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Retract Servos

Old 01-19-2014, 12:54 PM
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Redback
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Default Retract Servos

Need some thoughts on retract servos.

I picked up a NIB World Models Mustang ARF (46 size) from a guy who was selling up. It has mechanical retracts already fitted.
One of the attractions for buying was that I already have everything I need to get it flying, including a Hitec retract servo. There is however a problem (isn’t there always!).

The retract servo travels 180 degrees and there appears to be no easy way to get this to work with the linkages fitted. The instructions (such as they are) show a retract servo with less than 180 travel. Being a retract servo Tx end point adjust gets ignored.

My options seem to be to either get a servo with less travel (if there is such a thing) or use a standard high torque servo and adjust with the transmitter. I am also considering whether to fit a slow blow fuse into the servo lead to protect against a stall caused by a retract jamming.


Appreciate any thoughs, ideas, insights etc!


Terry
Old 01-19-2014, 02:14 PM
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TLH101
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http://www.youtube.com/watch?v=DRjjbEY3Q34
Old 01-19-2014, 02:50 PM
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chuckk2
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Odd! the 180 deg retract servo I have (not Hitec) does respond to travel and offset from the TX.
In order to deal with a possible stall condition, I'd recommend using a servo saver.
Old 01-20-2014, 08:14 AM
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LesUyeda
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Most RETRACT servos are lock to lock, not proportional, therefore no travel and offset possible.

Les
Old 01-20-2014, 08:45 AM
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There is no adjustment on a retract servo, but there is adjustment on the servo arm as to how much movement is transferred to the other end (the gear). Outer most hole=more movement, inner most hole=less movement.
Old 01-20-2014, 08:45 AM
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speedracerntrixie
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You should be able to move the linkage in on the servo wheel until the servo travel matches the retract actuator sweep.
Old 01-20-2014, 08:52 AM
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Ain't that what I said!! You can also use a round servo wheel to offset the holes to give no sweep for near 40% of the 180° movement.
Old 01-20-2014, 08:54 AM
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speedracerntrixie
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Yes Robert it is, we were typing at the same time, you were just a little faster, check the time stamps LOL
Old 01-20-2014, 08:57 AM
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I know! Just funning with ya!!
Old 01-20-2014, 10:00 AM
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KaP2011
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Spectrum has a nice retract servo that has adjustable end points. I'm useing two of these in a TF P-51 and a H9 Spitfire. They work great and have been trouble free. Not a bad price either.

http://www.horizonhobby.com/products...ervo-SPMSA7040
Old 01-20-2014, 10:13 AM
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Another good option

http://www.valuehobby.com/radio-syst...hd-1501mg.html
Old 01-20-2014, 10:27 AM
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Not sure if different retract units may have been installed but... I have a World Models Mustang 46 that I am just getting ready to fly again. It has a Hitec retract servo for the gear, which is the stock gear. It uses a servo arm with 1" of total throw (each control arm mounted 1/2" out from the servo). I would measure the needed throw for the retract units. I would guess its 1" or very close to it. If not, just divide the throw by half and mount the control arms that far out on the servo arm.
Old 01-20-2014, 01:51 PM
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KaP2011
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So far, to my knowledge anyway, every mechanical retract I've installed required 25mm of throw. Most retract servo arms are going to have holes at 12mm or 13mm. I know that hitec does.
Old 01-21-2014, 03:55 AM
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+1=ace your still the man,thanks for takin my comment
Old 01-21-2014, 06:11 AM
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aghost
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I had five of these WM P-51s over a ten-twelve year span, but not one at the moment. All with non-adjustable mechanical retracts/tx/rx, etc. As I remember (??) it worked if one had the servo arm slightly past center when the retracts are up. Then when the servo cycled it stopped just before it got the center, so the wires/linkages did not hit each other, just kind of parallel to one another across the servo. Kind of a fine adjustment.

Brian
Old 01-21-2014, 07:14 AM
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chuckk2
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With the advent of computerized TX adjustable travel time, throw(span) and offset, the older design two position retract servo is likely to loose it's popularity, However, it may still be useful for other secondary tasks - - gear and bomb doors, canopy open close, and such.
The electric retracts are another nail, in that many have circuitry that "trips" due to a jam or end of travel, and limits the current draw under such conditions. The electrics often include limit switches for end of travel, and circuitry for a time limit on travel, possibly also with a current limiter. I believe that electric retracts or retract servos that can draw high current, and possibly other functions not essential for controlled flight should use a separate power source, such as a BEC, rather than be powered from the RX power.
Old 01-22-2014, 04:28 PM
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Redback
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Thanks for the replies guys. The problem is not to do with getting the correct travel, but in stopping the pushrods fouling each other in the one position. Previously I have got round this by bending the pushrods, but with these preinstalled reatracts this will be challenging.
Current plan is to try a standard servo (I have a Hitec 645MG) and measure the operate current with a view to installing a fuse as stall protection.

Terry
Old 01-22-2014, 06:48 PM
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aghost
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"but in stopping the pushrods fouling each other in the one position"

Can you tried this?

Have the servo arm slightly past center when arms are extended (the retracts up I think(. Then when the servo cycles, it stops just before getting to the center, so the wires/linkages do not foul each other, just kind of parallel to one another across the servo.

Brian
Old 01-22-2014, 06:57 PM
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Redback
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It's an option, however it does mean that at the other extreme the servo goes past the centre and starts to reverse direction.

Generally a PITA but makes me love my Sierras more!!

Terry
Old 01-23-2014, 02:56 AM
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dadragon
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Terry! you do have 2 very good thoughts ( sierra and 645mg's) to me (arf's ) if one flys planes one should build planes.its just me I guess.I hope you get it figured out.my last 2 war birds was hydraulic flaps and landing gear.
Old 01-23-2014, 05:57 AM
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aghost
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As for starting to reverse when going past center. - Yes, it will, but there is generally enough slack in the retract that it will not unlock unless you really go farther than necessary. Just do enough that they don't foul on the other end.

+1 on the Sierra's. Got three sets of them.

Brian
Old 01-25-2014, 06:41 PM
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Using a separate batt pack for the retract servo is a good safety option.
Old 01-25-2014, 08:37 PM
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Tried the 645s, initial results were promising. Got the end points set up and used the servo slow feature on the Tx so the retracts didn't snap up and down. Set up an ammeter in the battery lead and ran some tests, Was drawing about 250mA to get the gear up, somewhere in the 100s for down.
Despite this I was still concerned that in a practical situation I could end up either running down the battery, or causing a voltage drop and consequent brownout. Didn't want the weight of another battery so I decided to take a look again at the retract servo. At this point I found that it would not fit, the rails are 40mm apart and the Hitec is 45mm long.

At this point, time for a reality check! This is an ARF that I bought for $100 to use as a knockaround plane, not a scale masterpiece!

So, I joined the two pushrods together with a cable tie to keep the wheels locked down and moved on to the next stage of assembly. Retracts have been put on hold (at least for the time being), if I want retracts I'll fly my ESM Bf109 with the Sierras!

Maybe if it turns out to be a flightline favourite I will revisit!

Thanks to all that tossed in ideas.

Terry

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