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Thread: Engine/Propeller Combination S7 sti?

  1. #1
    ratc's Avatar
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    Default Engine/Propeller Combination S7 sti?

    For those who have completed aircraft and to those building, what considerations have you made towards engine and propeller choice and why?

    I know a lot will depend on mission profile, an element which I would also be interested to hear.

    I'm based in the UK, where on the worst day we would never see density altitudes above 5000 ft, even in the highest terrain. The jury is out as to whether I would every use the full potential of the 915is and 3 blade MT constant speed propeller? The ability to cruise at 10,000 ft with reduced fuel burn and maximum efficiency does have its merits but defeats the LOW and SLOW profile.
    The cost is eye watering, another consideration. You could buy a lot of fuel for the difference between the 912is and the 915is.
    I do like the idea of a constant speed propeller in that it does give access to the full potential of the engine, albeit with added complexity.But with which engine?

    I'm sure there are aspects that I've not considered. ANY inputs would be gratefully received whilst I chew over this most difficult of choices.

    Many thanks.
    Ratc

  2. #2
    Denalifox's Avatar
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    Default Re: Engine/Propeller Combination S7 sti?

    Im in the exact same boat. I have had a year to mull over what combo to use. i THOUGHT i finally figured it out and then I start thinking again. My considerations into what Im going the way Im going are highly influenced by where I live. Ive seen alot of guys say start your FWB build and then start considering your FWF set up. FWIW, Im going with a 120hp O-200 with Airmaster propeller. My DA allows it up here in AK.

  3. #3
    Senior Member jiott's Avatar
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    Default Re: Engine/Propeller Combination S7 sti?

    I have been running a 912uls with Whirlwind 3-blade ground adjustable prop on my 7SS for over 6 years (800+ hours) and it performs great here in the Pacific Northwest, Cascade mountains and occasionally Idaho backcountry. Never been to an airstrip where I have had the slightest worries about engine power. Granted I am a 75 year old guy, not very aggressive, not competitive, but I am no wallflower either. That said, if I was starting a new build today I would change a few things based on my 6 years experience and observing and flying with Kitfox friends with other engine/prop combinations. For me, my perfect new build would again be the Kitfox 7SS (not the STI); the engine would be the Rotax 912is to get the increased fuel efficiency and no carburetor syncing or icing; and the prop (if I could afford it) would be the Airmaster 3-blade Whirlwind constant speed to take full advantage of all the engine power. I believe this combo gives the most performance advantages and lightest weight and lowest cost providing you don't fly at consistently high altitudes/density, are not into STOL drags, and don't need the "Hey Look at Me" admiration of peers. Just my own opinion, yours will certainly vary.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  4. #4
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    Default Re: Engine/Propeller Combination S7 sti?

    Okay here is my biased opinion. I will try not to ruffle anyones feathers.

    Edge Performance 155hp STI. Fixed pitch nr 79" 3 blade. Weight/Power is everything. You will have more horsepower and significantly lighter than the 915 with a constant speed. Constant speed on a STI wing is not worth the cost and weight penalty. The STI wing is pretty draggy. I am running 29" tires and STI wing. I seem to run into drag wall about 100mph. You throw more power at it and you can go a little faster but fuel burns climb rapidly and speed barley increases. I know it has been said you can get 115mph-120mph out of the STI with the 915 with constant speed but fuel burns are up around 10 gallons an hour, but I would guess no one flies around like this. At that power settings you are pushing the engine pretty hard IMO and intake temps are going to get up there. If you watch the "videos" the STI with the 915 trues out about 105mph on 6 gallons an hour. When I run 5200-5400 rpm, I true out at 105mph and I am burning 6.5gallons with the edge engine at 8500msl. I richened mine up on the topend to keep the egts a little cooler. 1500f and higher egts just seem crazy to me. I can push it up to about 8gallons an hour before I run out of prop, this is where a constant speed prop would help but my speed only goes up about 5mph and intake temps goes up and Manifold pressure is now up in the 30-32 range. Just harder than I want to push the engine in cruise. 47" for takeoff if you want to takeoff short.


    Here are a few arguing points.


    915PROS: The engine is supported by kitfox factory. You can buy the engine from kitfox and they have FWF kit. They will help you on the install and you will have a factory warranty on the engine. Its a great engine with lots of innovations and safety's built into the engine. I don't think you will be disappointed.

    915CONS: Weight and Cost.


