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Beechcraft D-18 Scratchbuild

Old 02-14-2012, 02:33 AM
  #1  
pmullen503
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Default Beechcraft D-18 Scratchbuild

Here's my entry for Fun Build 5 over on RCgroups: 1/11 scale Beechcraft D-18. I found some excellent 3 views so other than scaling up the 3 views and drawing in some structure there was very little to do to get a workable plan.

I plan to do the fuselage in heat formed FFF and wings using the "laser method" like I did for my Do-335 and my Lockheed Vega series.

Wingspan: 52"
Motors: Turnigy 2209-28
Battery 3S 2200 mAh
Weight: 50 oz
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Old 02-14-2012, 02:41 AM
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pmullen503
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I started with the tail surfaces. The tricky bit was getting the rudder linkage buried in the stabilizer. I wanted the linkage to be easily adjustable and low profile enough to cover with scale looking covers.

I started by cutting the stab outline out of FFF. Then I glued down 1/16 light balsa around the perimeter. In the center I added a more balsa to form a track for the Sullivan flex push rod (#507). The slots in that piece make pockets for the hinges.

The balsa strengthens the otherwise fragile edges of the foam, gives me a handy center line reference when shaping the airfoil, automatically centers the hinges and stiffens up the whole stabilizer.

Another layer of FFF was glued down and then sanded to an airfoil shape. The rudders were done the same way except I used 1/32" balsa for the center layer.

The stab was traced onto the rudders and the foam cut away down to the balsa center layer. The rudders were glued on. This provides a nice strong connection between the stab and vertical stabilizer. In my experience, twin rudders are always catching on something and take a beating.

The hinge surfaces were faced with balsa. The elevator got a 1/4" balsa LE. The rudders were just sanded round. After everything was finish sanded I gave it two coats of Liquid Sheeting II (aka Styrospray 1000). This is a two part polyurethane coating for foam that cures to a nice hard shell. It's roughly comparable in strength to an equivalent weight of epoxy and glass. It's great for something like this that would be a pain to fiberglass. WOWplanes does entire aircraft with it.

A 5g. servo was mounted and connected to the push rod. I still need to fabricate the fixed covers for the rudder linkage out of pop can aluminum or maybe I'll vacuform something. You can see the linkage cover in the full size D-18 in the photo.

It came out pretty good. A little heavier than I would like (82g.) but the linkage moves freely and is adjustable. The rudder to stab connection is very solid after the LSII coating. A little filling and some fillets and it will be ready for paint.
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Old 02-14-2012, 03:37 AM
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mclarkson
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Looks like an excellent start! Best of luck.
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Old 02-14-2012, 06:11 PM
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Bill G
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Always good to see a scratchbuild, which had by far become my favorite class of builds. The Beech twins in general are always good looking. I really would like to build a few of their twin models someday. The Dayton Wright RB1 was fun to enter in the last contest. It was loosely based on plans, but almost a scratch build. Best wishes with the contest.

I imagine the foam tail is reasonably lightweight which should help, as some twins can end up tail heavy. I have a few twins with more lead than I care for, in the nose and/or cowls. The hidden cable should be a good rudder method, as it looks better than an exposed cross-bar that I've used before.
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Old 02-14-2012, 07:30 PM
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Thanks, I've been watching your Comet build with great interest.

The tail is heavy and may yet be rebuilt. Wouldn't be the first time. Multi engine projects are always heavier than similarly sized single engine projects so adding dead weight to balance is something to be avoided.

Pat
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Old 02-15-2012, 12:35 AM
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Here's how I did the wing. Iím using the so-called ďlaser methodĒ. It offers certain advantages over built up and foam cored wing construction. Like hot wire cut foam cores, you donít have to draw and cut ribs, you just need root and tip templates. On larger wings there is a significant weight savings over a solid foam core. (Though at this size the weight savings are small). Where I live, anything over 2Ē thick EPS is not readily available. So being able to make the ribs out of cheap, easily available foam board is also a plus. Another big advantage for the scratch builder is that you can drop in ribs wherever you want. (Software like Profili will draw all the intermediate ribs for you as long as they are evenly spaced.). Adding spars is easy because you cut the slots all at once with a hot wire. I find it easier cut holes for wiring and such with this type of construction than to rout channels in solid foam cores. The building boards apply clamping pressure only where the ribs are when skinning. Dihedral and washout are built into the templates.

