I am using the recommended Behotec C50 retract option from Composite-ARF and the quality of this gear is fantastic. A few people have been a little confused when putting these units together and it is easy to think something is wrong. To avoid you encountering this, the following answers some of the  points that have caught people out....

Not all of the retract units are the same - there is an actual nose gear unit. It can be spotted by the recess machined into the trunion as in the pic below. This is for the supplied collet to retain the pin from the front oleo.

The set screws to hold the main gear legs on cannot be seen until the gear units are extended. They are found as in the picture below..

The wheel axles press in from the wheel side first. Ensure the flat side rather that the rounded side of the oleo fits against the brake unit. The set screw in the bottom of the oleo holds the axle in place. See the pictures below...

I'd recommend you only lightly secure the items together for now. Once you have completed the retract work you can then loctite any bolts/set screws and grind any flats that are needed.

OK, now we have our units sorted and temporarily together we can install them. First of all, take the main gear units and countersink the bolt holes so as to allow the supplied 4mm countersunk bolts to sit flush (or as near flush as possible) with the retract. If you do not do this, the gear well cover will not fit.

I would recommend that the gear units are not fitted 5-6mm from the outer rib but, rather, are fitted against it or approx 1mm from it. This will allow you more room for fitting the inner gear door cylinder when you get to that stage. Be aware that this will alter the outer gear door setup slightly so I suggest you read that section briefly before you make your decision. Place some masking tape on the bearers, align the retract unit and leg so that it sits squarely in the retract bay and mark the bolt hole positions. Drill through the bearers with a drill large enough to accommodate the captive nuts. If you have fitted the retract units further outboard then you may need to trim one of the captive nuts as per the photo.

Use a 4mm bolt and a large flat head washer to pull the captive nuts fully home into the ply mounts. You can apply a little epoxy on the nuts before they are pulled home. If you do this then ensure you grease the bolt. This way no glue can get stuck in the threads. Once dry you should find that your retract units can be securely fitted with the supplied 4mm countersunk bolts.

This is the trickiest part of the entire build but isn't actually hard - just fiddly. Start by tweaking the gear door so as to give about 1mm clearance all round. Next sand back the lip at the wing root using the main wing skin as a guide for the shape as in the picture below.

Glue the phenolic door hinges in place as per the manual i.e. 13mm from the edges and about 0.5mm from the door edge. Using a dremel sanding drum/cone or similar, CAREFULLY remove the foam material in the area of the bottom of the door ram until you can just see the top wing skin. Doing this will gain you a vitally important 1-2mm of extra depth.

Temporarily fit the cylinder to the door hinge so that you can see how thick the ply mounting block needs to be. The cylinder will hang vertically from the hinge and we, ideally, want to keep it vertical so there are no twisting loads on anything when the cylinder operates for real. The block thickness will be about 5-6mm and you can make the block up from a couple of lengths of the 3mm x 12mm strip supplied in the kit. Don't forget to drill a hole for the air hose through the block either!! Mount the ram to the block so that the metal bracket is 'higher' than the bottom of the cylinder rather than the metal bracket being lower than the bottom of the cylinder. Also, mark and cut a slot in the root rib for the unused metal bracket lug to protrude through. Now, attach the ram to the door hinge and hold the block and cylinder as close to the root rib and as low as possible (use the hole in the root rib to get to the unit whilst the door is closed). If you have got the cylinder mounted as low as it can go then you should be able to push the gear door fully closed. Equally the door should also open enough to allow the main gear to cycle - test this too before gluing!! You will see that there is only a few mm of clearance which is why mounting the retracts further outboard really helps! When happy with the position, mark it as you may just forget to do the next step which is to drill a hole in the spar for the air hose to pass through!! Now, tack glue the block and ram in place and check the cycle of gear door and main gear once more. Finally, secure the block in place with some epoxy and milled fibres.

To allow for easy maintenance I chose to apply some black tape over the metal hinge tube pins. This means that the hinge pins can be easily removed as and when necessary but they will not slide out during flight. Just a little simpler than gluing them in place I think.

Place the gear cover plates in place and sand where necessary for a nice snug fit. You will find that you need to cut a slot away from the outboard edge of the cover to allow the aileron pushrod to pass through. Once this is done, tape the plate in position and drill 6 or 8 holes approximately 3-5mm in from the edges. Remove the plate and glue small ply squares in position. When dry, re-drill the holes throught the ply. This stage is now complete!

Whilst fitting the retracts as far outboard as possible made our life easier whilst installing the door ram etc, it does mean that the gear well cover plate needs a little modifying to allow the retract leg to rotate the full 90 degrees. This is turn means that the outer gear doors come up a little short when in the retracted position. You can either lengthen them with some fibreglass board or leave them as is. This is a trade off of making the inner gear door ram work a lot easier so it is up to you what you choose to do!

Anyway, the outer gear doors (the strut doors) are hinged to the gear cover plate with a standard plastic pin hinge. be sure to roughen the surfaces up before gluing and also tape the strut door in place when gluing so as to be sure it dries with everything 'square'

First we need to build the aileron mounts. These are a pretty cool idea. The mounts are bolted to pre-installed mounts in the wings. Three bolts and you can whip the hole thing out! All you need do is cut the ply parts from the sheet and cyano them together. Be sure to make a left and a right!!! Then run a fillet of epoxy or Hysol along the joints to strengthen the structure. Very simple!

Next, fit your servos in the mounts and the mounts to the wings. It is best to centre your servos and fit the cut down SWB control horns before the servo is bolted to the wing as it just makes setup simpler. Nothing more annoying than connecting your R/C gear for the first time and having servos go to all sorts of weird positions because they weren't centred! Make up the aileron pushrods as per the instructions and remember to apply the grease to the metal clevis where it passes through the SWB horn. You will find that you need to remove some of the wing skin to allow the linkage full movement but this is easily done with a blade and then a needle file to tidy it up.

The flap servo and linkage is very simple but a little fiddly to get in. Again, centre your servo and attach the cut down SWB arm. Make up the linkage and attach it to the servo arm so that the ball joint is mounted 'underneath' the arm - this will ensure your linkage stays in line with the flap horns. The slot in the trailing edge will, more than likely, need lengthing to allow for the max flap movement and also widening to allow the metal clevis to pass through the wing skin. This can be done quite easily with a needle file or similar. A 'keyhole' shape does the job and looks quite neat.

You may find it useful to know that my linkage was approx 73mm from clevis pin to clevis pin. The maximum movement you should ever need is 85mm (measured at the trailing edge)

Next we can mount the wings. Start by drilling the hole in the ply wing root rib. It is very important that you drill the initial hole where it says in the manual. It may seem odd at first but if you don't you will not be able to get the wing bolt, door ram and wheels all in place so make sure you follow the instructions! You may find that in order for the captive nut to sit flush on the ply plate you need to cut off the front part but these are easily cut with a small hacksaw.

Drill the root rib hole 6mm. Slide the wing on the tube, mark the position of the hole on the fuselage, remove the wing, drill the hole in the fuse 7.5mm, refit the wing and use the bolt to pull the captive nut into the ply a little before removing everything. Now, simply pull the captive nut into place using the wing bolt and some strength! I found that two large fender washers were ideal for 'pulling' the captive nut into the ply plate inside the fuse. Do not try and pull the nut into place with the wing on - you will damage the root rib.

When you're happy with the position of everything, run some epoxy over the captive nuts to hold them securely in place. Avoid getting any glue in the thread of course! If you dip the bolt in vaseline and screw it in to the nut then you can avoid any chance of gluing things accidently.

The final task for the wings is to get the servo leads and airlines organised.