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Dornier Do-335

Old 06-11-2011, 10:34 PM
  #1  
pmullen503
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Default Dornier Do-335

I've always wanted to do one. The Pfeil is just super cool. As a modeling subject is has a couple significant problems. The rear engine makes it difficult to get the correct CG and it has to haul around a relatively large fuselage and tail for it's wing area. Most models I've seen done tend to be pretty heavy. The scale is about 1/10, 54" W.S. and about as long.

After invaluable assistance from J Morgan and Stacker I've decided to give a shot. Generally, I start by building a simplified prototype before spending a lot of time adding scale details, retracts, etc. just to see how well it flies before attempting a "scale" build. That's what I'm going to do here.

Instead of two identical motors I'm going to use a smaller motor for the rear. The power distribution will be about 2 to 1, front to rear. That should help in getting the CG right. To save weight I'm going to make it a belly lander (though there will be the structure in place to add fixed gear later). To speed up the build I'm taking certain liberties with the wing shape. The lower fin will be removable. The cockpit will be painted. Not only to simplify things but to address structural issues in that area of the fuselage. I'm shooting for a wing loading of 17oz/ft^2.
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Old 06-11-2011, 10:44 PM
  #2  
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Default Fuselage

I'm going to use a formed FFF fuselage. The method was originally developed by Harpye. I'll use a solid form and do the forming in an oven but otherwise the method is the same. It's light, strong and really a time saver if you plan to do multiple copies. Using a solid form allows me to do this with little more than a scaled up three view.


The methods I used are described in more detail here and here but the steps are:

1. Trace the side view onto a pair of 1/2" plywood backer boards.
2. Use the cross section from the blown up 3-view to make templates to rough cut the fuselage sections about 1/4" over sized.
3. Glue the rough cut sections to the backer boards and sand to the final shape using the templates to check progress and make sure both sides match. I make hot wire cuts in the sanded form and trace the outline onto cardboard for areas where there will be structure but no cross section on the three-view.
4. I covered the form with fiberglass and epoxy for durability. It's not strictly necessary if you only plan to pull a couple copies.
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Old 06-11-2011, 10:54 PM
  #3  
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First I cover a piece of FFF with packing tape. Cover the side with the film on it. The film comes off with the tape later. The stuff I use is PVC tape which shrinks at a lower temp that polyester tape. Both work, PVC works better. I overlap the tape at least 1/4". Next, I cold form the taped FFF into curve by rolling the back side with a broom handle on a couch cushion then bending it over the edge of a table with my palms. (If you crack it, toss it and start over.)

Then I tape it to the form. First a few pieces of tape to hold it in place then keep splitting the difference to spread the excess foam evenly over the edges of the form. (Every piece of tape was folded over to make a tab to make removal easier.) When it's all taped I pop it into the oven made from 1" foil faced foam. The oven is 16"x16"x4' and it just barely fit. Bake for 15 minutes at 100'C. The tape and/or the foam releases some fumes so I do it in the garage with the door open.

Let it cool and pull off the tape holding the shell to the form (leave the tape on the shell itself). Trim the foam flush with the backer board and mark the former locations. I made small saw cuts at the former locations when I cut out the backer boards; marker eventually rubs off.

The finished 1/2 fuselage shell, minus the tape, weighs only 46g.
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Old 06-13-2011, 04:43 PM
  #4  
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Now that the shells are made it's time to cover the inside and add internal structure. This is true monocoque construction; the exterior skin bears all the loads on the structure. To make the FFF stiff enough, and strong enough to carry the loads I cover both sides.

Normally, I'd cover the inside with 1/2 oz. glass cloth and epoxy (I did one set like this and send it to Stacker for eval.) but I thought I would try covering the inside with paper and WBPU. The paper covered shells are lighter (60 vs 78g each) than the fiberglass but the fiberglass is probably much stronger.

The strategy for motor and LG mounts is to spread the loads to as much skin as possible. The skin will have excellent tensile strength but not much compressive strength. The idea is to tie several formers together with bamboo or carbon fiber to spread out the loads. I use plywood for any part that has screws attached. Otherwise, foam with balsa laminated to both sides makes a very strong, yet light former material. In the nose I'll add balsa stiffeners after I cut the hatch. I still haven't figured out where everything will go in front. I like to put a FFF former every 4-5". Normally, I'd glue the sides and formers in one step but because there's so much going on and I wanted to run the wires and work out the pushrod runs, I glued the formers into one half (the longerons are there to keep the formers lined up). I ran the wires, mounted the servos and ran a flex cable for the rudder. Then I glued on the other side. When the glue is dry, I block sanded out the ripples in the FFF.

