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Thread: HP loss w/ DA

  1. #1

    Join Date
    Apr 2019
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    South Florida
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    Default HP loss w/ DA

    Hp loss = (DA x .03 x sea level hp)/1000

    So on a stock 912is at 12k DA you now have a loss of 36 HP. So at 12k DA you now have 64 HP.

    I am keenly aware of the high DA's that occur in the northwest --- but at the time I did not care. Now I have aspirations of exploring out there in a KF, I thought I didn't care - how bad could the DA hit be? Other guys do it! But then I did the math (backed up by my 12 year old) -- and at 36% power loss? I care.

    Heck even here in Florida we get hot humid days and our DA can get up a little --- considering floats and the implications of even 4'k DA...

    For those that do significant flying out west do you find that having turbo to normalize is significant? Seems to me that it makes a pretty significant difference (36% difference give or take depending on your exact engine).

    I am trying to avoid building a plane and going, "huh. I really should have bought a turbo-normalized engine". That said -- it's a hefty price tag (and weight) jump to get into a turbo'd rotax.

    Thoughts? (is this one of those "if you have to ask..." type questions?)



  2. #2
    Senior Member
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    Nov 2013
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    Default Re: HP loss w/ DA

    I've just got one season under my belt with my 912ULS in Idaho. Two things. There hasn't been any place I've wanted to go but couldn't and to fly in the mountains I've adopted the mindset that I'm heading out of the hills by 11am in the summer. The thermals and winds start picking up about 10am. There no way I'd plan to be in the mountains at 3pm on a summer day.

  3. #3
    Senior Member jiott's Avatar
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    Default Re: HP loss w/ DA

    Ditto what Guppy said. I also have been flying in the NW for 6 years with a 912uls and have never had to scrub because of DA, or even had a close call. Of course a turbo would be nice and gives a better safety factor, but it certainly is not mandatory out here in the NW.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  4. #4
    Senior Member efwd's Avatar
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    Jun 2015
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    Yorba Linda, CA
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    Default Re: HP loss w/ DA

    Only one season on my 912iS but while alone in the plane and at 9000ft DA I had no problem going into Big Bear Lake East of LA. With two up, I definitely feel it but on the improved airport I won't skip that trip. I agree with the previous points.
    Eddie Forward
    Flying
    SS7, 912iS, Garmin G3X

  5. #5

    Join Date
    Oct 2019
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    Lake Wales, FL
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    23

    Default Re: HP loss w/ DA

    I think everyone goes through this dilemma when planning a build. In fact I've asked this same question before. After pulling my hair out over this topic for the better part of a year now, I've settled on the 912is. The engine is incredibly reliable, is widely supported, is super efficient, and (as seen here) makes plenty of power to get you where you need to go. Also mo carbs = mo problems so the 914 was ruled out from the start. Ask anyone who owns carbed boats or motorcycles because plane people are biased.

    I too worried about the lack of power at high DA but GuppyWN brings up a good point, you shouldn't be flying in the mountains when it is really hot for more reasons than just high DA. Just do your flying in the mornings or late evenings like everyone else when it is cool. I had to remind myself that I'm not a medevec team, and I will have time and flexibility to plan my trips around the conditions.

    The KF was designed to fly just fine on 80hp. Of course it would be nice to have 135hp all the time, but I've decided that it's just not necessary to have a super awesome and capable plane. The added cost is pretty huge when you add it all up all the bells and whistles to make the 915is go. And yes you can go many cheaper routes than the 915 to get hp, but then you start getting into elevated risk scenarios due to unknown reliability. Zipper and Edge kits have all gone through various failures. I would rather have a reliable and predictable engine in the mountains than extra hp. I know for a fact that I will find myself saying "I wish I had more hp", but I'm ok with that. I feel like pretty much everyone says that about pretty much every plane.

    So in the end you can spend the money and buy a fire breather, but ask yourself how often you will realistically be in a scenario where you will actually NEED the extra hp. I like to fantasize about landing on top of some tiny ridge at 10k ft with all my gear, but am I really going to do that? Are you trying to win STOL competitions? Am I really going to try to cram into some tiny strip at elevation at max gross? For me the answer is no. Pretty much all the strips in Idaho can handle say a Cessna 180 given a good pilot, and a KF will eat that plane for breakfast even with only 100hp. Maybe in 5 years I'll put the latest big hp rotax motor on (hopefully they have a turbine by then).

