PROs and CONs of Engine Choices

[jfrantz]

Looking at all of the historical, current, and upcoming engine choices, its really hard to find something that is the best "all around" choice.

I would really like to buy a flying Kitfox in about 2 years and get to know the plane and capabilities and all that before building a new Kitfox - but the used market is pretty thin and I may just end up building first.

What I am trying to figure out is the best engine choice, or better yet, the best all around build setup.

My flying mission would include a lot of normal airport to airport flying, local area stuff, lots of 200-300 XC trips and many of those being for seasonal camping trips to places with good condition turf (Cavanaugh Bay, Sullivan etc). I would also like to ability to install some larger tires and get into the backcountry like so many kitfox pilots do. 29" tires would be a seasonal thing also - as would floats in the future. Having the ability to convert multiple times in a single flying season is what really sells me on the Kitfox.

Ok - so I am currently thinking a standard Super Sport kit, but with STi gear and shocks/TW like what AirFox has done. His build is very close to what I think would be perfect for my mission.

The STi wing is awesome and performance is great - but I think I would stick with a standard wing to facilitate better cruise speeds and I would plan landing ground roll and locations accordingly.

Now to the engines. Ive never flown behind a Rotax, but I trust the engine and its long history of success in the experimental community. That being said - I do like the Titan X340 option used on the STi. My questions there is, would it be a good choice on the standard SS kit? My initial thoughts are W&B, useful load (EAB, not LSA), and prop clearance if on standard gear.

I really like the factory SS with the Lycoming 233, but I can't find much data on that since the testing. It seems like a Rotax and 233 are pretty darned close overall - but the Rotax has better fuel economy and is much more 'standard' in the Kitfox community.

I am really thinking here and I think a 912iS or Titan 340 will be the competing engines here. The Titan on a standard SS with STi gear and 26" tires seems like a good fit in the (easier) backcountry strips and still have lots of power for good cruise numbers - but could also pull power and cruise closer to 100TAS and see really great fuel numbers.

Anyways - theres my brain spaghetti for the afternoon. Long story short - there are way too many options and they all have their merits - and I think that is a compliment to the Kitfox community and the versatility of an awesome kit plane.

Jake

[efwd]

Hi Jake
I wanted my aircraft to be LSA compliant. I really wanted the radial engine. I chose the 912iS. John M of Kitfox told me power to wt ratio is best with the 912. I toured the factory with John and he assured me I would decide on the Rotax when all was said and done. When I flew behind the 912iS the next day I was surprised to say the least. As dumb as this sounds, I really didn't like how a Rotax sounds, but with Noise cancelling headsets I couldn't tell what kind of engine was up front. Certainly there are those with experience who could tell even if deaf. Anyhow, that's how I came to the 912iS. Yours and my flying needs read identical.

[jfrantz]

Thanks for the input - I do feel that I will eventually find myself behind the Rotax, I am just way comfortable and familiar with the Ly/Con engines and it will take some getting used to.

I do really like what Vans has done with the 912iS in the RV-12. Very impressive cruise performance - just not a very STOL-y airplane.
I like the economy of the 912iS and the injected configuration.
Are there any negatives of an injected/FADEC engine in the backcountry? It seems like a 'breakdown' or electrical issues would be a bit more difficult than a carb issue in the bush. Also - there are kits out there to bump the HP of the 912 - would the 912iS be able to have the same power upgrade? I imagine a tune or flash would be required also.

[Esser]

Looking at all of the historical, current, and upcoming engine choices, its really hard to find something that is the best "all around" choice.


I hate to be this guy but power to weight to fuel burn to longevity history it’s REALLY easy to find an all around choice and that choice is a Rotax every time.

[jfrantz]

The more I read and research Kitfox and talk to builders/flyers, read trip reports and build logs etc etc etc.. the more I find that a Rotax really is right.
Now... which one.. the new 912iS seems to be a logical choice - but a turbo model sounds appealing too. The 915iS is pretty $$$ and the benefits would most likely be worth it- I also understand it requires a c/s prop? How does owning and operating a turbo Rotax (maintenance/inspections/parts) differ from a naturally aspirated Rotax?

[Swanny]

I'll add my overly lengthy and fairly limited knowledge based opinion for whatever it's worth.. I am building an STi and having the same inner conflict with myself. I currently fly a M20K 252 Mooney. So for me, everything about the choices screams turbo. The confidence in knowing your making all available power no matter the condition is a very reassuring feeling. Your ground roll and climb out typically are not as proportional to altitude as they are with an NA motor. Plus modern technology is pretty dang reliable these days. However, let us all be honest for a second. These planes are not fast, they are not built to fly cross country at 200kts. They cannot get you from coast to coast within daylight hours. Therefor, why toss in the extra turbo'd HP right? Dollar for dollar the 912ULS wins everytime. The 912is is almost the perfect little engine for these planes. Modern fuel injection (which isn't that modern in the grand scheme of all things petroleum powered... it's 2018 people.) If the Titan wasn't such a heavy install, it would be my first choice (and was). The 912ULS is the lightest of the installs and by far simplest. So I see a very strong argument for going that direction, the KISS principle is a real thing... Personally, I don't know how I am going to afford it, but I know how much time, hard work, heart and soul will be put into this build, so I would kick myself if I did not put the latest and greatest into this plane.. I am sold on the 912is. But, the wife says we are putting the 915 in it soo... happy wife, right?!

Ask yourself this, if you were to buy a brand new car tomorrow, would you want it to be carbureted with a 8 track and AM/FM radio?

