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Thread: Center of gravity and retractable skis

  1. #1
    skukum12's Avatar
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    Center of gravity and retractable skis

    My 12 is still very nose heavy. Of course I can load survival gear aft and other considerations. My question is this, has anyone experienced ill effects of retractable skis and flight characteristics? Or do the aerodynamic effects of the ski negate any nose heavy tendencies?
    "Always looking up"

  2. #2

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    Adding Fluidyne or FliLites in place of 31s is going to put about 70# added weight on your mains. That's a guestimate since I no longer have my -12 W&B. (My Cessna difference between Fluidynes/850s and 29s is 101# with Magnum tail ski in the mix.) Airglas hydraulics add less. In any case hydraulic skis are heavy and draggy and don't perform on the snow as well as straight skis but landing and parking on dry ground makes the compromise worthwhile for some of us.

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    My PA-12/18 hybrid (homebuilt) is also quite nose-heavy. Like you, I find ways to compensate for that.

    I think you're referring to the change in CG as the retractables move fore and aft. I've not noticed any significant trim changes with my Datums.

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    I am at the front of my empty CG with my Federal AWB 2500. Plane performs great with skis up or down, I do have to retrim between up and down mine weigh a bit more than the Datums. If you are running out of nose up trim on wheels you may need to cut the tail or just put more gear aft for heavy skis
    DENNY

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    beaver18's Avatar
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    I had Flilite 3000 before my Trickair and foud some interesting flight carecteristic when empty gas and no passenger or baggage the tailplane will stall before the wings ( have VGs ) at a much higher speed than normal stall speed be careful.

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    Beaver 18
    Do you use full nose up trim on landing?
    DENNY

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    skukum12's Avatar
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    Denny, most landing configurations I am already using full nose up trim plus a blast of power in the flare (29 bushwheels). Adding a bunch of weight that far up spooks me, not to mention the shifting cg of the skis.

    I have been given quality ideas about the tail, what specifically does your "cutting the tail" entail?
    "Always looking up"

  8. #8
    beaver18's Avatar
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    Quote Originally Posted by DENNY View Post
    Beaver 18
    Do you use full nose up trim on landing?
    DENNY
    No just normal flight needed trim no more no less ,found that one day with low fuel and everyting else empty comming in for landing and about 3 feets of the ground and the thing stalled without warning at a higher airspeed than usual no big deal but made a touch and go to confirm with more precaution to confirm

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    Loved my FliLites on the -12 but my old short mount -12 didn't have W&B issues. . My perception was that the skis contributed to ground effect lift. I don't have that feeling with other skis.
    Last edited by stewartb; 12-11-2016 at 09:05 PM.

  10. #10
    Scooter7779h's Avatar
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    We just weighed 53 on FliLite 3000s last Sunday (SBs old -12). I will report back with numbers they are at the hangar. It is not nose heavy at all with short mount. I don't trim at all from cruise trim setting to slow full stall landing.

    Quote Originally Posted by stewartb View Post
    Adding Fluidyne or FliLites in place of 31s is going to put about 70# added weight on your mains. That's a guestimate since I no longer have my -12 W&B. (My Cessna difference between Fluidynes/850s and 29s is 101# with Magnum tail ski in the mix.) Airglas hydraulics add less. In any case hydraulic skis are heavy and draggy and don't perform on the snow as well as straight skis but landing and parking on dry ground makes the compromise worthwhile for some of us.
    =========
    PA-12 fan

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    beaver18's Avatar
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    One thing for shure i wont be going back on Flilite they are heavy and performance wise they dont come close to my Trickair .

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    skywagon8a's Avatar
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    skukum12, You haven't mentioned it, do you have extended landing gear? IF you do, the wheels/skis will be further down in relation to the vertical CG. This will increase drag moments giving the impression of being more nose heavy. Actually it is more nose down drag requiring more nose up trim. The easiest solution is to add ballast or baggage aft.

    Denny's "cutting the tail" is a serious alteration.
    N1PA

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    Things I think about with respect to skis, and I'm in decision mode myself. Not whether to get hydraulics again but which ones to get.

    On snow performance in the worst conditions. Any ski works in good conditions. Which one do I want when the LZ is short and punky with overflow and night is falling so I need to get out now?

    On dirt performance. Any wheel ski works on graded pavement. Which one do I want for landing wheels down on unimproved and ungraded strips? Skis-up ground clearance is important.

    Durability. Which ski will take the inevitable hits from rock hard drifts and frozen snowgo tracks you didn't see?
    Last edited by stewartb; 12-12-2016 at 08:02 AM.

