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Old Mar 12, 2009, 04:49 PM
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Mechanical Retracts: Install Basics

I have been asked by some to do a thread on the installation of mechanical retracts. I have assembled 4 differents brands of retracts and the two most popular retract servos, JR 791 and HiTec H75BB. One of the retracts is a 90 degree Hobbico/H9 rotator, one is a Kyosho standard retract as supplied in their 50 series warbirds, one is a Dave Brown ( I believe), and the other is a cheap Chinese one off of a internet ARF. I will discuss how to measure the pushrod throws of the retracts, both out of the plane and mounted (in the case of ARF pre-installed units). I will cover installation of the retracts, servo, and the construction of the pushrods themselves. I will be assembling a wing kit for a Kyosho P51 and installing the retracts. This wing is considered "retract ready" but the wells are not cut out. I will be taking pictures of each step to help in the understanding of the process. A lot of the same procedures work on the larger mechanicals but I fly mostly 50 size aircraft now and those componets are what I have on hand. What I will be posting is not gospel and I'm sure there are other ways to install retracts. I will be discussing what has worked for me and what I have found the least complicated to do. Please feel free to speak up when you have a experience that has worked for you. The more workable ideas the better it is for all.
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Old Mar 12, 2009, 10:25 PM
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Thanks...
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Old Mar 12, 2009, 10:47 PM
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1st Picture, Here are the retracts:
Top- Hobbico/Hanger 9 90 degree rotating
2nd- Kyosho stock reverseable (adjustable for inward or outward retraction)
3rd- Dave Brown aluminum frame
4th- Plastic bodied Chinese ARF (bad construction)

In these series of pictures I show how to measure the travel of each specific retract. You can either put tape, mark with a marker, or what I prefer, a piece of fuel tubing slice down the side to pop over the pushrod. When you push the control arm all the way in it will automatically position the fuel tubing. When you pull the control arm all the way out the distance from the fuel tubing to the retract body is the total travel you need to get out of your retract servo. All of the retracts other than the Kyosho use about the same amount of travel, 27-28mm. This comes out to about 1 1/16". The Kyosho is the only retract that requires quite a bit less and this probably explains why this retracts seems to function really well. The Kyosho requires about 22mm or 7/8ths".
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Old Mar 12, 2009, 11:21 PM
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Here are the servos. There are three rows but only 2 servos. I didn't have a Futaba but I had a standard Futaba servo wheel. Above the servos are some of the manufacturer's supplied servo arms that will never give enough travel for any of the retracts in this test. The round wheels are the stock ones supplied with each retract servo and they too, do not supply enough travel, even the 22mm for the Kyosho. The largest distance on these two wheels is 20mm which is still not enough. The 20mm is on the JR, the Hitec is 18mm. As you can see, under the JR there is only one stock arm that could be long enough to supply enough travel. The other arm is a Dubro HD. When you buy a JR 791 I suggest to try and by a round blank wheel at least 1 1/4" in diameter. You can drill the holes you'll need. The HiTec servo has more standard servo arms that will work. The 4 arm one works for the Kyosho retracts using arm 1 and 3, middle holes. You will note the large red arm. This is a stock arm for HiTec car servos. It is the cats meow as it is large enough to work all the retracts and the holes are numbered in mm the distance from the center. The Futaba wheel, like the JR and HiTec is too small to get the job done.
To determine what size arm you will need you will have to take your total distance and divide it in half.
Example: 27-28mm = 13 to 14mm
If using the red Hitec arm you can't go wrong. Use either the closest lower, or the closest higher number. My suggestion is to use the lower number when using 4-40 pushrods and or 6volts. When using 2-56 pushrods you can go with the higher number usually with either 4.8 or 6volts because of the "flex" of the 2-56 rods.