Start by drawing the wing outline and rib locations on the building boards. Iím using ĺĒ melamine coated shelving. Then glue down the rib blanks. (I used hot melt glue) In this case, I did the ribs from the nacelle to root first. Iím adding two 1/8Ē lite ply dihedral braces to carry the landing gear loads so I cut those slots first. The motor/LG mounts will be a pair of 1/8Ē lite ply plates sandwiched between (3) ĺĒ ribs. The lite ply and foam ribs were clamped together to ensure the correct spacing while the ribs were glued down.

Once the dihedral brace slots were cut, I glued down the rest of the ribs and cut the main spar slot. I added a block of foam for the aileron servo too. Before adding the top building board, I used two straight edges clamped to the root and tip to make a hot wire cut to make sure all of the tops of the ribs were straight and level. Then the top board was glued to the ribs (Gorilla Glue in this case Ė too many ribs to glue at once for hot melt). At this point, you have the ribs (spar and brace slots cut) glued to the top and bottom boards.

Next, add the root and tip templates and make the first hot wire cut.
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Old 02-15-2012, 12:41 AM
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After the top cut is made, lift off the top building board and glue down the top skin. I pre-form the DT foam skin into a curve by rolling it with a piece of plastic pipe on a couch cushion. The harder you push and the smaller the pipe, the tighter the curve. Apply glue to the ribs and place the skin on the ribs. I use white Gorilla Glue. If you apply just the minimum needed to attach the skin you can hot wire through it with no problems.

Replace the top board and weight it down to clamp the skin in place while the glue sets.

With the soft DT foam, pre-forming is needed. Otherwise, the weight needed to get the skin clamped to the ribs will dent the foam. (Not a problem with balsa skins.) The building board only applies clamping force to the ribs so the skin doesn't sag between ribs.

I used Dollar Tree foam and Iíll cover it with kraft paper and WBPU. Balsa also works great with this method and makes a really strong wing. The foam and paper will weight about 25% less than 1/16Ē balsa and a light glass cloth/epoxy finish. Iím also going to splice in a section of LE to handle the area between root and nacelle and that will be easier with foam so thatís what I used.

Once the glue is dry remove the top building board and trim off the excess skin at the root and tip. The excess at the LE and TE will be trimmed off when the cut is made for the bottom of the wing.

Add the templates for the bottom of the wing. Before replacing the top board apply several pieces of double-sided tape to the rib cut offs stuck to the top building board. Carefully replace the top building board using the spar slot cuts to make sure itís properly lined up. The idea is to temporarily attach the wing to the top board to hold it in place when the bottom is cut off.

Make the cut for the bottom surface.
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Old 02-15-2012, 12:46 AM
  #8  
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Now it's time to add internal structure and the holes for the wiring.

1/8" lite ply for the dihedral braces was slid into placed and marked. The braces are cut out and glued into place.

The main spar is a sandwich of 3mm bamboo top and bottom with 1/8" balsa between. It's made by dropping a 3mm round bamboo stick into the spar slot and sliding 1/8" balsa into the slot after it and marking. The balsa spar was cut (subtracting 3mm for the top bamboo stick) and the bottom bamboo stick, balsa and top bamboo piece glued into place. I've used this type of spar on larger planes and it's more than adequate.

I cut the holes for the wiring with a sharpened brass tube. Unlike with a solid foam core, you can see exactly where the wiring holes are going. On longer wings I've clamped templates to the ribs and cut the wiring channels with the hot wire.

The bottom skin is then glued on in the same fashion as the top. When the glue was set I set up templates to trim the LE and TE. Since the flap and aileron edges are a straight, continuous line, I made a 45 degree cut at the TE and capped it with 1/8" balsa. I'll make ailerons and flaps from balsa sheet rather than trying to use the foam cut offs. The LE gets a 1/4" balsa cap. It's possible to do the LE in foam but the balsa cap adds needed dent resistance.

The D-18 wing should really be done in 5 sections, the center and root to nacelle and nacelle to tip for both wings. But when you look at the full size airplane, there is no obvious change in shape for the back half of the airfoil. That and making the skin continuous should make a stronger wing, lead me to attempt to just splice in a new airfoil section to match the plan view.

