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Old Jun 28, 2011, 01:14 PM
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United States, WI, Fond du Lac
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Originally Posted by stacker View Post
Pat,

DO335 is lookg great! You are moving along at a fast clip.

Keep it up -- Stacker
As you know, I've been planning for many months so it's moving along quickly, at least to this point. (IIR your 219 came together quickly until you got to the landing gear).

I need to start bolting stuff on to see how close I am on the CG. My spreadsheet says I should be fine, but it's been wrong before. Might even be able to place an esc right behind the firewall. Now I've got to make hatches and figure out how to stuff all the gear and wires into the fuselage. I'll squeeze the front motor esc in the cowl but the rear motor esc will end up inside the fuselage. I'll probably have to add a heat sink to that esc. The weight is good though; under 30 oz. for the bare airframe. I should be able to bring it in under my 60 oz. AUW target.

Pat
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Old Jun 28, 2011, 03:05 PM
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Chapel Hill, TN USA
Joined Apr 2001
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Looking really good Pat. I had to literally stuff everything possible up as far forward as I could to achieve CG. Looks like you're doing a better job of it. Won't be too long now.

J
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Old Jun 28, 2011, 04:33 PM
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Originally Posted by J Morgan View Post
Looking really good Pat. I had to literally stuff everything possible up as far forward as I could to achieve CG. Looks like you're doing a better job of it. Won't be too long now.

J
Thanks John. But of course, I had your experience to draw upon. I remember how the battery filled the nose on yours .

Using a lighter motor in the back really helps; like 1/2 of the weight of one you used. It's the equivalent of about 3 oz. of lead in the nose in terms of it's effect on CG. A smaller motor meant lighter wires too. Almost 10% of the airframe's weight at this stage are the wires for the rear motor!

Pat
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Old Jun 29, 2011, 01:46 PM
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Here's a shot of the belly under the wing. I used the piece of the fuselage from the wing cutout. I peeled of the paper laminated on the inside of the shell to make it easier to sand. I held it in place on the wing and slid a piece of sandpaper between the wing and the cutout and sanded it down until the front and back matched with the rest of the cutout. The ends were faced with balsa and the piece was glued to the wing. The access holes for the wing screws are a bit smaller than the screw heads so the screws are trapped; no more lost screws!
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Old Jun 29, 2011, 02:15 PM
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Here's how I glue point hinges. I like using them but it can be tough to use them without getting glue in the hinge.

I drill my holes with a Robarts jig. I drill them deeper than needed so there is a place for excess glue to expand into. Then I wrap some sticky label stock (sticky side out) around a 3mm dowel to make a cylinder. Insert the cylinder into the hinge hole about 1/2". Squirt some Gorilla glue into the cylinder and use the dowel like a piston to push the glue down into the hole. I spin the dowel to distribute the glue. Then withdraw the dowel and pull out the cylinder. The glue is now down in the hole with none on the sides near the pivot point. Insert the hinge point and let it cure.

Don't put too much glue in or it will travel up the pin and foul the hinge.

This also works with epoxy but I prefer PU glue when gluing into foam because it expands and locks the hinge point in better. I also recess the pivot point into the movable surface about 1/2 the thickness of the surface. That allows a smaller gap.
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Old Jun 29, 2011, 02:25 PM
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When I make elevators or ailerons instead of capping the ends with balsa, I just seal them with epoxy. Spread some epoxy on the end and warm it with a hair dryer. Bubbles will form from air trapped in the foam or balsa. Rub in the epoxy and warm it again. Rub out any new bubbles. Do this a couple times until bubbles no longer form. Let the epoxy cure.

The warming and rubbing drives the epoxy into the pores of the wood and foam and hardens the ends to a surprising degree. It's quick and you don't have to blend the balsa end caps into the control surface.
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Old Jun 30, 2011, 08:58 AM
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Nice idea Pat. I'll have to try that.

J
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Old Jul 01, 2011, 10:40 AM
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Battery hatch.

Before cutting the battery hatch I bolted on the motors and other parts to check the CG. I held the batteries on the outside with rubber bands and moved them around until it balanced at 25% MAC. Good thing too, because I needed to move the batteries back farther than anticipated.

I cut out the hatch and lined the edges with 1/16" balsa. Cutting a big hole in the monocoque fuselage weakens it at that point. It's stiff enough to handle torque loads but it's compressive strength is severely compromised. Think of the cardboard tube from a roll of paper towels. As long as the tube is intact you can push on the ends and it's very strong. If you cut a large hole in the middle and pushed on the ends the sides of the hole would bend outward and the tube would collapse. Same thing going on here. The solution is to add some structure with compressive strength to carry the loads past the opening to the rest of the fuselage. The wing saddle performs that function for the wing opening and for the battery hatch I used bamboo skewers.

I cut through the paper lining of the inside of the fuselage around the hatch and carefully peeled the paper away. Then I glued in pieces of birch veneer to the sides that run from the firewall to the plywood front LG mount. The veneer does two things; the edges stick up above the cutout to align the sides of the hatch and it provides a large glue area to the sides of the fuselage. Then I glued a pair of bamboo skewers to the veneer. They also connect the firewall and the LG mount.
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Old Jul 05, 2011, 10:58 AM
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I made the lower rudder removable. There are a pair of 1/16" plywood plates, one attached to the fuselage and the other to the rudder. Magnets glued into both will hold the rudder on and pins will keep it straight. The hope is that the rudder will pop off on landing with no damage. The rudder is a hot wire cut core covered with birch veneer. It's really tough.