DO NOT GO OVER 1MM IN TOTAL DISTANCE!!!!!! This could jam and burn out your servo or drain your flight pack.
If not using the red arm then you will need to search for a servo arm with the proper distance between the same two holes directly 180 degrees opposite each other. You cannot use the outside hole on one arm and the middle hole on the other arm. Once you have found the proper arm, you are ready to start installing the retract. I will be covering how to measure the total required throw for a pre-installed retract a little later on.
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Old Mar 12, 2009, 11:43 PM
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Push rods and set ups:
On 40 size planes using small or foam tires you can use 2-56 pushrods. On larger planes or when using scale or large tires I recommend 4-40 pushrods. The pushrods have a specific bend in them. This bend is to clear the tire and mostly the axle of the retract when it is retracted. On the smaller size planes with retracts the chord of the wing tends to be thinner and the servo bay is usually placed behind the centerline of the retract. The bend in the pushrod is designed so that when the retract servo is rotating and the pushrod is moving in a circular/linear motion it does not contact the axle. In thicker chords the bend can be positioned not to go around the axle but below the whole axle and tire allowing a deeper wheel well. This is better for larger scale applications where we try to hide the pushrod. Try and keep the bends as small and subtle as possible to hinder any flex that might occur. Even 4-40 pushrods will flex with big bends and long lengths. You can use your preferred attachment to retract control arm. I like to use Z bends or metal clevis'. On the smaller planes, like the Kyosho P51 here, the bends will go to the rear of the retract bay because the servo bay is behind the centerline of the retracts.
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Old Mar 13, 2009, 12:15 AM
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Now that we got the right servo arm and the pushrods made it is time to install the retracts. Most ARFs and kits will show where and how large to make the hole for the tire and where to mount the retract. The P51 wing here is called retract ready as it has the mounts and the relief for the strut coil already installed. In the instructions it gives the distance to the center of the well hole and the size. I like to go just slightly larger with my protractor as when I cut I usually cut just inside the line. The P51 already has a section ready to cut out for the retract hense the retract ready claim. Once this piece is removed I measure between the mounting rails and find a distance of 20mm. I put a centerline mark at 10mm at each end of the rails. I then measure our the specified distance (170mm) and make my mark. I then set my protractor and draw my circle. Before I cut, I measure the circle to make sure it is of the recommended size. Not a big deal if not using plastic wheel wells. If you are installing plastic wheel wells you want the hole to be the right size. It is a pain to have to trim all the way around if it isn't. Measure it first then cut it only if it is right. It will save a lot of time later. Use a sharp blade when cutting the sheeting and I like to cut slightly up to prevent splitting the sheeting when cutting accross the grain. If you are installing retracts in a kit of course this doesn't apply to you as you will construct your well as you build. You will only have to measure your strut for the proper length to your well. I don't think I need to explain how to measure for the length of the strut but I do always cut them a half inch longer than the center of the hole. One the axle is put on and centered in the well the strut can be trimmed. File flats and loctite the stub axles if required by your retracts.
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Old Mar 13, 2009, 12:21 AM
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Wish someone could have helped me out with the mechanical retracts sooner. Anone reading either my VQ FW 190 or Kyosho Spitfire 90 assembly threads know what troubles they have caused me! I look forward to trying them out again, using the methods described here. Thanks Chistech!
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Old Mar 13, 2009, 12:37 AM
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People wanted lots of pictures so here they are. The accompanying text to clarify each picture.
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Old Mar 13, 2009, 12:47 AM
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Ok, I'm done for the night. It's 12:45 and I've entered all the info and pictures of my work done tonight. It's taken longer to do this thread than to install the pair of retracts. I will finish installing the aileron servos and horns tomorrow and glue the wings together. I put the wing halves together with the joiner but no glue just to show the retracts installed. More tomorrow.
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Old Mar 13, 2009, 03:34 PM
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Great thread. Thanks. Subscribed!

I've got a pair of electric rotating lados ordered for a p-40 but I can't afford that often.
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Old Mar 13, 2009, 08:46 PM
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Nice, thanks
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Old Mar 15, 2009, 04:49 PM
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Well, the mechanical retracts on my Spitfire now lock both up and down....I think.... I had previously been able to get them to lock down, but neither locked up and when I'd go into a dive, or other high G maneuver they would drop down. I removed the little disk from the retract servo and replaced it with a double arm. The difference in the hole location was only about 1/8 inch, but that can mean a lot when dealing with retracts. I hope to find out if they are working now tomorrow, but that all depends on getting the engine tuned and running good again.
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Old Mar 15, 2009, 06:27 PM
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KC, if your retracts are working on the bench in the wing properly now they will work properly in the air. When they are fully up they should be locked up in the well and you should not be able to pull them down. When they are locked down you should be able to see the lock block all the way out to the inside end of the retract housing they should not fold or collapse. The lock block is attached to the end of the control rod. I will be covering this and more in this thread. I've been busy and unable to work on the retracts and this thread.
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Old Mar 15, 2009, 07:20 PM
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Servo Installation: Most of this procedure applies to both ARFs and kit built aircraft. Again, pictures are of a ARF P51 wing. These days most are installing retracts in ARF's. Some ARF's come with retracts installed or retract ready. In previous posts I showed how to install retracts of your choice in a retract ready wing. Now with the retracts, pushrods, and wheel wells installed the wing panels are essentially ARF wings with installed retracts. The wings need to be assembled together ready to accept the retract mount. Do this per your models instructions. If your servo tray is intended to center mounted, always try to mount the center of the servo output arm directly in the center of the wing. This allows both pushrods to be exactly the same length which keeps the pressure of the pushrods equal to both retracts. If your retract servo is designed to be mounted offset of the wing center, (some are mounted way off center) I recommend using 4-40 pushrods only. Even if the pushrods are short, 2-56 pushrods will flex and different lengths will flex differently. I have seen this happen and one retract will retract up and lock and one will not go up all the way. Use 4-40's and this problem will most likely go away.
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Last edited by chistech; Mar 15, 2009 at 10:03 PM.
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