I glued on a 1/8" balsa rib extension at the root and a brace at the LE. I cut away the top and bottom skins to make room for the new airfoil section. The I pre-formed more DT foam into the new airfoil section. You must cover the outside with packing tape before forming the foam to keep it from cracking. Once the replacement piece was formed I glued it into place. It came out better than I had hoped. The outboard part of the splice will be hidden by the nacelle and the rest blends nicely.
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Old 02-18-2012, 05:47 PM
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Here's a shot of how the wing panels are joined. I used the building boards to build a wing holding jig. The boards themselves are lined up and set with the proper spacing between the wing halves and hot glued to the bench. The root and tips were traced onto pieces of foam (the dihedral angle is built into the tip supports) and the foam pieces cut and glued to the building board. Now I can drop the wing halves into the jig to trim the braces. I cut one side and used it to mark the other. After a few minutes of trimming, the two halves fit nicely. The braces were joined with thin plywood plates The center section will be cut from solid foam and fitted around the braces.
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Old 02-18-2012, 05:52 PM
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Here's how I did the wing tips. Wing tips seem to take a lot of abuse so I added a strip of bamboo around the edge. The bamboo comes from a bamboo blind I found in the home center clearance bin a while back. I now have a lifetime supply of 3mm round sticks.

I made a form from some scrap 1/4" plywood. I printed out the wing tip about 10% undersized to account for springback. I taped one end of the bamboo dowels to the form and slowly bent them around the form while applying steam from a tea kettle (wear heavy leather gloves!) After the bamboo sticks were bend around the form I taped down the other ends and let them dry over night. When released from the form they will spring back some but the shape is now fixed.

Then I cut the wing tips from foam and glued them in place along with a 1/16" balsa tip rib. After rough shaping them, I cut a shallow groove in the foam and about an inch along the leading edge balsa and glued the bamboo in place. When the glue was dry, I did the final shaping of the foam. A little filler here and there and the wing halves will be ready to cover.
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Old 02-18-2012, 05:58 PM
  #11  
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I covered the wings with 30lb kraft paper. I first gave the wings a coat of WBPU (I used Carver Tripp "Safe and Simple") and let it dry to raise the grain of the balsa and give a harder surface to the foam. Then I sanded with 240 grit paper.

I cut out the paper and misted it with water to wet it. Not soaking wet, just enough to get it to relax and stretch out. Then I gave the side of the paper that will be against the wing and the wing itself a coat of WBPU and applied the paper. The paper is overlapped at the TE. I added an extra layer of paper where I spliced in the area between the nacelle and the root to toughen it up because the LE is just foam in that area. When everything was smooth, I gave the wing a final light coat of WBPU and hung the wings from the dihedral braces to dry vertically in still air. It's important to let the wings dry evenly on both sides to prevent warps.

For the tips I used small binder clips to clamp the paper down while it dried. I'll cut away the excess when it's dry and cover it with a strip of bias cut fiberglass cloth and WBPU. I've cut slits in the paper and overlapped it in the past, much the same way you would do a heat shrink covering, but I still got wrinkles and bumps and ended up covering the edge with a strip of fiberglass cloth anyway. I think I get a better result doing the tips this way. You can see in the last picture how nicely the paper molds to the edge.

The weight of the two wing halves is 238g. My "budget" is 300g for the completed bare wing so I've got about 60g to spend on the center section, flaps, and ailerons. Because the wet paper shrinks as it dries, the wing really stiffens up. I dropped it back into the wing holding jig to check for warps and it's good.
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Last edited by pmullen503; 02-19-2012 at 04:39 PM.
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Old 02-23-2012, 03:59 AM
  #12  
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Center section. I rough cut pieces of solid foam on a bandsaw to fill in the spaces around the dihedral braces. The foam pieces are a drop in fit. The gorilla glue will expand to fill any gaps. I'll sand the center section to shape and cover it with more paper and WBPU.
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Old 02-23-2012, 04:06 AM
  #13  
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Fuselage. I'm heat forming the fuselage shells from FFF. I learned of the method from Harpye's builds. The basic principle is to heat the foam in a mold to the point where the foam softens but doesn't collapse. When the foam cools, it retains the new shape. The form acts as the male mold and the tape acts as the female mold. I use a solid form which allows me to build from a three view rather than needing a 3D CAD model and I use an oven to bake the foam rather than a heat gun. The formed shells are very light and rigid minimizing the need for internal structure. You also get a nice smooth surface; except for the seam between the halves, no joints to fill! Once you have the form, you can make multiple copies for your buddies or rebuild it in the future.

I've described the method before in more detail in this thread:

DO-335

Here are the highlights:
Start with a pair of backer boards made from 1/2" plywood cut to the side view (minus the thickness of the foam). Rough cut blocks of foam using the cross sections from the 3 view. Glue to them backer boards leaving a gap between the blocks just large enough to slide in the cardboard templates. Sand the form to shape. I drew lines on the cross sections about 1/4" inside the outer edge to account for the thickness of the foam. I don't necessarily sand to this final line on every template. I'm more concerned with getting nice smooth curves and the right shapes. Even on a good 3-view like the one I'm using, there's no guarantee that the cross sections are all correct. When I get close to the final size I stop, slide fresh cardboard into the slots, mark the actual shapes, and from then on I work to made both halves match each other.

