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Thread: PROs and CONs of Engine Choices

  1. #21

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    Default Re: PROs and CONs of Engine Choices

    Quote Originally Posted by Danzer1 View Post
    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.
    Last edited by nyrikki; 09-13-2018 at 06:39 PM.

  2. #22
    Senior Member aviator79's Avatar
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    Default Re: PROs and CONs of Engine Choices

    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?
    --Brian
    Flying - S7SS

  3. #23

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    Default Re: PROs and CONs of Engine Choices

    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.
    Last edited by nyrikki; 09-13-2018 at 06:41 PM.

  4. #24
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    Default Re: PROs and CONs of Engine Choices

    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

  5. #25

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    Default Re: PROs and CONs of Engine Choices

    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/...A4?asPdf=false
    Last edited by nyrikki; 09-13-2018 at 07:14 PM.

  6. #26
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    Default Re: PROs and CONs of Engine Choices

    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.

  7. #27

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    Default Re: PROs and CONs of Engine Choices

    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/...8/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.

  8. #28
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    Default Re: PROs and CONs of Engine Choices

    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

  9. #29

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    Default Re: PROs and CONs of Engine Choices

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

    http://www.continentalmotors.aero/up...ons_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.
    Last edited by nyrikki; 09-13-2018 at 09:55 PM.

  10. #30

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    Default Re: PROs and CONs of Engine Choices

    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/...AO?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.
    Last edited by nyrikki; 09-13-2018 at 10:37 PM.

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