As far as STi or SS7.. Only you can deside that one. I argued with myself there as well. Dollar for dollar option for option it was cheaper for me to go the STi route, I'll make up the speed difference with the 915.. If I built a second plane, it would be a ULS powered speedster.

[Esser]

The more I read and research Kitfox and talk to builders/flyers, read trip reports and build logs etc etc etc.. the more I find that a Rotax really is right.
Now... which one.. the new 912iS seems to be a logical choice - but a turbo model sounds appealing too. The 915iS is pretty $$$ and the benefits would most likely be worth it- I also understand it requires a c/s prop? How does owning and operating a turbo Rotax (maintenance/inspections/parts) differ from a naturally aspirated Rotax?

There really isn’t too much difference. I have a 914 in my plane that I after market fuel injected. I haven’t flown behind it yet but I’ve flown 20 hours behind the 914. Just have to watch some operating parameters a little more closely temps etc.

For maintenance, once a year you have to check your waste gate cable and adjust it if required. That’s it for extra maintenance.

My wife and I flew two kitfoxes for 20 hours each. She was in a 912iS powered plane with a constant speed prop and I was in a 914 powered Kitfox with a fixed pitch and a heavier instructor. After watching the side by side performance of the two planes, I would always go with a turbo if that was an option

[aviator79]

I only have about 8 hours behind 912iS with Stick and Rudder. I am in the process of installing a 914 on my plane. I had dinner with Paul L. and one of his instructors, and I asked them directly what engine they would choose. Paul said that the 912iS mated to an Airmaster prop was the best value in terms of performance per dollar. He said that if your budget forced a decision between a turbo or the Airmaster, go with the Airmaster. He then said "But (dramatic pause)... If you really want the best performance, the turbo and the CS prop is amazing." His instructor, Gary, grinned and bobbed his head enthusiastically. I figure he knows better than just about anyone.

In my case, if I lived at sea level, I would have saved some money and gone with the 912iS+Airmaster. However, our density altitudes often run between 9-10k feet in Los Alamos, and I wanted all the power I was paying for. I was tempted by the 915, but you have to draw the budget line somewhere.

[jmodguy]

And now for something a little different...
I went with the 340. It's not a Titan though, I built it from parts I bought from Airpower. Saved half the cost of buying one outright and I know how my engine went together!
Is it heavier? yup It also puts out gobs more power and torque. I made it light everywhere I could and went with the FlyEFII system, cold air sump, taper fin cylinders, 9:1 pistons and an 80" GA200 STOL prop from whirlwind. My main gear is by John Roberts of AV Weld with 22" Dessers and I have 14" of clearance prop tip to ground in a level attitude. I will be a bit over 900 lb when its all said and done. I am keeping the rest of the airframe light as well, such as no side panels and other niceties.
Dare to be different! :cool:

[nyrikki]

Are you far enough along to know what your CG is in this configuration?

I am moving back to a higher altitude area and am still trying to make the 914 vs 340 decision.

As the cost is similar and my pessimistic 66% of rated horsepower number makes them about a wash at my target altitude it is a hard choice. I don't think it is an issue on the 7 but a couple of people with o-200 IV's have told me they were a bit nose heavy with the smaller original tail feathers but those individuals also didn't have in flight adjustable trim either.

[aviator79]

Correct me if I'm wrong, but the Titan X340 engine is about 100 lbs heavier and burns about twice as much fuel as a 914. At sea level, you get some payoff for the weight and the fuel burn by having a lot more punch. But if high-altitude ops are what you're after, the turbo seems the more efficient route from a weight and fuel perspective.

[jmodguy]

Nyrikki
I am not ready for w/b but I know it will need a little weight in the back. I’ve added a few pounds in tailwheel and shock spring and a 3/8” steel plate for a battery hold down. Making a cowl and hope to shed a few pounds there as well.
In a few weeks I should have a w/b answer

Yeah its heavier and burns more gas. For what it’s worth, I don’t see anyone spending 6 hours in a Kitfox seat...

[aviator79]

Yeah its heavier and burns more gas. For what it’s worth, I don’t see anyone spending 6 hours in a Kitfox seat...

Just to be clear, I'm not criticizing jmodguy's engine choice. His build is beautiful and the engine/cowl look mean. I'm just saying that at high elevation the tradeoffs are different, and probably favor the turbo.

[nyrikki]

Correct me if I'm wrong, but the Titan X340 engine is about 100 lbs heavier and burns about twice as much fuel as a 914. At sea level, you get some payoff for the weight and the fuel burn by having a lot more punch. But if high-altitude ops are what you're after, the turbo seems the more efficient route from a weight and fuel perspective.

I can pretend that I have some rational reason like the 914's non-linear power delivery in the boost range or that it's fuel burn is also higher in the takeoff power range (~9gph) but I'll just be honest that I prefer the sound of a slow turning aircraft engine :)

If my primary mission altitude was lower a 912uls or is sport would be fine...but because of the sound and lie I tell myself about the costs of the 5 year rubber replacement on the rotax justifies 100# of increase weight.

Outside of the increased induced drag from the extra weight the fuel burn at cruise may not be that large under typical slow and low flying.

The x340 burns 5.5 gph at 75%, but I would be running slower than that as my friends will be in super cubs.

Please correct my assumptions if they are wrong but here is a poster with a O-290D 2200RPM for 118mph @ 5.1gph
(http://www.teamkitfox.com/Forums/showpost.php?p=55020&postcount=58)

At the high 90's speed range, assuming a 4700 rpm for a 914 at that speed (which is very approximate and shouldn't be used for any planing) it appears to be burning about 4.5 GPH based on the rotax spec sheet.