    My kit ships in 2 weeks. Getting stoked for the build. Sorry if this is too long.

  6. #6
    Senior Member HighWing's Avatar
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    Default Re: HP loss w/ DA

    Yes, But. All things are not the same. Back in the day when we were flying our annual flights of six to the Idaho back country, we over-nighted as usual at Johnson Creek. The next day when we left, I was on the right wing of one of the guys, Mark and his teenage son in their Model IV with Rotax 912UL, as we climbed out. Sadly, I didn't recognize what he was focusing on until too late, but his goal on that departure was to climb straight over the distant ridge through what we called the gun-sight. Since the river ran down toward the left, it was typical of all departing aircraft to hug the slope to the right to get a bit of wind lift, then turn downriver to finish the climb. He made it over the ridge straight ahead, but not me, at the last minute, I had to turn down river as I needed about 100 more feet to comfortably make it.

    The reason I often tell this story is because even with the 80 Hp 912UL the Rotax pulling a Kitfox makes for a really aggressive machine. I recall in the How to Fly A Kitfox book of that day, reading the comment that the Kitfox is a really "high performance" airplane. Not that it is necessarily fast from here to there, but it is a really good climber. In the picture below, I took the liberty of photo-shopping an image of an airplane showing where we typically saw the typical certified airplane disappear from view during its climb out.

    The large notch on the ridge straight beyond the runway is the "Gun-sight". As the river turns west as it descends, the ridges on both sides also turn to the west and the next dip in the ridge reflects that descent.

    I should mention Johnson Creek Elev. 4900 ft.
    Attached Images Attached Images
    Last edited by HighWing; 03-14-2020 at 06:29 PM.
    Lowell Fitt
    Goodyear, AZ


    My You Tube Channel

  7. #7
    Senior Member aviator79's Avatar
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    Default Re: HP loss w/ DA

    I don't think you can talk about turbos without also talking about props. In my opinion, the turbocharged engine badly wants a constant speed prop. If your decisions are budget-based and you can afford either a turbo or a CS prop (but not both), choose the prop. The exception would be if you absolutely need to take off in 50', which is a very slim minority of pilots. It's true that nobody ever wishes they had less power, but whether you really get value for your dollar with a turbo depends upon where you're based and what your typical flight looks like. It does seem like a lot of new builds are being done on a "Money-is-no-object" basis (I'm sort of guilty). But don't let that push you into thinking you need things you don't. To break it down:

    Normally-aspirated/Fixed-pitch - A perfectly capable airplane that will go in and out of almost anywhere you're likely to go.

    Turbo/Fixed pitch - You'll likely pitch your prop for climb. Incredible takeoff/climb performance, but when you push the nose over in cruise, you'll have to pull the throttle back into unboosted territory to keep the engine turning below redline, so the turbo does nothing for you most of the time. Also, in turbulence, you'll be chasing RPM with the throttle all the time as the Kitfox rides every up and downdraft like a rowboat in an ocean squall. When you need to climb though, you'll have performance to spare.

    NA/CS: Really comfortable airplane to fly. No chasing the throttle setting, and you get to squeeze all the cruise performance out of your engine. DA will affect you, but will seldom keep you from going where you want to go. And in cruise, you'll absolutely run away from your FP friends, even if they have turbos.

    Turbo/CS: The best of all worlds. Outstanding takeoff/climb performance, and great cruise speeds. You get to make all your power at altitude, and your prop will let you use it. Even in the mountains, you have altitude options to try to find smooth air and favorable winds.

    I chose a turbo (w/Airmaster CS prop) because I fly out of 7200' in New Mexico. Summer DA's are in excess of 10k', so I get my money's worth from the turbo. If I were closer to sea level, I would lean toward a 912iS unless money really were no object. The best thing you can do is go fly Stick and Rudder's 912iS/Airmaster plane and their 914UL/Fixed-pitch plane in the same day. Unfortunately, they don't have a 914UL/Airmaster plane, but you'll be able to appreciate the entire decision space if you fly both of those airplanes.
    --Brian
    Flying - S7SS

  8. #8
    Senior Member
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    Default Re: HP loss w/ DA

    All good points by Brian. Although, if you are doing a STI wing I have yet to see the benefits. It just adds weight and doesn’t do much for topend speed.
    Dustin Dickerson

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

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