rotax 80hp vs 100hp auto conversion

[tx_swordguy]

I am interested in buying a kitfox in the future and have noticed some autoconversions such as the Honda cam 100 hp on some of them. I know the lower hp Subaru is just too heavy to be considered. I am curious to know opinions on hp to weight differences between the rotax 912 ul 80hp and the 100 hp conversions. The conversions obviously weigh 100+ pounds more but I am wondering if the hp difference in the two motors will be about equal in performance with the weight differences. I am wanting the performance of the 80hp rotax is why I am asking and am worried about the weight of the conversions hindering performance.
Mark

[DesertFox4]

Mark, hold out for a Rotax powered Kitfox. So far I haven't heard of an auto conversion that has worked as well. A couple guys are going to install Corvair conversions. We are hoping they work well. Good luck with your search.

[jtpitkin06]

If you enjoy “thinkaneering “ and fabrication, then using an auto conversion engine can be a rewarding and challenging experience. If you just want to build it and get in the air, then stick with a Rotax.

To answer your questions on performance vs. weight consider the following:

We hear it all the time how the Kitfox performance will decrease if you use anything but a Rotax because of the increase in weight. With the same kind of thinking one would never want a 180 hp C-172 because the engine is so much heavier than the 145 hp version. Yet, clearly, the 180 hp version is superior to the earlier versions.

So how does this apply to the Kitfox?

Yes, the Rotax works well in the Kitfox. So do lots of other engines. The factory has installed Rotax, Subaru, Rotec, Lycoming, Continental and others. Some are better than others.

The first thing that pops up on any discussion of engines is weight. So just how much weight are we talking about? It’s not anywhere near the 100+ pounds you stated. The Lycoming O-233 (lightweight version of the O-235) comes in at roughly 215 pounds and the Continental O-200D is about the same. The Subaru and the Corvair are about 230

Does this hurt performance? Maybe… maybe not. Most of the alternative engines produce 100 hp or more continuous. The Lycoming pumps out 115. Most of the Rotax 912 engines are 80 hp. The 912 ULS is rated at 100 for five minutes, then you must pull the power back to 95 hp.

I’d like to see a published side by side performance comparison of a Kitfox with a Rotax compared to some other engine of equal power but heavier. You will hear lots of verbal claims but I haven't seen anything documented.

As to the weight of a Rotax the figures vary but it’s safe to say the installed weight Rotax is more than 160 lbs. Yeah… I see published figures of 140.6 lbs dry weight, but last I checked you can’t fly it without coolant and oil.

For discussion, let’s say the Rotax is 70 pounds lighter and the empty weight of the Rotax equipped Kitfox is 750 pounds. We’ll say the empty weight of the alternative engine equipped Kitfox is 820 pounds. On the surface it looks like the alternative engine equipped airplane is almost ten percent heavier. But, you don’t fly it empty. Let’s load up both airplanes with 350 pounds of body fat (one 200 pounder and one 150 pounder) and 25 gallons of fuel. The Rotax airplane tips the scale at 1250 pounds and the alternative engine airplane hits 1320 pounds to stay in LSA limits. That’s just 5.6 percent heavier with anywhere from 5% to 40% more power available.[80 hp Rotax vs 115hp Lycoming]

With all that power, I’d bet on the Lycoming performance every time.

What all this means is a 70 pound heavier engine can usually be accommodated without a power to weight ratio penalty if it produces at least 5 percent more power.

Which brings us to the real question. Just what are you planning to do with this aircraft that requires excess performance? The Kitfox jumps off the ground in 300 feet.

The most common reason for building or buying a Kitfox with an auto conversion engine is cost. You do not get any factory support. You are on your own.

The most common reason for an alternative engine from Lycoming or Continental is personal preference over Rotax. You get some Kitfox factory support.

The most common reason for installing a Rotec radial is because it “looks cool”. You get some factory support.

As a builder who has gone the rogue route of using a Corvair I can say with certainty it's been a lot of fun for me but not an easy road. Everything firewall forward is custom work. For tinkerers and fabricators I say its a great project. However, if you have trouble assembling furniture from Ikea, then stick with the Rotax.

Hey… whatever wiggles your rudder.

John Pitkin
Greenville, TX

[kitfox2009]

Hi John
Don't all the engines you have referred to only produce their rated horse power at maximum RPM?
I wouldn't think many Lycomings or Continentals cruise at 2750 or so,do they?
Wouldn't they also loose 15% or so at normal cruise ?
Just asking.
Don

[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.