If you have bought a kit, it will usually come with some instructions, no matter how rudimentary which will give you a good idea of the sequence of building your model. I strongly recommend, if it is possible, that you get the mechanics flying as a pod and boom machine first. You can use a canopy and landing gear from another model if you don’t have any, or fly it without a canopy. All you need to do is the basic setting up, and this will ensure that at least the worst of the setup is done before you fit the fuselage. It’s a real pain to have to take the mechanics out just to reverse one servo or make a minor adjustment to the mixture. Also, there is nothing quite so exciting as a beautiful scale helicopter on its maiden flight, with a reversed gyro. It’s guaranteed to raise your pulse rate when you find out about that one.

When you have your mechanics sorted so it has a working pitch and throttle curve, look at them again from the point of view of any changes you are going to have to make to them to get them inside the body. Will the radio tray have to go so the electronics is going under the cockpit floor? Where will the gyro and battery go? Try and make everything a complete unit so that when you do have to remove the mechanics for maintenance, you aren’t left with a tangle of wires and fuel lines in the body, which will be a struggle to reconnect on re assembly. Any part of the re assembly process that is a struggle, will almost certainly result in something getting broken sooner or later.

If the fuselage is in 2 major parts, tail and tub, then I suggest you do as much work on the tail assembly as possible before joining the two together. It’s much easier to work on a small section. If the tailboom is already joined to the fuse or its all one molding, then you are going to be working on a large item and you will need plenty of room to do it. My giant BK117 came in a 7-foot long box and was one enormous fiberglass molding that required special handling while assembling it. In this case, I took the seat off an old office chair and fitted a 3-foot square piece of wood in its place. I covered this with some old carpet and that made a perfect swiveling workbench that was just the right height to work on while sitting down. I did have some trouble explaining to my wife about the 3-foot square hole in the carpet under the bed in the spare room though!

If you have a kit, then it will either come with some wooden formers for mounting the mechanics to the fuselage, or drawings of how to make up a chassis. If you don’t, its time to start doing some eyeballing. Remove the blades, landing gear and tailboom from the mechanics and put them into the tub. Chock them up with bits of wood and stuff until the head is at the right height and position in the tub. Take your time doing this, the whole tailboom alignment process is affected by this operation. Does it all seem to go together properly? Will it work?

The next step is to make provision to fit a bearing in the end of the tub where the tailboom will attach. This has to be accurately done or vibrations and drumming will be the result. There are 4 possible ways of driving the tail. You can keep the tailboom in place and use it as a support for the drive system. You can use a separate wire drive inside the tailboom. You can use a torque tube inside the tailboom, or you can use a flexible cable drive. If you go for the last, you are on your own.

I prefer to use a tailboom with lots of bearings inside it from a pod and boom machine and fit it inside the scale tailboom. On my BK117 I used Vario gasser mechanics and the tail boom had a Robbe Millennium tailboom inside it with 5 bearings. If you are building a model with the gearbox at the bottom of the tailboom such as a Jet Ranger or Bell 222, you can fit the tailboom into the mechanics, remove the tailrotor gearbox and then slide the fiberglass tailboom over it all to see how everything lines up. That’s the easy way.

If you want to use a wire drive or torque tube drive without the mechanics tailboom, then you will need to make a support for the end of the wire drive tube or the tube drive. What we are trying to do is to avoid any misalignment between the mechanics and the tail rotor gearbox or 145 degree gearbox. You can either fit an empty tailboom into the tailboom mounts on the mechanics or then adjust everything so the scale tailboom is in the right place, or you can make up a marker system.

Make an extension for the tail drive shaft that is long enough to project out of the tub. Fit it into the coupler and secure it with some tape. This can be a wire for the wire drive or a pencil or old tube for the tube drive. Rotate the head to turn the coupler and adjust it until there is no radial movement when turning the head. Now you know exactly where the tail drive is going to come out. Fit the mechanics back into the tub and realign it all so the head is right. Now take a big step back and a long look at it all. Is the mast at the right angle? Should it tip forwards? If it should, will the tail drive go down the tail boom? Don’t rush this bit, get it right.

