I was inspired by J Morgan's excellent Do-335 build. 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.

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.

Specs.
Scale 1/10
WS 54"
AUW 61oz
Wing Loading 17.6 oz/sqft
Power front RCtimer 3530 1100kv, Rear Turnigy 2209-28 1050kv ~420 watts total
Battery (2) 1800 or 2200mAh 3s

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.

J Morgan graciously sent the form he used for his fiberglass Do-335 which I had hoped I could use but I decided to make a new one from scratch. Having John's form there a guide while I was shaping mine was a great help.

The methods I used are described here http://www.rcgroups.com/forums/showthread.php?t=1240702 and here http://www.rcgroups.com/forums/showthread.php?t=1300153 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.

First I cover a piece of FFF with packing tape. 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.

Once 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 covered shell 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.

nice work i will be watching for the finished plane. looks great so far.

Pat,
DO335 fuse looks super so far.

Take care -- Stacker

Originally Posted by stacker

Pat,
DO335 fuse looks super so far.

Take care -- Stacker

Bet it looks familiar too............

Pat

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'm sick of working on it. I've got more time into the scoop that it took to make the fuselage forms!

I had planned on building one of these over the winter, but due to issues with my soon to be exwife I never even got started. I want to build one in the 80"-90" ws range. I will be watching your build.

Originally Posted by tommygunner

I had planned on building one of these over the winter, but due to issues with my soon to be exwife I never even got started. I want to build one in the 80"-90" ws range. I will be watching your build.

I should have mentioned this earlier: The scale is 1/10, 54" W.S. and about as long.

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 significant. (The threads cited in post #2 describe the same process.)

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 built 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.

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. I made a test wing with this spar and it worked well. More flex than a hardwood spar but it didn't break. Almost an ounce lighter too. 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 and glued on the bottom skin in the same fashion I did the top.

Once 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.

This looks like a lot of work but it's 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 will come in around 3 oz./sqft ready to paint. 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.) That should give a nice thin, durable TE.

In the past, I've shaped the tips from solid 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.

Wing tips installed. Having the sheet of balsa buried in the foam made it easy to match both tips.