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Engine selection for float plane

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Chris In Marshfield

Joined: 26 May 2006
Posts: 87

Posted: Sat Jul 07, 2007 7:21 am Post subject: Engine selection for float plane

Good morning all,



I've decided upon the Light Miniature Aircraft Super Cub replica as a building project, and I want to ensure that the aircraft is suitable for safe, reliable float operation.  The designer has told me that it would work well on floats, so there's the first hurdle, and building manual describes that the airframe has been updated in the plans to accommodate larger/heavier engines than the plans originally called for some 25 years ago (originally the 65hp Rotax 532).



A number of builder are installing engines from the A-65 through the O-200 on their aircraft, and the designer also suggests that there has been interest in the Rotax 912.  I'm presuming that all are suitable engines for a floatplane, however I'm curious if some are better suited than others.



As I recall, Paul S. is using a Rotax 912 on his aircraft and that seems to be a popular choice on a number of LSAs that are going on floats these days.  With the 912 being a newer engine, however, I suspect that the availability of them on the used market is lower, and I've noted that the prices are quite a bit higher than a quality used Continental.



Are there any schools of thought in this regard?  Does 100hp from a 912 equal 100hp from an O-200 equal 100hp from a VW conversion?  If the engine develops rated horsepower with the same length prop, does it really matter which one you choose?



Thanks and best regards,

Chris

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_________________
Chris Owens
Waunakee, WI

av8rps(at)tznet.com
Guest

Posted: Mon Jul 09, 2007 8:47 am Post subject: Engine selection for float plane

Chris,



My kitfox amphib is flying better and better with that little ol 80 hp 912

Rotax 912, as I continue to tweak it for performance.  And being that it is

a heavy model IV (has everything but the kitchen sink in it), the

performance is nothing short of amazing.  In perfect conditions (72 degrees

with a 10 mile per hour headwind, a little chop, and only me with 10 gallons

of fuel) I can get it off the water in 5-6 seconds, and climb out at a 1000+

fpm at a pretty steep angle (30-35 degrees I'm guessing).  And as I've said

before, this little plane will go 120+ miles per hour flat out.  ON AMPHIBS!

Load it up with a 200+ pound passenger in the same conditions, and the

takeoff run will be in the 13-15 second range.  And that's been proven as

lately as this last weekend by me taking a few of my friends flying.  Oh

yeah, I also did a solo takeoff the other day with full fuel (28 gallons)

and it took 10 seconds off the water without even trying hard.  And I took

off crosswind, with a 90+ degree temperature (density altitude was 3600 feet

as I recall).  Fuel burn with the 912 is nothing short of amazing - I

average 3.2 gph in all of my flying.  And my typical flying speeds are

95-100 mph.  I also do A LOT of water work.  So I'd burn less if I could

resist playing more.  I do have the advantage of using an inflight

adjustable IVO prop, but I am going to compare a warp ground adjustable one

of these days as I really like the water resistance that blade offers for

seaplane use.  Personally I don't believe I will see less than a 5-10%

performance reduction.  And that's not gonna kill me as I've alway said if I

can have a seaplane do 100 mph, it becomes a practical cross country

airplane you will use to go places.  This kitfox has proven to be all that

and then some.  It is an awesome little seaplane!!  I have no regrets.  In

fact, it makes such a good seaplane with that 80 hp 912 and the IVO prop,

that even though I have a 100 hp 912 sitting ready to bolt into it, I think

I'm going to save that for another project.  From all I can tell, I really

don't need the 100 hp on this airplane.  Sure, more power is good, but that

80 hp 912 is one sweet motor, and a mizer on fuel.



So, would I use an aircraft engine in a Kitfox?  No way.  Not if I were

planning on using it on floats.  Particularly if it were going to be an

amphib.  Don't get me wrong, I like the old tried and true aircraft engines.

They are well proven and are less expensive to obtain than a good 912.  But

they make airplanes like a Kitfox too heavy.  That's not a huge thing on

wheels, but it is a very huge thing on floats.  That's not to say it won't

work, but why accept mediocre performance when you can have stellar

performance on less fuel per hour?  The small diffence in price between a

used 912 and a used o-200 will be well worth the difference in my opinion.