So my assumption, which is biased and most likely inaccurate with a lower cruise speed, EFI and CDI I should be somewhere within 10 to 15% of the fuel burn between the two engines at those slower speeds. This is still significant though as 15% of a conservative 20 gallons of usable after takeoff and climb is ~3 US gallons.

I am sure there is complex variables like flame front speed and other considerations that my wag isn't capturing and obviously there are losses from fuel vaporization from air cooled heads on the titan but the 914 is also lacks an intercooler which will also have an impact that may make that a wash. CDI, electronic timing and mixture control will have a big boost on efficiency too while also increasing weight due to high pressure pumps return lines etc..

It is all mission based and I have more data to collect before I decide but outside of the extra weight and resulting drag it seems to be a wash if you consider I will probably just equip less gear and reduce my baggage but fly both engines at just below max legal take off weight.

If anyone does have reliable numbers especially with the FlyEFII systems it would be appreciated as I am sure confirmation bias and my desire to get that direct drive sound are coloring my assumptions but a ~2100 rpm cruise seems like it wouldn't be a massive change in consumption but with the benefit of a massive increase in reserve power.

[aviator79]

Very informed and honest assesment.

[nyrikki]

OK I found more data in my docs and wanted to ask for more input.

The 914 at 75 % continuous performance is 5.4 gph and continuous HP is ~98.5 HP and 75 % ~74 HP. Rated at 80 HP continuous the x340 burns 5.5gph at 75%.

From the Rotax 914 user manual PDF
(https://rotax-docs.secure.force.com/DocumentsSearch/sfc/servlet.shepherd/version/download/06812000002WNDDAA4?asPdf=false)

While the docs are harder to find now that conti owns the titan brand the earlier EFI/CDI claim was 5gph at 80hp but 5.5 seems more typical from guesses from the carbon cub users and some loose math.

Obviously not having a continuous prop and the induced drag from the extra weight will impact this significantly but the burn rate is pretty close at lower power ratios may actually slightly favor the 340 at lower airspeeds. Purely because of timing + efi.

Feedback would be appreciated as my soon to be home airport is 7142' MSL and the 914 is only normalized for takeoff to 8000' I am concerned that some work will be in the less than linear part of the power band as the ECU will be limiting boost on fairly typical days DA and obviously I will be past that value at pattern altitude.

Hopefully someone will talk some sense into me to shoot for that useful load, but it is unclear if that 8000' limit is just a limitation of the turbo and power drops in a predictable fashion or if the ECU is limited on pressure differential and power could drop by a large fraction when the waist gate is opened.

I can deal with the implications of DA but with the reported non-linearity of the boosted power band of the 914 and the fact that I will hit their normalized limit when getting to pattern altitude even under standard conditions it is a bit concerning.

I may be overthinking this as the SS7 with the 912ULS is set to a service ceiling of 16K which I assume is when climb drops below 300fpm and the 914 ceiling has been reported at 25,000' so it is really concerns about an abrupt change in behavior by the waste gate being opened.

To be honest the 912ULS may be fine but without hard climb numbers at around 10K DA I am planning on erroring on the expensive/heavy side.

[Danzer1]

The x340 burns 5.5 gph at 75%, but I would be running slower than that as my friends will be in super cubs.

Please correct my assumptions if they are wrong but here is a poster with a O-290D 2200RPM for 118mph @ 5.1gph

Greg - sorry to be picking on you today. The o-290 at 5.1 gph at 2200 is very believable but I doubt you could get anywhere near that with the x340 at 75% which you suggest is 5.5 gph. Would love to see data on that.

A bone stock 340 (not Titan as they have not released HP curves that I know of) normally aspirated, magnetos, fixed pitch, specs at .46 lb/hr at BEST economy which is at 125 bhp which equals 9.58 gph. I know the Titan is better and especially if injected and with electronic ignition and a constant speed prop - but 5.5 gph would be a fantastic accomplishment at 75% power.

YMMV, Greg

[jmodguy]

Just to be clear, I'm not criticizing jmodguy's engine choice. His build is beautiful and the engine/cowl look mean. I'm just saying that at high elevation the tradeoffs are different, and probably favor the turbo.

Ha ha! No I dont think you are criticizing. I’ve been told I can come across a bit gruff 😬

Turbo 340??? Hmmm 🤔

And thanks tor the kind words

[nyrikki]

Greg - sorry to be picking on you today. The o-290 at 5.1 gph at 2200 is very believable but I doubt you could get anywhere near that with the x340 at 75% which you suggest is 5.5 gph. Would love to see data on that.

A bone stock 340 (not Titan as they have not released HP curves that I know of) normally aspirated, magnetos, fixed pitch, specs at .46 lb/hr at BEST economy which is at 125 bhp which equals 9.58 gph. I know the Titan is better and especially if injected and with electronic ignition and a constant speed prop - but 5.5 gph would be a fantastic accomplishment at 75% power.

YMMV, Greg

Yep, the other was my mistake and don't feel bad about it. I am far happier to find I am in error and personally don't consider myself omnipotent.

The problem is that engine manufacturers play tricks and the consumption on the rotax is not based on peak but on continuous rating. In the case of an engine like the x340 you need to think about what it's cruise output is and not think 85% of the 180hp but that it has a large amount of reserve power and a lower continuous in this application.

To avoid fractions or decimal points I'm going to mix units of measure here but a general wag number you can use on traditional air cooled, magitoed engines is that they will burn 27 litres of gas an hour to produce 100hp.

