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Lunak LF-107 3.7m

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  • Lunak LF-107 3.7m

    Time to buy servos for the first time in 10 or so years. Approx. how much torque is required for servos in a model this size?

    Thanks Tom, nice meeting you

  • #2
    For the ailerons, you need to see what they provide for mounting access. I use MKS and JR servos. If there is space, then put in JR DS198HV or a MKS6130HV. on the flaps and ailerons. I have some variation of the MKS 6110HV on spoilers on several sailplanes. It all depends on if they are mounted vertically or horizontally. I don't have any KST (I keep number of different servos to a minimum) but they also offer similar sized servos. Any mini sized servo is probably fine in the fuselage. JR S3411 or MKS HV69. This is assuming you use something like 18650 or 21700 2-cell flight packs. There is nothing unusual about that sailplane and almost anything you pick that fits will probably have way more power than required.

    If you search for servo torque calculators, you can probably find a few spreadsheets out there on the Internet.

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    • #3
      Good morning John, you're digging right into this thing without delay!! You won't find more help with this build, or any other for that matter, than on these boards. Super helpful and knowledgable crew that has really helped me along with my addiction.

      I just sent the build directions to you from my work email, check your inbox or spam. Directions suggest 4 mini HS-225 for the flaps and airbrakes, then two HS-85 for the ailerons. Can't wait to see this finished up, will be happy to arrange a tow/tug at Farmington or Bloomfield for the maiden. Bloomfield would be better, larger field to some degree and able to get you a but higher in the air given Farmington's proximity to Robertson airport.

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      • #4
        Thanks,
        just printed them, only took 2 hours to get all the old masking tape off…
        Found the instrument panel so I think I’ll order that. I’m going to make a scale tow hook, I just happen to have some full size ones to copy. Simple seat pan out of some molded foam board and I need a wheel. I need to make a list I guess.

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        • #5
          >>Directions suggest 4 mini HS-225 for the flaps and airbrakes, then two HS-85 for the ailerons.<<

          Mike's post is a good lead -- those specifications in the directions above are from a time long past. Very easy to do a HV setup on 2S LiPo and go with modern day metal gear much better torque choices. The JR 198HV is a a really good servo for ailerons also the KST X10 @ $45 is a vastly better call than the HS85. Don't skimp on the elevator servo and remember that a tow release needs serious torque to release in windy conditions or when a tow goes to $#!+.
          Team PowerBox Systems Americas... If flying were the language of men, soaring would be its poetry.

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          • #6
            Thank you
            yes the directions definitely date themselves. But it also gives me an idea of what torque range servos I’m looking for.
            I want to learn more about the power box set up, I was using regulators on the helis, but I’m sure the systems have gotten much better.

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            • #7
              You can actually cover about everything, wing, tail, with the JR DS198HV. It is shipped with an extra case top and bottom you can swap in to make it a conventional servo mount instead of a wing mount.

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              • #8
                Thanks
                I’ll be working on the servo set up tonight.
                I’m still getting over how much room there is inside these things, could stuff an old Kraft brick in there just for the nose weight.

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                • #9
                  Click image for larger version

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ID:	47602 Mocked up the servo tray, seat pan/stick. Working out the elevator linkage, seriously considering installing a joiner and driving it with a cable loop. 2 servos and long push rods don’t sound appealing to me. Click image for larger version

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                  • #10
                    How does the stab attach to fuselage? Pushrod should not be long from the wing.

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                    • #11
                      It has a tube that passes through with locator pins. Screw on one side to secure it.

                      On the full scale Cherokee II and my J2, it’s just a cable run from the stick around a pulley on the tail post and back to the stick, with a link in the wire to secure the elevator to. Pretty much a pull-pull set up for elevator, works really well at 1:1 scale.

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                      • #12
                        Click image for larger version

Name:	D3995A5B-029B-48F1-A81B-BE017FD641CD.jpg
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ID:	47609 This is the section on the Cherokee plan showing the elevator cable/linkage

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                        • #13
                          Two piece tail makes one linkage complicated. Full scale cable loops are not easy to set up in small models. Access panels big enough to work through on a model are also a challenge. With pushrods, you have access to everything.

                          Full scale is constrained by certification processes. In model sailplanes the current state of the art is servo buried in the surface with lds linkage all hidden inside. Depending on the stab construction, you could possibly install a servo in each side. Servo, linkage, and mount around 30g each side.

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                          • #14
                            If you have never looked at LDS linkages....
                            https://tomas-liu-studio.com/

                            https://www.youtube.com/watch?v=J2QW-IQCEno
                            mlachow
                            Senior Member
                            Last edited by mlachow; 04-30-2022, 12:54 AM.

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                            • #15
                              Thanks, no haven’t seen those, I see a lot of potential with some of the things we have available now.

                              A long way from “make the fuse just wide enough to fit the servos in”

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                              • mlachow
                                mlachow
                                Senior Member
                                mlachow commented
                                Editing a comment
                                Now it's make the fuse wide enough for an 18650 cell to fit. The RX and servos are no problem.
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