The weight at this point is 298g and it's stiff as a board. 30% of the total weight is wire, servos and pushrods.
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Old 06-13-2011, 10:49 PM
  #5  
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Looking good!
If you need any additional data please feel free to ask; have many pics of the NASM DO-335
Dave
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Old 06-13-2011, 11:12 PM
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Originally Posted by HO-229 View Post
Looking good!
If you need any additional data please feel free to ask; have many pics of the NASM DO-335
Dave
Actually, I could really use some photos of the ventral scoop/lower rudder area. The 3 views I have are all different and one is obviously wrong. I'm shaping that area now and it is a real pain. I've got more time into the scoop than fuselage molds!

BTW, I used to live in Poolesville, MD if you know where that is. Also, your wooden 212 molds would be perfect for forming FFF...............

Pat
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Old 06-14-2011, 06:42 AM
  #7  
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Just picked up your thread, don't know why I didn't see it before-great to see a different scratchbuild subject and build method.
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Old 06-14-2011, 10:35 AM
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Enjoy
Dave
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Old 06-14-2011, 10:44 AM
  #9  
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Originally Posted by baz49exe View Post
Just picked up your thread, don't know why I didn't see it before-great to see a different scratchbuild subject and build method.
Me too How did i miss this build Very Nice work Dave
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Old 06-14-2011, 04:07 PM
  #10  
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More work on the fuselage.

I cut out the cockpit area, cut a block of foam to fit and shaped it. I find it easier to pull the block out, shape it, drop it back in, check it and shape some more. When it's close, I glue in the block and do the final shaping. If I were going to vacuform a clear canopy, I'd use a block of basswood and use it for the vacuform mold.

The reason I didn't do a clear canopy is structural. When you cut out for the canopy and the wing, there's only a 2" strip of FFF on either side connecting the nose to the rest of the airplane. I would have had to add two or more carbon rods from the firewall to mid wing or beyond to handle the loads, especially the nose gear. I would have laminated the sides in that area with 1/64 or 1/32" plywood as well. So for simplicity's sake, no clear canopy.

The scoop had to be an add on because it would be tough to do with the heat forming process. This method works great on nice, gently curving, convex shapes. The scoop area has some sharp bends and concave areas where the foam would pull away from the mold.

I started by rough forming the foam pieces and hollowing out the scoop. The scoop has a 1/8" plywood lip and a 1/16" balsa web slotted into the foam to carry impact loads back into the bulk of foam scoop. Otherwise the scoop opening would be very fragile (the prototype is a belly lander). A corresponding hole was cut into the fuselage to allow cooling air in. After bonding it on to the fuselage I did more shaping. I'm not sure if I'm done with it or not. It's not perfect, but I sick of working on it. I've got more time into the scoop that it took to make the fuselage forms! (Dave: Thanks for the photos. The third one especially is the view I was missing of that area.)
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Old 06-14-2011, 07:14 PM
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Looks very nice… The Germans had several versions of this aircraft all very exciting! I have drawings for all the flavors but as you have noticed I tend to over extend myself with lofty builds.
I am sure you noticed the vertical exit vents at the end of the fuselage.
Somewhere on my home PC is an extensive walk around photo collection of the NASM DO-335 but these photos are all I could muster at this time.
Look forward to seeing it complete; keep up the great thread!

Been to Poolsville MD but it has been many years ago, our family lives at the intersection of the Patuxent River and the Chesapeake Bay (Southern Maryland)
Dave
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Old 06-14-2011, 07:30 PM
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Originally Posted by HO-229 View Post
Looks very nice… The Germans had several versions of this aircraft all very exciting! I have drawings for all the flavors but as you have noticed I tend to over extend myself with lofty builds.
I am sure you noticed the vertical exit vents at the end of the fuselage.
Somewhere on my home PC is an extensive walk around photo collection of the NASM DO-335 but these photos are all I could muster at this time.
Look forward to seeing it complete; keep up the great thread!

Been to Poolsville MD but it has been many years ago, our family lives at the intersection of the Patuxent River and the Chesapeake Bay (Southern Maryland)
Dave
Dave,

I did notice the vents in back and plan to use them. I'll graft those on after I fiberglass the outside and cut an air hole. I was so concerned about the strength and rigidity of the rear mount area that I wanted to keep the fuselage as close to a round tube as possible. That shape makes the most of the monocoque construction's virtues. Adding the tail surfaces will compromise it further hence the over engineering of that area. Still, sooner or later I'll get a full throttle prop strike with the ground and I'll find out if it was over engineered or not....