If you look at the spec sheet for the carbon cub you will see they claim 5-6 GPH at 80hp on the carb version of their engine.

http://cubcrafters.com/carboncub/ss

If you look at the 912 user manual
(https://rotax-docs.secure.force.com/DocumentsSearch/sfc/servlet.shepherd/version/download/06812000002o3NWAAY?asPdf=false)

And look at page 5-6 you will see the max continuous is 90HP and 10 in reserve. Their 75% cruise is at 71.84 HP and not 75 HP.

If you look at a similar 80HP output from cubcrafters they are burning about 6 gallons per hour too looking based on chart from the previous page on the rotax doc.

The rotax can be lighter because it spins faster but that gear train also has losses too.

Historically simple aircraft engines have timing and mixture problems that dramatically reduce their efficiency at lower RPMs and power levels and even with CDI and EFI you do lose some efficacy but it just doesn't change the fact that there are ~115,000 BTU/gal of gasoline.

The power to weight ratio of the Rotax is far better than a Lycoming based engine but what is actually delivered per volume of fuel is incremental and as I mentioned gear reduction is not zero cost. Do to the increased weight of the x340 you will typically have a higher angle of attack which will impact things slightly but when loaded to similar weights and CG's it should be expected that a 912 and a x340 would burn similar amounts of fuel at the same airspeed.

Don't make the mistake of assuming 75% of 180hp when you will be using less than half that number for any cruise speed where range is important. You have to consider similar airspeeds to compair burn rates especially in draggy airplanes like a cub or a kitfox as the energy required to go faster is exponential in nature.

Full liquid cooling and variable valve timing is probably what would be required to gain a significant amount of efficacy past EFI and electronic ignition as fuel vaporization is a large limiter on efficiency of an air cooled engine.

[Danzer1]

Greg, I hear what you are saying but you stated:
The x340 burns 5.5 gph at 75%

So at what rpm and HP is that 75% at - on an x340?

Per your own equation:
they will burn 27 litres of gas an hour to produce 100hp which equals 7.13 GPH, so I can only conclude that to be at 5.5 GPH on a x340, the HP must be somewhere less than 100 which would certainly be less than 75%.

Further the Cubcrafters data states "at optimal conditions" but nowhere states what those are or any percntage power associated. But knowing 320's and 360's (340 being in between) 5.5 GPH would likely be at 50 to 55% at best.

Not arguing if 5.5 gph can be achieved, but not anywhere near 75% as stated, no matter how you slice it! IMHO

Greg

[nyrikki]

Greg, I hear what you are saying but you stated:

So at what rpm and HP is that 75% at - on an x340?

Per your own equation: which equals 7.13 GPH, so I can only conclude that to be at 5.5 GPH on a x340, the HP must be somewhere less than 100 which would certainly be less than 75%.

Further the Cubcrafters data states "at optimal conditions" but nowhere states what those are or any percntage power associated. But knowing 320's and 360's (340 being in between) 5.5 GPH would likely be at 50 to 55% at best.

Not arguing if 5.5 gph can be achieved, but not anywhere near 75% as stated, no matter how you slice it! IMHO

Greg

75% of the rated continuous horsepower, which is about 80HP to meet S-LSA rules. Same standard as the rotax is held to.

5.5 GPH is conservative using the same standards as are afforded to rotax.

Edited to add: The x320 will use a longer two blade and prop efficiency complicates the math too much to share in this format.

[aviator79]

By non-linear throtttle range on the 914, do you mean between 108 and 115% throttle? As I understand it, this is non-linear at any altitude, and there is no reason to operate in that range. You use 115 for takeoff, and then you reduce below 108. You don't try to set power in that range. I operate out of a 7172' field, and know a guy who flew a 914 up here. He actually did switch engines. His Pegazair is a bit heavier, and he wanted more power. He did have good things to say about the 914 performance, and did not complain about any throttle nonlinearity. He did say that the TCU would sometimes dump boost during full power climbs on warm days on account of high airbox temps. He recommended an intercooler if you can install one. I am considering it, but I think it's pretty tough to fit one in the KF cowl unless you go EFI and ditch the airbox. I know Paul at S&R uses max continuous boost for normal takeoffs, and only uses full boost when a maximum performance takeoff is required. This prevents an unexpected boost reduction at a critical phase of flight. I'll give you that you wouldn't worry about any of this with an NA engine, it makes less power at high DA in a much more predictable way.

Your new field elevation seems shockingly close to mine. Where are you going?

[nyrikki]

I am moving closer to family in SW Wyoming, thus my obsession with head winds.

It dumping boost during takeoff is the concern, thanks for the feedback.

[Danzer1]

75% of the rated continuous horsepower, which is about 80HP to meet S-LSA rules.

There is no such requirement for S-LSA or LSA, they must stall below 45 kts and have max speed of 120 kts at max power. So Cubcrafters has determined that to not exceed 120 kts with an x340 they have (theoreticlly) "capped" the "maximum HP" at 80 HP.

No where does the reg or Cubcrafters state that 120 kts is at 75% or even near 75%. 75% power of a 180 HP engine is not ever going to be 80 HP no matter how you try to slice it.

Greg

[nyrikki]

120 kts and the 1320 - 170 per seat - 1/2 hp max empty weight.

If max continuous was 180 HP they would be limited to a 890 empty weight but can sell a S-LSA up to 940 pounds with 80 continuous.

While the rotax numbers are a bit lower on this aspect the max empty weight reduction is based off 1/2 the max continuous and not the peak HP.

