Let me start first by admitting that I have very little experience in laying up fiberglass components, so I'm certainly no expert at it. I've found methods that work OK for me, but there are a lot of guys out there that can certainly do a better job. Partly because I'm cheap and lazy and don't always go out and buy the right materials for the job ;-)
Technically this process is best done with Epoxies that are designed for making glass components. They cure much harder and don't need to be laid up as thick. I didn't have any, too broke and impatient to order it, and I had a overabundance of 30-Minute Epoxy on hand, so there you have it.
Anyhow, I start by covering the gear area with a plastic film covering like Monokote or Ultracote. I used Ultracote this time, and found that it didn't work as well as Monokote. When removing it from the glass, the adhesive film wanted to separate from the color coating... messy.
So, get your covering ironed on, then cut out a few pieces of heavyweight fiberglass cloth to shape/size. I tape the first layer down to keep it in place until we get some epoxy on the scene. Note that before taping the cloth down, I use my template to sketch out the rough area of the gear cutouts so I have a good idea what needs good glass coverage.
Laying up the glass
With the first layer of glass cloth down, I begin screeting the epoxy through the cloth, evenly across the entire area. I thin my epoxy slightly with denatured alcohol. This will help it to spread thinner and soak through the cloth. I've found the easiest way to do this is to pick up a cheap deck of playing cards and use one as a squeegee to apply with... Cheap and disposable.
I let the epoxy dry, then wipe it down with alcohol and lightly sand it. Then I apply another light coat of epoxy and repeat the wipe down and sand process.
Once the first round of glass is complete, you may see some low spots or wrinkles. These can be taken care of with some spot putty at this time provided they are very light. You don't want to build up any substantial thickness of putty in the doors... just minor imperfections.
I repeat the process of adding cloth, epoxy, clean and sand a couple more times. No need for putty anymore... maybe a little after everything is done to hit any final blemishes. Ultimately you are looking to build up something that follows the contour of the wing, is as strong as you can get it with as little weight as possible. Simple right? Well, that's the obvious challenge we always face as RC modelers.
Once again, I admit that this could have been done with less epoxy (and a little less weight) if I had used the proper type of epoxy. But, I've used this method before and it's really not all that bad.
Once you are all done laying down your fiberglass, have everything sanded down nice and smooth, you can do your final spot putty work and finish sanding. Next you're ready to pull this all off of the wing and move on.
Shaping things up
Next I pulled everything off the wing, flipped it over and removed the Ultracote. Like I said earlier, this didn't work out as cleanly as it had in the past with Monokote. Here's a hint... hit the covering with a heat gun as you pull it off and the covering will soften up. This seems to make it come loose from the glass a bit easier. Otherwise it tends to separate and strips into small pieces.
Now I go ahead and rough cut out the shape of the gear doors and access hatch as one big piece for each wing. Be sure to allow for plenty of excess around the edges, including the full shape of the "finished" doors (vs the cutout) as well as the full area of the access hatch.
I still felt the gear doors weren't quite stiff enough for my liking, and I wanted something with some texture on the backside to glue other components to. So, I decided to laminate a sheet of 1/32" aircraft ply to the backside of each door. To do this, I taped the AC ply to the wing the epoxied the doors over each piece taping securely until cured.
Once cured, I pull everything off and begin cutting the shape down closer to final. The next steps will be cutting out the final shape from the sheeting in the wing. This will be somewhat of a "back and forth" process between the wing cutout and the door shape as you trim and adjust each to match the other while trying to stick to the actual scale lines of the real 190. You'll find later that some lines will just have to be sacrificed.
Special Note:
In retrospect, I now think it may have been better to skip the Ultracote process entirely. Since I shaped the AC ply to the wing, then glued the glass doors to the AC ply, I suppose I could have just started with the AC ply laid down in the first place. In other words, replace the Ultracote with the AC ply and build your glass up right on it. The shape might not have followed the wing quite as closely, but the AC ply will end up reshaping the glass to match it anyway. Besides, it would have been a lot easier and less messy.
Mounting to the struts
Now on to actually mounting the doors to the struts. Lining up the doors to the cutouts in the wing, I positioned the two Shindin standoff brackets to where I felt they needed to be. They ended up being very close to the scale positions of two of the full scale standoffs. Basically the top and bottom standoffs will be functional, and the two middle ones will be added as non-functional scale details later.
I then cut out the actual doors from the retract access hatches. In the pics at right you'll see that these are the lines that had to be altered for model functionality. The position of the pivot point in model retracts is not the same as full scale retracts. Also, you have the aluminum retract box inside the wing that you have to avoid. Therefore, the top end of the gear doors had to be shortened and reshaped slightly so that they would pivot down into the wing and not run into the retracts themselves.
Once all this was worked out, I tried mounting the doors to the standoff brackets. All along I've been having problems getting the doors to close flush to the wing, as just about anyone who has ever modeled a 190 has experienced. To help reach that end, I ground about 1/16" of aluminum stock away from the top side of the standoff brackets. I was careful not to get into the set screw, and left plenty of threads for the two mounting screws. I also added a slight contour to match the shape of the wing.