Once the mold is done, I cover the film side of FFF with strips of 2" PVC packing tape, overlapping each strip about 1/4". The taped FFF is then wrapped around the form and taped down with more PVC tape. I screwed a 2x3" board to the back side of the form to keep it from warping. It also provides space for the tape to pull the excess FFF as it's heated and the tape shrinks.

The whole thing is baked at 100'C (212F) for 15 minutes in my foam box oven (described in the thread cited above). After it cooled, I trimmed off the excess foam and popped it off the form. The finished halves weighed 85g total. I'll leave the tape on until after the internal structure is added and the halves are joined. The bare FFF is soft and easily dented and keeping the tape on until the last minute keeps it from getting too beat up.
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Old 02-25-2012, 03:16 PM
  #14  
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nice build
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Old 02-25-2012, 06:21 PM
  #15  
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Here's a shot of the internal structure. I glued the formers into one side to run the elevator pushrod and work out what I'm going to do with the tail wheel. I was hoping to cram in a small retract in unit but it doesn't look like I'll have enough room. I'll probably end up with a fixed tail wheel.

I plan to make the whole nosecone removable for battery access, probably at the second former. I'm doing the early, short nose version but if it ends up too tail heavy, I'll make a new nose section for the later, longer nose version so I can move the battery farther forward.

A smallish tail dragger like this is going nose over when landing on grass so I'll make a molded fiberglass nose that can be easily replaced when it eventually gets beat up.

Once the halves are glued together the tape can be removed and the fuselage block sanded smooth. It doesn't take much sanding, just enough to remove the ripples in the FFF and any marks that the tape leaves. The fuselage weighs 122g at this point and less that 2g was removed by sanding to get it smooth.

Note the beach towel covering the bench. The bare FFF is really soft and dents easily at this stage. I gave it a good coat of WBPU to harden it up. Besides making it more dent resistant, the WBPU makes the foam harder than the filler so it can be sanded flush with the foam.

There's a little filling needed here and there along seam. The windscreen area will need more work to get it ready to make the mold for vacuforming the windscreen.
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Old 02-26-2012, 02:32 AM
  #16  
pmullen503
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Here's how I'm doing the mold for the windshield plug. I've had mixed results using foam for vacuuform plugs and prefer to cast them from plaster or shape them from solid wood. To cast the plaster plug I'll need a female mold of the fuselage.

First, I covered the windshield area with shrinkable window insulation film. It's a pretty common item at home centers in cold climates. It comes in large sheets that you tape around the inside of your windows and shrink it tight with a hair dryer. I use it to create a barrier between the foam and fiberglass. Cut a piece to size, tape down the edges, and shrink it tight.

Then I applied several layers of glass cloth and epoxy. I used a full sized piece of 4 oz cloth and scraps of 6 oz. to build up 3 or 4 layers.

As is, the fiberglass would not be stiff enough to pour the plaster plug so I added chunks of foam to make a frame. I hot glued a couple sticks to the foam frame and used them to hot wire a nice flat bottom to keep the mold level during the pour. I've learned the hard way to make the supporting frame from something with a little flex to make it easier to remove the mold from the fuselage and the plaster from the mold later.

After the epoxy cured the mold was removed from the fuselage. The shrink film leaves a very smooth surface inside the mold. (the white line in the lower right corner is a reflection of the fluorescent light overhead!) I'll add dams at the ends and make foam plug to take up some space inside the mold. As is, it would take a couple pounds of plaster to fill the mold!
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Old 02-26-2012, 02:37 AM
  #17  
pmullen503
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Here's the poured plaster plug for vacuforming the windscreen. Even with a foam plug in the center to take up all but 1" to 1 1/2" around the edges, it took 3 pounds of plaster to fill the mold! Once it's fully dry I'll do a little shaping to get a more defined edge between the front of the windscreen and the fuselage, right now it's more of a radius.
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Old 03-03-2012, 12:41 AM
  #18  
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I covered the fuselage with the same paper used on the wings. My "budget" for the fuselage at this stage was 200g and it came it at 192g. I marked out the cabin window locations then cut them and painted the inside brown to hide the foam. I cut away a rectangular area and recessed a piece of clear PETG to sit flush with the rest of the fuselage and glued it in place with canopy glue. Then I covered it with another layer of paper and WBPU. After the fuselage is painted I'll cut away the paper to reveal the window. The paper is translucent enough to find the window cut outs if I shine a bright light through the windows.