Obviously E-LSA or E-AB doesn't have this same 1/2 HP penalty.

But I linked to the official Rotax docs, which shows that the consumption numbers in the marketing material and the typical 75% cruise numbers are based off that continuous rating.

Here is a link for the 914 page 1-14 will show it is burning 33.0 l/h (8.7 gal/h) at max power as well as show the rates for max continuous and the 75% based number based on that max continuous number.

https://rotax-docs.secure.force.com/DocumentsSearch/sfc/servlet.shepherd/version/download/06812000002WNDDAA4?asPdf=false

[Danzer1]

Greg this is going nowhere - not arging the regs, not taking about the 914 either.

You stated the x340 made 5.5 gph at 75% - you have not addressed that - it does not and can not. You can manipulate the numbers any way you want to "meet" the S-LSA fed requirements, but fuel consumption based on BHP isn't going to change - all I asked was for you to provide backup for your claim of 5.5 gph at 75% power on an x340 (any 340 for that matter) and 80 hp is not 75% on any 340, at any rpm, at any altitude - so that's not the answer. You can call it 80 all day long to "meet the regs" but it either isn't 75% or isn't 5.5 gph! Take your pick.

[nyrikki]

I have addressed it several times but yes we are at an impasse.

I have made the point several times that it is unfair and unrealistic to compare burn rates at different air speeds.

I didn't set the rules but I need to comply with them.

We should probably open another thread but the sticker on the left side of the dash here shows how they set max continuous RPM based on altitude and how it goes up with altitude which makes me think that it is the weight penalty that drove their decision.

http://cubcrafters.com/c/wp-content/uploads/2018/08/DSC_3694.jpg

I am stuck under the sport pilot rules and assume that this precedent will be fine with the DAR if I just don't go with a big bore 912 or a 914. At a high DA the ground is moving a lot quicker anyway and so I'm not adding more HP to go faster or burn a lot of gas.

The 141 wing Rans S20 with the Titan is too fast and would need a POH limitation for rpm and speed and I assume the ss7 would too. It may be counter intuitive but a tandem aircraft is to narrow to have good aerodynamics and while a CC may not hit 138mph a kitfox may.

There is little benefit to chasing that number in my mind. If you can convince me that two kitfoxes at the same weight and same airspeed with different engines are burning massive different amounts of fuel you will change my mind.

But holding the titan fuel flow at 139HP while allowing the rotax 914 at 74HP or a 912uls at 67HP just to match the common 75% marketing term isn't very useful for understanding real fuel burn rates.

It will take similar power inputs at the same weight to go the same speed, there is nothing special about the 340 that will require it to go faster but it will burn more fuel with the same load.

For most people the IAS is far more important that some percentage of the throttle lever.

Feel free to point out where I have missed my numbers here but use IAS and not some massively different airspeed and thus drag.

In the case of the 912ULS on a ss7 that last 25% of continuous power gives you ~5kt of speed at the cost of ~1.5GPH higher burn. That is about a ~25% increased burn rate for about ~4% increase in speed and it only gets worse the faster you try to go.

While the weight of the x340 is a cost it isn't really comparing apples to apples to assume that just because you have 180hp doesn't mean you are going to chase those diminishing returns and get anywhere close to 7-8+ GPH.

120mph and about 80HP is what I am using for a typical cruise speed, and at those power levels my assumptions on fuel burn seem to work out but feel free to explain where I am wrong here. Note that those are also conservative numbers as the official spec for 120mph cruise is at around 68HP continuous.

I won't be cruising anywhere near 138HP of output.

[Danzer1]

Greg - I understand all of that but none of it answers the question, nor is it relevant to your statement which was "The x340 burns 5.5 gph at 75%", regardless of speed, weight, rpm, etc. - it doesn't!

All I'm pointing out (the rest of your choice dilemma aside) is: to prevent other from believing the same myths that are passed around by most S-LSA builders that are "adjusting" the numbers to "conform" and leading people to believe they can achieve those types of numbers - you can't.

You haven't substantiated your statement, but danced around everything else, so I'm going to assume you can't. If anyone believes a (current) x340 can achieve 5.5 gph at 75% (measured per standards) show me the dyno and I'll buy the company that makes them.

All your other points are valid, but does not substantiate your claim. 75% probably close to 10 gph (normally aspirated, mags, fixed pitch). 5.5 gph probably 45%. You can't have your cake and eat it to!

Lets move on! Greg

[nyrikki]

The max continuous HP is 80, can you provide any documentation that shows differently?

http://www.continentalmotors.aero/uploadedFiles/Content/Titan/Titan_340CC_Operations_Manual.pdf

If you can show me how this engine burns 10gph to make 60 (75%) I will avoid it.

But these engines are also not sold as a normally aspirated, mags, fixed pitch so that is what you call a straw-man. I am talking about EFI, electronic ignition engine.

Feel free to show me a normally aspirated, mags, fixed pitch x340 if I have missed it and I wouldn't buy one.

But I agree we are at in impasse. IIRC the x340 doesn't even have the gears to run a mags if you wanted to which is one of the reasons why it is ~20lbs lighter than a o320.

But I will concede if you want to continuously produce the max power or near that power with this engine it will be north of 10GPH. But you would probably want to also consider a far more aerodynamic airplane if you want the modest increases in speed doing so would offer on a kitfox.

But you can hit 5-6 gallons per hour at the 80 hp cruise but that will depend on what landing gear, tires and loading and other drag rate.

80 hp cruise will be ~76% of the speed of max power as it is the cube root of 80/180.