There's a reason so many light sport airplanes use the Rotax.  It just plain

works better than the other choices.



Fwiw - I have a friend that flies a Kitfox just like mine, but built much

lighter (lesser equipped, no shiny paint, etc), with probably the worlds

lightest Lycoming 0-235 in it.  And it's on straight floats.  So with 118+

hp on tap, on straight floats it should be a real rocket....right?  Nope.

It is a good little float plane, and flies well.  But it flies much like

most other aircraft engine equipped seaplanes.  You won't see it doing 5-6

second takeoffs.  It may (?) fly as fast as mine, although honestly I can't

say as we've never compared.  So even though my 80 hp Fox will outperform

that 0-235 airplane in most every aspect, to be fairly comparing an aircraft

engine against the Rotax, you would need to put the 115 hp 914 Rotax in my

fox, and remove the 80+ pounds of landing gear from my floats.  I will

guarantee you if I did that, I would have a seaplane that would awe most any

seaplane enthusiast.  I actually think my Kitfox would be "overpowered" if

you can imagine such a thing?



Last comment on the 912 choice;  As much as I love the 912's performance, it

does have it's limits.  Many of the newer airplanes are getting heavy.  So I

think we are starting to reach the upper limits of what the 912's horsepower

range can effectively fly.  While you can put a 912 in a Cessna 150 and fly

all day (has been proven fwiw), it is not going to be a rocket ship.  Until

Rotax comes out with a 150 hp 912, Lycomings and continentals will rule the

larger aircraft.   But for planes like my Kitfox, I think the 912 is the

perfect engine.  Horsepower is horsepower technically.  But only if at the 

same

weight.  Weight, and longer prop capability is wear the 912 shines imho.



And fwiw, I also fly a Lake amphibian with a Lycoming 0-360.  I love that

airplane too.  It is a good performer for a 180 hp four seat amphibian.  But

I always wonder how much better it might fly if Rotax were to build an

engine that weighed 100 pounds less and had the same or more power????



Paul Seehafer



Attached are a few pictures of a small seaplane fly in that was held in

Phillips Wisconsin last weekend (we had 17 airplanes total if I counted

right - some aren't showing in these photos).  The Kitfox, the Zenair, and

the Rans in the picture are all 912 powered, and can all fly 100 mph on

floats, burn half or less the fuel any of the other 100 mph floatplanes,

while still providing STOL performance and good load hauling ability.

Further testimony to the effectiveness of the 912's.  They do the job

well...

---

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Chris In Marshfield

Joined: 26 May 2006
Posts: 87

Posted: Mon Jul 09, 2007 1:21 pm Post subject: Re: Engine selection for float plane

Hi Paul,



Thanks for your input.  I greatly appreciate it.  A side benefit of the Rotax is the nice, warm winter heat source.  As a Wisconsiner yourself, I'm sure you can appreciate that as well 



Will you be at OSH this year?  I'll be camping with Al and Lori B. and plan to spend some time at the seaplane base.  I'd like to meet with you and see your Kitfox if you'll be there.



Best regards,

Chris

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_________________
Chris Owens
Waunakee, WI

av8rps(at)tznet.com
Guest

Posted: Tue Jul 10, 2007 3:16 am Post subject: Engine selection for float plane

Yes, I will be at Oshkosh (I'm the seaplane base chairman, so if I didn't 

show there would be quite a stir I'm sure 



Not positive I will have my fox there as there are logistic issues with 

everything else I have to take, but I am hoping to do so.  If I do figure 

out how to get it there, I would be happy to give you a ride and show you 

how the 912 does.  I'd also love to talk with you about your building 

choice.  That design has always somewhat intrigued me, but I never took the 

time to really research it. I'm sure you could fill me in.



You most definitely hang with a cool crowd.  Al and Lori are GREAT people, 

and good friends of mine.



Paul



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