reduction box. I'm not smart enough to know if the suzuki would swing a big prop but it would be on my list as an engine to try. Heck, once you run at 6000 rpm, it's no big deal to run at 8000

[av8rps]

Yeah, 6 grand or 8 grand, once you get past the idea that 2750 is screaming, high rpms won't bother you much. Heck, some of these 912's at only 5000 + are already almost turbine-like for smoothness.

I wonder if the Suzuki sled engine is the same one they run on the Hayabusa bike? The bike engine is pretty much bullet proof, has a huge powerband range for torque, is very light for its stock power of 197 hp (and that's with a catalyst), can make as much as 400+ hp with turbocharging, and a wrecked bike can be bought for about the price of 2 Rotax ignition boxes.

That stock engine connected to a Rotax gearbox would probably be incredible in a Kitfox...

If you want to learn more, wikipedia "Suzuki Hayabusa" and read all about it. Especially the part near the end where they explain use of the engine in other applications than cycles. The Hayabusa Smart Car or Miabusa Miata are just two interesting, but insane examples of unadulterated raw horsepower in a very small package. The Hayabusa cycles are so fast that laws were created to keep their speeds under 200 mph!

"Hayabusa powered vehicles" is also a fun thing to google... some of those vehicles are beyond nuts.

So imagine a 700-750 lb Kitfox with a modest (?) 250 hp Hayabusa turbo under the cowl? Aw heck, throw on an 80 inch constant speed prop right away so you can do some real fun VTOL stuff. After that STOL will be for sissies...

[mscotter]

huh, interesting, but what would it weight? I just did a search and one site said ready to run engine (i.e. with exhaust, airbox, etc) would be 220lbs. Then there's the connection to prop to consider. Would you need a reduction drive? Or could you use the bike's gearbox? Still you'd need some kind of additional custom machined parts to attach prop, so the weight is climing fast...