As a rough estimation a 120mph full out 180HP max cruise speed would give you around 90mph at 5-6 gph if you have electronic ignition.

[nyrikki]

To make this easier for still back of the envelope type calculations and avoiding the cubed root math.

At the same total mass, doubling the horsepower of an aircraft will allow it travel 1.26 times faster.

So if you change the prop to handle more power at the same mass a plane that can attain 100 mph with 100 hp would only go 125 mph with 200 hp.

While there is a bottom limit to this due to the back end of the power curve, dropping just a few mph will dramatically improve efficiency as induced drag is typically fairly small at these top speeds.

Unfortunately brake specific fuel consumption numbers aren't typically published in useful form but the figures on page 5-5 of the 912i series engines will help visualize it.

https://rotax-docs.secure.force.com/DocumentsSearch/sfc/servlet.shepherd/version/download/06812000002WNDrAAO?asPdf=false

Edit: I should note that if you add 31" tires or other draggy accessories you will have to drop 10mph or more to get close the 5-6 gph number and may get down far enough on the power curve that this rule no longer holds.

[aviator79]

I am moving closer to family in SW Wyoming, thus my obsession with head winds.

It dumping boost during takeoff is the concern, thanks for the feedback.

Not so much on takeoff as imitial climb. I know another guy with a non-intercooled 914 in Albuquerque that draws air from u deer the cowl with no cool air inlet. He's occasionally on the forums. I'll ask him if he's observed anything similar, as his should be the worst case scenario for airbox temps.

[Danzer1]

The max continuous HP is 80, can you provide any documentation that shows differently?

X340 liine card: 16333

The "340CC" manual was written to meet LSA requirements - the engine is stillcapable of the x340 specs (there are no limiters". You were refering to the x340 NOT the 340CC "manual".


But these engines are also not sold as a normally aspirated, mags, fixed pitch so that is what you call a straw-man.

The 340 CC you are now refering to IS carburated AND fixed pitch - so it is not a "straw man".

Please don't insult my intelligence and tell me that CC buyers are spending north of 200k and running them per the manual. They are sham numbers.

The x340 does 166/180 CONTINUOUS and 75% is way north of 5.5 GPH.

Greg

[jmodguy]

I should have some numbers for all in a couple months. I am running a “modern” 340 with the efii system.

[mr bill]

lets keep it civil.

[aviator79]

I asked forum member BobRS, who operates a normally-aspirated, non-intercooled 914-equipped S7 out of Albuquerque about the high airbox temperature. He says that he has not experienced his TCU limiting boost for high airbox temp.

He does report that hot starting is difficult, and he needs to let things cool down for 45 minutes or an hour prior to attempting a start, and believes this may be due to a hot airbox.

[Esser]

A 914 is not normally aspirated. It’s turboed.

I flew Stuck and Rudders 914. We didn’t start losing power until we did an extended climb for 5+ min. That was without using over boost. He also had a NACA duct that threw cold air on the filter that was directly on the turbo.

[aviator79]

Dur... I meant carbureted.

Why would you mistake what I said for what I meant? ;)

[av8rps]

A big factor that I'm not hearing about in this discussion is wing loading and power loading. While the power loading generally will be better with the bigger engines, many times the increased wing loading due to all the extra weight minimizes the extra power, resulting in similar, or even less overall performance. And almost always the lighter wing loaded airplane will be more fun to fly.

I like to fly seaplanes, amphibians specifically. And in that realm l have seen guys putting large engines in their seaplanes only to learn the smaller engined, lighter seaplanes will outperform them. Land planes are less effected, but the same general rule applies. Especially with small aircraft like the Kitfox that have small wing areas compared to something like a SuperCub.

But even in the SuperCub world big horsepower doesn't always help. A guy recently put a 260 hp 540 with a constant speed prop on his SuperCub floatplane. Yeah it was kinda cool, and made all kinds of noise...but it only marginally performed better than a 160 hp SuperCub. I call that the point of no return: Where the extra hp doesn't really offset the extra weight to the airplane, and the extra weight of the fuel needed.

This summer I was hanging out with the Just Aircraft gang and one of of them now has a couple hundred hours on his 340 powered Super STOL XL. He said he really loved all the extra torque of the 340, but that it also burns about twice as much fuel as his previous 912 equipped Super STOL. When I asked about performance differences he said a light 914 powered Super STOL is nearly as good on half the fuel. Now take that all with a grain of salt as that is only one mans opinion. But I think it is likely a realistic perspective of what to expect if going the route of the 340 on a Kitfox SS.

I personally would go for the 914 or 915 with the constant speed prop. Big bucks, but overall best performance IMHO. Again, I know it will be pricey, but you only go around once ...;)

[nyrikki]

As an update I am targeting the 912is Sport right now.

As most of you know the bing carburetors do do a fairly good job of altitude compensation until you hit main jets at high power levels which are not compensated and will be more rich.

With EFI this is less of an issue and even with the Bing carbs you could fly for a lifetime without having this be an issue...but we all have our own personal minimums and levels of acceptable risk.

Technical reason for those who care.


Bing CV carbs are almost certainly altitude compensating better than most pilots but only to about 50% of the optimal mixture due to Bernoulli's equation. So the higher you fly richer the mixture will get even at power levels below 75%. I am conservative on this topic but it is important to remember that on Bing CV carbs altitude compensation do not change the mixture the main jets.

