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  • Retract Servo?

    I am putting together a new KV 4.8 meter Ventus 2CX. Since the servo I originally selected for the supplied retract system will not work I started a new search. While discussing it with a flying buddy he said he has been using a Spectrum A7040 #SPMSA7040 in his ASW-20. It is a 160 degree servo with adjustable end points, most retract servos are end to end only with no adjustment abilities. It might work for my application, since I believe I can adjust the end points in my transmitter. However, it is an analogue servo and some will work with FHSS-3 ATX receivers, while others will not. The only way I know to find out if it works is to try it, $40 gamble. Do any of you know if it will work with my 10 channel FHSS-3 receiver? Below, I have posted my description and requirements of my application.

    "What I need is a low profile, 130 oz/in. to 200 oz/in. (minimum), metal geared, ball bearing and 6 V. or High Voltage digital servo. Speed does not matter that much, that can be adjusted by my SD-10G. I currently have installed a Savox SC-1251MG and with my servo tester I can get the required travel at 0.8-2.2 msec. pulse width. However, when using my transmitter, an SD-10G with the end points and maximum travel set a 150% that travel cannot be achieved. The manual states the pulse width range of my transmitter is 0.9-2.1 msec. pulse width.

    I cannot simply increase the servo arm length due to existing structure limitations and supplied mechanics. BTW, I could add a longer servo arm, but that would require fabrication of an adapter to replace the one supplied with my model; not beyond my abilities, but not preferred. The servo must be compatible with a Sanwa FHSS-3 receiver and if need be I would also require a programming tool, if a programmable servo will work. BTW, I have no experience with programmable servos. Is there a servo and what tool/tools are required that can solve my problem without fabricating the new adapter?"
    Mark

  • #2
    Would this help extend the throws on your current servo? http://www.chiefaircraft.com/pbs-6800.html

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    • #3
      Reading the description I don't believe it would. It appears to be for dual elevator control or matching the movement of two servos on a single surface so they don't fight each other.
      Mark

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      • #4
        Yes but it's also a programmer that controls center points and end points apart from the signal being sent by the receiver. You don't have to hook up both outputs. Question is does it boost the position beyond what is available from the receiver signal or just work within that range? Tech help desk might be able to provide specs for signal output ranges if PowerBox has a tech support dept. I would think they do.

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        • #5
          Hi Mark,

          I recently assembled an original KV Models (not Gliders Paradise) 5.33m Duo Discus. The instruction book supplied in the kit specified the HiTec HS-75BB servo for the retract. The retract installed in the model had a beefy red plastic servo arm already attached, as well as a mounting bracket for that servo. The HS-75BB is an analog retract (fixed travel) servo with a 3- pole motor, plastic gears and a single ball bearing. Torque specifications on the servo looked a little puny, 114 oz in at 6.0V. But, with everything set up for it I thought I'd give it a try. Turns out it works beautifully. I had to make one small adjustment in the linkage to get the uplock hook positioned correctly, but the overcenter downlock was set perfectly. The helper springs reduce the force required from the servo to raise the gear, so 114 oz in of torque seems to be plenty for smooth operation.
          I seriously doubt the 75BB is a HV servo so I have a MKS 2Amp Voltage Regulator in the servo lead to keep the voltage at 5.0V.
          All this said, I don't know whether the retract in your Ventus came from the same manufacturer, or is even the same model if it was the same manufacturer. I just though my experience might be of some help to you.

          Tom McP

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          • #6
            Tom, what you describe is quite similar to mine. It too had a beefy red plastic servo arm already attached. I also had to adjust the arm length to get it even close to either of the locking positions. Still, with that done my transmitter and servo combination will not allow either one or the other locking positions to be engaged. But, as I said it does using the servo cycler. Attached are a couple of pictures I took of the retract system. My plan is to use an A123 pack I built as an isolated power source for the retract, may not need a reg. Reading the specs on the HS-75 it says it's analog and some of those are not happy with the frame rate of the SD-10. Does yours go the full 180 degree travel? Also, I assume the paddle in front of the wheel is some type of brake.