Pat

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Old 06-14-2011, 07:46 PM
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What is the greenish looking filler
Do you know of a bonding adhesive for foam that does not require air, reasonably light weight and can be procured locally at reasonable costs?
My experience with adhesives and foam has been they bond well near the seams but a few mm into the foam the adhesive is still fluid, most bonding agents seem to require a vent to cure properly; epoxy seems to be the only sure bet I have found and it is a bit heavy.
Dave
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Old 06-14-2011, 09:25 PM
  #14  
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Originally Posted by HO-229 View Post
What is the greenish looking filler
Do you know of a bonding adhesive for foam that does not require air, reasonably light weight and can be procured locally at reasonable costs?
My experience with adhesives and foam has been they bond well near the seams but a few mm into the foam the adhesive is still fluid, most bonding agents seem to require a vent to cure properly; epoxy seems to be the only sure bet I have found and it is a bit heavy.
Dave
Dave,

I use the white Gorilla Glue for foam. All it requires is moisture to cure. There's usually enough in the air but you can mist the surfaces with water if you were laminating big blocks of foam on a particularly dry day. You can also control how much/how dense it foams when it dries. More moisture-more foaming. You can get it to foam up almost as light as EPS, which is handy when you have to sand through a joint and don't want a hard spot. I use it to bond balsa skins to foam cores too. Apply and scrape off all you can, there's still enough for a good bond. I've even used it in molding pilot figures by premixing with water and pouring into a silicone mold (very messy, I don't recommend it unless you are as cheap as I am).

The green line is from a water based highlighting marker I used to color the edges before I glued the shells together. Since you aren't building on a plan, you have to use landmarks on the shells themselves to position cut outs etc. The vertical centerline is obviously a useful landmark. The trouble is that I've gotten good enough at this stuff that without color in the joint, it can disappear after sanding; and that's bad! Premarking the joint with a highlighter stained the foam enough that the centerline is still visible.

Pat
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Old 06-15-2011, 03:13 AM
  #15  
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Pat,

Very cool construction techniques, I am enjoying your work. thanks for sharing. And, the subject matter is very cool too. I look forward to watching your progress. Good luck!

Forge

Ps. Dave, those are some great photos of the Dornier, looks like a really nice museum!
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Old 06-15-2011, 03:10 PM
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Hay, what are you using to fill in around the scoop? Is that some sort of spackle?
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Old 06-15-2011, 04:54 PM
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Originally Posted by flyboy_fx View Post
Hay, what are you using to fill in around the scoop? Is that some sort of spackle?
Yup, just plain lightweight spakling compound. I use it straight over foam but I like to add a little WBPU to harden it up a bit over fiberglass. Otherwise it'll sand too easily and I get a low spot in the filler.

Pat
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Old 06-15-2011, 05:39 PM
  #18  
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Default Wings

To make the wings I use the "laser method". I'm not sure who first developed it but I really like the method. It's primary advantage it that it builds a really accurate, lightweight wing. From a design stand point, it allows you to easily add structure to a tapered wing without having to develop every wing rib. It has all the advantages of a hot wire cut foam cored wing but without the weight of a solid foam core. Once you get into the 4 ft or larger W.S. the weight savings become really significant.

Start by laying out the wing on the building board. Wing outlines, spar and the rib locations are penciled in and rectangular foam rib blanks are stuck to the board (about 3" tall in this case). I've done a number with double sided tape but this time I used dots of epoxy to stick the rib blanks to the board. If they are crooked it doesn't really matter except for the root and tip blanks. Take care to stick them down straight with a good 90 degree angle to the board. I use FFF for most ribs but used 3/4" foam for the root, tip, and the area where the LG mounts will be. There's also a partial rib where the end of the aileron cut out will be.

I used template to make slots for the main spar and dihedral brace. If I had designed a built up wing I would have had to design these slots into each rib. Here the hot wire makes sure they're all where they need to be. Next I clamp guides to either end and cut off about a 1/4" with the hot wire to make sure the top of the rib blanks are flat and co planar. Finally, I glue another board to the rib tops with more dots of epoxy.

Now you can add the root and tip templates. The templates have the dihedral and washout angles build in to them. You pretty much have to use separate templates for upper and lower wing surfaces. I reference everything from the spar slot rather than the TE or LE. Then you make the cut for the top surface with the hot wire. I used a home made Feathercut apparatus to accurately cut the taper. Otherwise, I'd have cut the building boards to match the wing outline so I could eyeball the taper. The taper on the Do-335 is around 2 to 1 which is tough for me at least, to cut by hand.
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Old 06-15-2011, 06:20 PM
  #19  
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Default Wings continued

Now that the top cut is made it's time to sheet the top. I'm using Dollar Tree foam. You can use balsa or Obeci veneer, even cardboard or coroplast. I pre form the foam skin to the top shape. Apply glue (white Gorilla glue in this case), place the foam skin in place and replace the top building board and pile some weights on top. Here's another huge benefit to this method: The building board only applies clamping pressure where the ribs are. You can get away with sheeting with a soft material like DT foam without it sagging between the ribs!