Once it is set up correctly, you need to check it will be in the right place in the tail. Get the little bubble spirit level that comes with the Robart pitch gauge and check the table is horizontal. Then check the output extension is horizontal. Measure the height from the table to the center of the output driveshaft. Fit the tailboom onto the tub, I have assumed you have trimmed it to size, and tape it in place, supporting it underneath so that the driveshaft extension remains horizontal. Now measure up from the table on the tailboom to where the driveshaft will end when the final one is in place. Is it in the right place for the tail rotor gearbox or 145 degree gearbox if the rotor is in the fin?

Once everything is lined up, slide the scale tailboom off, and put 4 pieces of tape in a tic tac toe fashion around the mechanics tailboom or wire drive or tube drive. To save typing I am going to call this our output marker in future. The square in the center is exactly where the drive shaft will exit the tub. Drill a hole in a piece of ply, big enough to go round the output marker and slip it over it. Then butt it up to the fuselage and mark round the outside of the fuselage onto the wood. This is now a template for the drive support as it leaves the tub. Trim it to size so it fits inside the tub and tack it in place with some CA. Reassemble everything and check it all again.

Now you can take some measurements inside the tub to see where the chassis is going to sit and how big it needs to be. If the instructions don’t tell you how to make the chassis then you need to design it. Bear in mind that the mechanics hold the body up, the body does not hold up the mechanics, so you only have to make something strong enough to support the weight of the body. I favor making two half moon shapes to go across the tub and two longerons to go front to back and then a flat plate across the top to mount the mechanics to. Use 1/8" plywood with a couple of ½" square engine bearers to go where the mechanics bolt to. I like to use captive nuts under the engine bearers to make assembly easier.

Keep trying the mechanics and the chassis in the tub until the output system lines up perfectly with the bearing support at the back of the tub. Mark where the chassis will sit inside the tub. Clean the glass fiber inside the tub with 60-grit sandpaper to key it. Mark it up again. If the scale landing gear goes through the tub rather than bolting underneath, you need to make some supports for it on the chassis. Mark the landing gear onto the chassis so you can see where it is going to go. Make sure it doesn’t foul anything. Fit the chassis into the tub and mark where the landing gear will come through the tub. Drill a pilot hole in each of the 4 locations and then leave everything as it is. We will fit the landing gear later.

Sand the chassis smooth and give it some coats of polyurethane fuel proofer. Now you can do some assembling. I prefer to use an adhesive called PFM to fix wood to fiberglass. It is flexible and resilient and will absorb vibrations without transmitting them to the tub. Blob PFM on the chassis and line the chassis up inside the tub. Fix the mechanics in place to ensure everything is lined up and leave it to set for 24 hours.

Meanwhile, you can start work on the tailboom. Make sure the tailboom fits the tub correctly and there are no lumps of fiberglass in the joint either outside or inside which will stop the clean mating of the two parts.

If this is a simple tailboom with the mechanics tailboom going inside, it is only going to be necessary to get the length right so that the tail rotor is in the right place. When you cut the boom and driveshaft to length, be sure to leave a small amount of play in the length of the wire drive or tube drive. One tip when fitting a wire drive is in the grinding of the flat on the end of the wire drive. If you grind it so that it only just clears the set screw, it will be impossible for the wire to work its way into the tail rotor gearbox and out of the main rotor gearbox with the resulting instant cessation of tail drive.

If you are using a wire drive or tube drive, you need to mount the tail rotor gearbox to the scale tailboom. There will probably be a piece of ply and a place to fit it already in the design.

If the rotor is high up in the fin, then either provision will have been made to fit a 145degree gearbox or you will have to make provision. Unless you have machining facilities, use the Robbe gearbox for wire drives and the Vario gearbox for tube drives with dog bone couplers. I prefer the Robbe ones as they have metal gears but I also like tube drives, so I end up machining parts on my lathe to allow the use of Vario or Xcell dog bone couplers. You need to line up the drive shaft and output shaft from the 145-degree gearbox and decide exactly where it will be located. Try for the very minimum angular error in the drives, errors will only introduce wear.