The plates are secured with bamboo dowels glued deep into the foam. I thought I'd include a shot of how I size the bamboo to get a tight fit in the holes in the plywood plates. I drilled a hole in a piece of steel slightly smaller than the dowel. Then I drive the dowel through the hole in the steel. The excess gets trimmed off and the dowel will now fit tightly into a hole drilled with the same bit.
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Old Jul 18, 2011, 10:08 AM
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Cowl.

Finally got the cowl made. I'd been putting it off because I wasn't sure how I wanted to do it. The trouble with the Do 335 cowl is that it's larger in the middle than at the ends. I knew that I wanted to make at least 2 copies, one for the prototype and one for the "scale" version later. I could use the lost foam method but the mold would be destroyed getting the cowl out. I could do a 2 part mold but that's a lot of work. I like shrunken pop bottles for cowls because they are tough, fast, and cheap but I had to make a special mold.

The mold is made from 5 pieces: Two thick outer slabs of wood and a thinner center section made from three wedge shaped pieces. The block was assembled and held together with screws. The shape was turned on a lathe. The bottle was shrunk over the form, trimmed, and then the screws were taken out. The middle section of the center pieces was tapped out and the other two pieces of the center section were removed. That made room for the two thicker sections to be pulled out. The mold was reassembled and I pulled another cowl for later use. It worked great.

Getting the front edge of the cowl to curl in is always a problem because the bottle won't shrink into a depression. Once the bottle is shrunk down around leading edge of the cowl, I applied pressure with the lathe's tail stock to push the plastic into the undercut cowl opening (you could do the same thing on a drill press if you don't have a lathe).

A plywood ring was glued into the cowl with canopy glue and the cowl secured with screws.
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Old Jul 18, 2011, 10:43 AM
Yeeah buddy!
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Northern Colorado
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Great job on the cowling!
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Old Jul 19, 2011, 10:06 AM
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Rear spinner.

I looked around for suitable spinners and found nothing for the front. For the rear however I found something close:http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXBFAP I hate making spinners so I went with it. I needed to make a suitable prop adapter. Most 3.17mm adapters are too short to mount a spinner but this spinner: http://www3.towerhobbies.com/cgi-bin...?&I=LXSUA5&P=7 has a nice deep adapter. But it comes in limited sizes so I decided to graft it's adapter onto the larger spinner.

I turned off the flange of the Dubro spinner back and trued it up. Then I turned a matching hole in the backplate of the Thunder Tiger spinner. I glued the Dubro collet adapter into the larger spinner's backplate to make the hybrid spinner. I'll probably add a couple pins or small screws to make sure it doesn't come loose.

The hybrid spinner runs almost perfectly. The gap between the fuselage and the spinner is greater that I'd like but I can fix that later. Other than the tip being too pointy, it's a good match to the scale spinner and it follows the lines of the plane. The front is a different story. I'll probably just go with whatever I can find at the LHS, at least for the maiden.
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Old Jul 22, 2011, 11:18 AM
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Ready to maiden...almost.

I was hoping to maiden the Do335 today but ran into a problem setting the CG. Darn thing came out nose heavy with my intended batteries. I had hoped to use a pair of 20C 2200 mAh batteries but I can't place them back far enough without some major surgery. I can get it to balance with a single 2200 mAh against the firewall and the lower rudder and rear spinner removed but my estimated 35A static draw is a little too high for a single battery. I have a heavier motor I could bolt into the rear but I hate to increase total weight for the maiden. I have a pair of 1800s and it will balance with those but the connectors don't match so I'd have to get new connectors and solder them in. I could order some 3000s or 3600s which would also need new connectors. (that would get me under my 60 oz goal)

I am bummed out. We have very light winds today. I suppose a 2 or 3 minute hop with a single 2200 might work...........

Specs with (2) 2200mAH

Weight: 60.7 oz
Wing loading 17.6 oz/sqft
Static thrust >1:1

PS. I know the camouflage colors are messed up. I'm color blind and I normally let my wife pick greens and browns for me but she was out of town. She just gave me one of those "looks" when she got back and I asked her how close I got.........
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Old Jul 22, 2011, 11:22 AM
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Great job Pat! Food luck on the maiden.

J
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Old Jul 23, 2011, 04:19 AM
Wilde Sau
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Germany
Joined Sep 2009
3,604 Posts
Very nice build!

Your CG outcome helps a lot with my new project: converting a small foam Guanli/Lanyu Do335 to brushless. I was also concerned about the CG as I want to use two similar motors for front and back, but reading now that your's came out nose heavy, I'll go for it...

LOL the paintjob looks a little bit like the one that comes stock on that Guanli bird :P
The light grey-green on your bird should be more dark, like a saturated a mid-green (RLM70 or RLM 82).
The dark green areas on your paintjob should be more like a dark brown-olive or black-green (RLM 71 or 81)
Can't see the underside, it should be light blue with a tint of grey (RLM65)
RLM 70goes together with RLM71, RLM81 goes together with RLM82. Both combinations used RLM65 for the belly.

Use this for color reference:
http://www.cybermodeler.com/color/rlm_matrix.shtml

Maybe this also helps:
http://anonymous-generaltopics.blogs...er-do-355.html
And this:
http://www.cybermodeler.com/aircraft...335_walk.shtml

Best wishes for your maiden,
Bernhard
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