With the warning that you should take these following numbers as examples and not as set in stone:


idle to ~ 25%: Idle Jet
15% to ~ 80%: Needle Jet
65% to 100%: Main Jet

Note: As the main jet is not altitude compensated on the Bing 64 CV carbs the mixture will be richer above ~75% than below (IIRC).

If we choose those random numbers of 100 % at 100' MSL and 75% at 7000' you will be producing somewhere around 60-65% or less at full throttle with a sea level sized main jet. The important part is that you should expect more power loss at takeoff power in any Bing CV equipped engine than you see lower in the power band.


To be honest isn't a huge concern if you plan for this additional drop in power on the top part and/or an occasional visitor.

The Bing CV carbs are probably safer than the problem of pilots failing to lean the engine for maximum RPM at altitude with a mixture control. But remember that a nice idle or acceptable if larger cruise fuel burn doesn't mean you will have the same performance at the top end when the main jets are the largest source of fuel.

The additional weight for FI and x340 is what drove my decision when I cataloged what I would want to typically take on a weekend trip. While the x340 would have more than enough power even rich I think that trying to lean to maximum RPM at full throttle during runup with 180HP on the front on the carb version would be a bit past my comfort zone in a light taildragger too. I am making no claim as to if there is actually at risk of a nose over but it is beyond my personal comfort levels.

As a reminder, this post is opinion and very specific to my decisions, desires, and wants so once again take all of this with a grain of salt. Hopefully this will help others decide to look into this themselves or possible remind people that they need to leave some head room for larger power losses at high DA with Bing CV carbs at high power settings.

Obviously if you only fly at altitude changing the main jet would also be an option as long as you remembered to be careful at lower altitudes where you will be too lean.

For the vast majority of people using Bing CV carbs a small increase in fuel burn is probably the only result of this behavior. For those with engines and a mixture knob remember to lean for RPM during runup above 5K MSL or AD.

[jmodguy]

I am assuming that all who prescribe to the economy of the Rotax as the best choice for our aircraft, all drive a Prius...:D:eek: I kid... I kid...

All kidding aside, I am not interested in the best fuel economy. The Kitfox flies just fine with a 340. Otherwise Kitfox Aircraft would not have shown the world what it was capable of with this engine.
I don't drive a Prius nor do I worry about what my MPG is at any given day. Does a 340 burn more gas? I am sure it does. Will it out climb a 912 is? I am sure it will.
To each his own, I choose a different path. Catch me... if you can! :cool:

[jrevens]

IMHO, a Kitfox is just not going to be a real efficient airplane as far as mpg goes, because it's never going to be a slick, fast airplane no matter how much horsepower you have. I shines in other ways that are important to most of us.


As an example of the mpg thing, I flew a cross country trip up to Idaho with my friend Stan. He flew his 80 hp Mod. IV Speedster, and I my 160 hp Thorp. While not going as fast as I could have, I averaged 180 mph and obviously reached our planned stops well before he did, where I waited and re-fueled along with him. I burned considerably less fuel at each stop. My mpg was close to 30. He could have burned cheaper fuel than I, if it had been available.

[aviator79]

In the same spirit of friendly ribbing, the factory liked that 340 so much that they replaced it with a turbochrged Rotax. :D

I kid as well; they use those planes as test mules. Removing the 340 is not an indictment of its performance. How many engines has 12KF had? Nonetheless, I don't think anyone would say the performance from the current powerplant on the STI is Prius-like.

jmodguy, your plane will be a beast, and a pretty one at that. What's great about building your own plane is that you get a plane that is really your own.

[aviator79]

Now that I think of it, because the STi has had both the 340 and a turbo Rotax, asking John or Brandon to give you an honest comparison might not be a bad idea. It's not quite apples-to-apples because the 915 has a bit more punch, an intercooler, and EFI. And then there's the price tag... Still, you can eliminate a lot of variables by holding the pilot and the rest of the airplane constant.

[nyrikki]

With product cycle times and development costs I would guess Rotax and a few of the kit providers are making bets on possible FAA rule changes (https://generalaviationnews.com/2018/09/02/new-opportunities-coming-for-light-sport-aircraft/) which would impact the release of a 915 without CV prop oil. As an in-flight adjustable prop is needed to take full advantage of the 915 Rotax may just consider the market too small to target especially as this isn't an issue in Europe.

Disclaimer that I have zero insider information and that my statements here are pure guesses and most likely wrong.

I would guess that as part of LAMA's request for early testing of the single lever adjustable props and other requests are related.

If you look at the factory STi with the 915 it does seem to be single lever control and is probably the system in the below presentation from RS Aerotech.

https://youtu.be/y7xU_2Zzbb0?t=300

As John has stated that there has been a shift to people wanting to buy S-LSA airplanes and a relative decline of people building kits. So being ready for potential rule changes and being able to have data to justify those changes is probably of value to them.

The RS Aerotech's presentation on a single lever solution requires a FADEC engine and you can see some info here although public info is fairly sparse. Look at the display on this sheet then look at the LCD in front of the right seat on the factory STi. Note that they are also using the same MT prop from this presentation too.

http://www.rsaerotech.com/tl_files/downloads/emu912is_downloads/Documents/SFS_MT_Presentation.pdf

Under the S-LSA rules the x340 weight puts is very close to the limits without going E-AB and I would bet being able to support LAMAs request and the offer for early access to a new sexy prototype engine justified swapping them out. Especially compared to the time and cost of pulling a new airplane from a production line with a waiting list. The x340 is not full FADEC or even EFI how kitfox sells them and I don't know if anyone is testing a single lever solution for that engine yet but it would add weight.

Reminder this is all purely a guess on my part.