Once you have decided where to put the gearbox, mark it out and cut a hole in the tailboom. I use a Dremel fine cutoff wheel that gives me a very thin cut line. I then glue wooden strips back inside the hole to support the cutout when I screw it back. If you have made the cut lines a bit too obvious, screw the cover back into place and fill the gaps with Bondo or similar. Let it set and then unscrew the cover and you should have a perfect fit. It may be necessary to ease the cover out with a knife blade. Make up a wooden chassis plate for the gearbox and fit it to the gearbox. Again, I like to use blind nuts for easy maintenance.

Now you can fit the tail rotor gearbox in the fin. This should also have some provision in the kit for mounting it. Glue the former into place with PFM and let it set. Make up the drive shafts and fit them and then fit the intermediate gearbox and fix it with PFM. Let it all set solid and you should have a drive train which is smooth but will accept minor deviations from true without causing any stress.

Now you can glue the tailboom to the tub. There may be an intermediate former in the tailboom to support the wire drive outer tube or a bearing for the tube drive. If there is, cut it to size now and fit it with some more PFM. Fit the tailboom to the tub with JB weld or other 24-hour epoxy. Don’t be tempted to use a quicker setting epoxy, it won’t hold so well. The 24-hour stuff has time to soak into the nicks and notches you put in the fiberglass gel coat. You did remember to scratch the surface with 60 grit didn’t you? Do all this assembly with the mechanics and drive shaft system in place to ensure correct alignment. Pack the tub and tailboom so that they are correctly aligned and tape them together and leave to set overnight.

The next job is the tail rotor pitch control. If the tail rotor is low and you have used the mechanics tail boom, life is simple. Stick with the stock tail rotor servo location, and keep the stock wire pushrod guides. You may want to upgrade the system by fitting a tailboom servo mount for added control rigidity. If you have not used the tailboom, you may want to go to a carbon pushrod from the servo to the pitch lever. This will not bend and is very light. It should not need any guides. If you have used a wire drive, it may be necessary to fill the scale boom with lightweight expanding foam to give extra support to the drive wire. If you do that, you will need to adapt the control rod by fitting it into a plastic tube and then fitting the plastic tube inside a brass or aluminum tube for rigidity. The plastic tube is to prevent metal to metal contact, which could cause interference in the radio system.

If you have the angle gearbox life is not so simple. I tried the plastic pushrods in plastic tubes and found that no matter how I ran it and secured it, I always got play and the control was always sticky. I am paranoid about getting a solid tail control on my scale machines and so I have gone to different methods to ensure the most accurate control system I can devise.

In the Huey, I mounted the servo in the tub and used a pushrod to a bell crank in the fin, which then went to another pushrod to the pitch control arm. This was very difficult to set up and adjust and it was in the days before heading hold gyros so I had to get it right mechanically.

In the Cobra and Seaking, I used a Multiplex high-speed micro servo and mounted them close to the pitch lever. In the Seaking I mounted it in the horizontal stabilizer with the push rod just poking out under the stabilizer and going straight through the fin to the pitch control arm. In the Cobra I cut another hole like the one for the gearbox and mounted the servo in the fin with a short pushrod to the control lever. This way of doing it is molded into the BK117 and that makes it so much easier.

That is the basic assembly done, now there is the detail work to do such as fitting the horizontal stabilizers if they aren’t the moving type on the full size machine. If there are retracts to be fitted, now is the time to sort that out, together with other major lumps and bumps like the sponsons and floats on the Seaking. Any other formers for strengthening inside the tub can be fitted. The floor for the cabin and cockpit can be made and provision for locating batteries, electrics and fuel tanks can be made. In other words, do most of the fitting out of the fuselage interior at this stage. I like to make the cockpit removable in one piece so I can detail it outside the model and then slip the completed unit into the body and secure it. Some flat sided models like the Seaking need extra strengthening to ensure the sides don’t flex and that the landing gear and sponsons are firmly attached, especially if they are supporting the whole weight of the model. As this is such a individual topic, it is difficult to give advice, its something you have to judge as the model is assembled. For example, on my Cobra, everything was fine until I attached the rocket pods to the wings, and then they wobbled like jellies due to the extra weight.

 

Chapter 3