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bandit
01-09-2012, 08:47 PM
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
01-10-2012, 09:43 PM
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!) :D

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
01-11-2012, 06:32 AM
Ditto... :)

DesertFox4
01-11-2012, 10:15 AM
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
01-11-2012, 11:07 AM
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
01-11-2012, 02:38 PM
Same question, KF2 with a 80 HP now with 582 ?

Monocock
01-11-2012, 02:50 PM
GDN

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

DesertFox4
01-11-2012, 04:09 PM
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
01-12-2012, 07:10 AM
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
02-20-2012, 03:44 PM
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.

kitfox2009
02-20-2012, 08:49 PM
I have an 80HP in my Vixen with a 70" ultralight IVO IFA. Today with 3/4 fuel,2x175# aboard and a 17 knot headwind we were at circuit height at the end of a 6000 ft runway. Reading 1000/1200 fpm at times.
I am plenty HAPPY with that. Not sure I would even want a 100HP unless I was flying out of unimproved strips! I like the "fuel sipping" UL.
Cheers
Don

runwayrex
02-22-2012, 07:07 AM
I have a Model 3 with the 80 horse Rotax. Been flying it for twelve years now. It has plenty of power and uses 87 octane mogas. I'm currently building a Model 7 and looking at engine choices for that. Has anybody put the 80 horse on a Model 7? I'm also following the development of the 87 octane Viking engine. The UL Power distributor says that all his engines require premium or 100 LL.

av8rps
03-05-2012, 10:05 PM
I have a Model 3 with the 80 horse Rotax. Been flying it for twelve years now. It has plenty of power and uses 87 octane mogas. I'm currently building a Model 7 and looking at engine choices for that. Has anybody put the 80 horse on a Model 7? I'm also following the development of the 87 octane Viking engine. The UL Power distributor says that all his engines require premium or 100 LL.

This has certainly been an interesting discussion. Here's my take on it;

First, can one ever really have too much power? Heck, if I could have 25% more power (912s vs 912ul) for the same weight, why not go for more power? If I was purchasing a new 912, I'd spend the little bit of extra money and go the 100 hp 912s. Even on a Model 2 or 3. But I'd enjoy the extra power in stol work and climb, and carefully watch so as not to overspeed that early airframe with all that power. Oh, and I wouldn't bother with the in flight adjustable prop as you wouldn't need it as that would really be overkill on an early, light Kitfox imho. :eek:

Second, fuel burn. I own a Highlander with a 912s and a Kitfox amphib with a 912ul. I like both engines, but when it comes to fuel burn, if I fly both engines the same, their fuel burn appears to be the same from all I can tell. And that would make sense when you consider the fact that two very similar engines, but one being high compression and the other being low compression, the higher compression engine is going to make more power per lb of fuel. SO, if you flew the same kitfox with the 100 hp 912s at the same speeds as you flew it with the 80 hp 912ul, you should burn the same or less fuel. I think the problem is that as a group us pilots are all the same; give us more power and we are going to use it. More power consumed, more fuel consumed. But fly either of those 912's around at lower power settings and both will sip fuel.

Third, will an 80 hp work on a Model 7? I can't see any reason it wouldn't. There is a guy that built a Just Highlander with gobs and gobs of mods, making it heavier than usual (over 750 lbs if I recall correct), and yet it flies just great with the 80. He is very happy with the overall performance and when comparing it against other Highlanders with the 100 hp 912s, he cruises just as fast, but it climbs a bit less than the 100. But he also is not using an IFA prop, nor has he done any of the readily available mods to his 912ul engine to get more power. So with all that said, I belive a 912ul would fly a Model 7 just fine. But if you did that and then one day wanted more power, just turn that 912ul into a 115 hp turbo charged 914 from one of the $8,ooo kits available from Rotax. That will give your Kitfox a mega kick in the pants!

OR, just install the high compression pistons in the 912ul and get within about 8 horsepower of the 912s. And if that still isn't enough, put an airbox from a 912uls on the carbs so they get cool outside air and you will kick the horsepower up another 3-5 hp, just like the 912s does (fwiw; a 912s without the airbox is 95 hp, not 100).

Oh, and one last comment; I have trouble finding non-ethanol regular grade unleaded auto fuel. So I always end up buying premium autogas for my 912 ul anyhow. So I figure since I have the high octane fuel in the tank, why not make the best use of it by having an engine that is high compression? If I did I should get more air miles out of that gallon of gas.

So if you find a good deal on an 80 hp 912, go for it! And if you regret that 80 hp 912 later, just modify it to bump up the HP. BUT, if buying a new engine, spend the little extra money for the 100 hp 912s, it's worth it.