  14. #14

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    Quote Originally Posted by skukum12 View Post
    My 12 is still very nose heavy. Of course I can load survival gear aft and other considerations. My question is this, has anyone experienced ill effects of retractable skis and flight characteristics? Or do the aerodynamic effects of the ski negate any nose heavy tendencies?
    Feels nose heavy? What's your CG? Are the two related? Perhaps you should get a handle on the feels nose heavy thing first before you spend a bunch of money on skis that will make the CG move forward. Empty CG on the forward limit is the best condition I could hope for on skis. That's a useful and verstile airplane to me. In fact that's my target for my new project.
    Last edited by stewartb; 12-12-2016 at 08:03 AM.

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    mvivion's Avatar
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    All the retractable wheel-ski equipped aircraft I ever flew had a weight and balance sheet that gave an empty CG for both skis up and skis down. Depending on the skis, lowering the skis shifts the empty CG a fair bit aft. Consider AWB 2500 skis or Fluidynes: You're shifting well over a hundred pounds of ski aft anywhere from six inches to almost a foot when you lower the skis.

    Never caused me any problems, but I always did my balance calculations considering the skis down position unless heavily loaded.

    Stewart, we've had this conversation before, but the Rodri Fernandez 8001 skis really shine in every category you listed. Plus, they weigh about the same as a set of straight skis. Bring $$, however.

    MTV
    Last edited by mvivion; 12-12-2016 at 02:39 PM.

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    CG goes forward when you retract the skis, but we get the point.

    I traded messages with Thomas last winter and discovered RFs aren't much different in price from Airglas LH skis. They're definitely less than new AirGlides. I'm just not sold on the durability. A friend took RFs off his Cub and replaced them with LHs because the installation durability concerned him. I don't need to buy skis this winter so I have time to think about it. I'd like to hear more pireps about RFs and LHs.

  17. #17
    skukum12's Avatar
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    skywagon, I do have the 3" gear, it's so common that it's thought of a standard equipment. Gonna call Steve Kracke today and find out if the 3" gear moves the axle forward (or aft) or just up and down. Here I go with the question of 1" or 1.5" aft of datum?

    stewart, the plane was weighed two years ago and had an 8.9 ew cg., 9 is the limit. After some changes, the new calculated # is 9.6 empty, 10.5 with me and full fuel. The plan is to weigh her again in about a month or so.

    I would love to see some more pireps myself of the LHs from Airglas, just the plate and some ram moving back and forth, lighter overall skis.
    "Always looking up"

  18. #18
    kase's Avatar
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    I had a cub with AWB2500s. When the skis were up the ewcg was in the 9 in range.

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    Skukum12
    Cutting the tail usually involves removing fabric from lower longerons and cutting them. This will allow you to adjust tail position relative to the wing (allowing better trim range) Once set properly inner or outer splice on the longerons than reglue fabric/tape/paint/shrink as needed. The change in longeron length is often very little. Skywaggon is correct this sounds simple but having a good understanding of the plane/ angles/goals involved make it a much harder job than my three sentence explanation did.
    DENNY

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    mvivion's Avatar
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    Quote Originally Posted by stewartb View Post
    CG goes forward when you retract the skis, but we get the point.

    I traded messages with Thomas last winter and discovered RFs aren't much different in price from Airglas LH skis. They're definitely less than new AirGlides. I'm just not sold on the durability. A friend took RFs off his Cub and replaced them with LHs because the installation durability concerned him. I don't need to buy skis this winter so I have time to think about it. I'd like to hear more pireps about RFs and LHs.
    Duh!! Thanks, I corrected my post.....too early in the AM to get east and west straight, let alone forward and aft....

    Mike

  21. #21
    Dave Calkins's Avatar
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    I knew what you meant to say and REALLY would have loved to correct you. But SB got you!

  22. #22
    Dave Calkins's Avatar
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    Fernandez skis move a long ways forward when retracted

    thank God they are very light skis!

  23. #23
    mvivion's Avatar
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    Quote Originally Posted by Dave Calkins View Post
    Fernandez skis move a long ways forward when retracted

    thank God they are very light skis!
    Indeed:

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    MTV

  24. #24
    skukum12's Avatar
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    Quote Originally Posted by kase View Post
    I had a cub with AWB2500s. When the skis were up the ewcg was in the 9 in range.
    Kase, how did she fly when light? Or were you simply loading aft IF there were any ill characteristics?
    "Always looking up"

  25. #25
    kase's Avatar
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    Flew fine. Just gave it a couple turns nose up trim when landing.

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