Edit to add tread from earlier this year that talks about this.

http://www.teamkitfox.com/Forums/showthread.php?p=71114

[jmodguy]

A quick look at the rsaerotech system gave me this...
$35k for the engine, prob closer to 40k installed, @$10-12k for prop, can’t seem to find EMU pricing but let’s say $1.5k. Don’t forget the 6year CS prop overhaul cost (@ $2k min for an MT). Maybe they offer a package discount? Anyone want to go to a Nassau to check them out??:D
With the added complexities don’t forget to consider failure modes and the cost of troubleshooting/repairing this system. Look at all the threads on troubleshooting basic 912 engines on this forum. This system may be simple to operate, but it will be costly to maintain and IMO cost prohibitive to install at $50k+.

[airlina]

Amen Brother, its all fun and games until you gotta fix something-in my simple mind simpler is better. I have an IO-240 up front of my Series 5 . So far 15 years and 850 hrs later its been bullet proof . No regrets here , but thats the beauty of a homebuilt, you build it the way YOU want it , to meet what YOU think an airplane should look like.Bruce N199CL

[nyrikki]

I should clarify that I was referencing guessing on Rotax and other S-LSA industry trends and motivations. The exponential pricing seems to indicate that they don't worry too much about the increased price and CV props are fairly popular in Europe where the rules are different.

My budget is below S-LSA prices but sufficient enough to avoid hand propping J3/7AC. I won't go with a 915+CV but I doubt the LSA rules will ever allow for a three lever config. "pilot's improper use of the mixture control" is pretty common on crash reports. So I doubt that adding a 3rd lever will happen for S-LSA. Note that is a guess and not based on any knowledge.

I don't own a car without a clutch pedal and haven't for decades so I get the sentiment about simplicity.

While I have no experience with CV props and can't even comment on that topic. On the subject of the rotax ignition. A shift in complexity can actually lead to less complexity for the system as a whole.

While my opinion won't be true for everyone. IMHO it would be easier for me to troubleshoot the 912/15iS than the 912ul(s) Ducati "Dual Capacitor Discharge Ignition". Despite being "electronic" it is not digital and while it may have a lower part count it is not necessarily simpler.

The magneto generator stator assembly is complex and the e-boxes are hard to test and the shared nature of components makes things challenging IMHO. Like a typical CDI system it is mostly passive components which sounds simpler until you have multiple functions on one stator and two potted black boxes without enough connectivity to fully test the thyristor circuit.

http://www.aeroelectric.com/Mfgr_Data/Rotax/Rotax_912_Ignition_Schematic.jpg

While the value obviously needs to be evaluated by each user the increase number in sensors on a computer controlled system can be easier to test in isolation.

With simple dual coil packs and hall effect crank sensors I wouldn't even need a schematic to troubleshoot the major components on a 912is/915is. Which is a good thing because I don't think Rotax has release a schematic but that is partly because they don't have to.

Obviously this is dependent on a builders preferences, budget, and experience. Personally I would find the shift in complexity of a computer controlled ignition to actually result in a lower overall complexity compared to the 912ul(s) dual Ducati DCDI system.

I wouldn't back away from a newer flyEFii or SDS system on a traditional aero engine but people will want to understand fuel maps etc...which does pose an additional barrier.

Your mileage may vary.

It is really cool to have the ability to build it the way YOU want.

[tjentzsch]

For me, when I get to it, the 912is will most likely be the one I choose. I live in Utah, so already I am high elevation. I would LOVE to put on the 915is, but 38k for just an engine YIKES!. My entire brand new 2016 Tacoma cost under that!

[nyrikki]

As to not include others in what probably edges on paranoia on my part I should point out that people do fine with the 914 and even the lower power 912ul or even the two strokes in Wyoming and Colorado.

I had a bad experience coming back west bound across the divide South of Lander a few decades back J-3 (pre-gps days). With the typical Westerlies it is not uncommon for there to be a sink on the leeward side and there is a gradual rise that can be very deceptive especially in the winter with flat lighting or fresh snow.

This has lead to several crashes in the area but a kitfox should do better than that old J-3. The slight increase in power at altitude from the iS plus Bing carbs tending to the rich side is a concern for me. Note this is a extremely conservative stance. The J-3 didn't even have a mixture control or the altitude compensation and had less HP to start with so...

I lost the link to the great web page that graphed historical DA. The concerns about the 8000' critical altitude for takeoff power on the 914 was the inability for official sources to describe how power fell off . If you watch the DA for EVW you will see that it cycles between 7200' and 10,000' daily for most of the year.

If you are down in the valley in Utah at a ~4,200 elevation this is less of a concern and the performance benefits may be worth it for you if cost is not a barrier.

My grandfather built a glider in the 1930's and had no problem flying it at 8000' despite the fact that he launched with his brother towing it with a motorcycle and even 45HP would be a stretch. So I chose to error on the side of known values and constancy.

None of these limitations I mentioned would pose a problem if someone simply knew their aircraft. It is simply past my personal comfort levels because I want to do the math.

I am only offering my opinions because this is a hard decision and most threads tend to be focused on single choices. Hopefully my posts help others evaluate their options but please temper them with with the above personal limitations and conditions. I am in no way an authority or subject matter expert.

If my home airport below 5000' I would personally consider the risk vs. exposure time to be small and less critical than other factors.

A 915 or x340 and potentially Vx being primarily limited by pilot comfort would be pretty cool though.

It would be nice if people circle back an update what personal choices or assumptions they would have changed after they have some flight time. I'll try to remember to point out my mistakes were.