            BTW, Alex at SUSA told me the model is a KV model. I have seen on the Gliders Paradise site them refer to KV in one of their descriptions. Mine came with no instructions only some settings (CG and throws) supplied by Vyroba Modelu.
            Mark

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            • #7
              Mark, Sorry its taken me so long to get back to this subject. Living East of Pensacola I've been a bit distracted by Hurricane Michael! I'll attach some photos similar to yours showing the Duo Discus gear. The HS-75 is a 180 degree retract servo with fixed end stops. Cycle time is a bit under 0.5 second. My radio is a Jeti DS-16 and I have had no trouble driving the servo. I ran the HS-75 with a servo tester as well when I was setting up the gear. The biggest difference I notice between our retract systems is the helper springs. I have two vs one on your unit. The lower attach point for the spring is different too. Mine pulls directly on the front link, yours on the brake cross shaft. The springs themselves are different. Yours looks like it has a stronger pull force than mine, but then you have one, I have two, so the total force is probably about equal.

              If you'd like me to take more photos of my retract with the servo installed I'll be happy to do that.

              Tom McP

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              • #8
                Tom, I hope you weather the storm well without loss. Here we just get flash floods and tornadoes. A F4 tornado missed me by less that 200 yards about 3 years ago. Very scary when the lights go out and you hear it coming! Good luck and stay safe.

                Thanks for the pictures it does look like they have a "family resemblance". Took out the retract completely yesterday and measured the servo arm rotation. With my cycler it's 136 degrees and with my SD-10G it's 116 degrees maximum. Since the HS-75 is 180 degrees that adds another concern, other than analog, 136 degrees is sufficient to engage the locking positions. Plus with no adjustable end points it might hit stall on each end, unless I can adjust the arm length. My red arm would not fit the Savox servo.

                I like your rudder servo installation, very clean fillets. Try as I might I can never get them that consistent. My plan was to locate my servo in the same spot. Additionally the 2CX looks like it uses the same pushrod. Have you decided what to do for rod ends? I'm not sure what to use, they may have hardware on the other side of the big pond I'm not aware of. I do have a spare carbon push rod and tried it in the sleeve and it might work.
                Mark

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                • #9
                  Hi Mark,
                  As it turned out , no worries here in Pensacola about Michael. Just a wet, windy day as we're on the Western edge of the circulation, thank goodness. Dodged a bullet this time for sure!

                  The red servo arm that comes with the retract is a perfect fit for the HS-75 splined output shaft (as one would expect it to be). There is a retract drawing in the instruction book that's difficult to decipher. It seems to indicate the length of the retract servo arm is 17.5 mm, center to output, which would then let you fit any servo with 180 deg travel with a custom arm. You could check the red arm if you still have it for the center hole to outer hole dimension.

                  The rudder servo photo I sent was one of a set. I'll attach a couple with the servo connected so you can see the final setup. I pulled out and discarded everything but the outer tube and replaced the provided pushrod with a carbon fiber pushrod. I used 2 mm couplers and clevises like an F3J rudder setup. Don't recall the OD of the CF rod, but it's a nice snug fit in the coupler. I can see if I wrote it down if you'd like the info.

                  Tom McP

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                  • #10
                    Tom,
                    Good you didn't have any storm problems. Thanks for all the pictures and information. I still have the red arm and used it to size the arm that fits my Savox servo.

                    I was thinking of doing the same on my rudder push rod. I have in my stock a couple of pieces of .062 x 48" pultruded carbon rod, an old .070 carbon rushrod that has a 2 mm coupler on one end that might be long enough (haven't checked it yet) and a .074 x 60" carbon tube too. I believe the .074 will be too tight a fit. My airframe came with the same type mud guard. How are you attaching it? Sorry about asking so many questions.
                    Mark

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                    • #11
                      Mark,
                      My rudder pushrod was made from a 0.050" OD X 60" carbon rod. I believe 0.050" was approximately the diameter of the plastic rod I pulled out of the outer sleeve. It comes down to the max diameter that will fit and slide smoothly in the outer sleeve and fit the couplers and clevises you intend to use. if a 0.060" OD rod will work by all means use it.

                      The retract mud guard on the Duo Discus is formed to fit completely over the retract frame and doesn't need to be attached. It's a formed fiberglass sheet that follows the shape of the frame and drops down over the back like you see in the front. Very nice.

                      Don't worry about asking questions. So much of what we need to know in building one of these airplanes comes from asking folks that have built one. You don't get a comprehensive building manual with one of these things. It's a learn as you go process.

                      Tom

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                      • #12
                        Tom,
                        The cover on my Ventus is form fit fiberglass as well. I might trim the corners slightly to get a more snug fit. Not sure how much it will do since it could lift slightly. The main panel joiner system will hold it down to a point.
                        Mark

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