Once the glue is dry, remove the building board and apply a few pieces of double sided tape to the root, tip and one or two of the center ribs. Carefully replace the building board back on the skin taking care to line it up with the lower board exactly. Add the lower templates and cut the bottom surface. Now you have the half sheeted wing stuck firmly to the opposite building board - holding it perfectly straight.

Fit the spar and dihedral brace. The main spar is vertical grain 1/8" balsa capped with 3mm bamboo. The spar blank in the photo will make both spars. I drop it in place and mark it at the rib locations. Cut the spar blank in half 3mm below the marks to compensate for the 3mm bamboo that will be glued to the bottom of the spar. The dihedral brace is done the same way except I used 1/8" Aspen (it's similar to Basswood). I forgot to add a block of foam for the aileron servo mount before I cut the lower surface so I had to shape a block and add it now. I used a sharpened brass tube to cut the holes for the servo wires. If I needed any other structure for landing gear, flaps, lights, etc. I would add that now. I don't need it here but to make balsa or plywood doublers for LG etc. I would have traced the top of the appropriate wing ribs after the first cut and traced the bottoms after the second cut.

When all the internal work was done I added the bamboo to the spar glue on the bottom skin in the same fashion I did the top.
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Old 06-15-2011, 07:31 PM
  #20  
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WOW
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Old 06-15-2011, 07:35 PM
  #21  
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Now that the skins are glued on, I trim the LE with the hot wire. I'll add a balsa LE.

I've included a couple shots of my version of a Feathercut. It's just a pivoting arm and some pulleys to guide strings with clips on the ends that pull the wire through the cut. By adjusting the distance on the pivot arm from the pivot point to the strings you can control how fast one string is pulled versus the other. Once you work out the proper distances, you connect everything up, turn on the hot wire and release the pivot arm. A weight pulls the arm down and with it the hot wire. Takes a bit of fooling around to get it set up but it makes a very reproducible cut- much better than I could do by hand.
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Old 06-16-2011, 04:06 PM
  #22  
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This is the fourth set of wings I've done with the laser method and DT foam skins. I'll cover the wings with kraft paper and WBPU. It looks like a lot of work but it really not bad. Somewhere between a built up balsa wing and a sheeted foam core. The reason to go through the process is weight saving. This wing should come in around 3 oz./sqft. A balsa sheeted foam core wing would weigh about 40% more and if I had sheeted this wing with balsa it would weigh about 20% more.

In the photos I've added and shaped the 1/4" balsa LE and started work on the ailerons. I'm going to cover the wing with kraft paper which will shrink as it dries imparting a lot of stiffness to the wing. But you either have to leave the DT foam TE quite thick or reinforce it somehow so it doesn't bend. I glued 3mm bamboo to the TE and sanded the foam down to it. (I found a bamboo roll up blind at the home center a while ago and now I have a lifetime supply if 3 mm bamboo sticks.)

In the past, I've shaped the tips from foam but the edges tend to get beat up quickly. I've been making tail surfaces for a while now from two layers of foam with hardwood veneer between. It allows me to sand the edges very thin and yet still be durable. I thought I'd try it on the wing tips. The center of the edge of the wing tips isn't a flat plane as in the tail surfaces, it's a curve. I derived the shape of the curve and hot wire cut it in a block of foam. I traced the tips on some 1/16" balsa and glued the block back together with the balsa in the middle. When the glue was dry I roughed out the shape on the bandsaw. I'll glue those onto the wings and do the final shaping before covering.
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Old 06-17-2011, 04:34 PM
  #23  
pmullen503
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Wing tips installed. Having the the sheet of balsa buried in the foam made it easy to match both tips.
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Old 06-17-2011, 07:29 PM
  #24  
CHELLIE
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Very interesting foam cutter, I like that

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Old 06-17-2011, 08:50 PM
  #25  
pmullen503
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Originally Posted by CHELLIE View Post
Very interesting foam cutter, I like that
I wish I had thought of it! It's simple, cheap, and works well. There are several plans on the web. Here's a link: http://www.charlesriverrc.org/articl...foamcutter.htm

And another:http://www.rcgroups.com/forums/showthread.php?t=462759
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