Kitfox 3 engine

[bandit]

Hi,

Would like to know if it is ok to put 100HP Rotax in a model 3 Kitfox? According to what I have read, they were only rated for 80HP Rotax. All responses appreciated!

Thank You,

Bandit

[DesertFox6]

Bandit -

There are plenty of folks on this web who can talk eruditely enough to the thrust/drag limitations imposed on the Model III's airframe with higher hp engines as well as to the yaw-control considerations bound to impact the lighter wing loading/gross-weight airframe with it's smaller, short-coupled rudder. A larger, heavier, prop, like an Ivo medium, is needed on the 100-horse Rotax and I'm just not convinced you need the extra weight, or "OOMPH", on a Model III. Think "Gee Bee R-2" and you'll see where I'm coming from.

I tend to lean toward the Bigger Picture anyway, so from the economy of effort perspective alone, I'd have to question "why?"

The 80-horse version hauls my Model IV Speedster, fully loaded at 1200 lbs., all over the western US at high density altitudes, climbs extremely well even on asphalt-melting days, cruises at 100-105 mph with the Ivo Prop in flat/climb pitch at 5200 rpm, 117 mph in cruise pitch, and does it all on 2.5-2.8 US gallons per hour and has for 11+ years in a row. (I still have the fuel logs and the receipts!)

The extra 20 ponies probably won't do much more than get you into trouble with a Model III if you don't keep a tight rein on them and the extra "hay" they'll eat will most likely, and unnecessarily, deprive you of a few $50.00 hamburgers. (Kitfox pilots don't DO $100.00 hamburgers to begin with!) I'd recommend the 80 hp engine with the lightweight ("ultra-light") Ivo prop, and a free pocket to keep the spare change you'll accumulate; the 80-horse is a real jewel!

I'll ask DesertFox4 to chime in here, 'cuz he flew his 80-pony Model III all over the map down here to the tune of a couple jillion hours and can make a truly valid comparison with his current 100 hp Model IV which you can see in several Kitfox company videos. I'll certainly defer to his judgement in this regard, but you invited all opinions and this one, too, is "free!"

"E.T."

[Av8r3400]

Ditto...

[DesertFox4]

Thanks DesertFox6.

Bandit- What DF6 said. The 3 is pretty much maxed out with the 80 hp. Rotax. It will hit VNE without trying very hard with the Ivo inflight prop. The 100 hp. might break something important. With a Rotax 80 hp you will be in that wonderful 1.8 to 2.8 gals per hour burn rate. The 100 hp. on my 4 never falls much below 4.8 gph. unless at taxi or idle. Usually it's 5.5 to 5. 8 gph. at cruise speeds.
The 3 is a blast with the 80 hp. though.

Like Clint Eastwood said " A man's got to know his aircraft's limitations".

[Monocock]

With a Rotax 80 hp you will be in that wonderful 1.8 to 2.8 gals per hour burn rate

Does the 80 really go as low as 1.8?

[GDN]

Same question, KF2 with a 80 HP now with 582 ?

[Monocock]

GDN

A Mk 2 with an 80 would be a superb machine. No problems at all.

[DesertFox4]

Does the 80 really go as low as 1.8? QUOTE]

Yes if you don't mind floating around at 65 to 70 mph and rpms under 4800.
That is what the gentleman that purchased my model 3 did. I didn't like to run it that slow so I would see 2.3 to 3.2 gph. depending on throttle/prop. setting.
Still , not bad compared to the 912S and much more efficient than the 582 with better performance.

[Monocock]

That's impressive economy, especially with your petrol prices. My 20 litres per hour is costing me the equivalent of $50 per flying hour with our fuel prices!!!

[wildirishtime]

There's a ton of scientific answers from true experts, but having owned a Model 3, you don't need more than a 582 for performance - with practice that plane can takeoff and land in 100-150 ft, climb 800-1000ft/min at sea level and thus perform flawlessly with the 582 in a Model 3. The ONLY reason to do it is 4 stroke reliability. Might consider other cheaper options to get 4stroke reliability if that's what you're going for as the additional horsepower itself will be wasted.