I tried this on a test piece and it worked great. The poly coated paper makes and excellent mask and peeled away leaving a clean surface and nice sharp edges.
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Old 03-25-2012, 02:27 AM
  #19  
pmullen503
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Motor mounts. The main structure is a pair of 1/8" lite ply plates sandwiching a piece of 3/4" foam (which conveniently, is the width of the retract units). The plates slide into gaps between the wing ribs and around the dihedral brace and back to the main spar.

I made them by jigging the wing into place on the work bench and sliding cardboard into the slots. Then I marked the motor center line and LG pivot point. From there it was just drawing the outline of the plates so all the parts end up in the right places.

Being able to slide the mounts in and out to work on them was a big help. I still need to add the actual motor mounting plates, cowl mounting rings and figure out where to mount the ESC's to get some cooling air flow.
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Old 03-25-2012, 02:45 AM
  #20  
pmullen503
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Nose cone. First I made the foam plug that I used for the mold. I glued 1/16" ply formers to the fuselage and a block of foam keyed with pins so I could remove and replace the nose cone while I shaped it. When I was happy with it, I covered the nose cone with a layer of 1/2 oz cloth and WBPU.

I sliced off the plywood former (I'll eventually glue it into the fiberglass nose cone) and added a FFF extension to the base to make the nose cone a little longer than needed. That way I'll had a little extra glass I could trim when it came time to fit it. I glued the plug to a base of FFF, gave it three coats of wax, and slathered about a 1/2" of plaster over the plug. I let that dry for a couple days.

Happily, the plug was easy to remove and came out in one piece.

I gave the plaster mold a couple coats of paste wax and laid up a layer of 3/4 oz cloth followed by 3 layers of 6 oz cloth in epoxy. (I'll need the weight to balance so no need to save weight here.)

When the epoxy had cured I broke off the plaster by smacking the mold with a hammer. Most of the plaster came off easily but I had to sand off the rest.

After trimming the fiberglass nose cone to fit, I glued the 1/16" plywood bulkhead back in with pins and magnets to secure the nose cone. The whole thing pops off to change the battery giving excellent access to the connectors. The surface was filled and sanded to get it ready to paint.
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Old 03-25-2012, 01:11 PM
  #21  
pd1
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I learned a lot about this type of construction from your thread.
Nice work.
The plane is coming out nicely too.

Paul
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Old 03-25-2012, 07:28 PM
  #22  
StephenW
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What an excellent build, I'm very impressed ,well documented and well constructed.
I salute your skill
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Old 04-04-2012, 10:16 PM
  #23  
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I vacuformed the canopy from .030" PETG and mounted it. Had to shoot some paint on it to see how it came out.
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Old 07-09-2012, 01:34 PM
  #24  
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Nacelle shells. The top and bottom are different profiles with the bottom longer to accommodate the landing gear. I made the lower form on the lathe. I glued a plywood firewall to a plywood piece cut to the plan view and filled it with foam. Then I turned it to shape.

I covered the form with the same heat shrink window film I used to make the canopy mold. Then I added a layer of 6 oz. cloth and a layer 2 oz. cloth. I used MAS low viscosity resin and slow hardener. I covered the wet epoxy with another layer of the heat shrink film and shrunk it tight. The slow cure hardener allowed me to heat shrink the film without prematurely curing the epoxy.

The result was better than I'd hoped. I've done some vacuum bagging of epoxy lay ups and it gives an almost perfectly smooth surface to the finished part. The trouble is that it only works on developable surfaces; compound curves are limited by the ability of the bag to stretch. The shrinkable film gets around that problem.

The result isn't perfect, there are some wrinkles at the tail end of the nacelle where the film couldn't quite conform to the mold but this will be MUCH easier to finish than without the film.
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Old 07-09-2012, 01:40 PM
  #25  
pmullen503
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Fitting the nacelle halves is pain because you have to compensate for wing's airfoil, taper and dihedral. Making an accurate mirror image for the other side only complicates the process.

Here's how I fit the nacelle halves. First, I cut and fit the right shell. Then replaced it onto the mold and marked the outline. To make the mirror image I transferred the distances from the center line of the mold to the marked line to the other half of the mold. Next I replaced the untrimmed, left shell onto the mold and used the new marks to draw the cutting line for other half.

This was very convenient with a round, turned form but it works with other shapes. You just half transfer the shape of the right side of the trimmed shell around to the left side of the form and vice versa using some convenient reference line.
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