    EP155STI PROS: A turbo charged engine is not for everyone. There is some tuning and close monitoring of the engine and in essence more MX on a turbocharged engine. I would prefer the edge engine over the 915 because I have the ability to tune it. I also prefer that everything is monitored by the G3X and everything is datalogged with the G3X. AFR's are bluetoothed to my panel mounted ipad, or whatever device you want. I know the 915 has its own engine control and monitoring, but I would prefer to use what I already have installed and not having to install a separate screen. If you want to run a midler boost curve (boost comes on more gradual) and less/more manifold pressure, or want to trim fuel from a cylinder to equalize egts, it takes about 2 min to adjust with a laptop.

    EP155STI CONS: Why is the EDGE motor so much lighter? Simplicity. That can be a good thing and a bad thing. The edge motor does not have the redundancy that the 915 has. This is good for weight, but the single channel ECU is a single point of failure. Of course I am running a VPX to power everything. If it dies, everything dies. Edge claims they have not had an ECU failure on the EFI systems and there is a lot of EFI systems flying. Also no factory warranty. Edge has stood behind there products, my muffler failed and edge re-designed a new better one. As far as ROTAX goes you are on your own. Of course we always say that a warranty does you no good when your brokedown in the backcountry. Edge products are top notch and well designed. From the machining to the welding to the wire harnesses they are all top quality. Everything went together and finish is great. Edge was very helpful on the install but you do have to understand this is not a factory supported engine, so support from the kitfox factory......your on your own.

    Another thing to consider. If I was a low elevation, I would highly consider the the 7ss versus the STI wing. I like boost so I would still go turbo. With the 7ss wing a constant speed does make more sense because now you can benefit on the top speed cruise and still have a very short takeoff and landing. I prefer the STI wing for our high elevation, and for shorter take offs and landings. I have got to imagine the STI does better when heavy also. So, I guess you really need to decide your mission and go from there. Scott Noble runs the 7ss wing and bush gear and tires. His plane does great at shortfield and he still cruises fast with his airmaster. He will be adding boost to it soon (right Scott?) and then he will have a killer combo.



    Last edited by Shadowrider; 01-09-2020 at 12:59 PM.
    Dustin Dickerson

    Building 7ss STI x 2
    Oratex
    29" shock monster
    EP912STI 155hp
    Garmin
    N33TF......FLYING!
    N53TF......FLYING!

  5. #5
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    Default Re: Engine/Propeller Combination S7 sti?

    I agree Jim. The more I fly mine the more I think the 912is would be the right engine. The bonus is I already have the Airmaster!

  6. #6
    ratc's Avatar
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    Default Re: Engine/Propeller Combination S7 sti?

    All fantastic information, thank you.

    Still much pondering to do!

    Ratc

  7. #7

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    Default Re: Engine/Propeller Combination S7 sti?

    Quote Originally Posted by Denalifox View Post
    a 120hp O-200 .
    Can I ask how your tuning the 100 hp to a 120 hp?

    Thanks

  8. #8

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    Default Re: Engine/Propeller Combination S7 sti?

    Just b/c cost is what cost is, has anyone looked at the Aeromomentum engines?

    http://www.aeromomentum.com/am15.html

    $12,495(117hp)
    $16,495(131hp)$16,495(147hp)

    184 lbs

  9. #9
    Denalifox's Avatar
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    Default Re: Engine/Propeller Combination S7 sti?

    Quote Originally Posted by Rik View Post
    Can I ask how your tuning the 100 hp to a 120 hp?

    Thanks
    high comp pistons, pmags, tuned exhaust, and port/polish. The tuned exhaust alone is worth 8-10hp. Possibly EFI. Not sold on that yet though. Not a fan of single injector per cyl.

  10. #10

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    Default Re: Engine/Propeller Combination S7 sti?

    Quote Originally Posted by Denalifox View Post
    high comp pistons, pmags, tuned exhaust, and port/polish. The tuned exhaust alone is worth 8-10hp. Possibly EFI. Not sold on that yet though. Not a fan of single injector per cyl.
    How many rpm will you have to spin it to?

    Reason I'm asking (totally different plane and application) a Q200 uses a IO-200 and they claim it's a continuous 90 hp engine. It gets hot on climb out they claim so it's like a 5 minute max 100 hp.

    Then I swear, every guy who has one claims 120-125 hp as they upped the compression from 7:1 to 9:1 and they all think it's an instant 20 hp?

    The pmag I am guessing are the electronic mag's that require a constant power source?

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