I decided to use countersunk 4-40 socket screws for a more scale look. As you can see here, the doors are nearly flush with the wing surface now. Not perfect, but acceptable.
Scale & Function issues
While I really wanted to use the scale "scissor mechanism" details that I got from Shindin, I ultimately had to abandon that plan for two reasons. The first reason is because the width of the scissors pushed the back half of the doors up into the air too high. No matter what I would have tried, I never could have got the doors to lay down flat with the scissors on unless the wing would have been deeper. The second reason is described below...
The doors of the 190 extend all the way to the bottom of the wheel rims... very close to the ground. With the doors mounted to the upper strut, this would push the doors down into the ground when the strut suspension compressed fully. Therefore the full scale 190 had the doors cut into an upper and lower door. The upper door mounts to the upper strut, moving up and down as the Oleo compresses. The lower door stays put along with the wheel by mounting independently to the lower strut.
With a 1/5 scale FW190, this puts the bottom of the doors right in the grass (if you fly off of grass like me) so the 3/4" strut travel would push the doors dangerously close to the dirt. Therefore, I finally decided two-piece doors were going to be required. This was not a "scale" decision as much as it was "functional/safety" decision. So, what does this all have to do with the scissors? Read on...
Making the doors two-piece sliders
The first step in making the two-piece sliding doors was to get some way of mounting the lower doors to the lower struts. I picked up couple standoff brackets from Tower Hobbies that were made by Robart for Mustangs. (Thanks to Mike "wulf190" Ufkin of the RCSB forums for the tip on the brackets.)
You'll need to mount these to ensure that you maintain clear lower strut for the full 3/4" travel of the suspension. This will put the two brackets very close together, which is the second reason I had to abandon the scissors. There just wasn't any room left to mount them to the lower strut.
To mount the doors to the brackets, I would end up running the set screws all the way down into the brackets, then run 4-40 countersunk socket screws through the doors right into the same hole as the set screws. You barely have enough threads before running into the top of the set screws, so make sure you end up using LocTite here.
The trick for finding the screw hole positions in the doors so that they line up to the brackets is as follows. Run a string vertically and horizontally across the wing to plot a position over the bracket screw hole. Then, with the doors laying under the strings, you get an X-Y plot to mark your screw hole. Drill a small pilot first so you can slightly adjust the position as needed when you enlarge the holes in the doors.
With the upper and lower doors mounted to the struts, you'll find you already have operational sliding doors. But, you need to make sure they stay put and don't collide or bind in the rigors of actual use.
On one side of the strut, I made a sliding track from two pieces of brass tubing. The smaller one, mounted to the lower door, slides inside the large one that is mounted to the upper door. A ply spacer will be needed on the lower door to get the tube up to the proper alignment.
I was initially concerned with the potential RF interference of having two pieces of metal sliding against on another. My research has led me to believe that this is not the case with brass, which is a non-ferrous (non-magnetic) metal. Technically it should not produce sufficient electro-magnetic resonance to produce dangerous RF interference. Many modelers use these materials successfully for sliding canopies and other moving mechanisms. I'll keep my fingers crossed for now.
The opposite side of the strut got an additional sliding track. This one is made from ply and only keeps the lower door aligned "front to back." That is to say, it will keep the leading edge of the lower door from lifting up in the slipstream, and/or getting hung up on the upper door. The brass tubes will maintain side-to-side alignment.
The video at left demonstrates the finished operational sliding doors.
That's pretty much it for making the sliding doors. This can be done a number of ways, with a multitude of sliding track configurations. You just have to pick one that you feel comfortable with fabricating and take your time to make sure everything aligns well. Also make sure you keep everything structurally strong as it will get a decent amount of abuse on landings.
Lastly, when the plane is completely finished, I'll add generous amounts of lubrication to the brass tubes... probably Vaseline.
Last steps for now
The last few steps for this phase are to add a little more scale dimension to the lower doors. The pic at left shows the raised scale surface that was made from balsa and glued to the doors. Make sure you curve the balsa when gluing to the doors or you will alter the shape/contour.
And finally, the access hatch was mounted using four socket screws. I'm screwing into balsa here, but this hatch won't get much stress on the screws. Plus, I always harden the balsa threads by soaking them well with thin CA.
Finished doors
That's pretty much it. Primary construction and mounting of functional two-piece scale doors is complete. I still have a good bit of scale detailing to do to both the doors and the struts themselves.
Obviously I also have to finish off the inside of the wheel wells, and retract cutouts. I have nearly zero room to add any scale detailing in there, but the styrofoam needs to be sealed away... it doesn't like gasoline very well!
The doors still don't close perfectly flush in all areas, but are very close. There nearly flush everywhere but the leading edge of the lower door. I still may have a few tricks up my sleeve to help minimize that final little bit of gap left over. Thanks again to all those on RCSB for their tips on making the sliding doors functional!
