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Old Jul 26, 2011, 01:05 PM
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United States, WI, Fond du Lac
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Is this dumb?

I thought I'd throw this out for a group opinion. I need to somehow stuff more or a larger battery in this thing. 3 minutes at cruising speed won't cut it. The trouble is more batteries means I need to do some surgery to move them back to around the front of the cockpit to balance properly. The trouble is that would prevent adding fixed landing gear. The way I did the front gear mount there just isn't room for a battery above it.

So I was thinking about adding a second battery hatch behind the cockpit and placing the second battery above the CG. The way everything is wired, it'll be easy to add the wiring. Adding weight at the CG shouldn't change the pitching moment much either. Is this a good idea? Am I missing something?
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Old Jul 26, 2011, 03:56 PM
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I see no problem at all with that Pat. As long as it at the CG you won't have any more balancing problems( until you add gear later).
That was a problem I had with mine. By the time I got to the balance point, the batt needed to go way up front and with the retracts in there..... no room. I ordered a special 3s-5000 batt(needed the weight up front) and it just barely fit in there.

J
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Old Jul 26, 2011, 06:03 PM
Wilde Sau
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Pat,

that should work pretty well if there is enough space!
Did you get those colors yet?

Cheers,
Bernhard
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Old Jul 26, 2011, 11:29 PM
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Pat,

that should work pretty well if there is enough space!
Did you get those colors yet?

Cheers,
Bernhard
Plenty of room. I've already made the cuts! The servo tray serves as the battery compartment floor and I had made it from 1/2 foamboard and left the paper on so it's strong enough. I had to remount the servos but that wasn't too bad. I may remount the rear esc to have a shorter wire runs and so I can easily check it's temperature.

As for the colors, I stopped by the LHS to get a couple small bottles of hobby enamels so I could make some chips to bring with me to the paint store. They had three brands of the same RLM color and they all looked very different to me. I noticed the same thing with RLM chips from different sources on the computer screen but I expect that due to monitor and scanner calibration. So I'm still looking into the color thing. I need to get a spray gun so that's an issue too.

I'm working on the engine scoops and the mold for the exhaust pipes so I won't repaint until that's finished anyway. This is just the prototype. It's job it to make sure my plans and structure are sound before I build the "scale" version. The good news is that it flies well and I think that once I get the battery capacity to fly it faster, it's going to be a fun airplane to fly. And with the new second hatch it will be simple to add weight to see how heavy I can make the scale version before the flight performance degrades to the point where it's not worth doing. (Though J Morgan's came out heavier than mine and it seemed to fly well so I'm not too worried about that.)

Pat
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Old Jul 26, 2011, 11:59 PM
Wilde Sau
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I know, they all look different. That's why I gave you those RGB codes, they should be good... forget about the manufacturers RLM designation, they are pretty useless...
I am working on a color chart for RLM to find out the best manufacturer matches. But it isn't ready yet...

Cheers,
Bernhard
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Old Jul 27, 2011, 03:14 PM
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Pat, give this a look.
http://www.simmerspaintshop.com/page...s-Germany.html

david
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Old Jul 28, 2011, 01:59 PM
Wilde Sau
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David,

that is just one of several lists I had found on a summary site.
Out of that site, I compared those lists to the original paintchips and posted my best guesses for the right colors one page back

Cheers,
Bernhard
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Old Aug 02, 2011, 12:49 PM
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Silicone molding on the cheap!

I made the air scoops and the mold for the exhaust pipes. The air scoops were just shaped from foam. I'll finish them with several coats of Liquid Sheeting II and hollow them out after they've cured.

I decided to mold the exhaust pipes. I'll need 8 copies; 4 for the prototype and another set for version 2 so it's worth the effort. The plug is made from aspen. The grain oriented so it runs roughly parallel to the pipes. Angled cuts were made on a table saw with the blade tilted 5 degrees to get the front profile and the top view cut with a scroll saw. Then back to the table saw to cut the rest of the block. There is a 1/16" flange that will hold the pipes together and provide a way to glue the pipes to the inside fuselage walls. The pipes are long enough to accommodate the fuselage wall thickness and allow about an 1/8" to be cut off the end of each pipe. I hope to slosh cast hollow pipes and cut off the tips to provide a place for cooling air to exit.

I sanded the edges of each pipe round with strips of sand paper. Not counting drying times for primer and sealer, the pipe plug only took an hour or so shape.

To make the mold I first embedded the plug about half way into modeling clay. Then I boxed the plug with strips of wood hot glued together. Holes in the clay provide keys to eventually align the two halves of the mold.

The best way to have made the mold would have been to buy some proper 2 part liquid casting silicone. But the cost is pretty high. Just to get started you can easily spend over $50. There are brush on latex compounds but I didn't have very good luck with it trying to make a two part mold.

I made the mold from silicone caulk. Silicone caulk cures by exposure to moisture. Normally only the surface cures leaving a flexible, partially uncured center. The trick is to get moisture evenly distributed through the silicone so it cures throughout. That's where the corn starch comes in. Corn starch is hygroscopic and holds moisture that can be delivered to the silicone when mixed with it. You must use 100% silicone. Low odor formulations don't work. When you mix the corn starch in it should release a strong vinegar smell. The ratio of corn starch to silicone is not critical. I use 50:50 by volume. More corn starch makes a stiffer, faster curing, harder mix.

The silicone and corn starch are mixed together and then slathered over the mold. I start by applying it with a Popsicle stick to coat the plug and then add more to fill the mold. Dust the top with more corn starch to keep it from sticking (it's really sticky) and press down hard to remove any air bubbles next to the plug.

When the silicone has cured (about 45 minutes), flip the mold over and pull out the clay. Go round the mold with a toothpick to remove the last bits of clay. Silicone really doesn't adhere well to anything but itself. To be able to get the mold halves apart you have to coat the first half of the mold with some type of release agent. I used vasoline. I took a blob of vasoline and heated it with a heat gun to melt it. Then I used a cheap bristle brush to coat the mold. You don't need much, use a thin layer.

Then mix up more silicone and fill the top of the mold. This is more difficult because the silicone will slide around on the greased mold. Press down to eliminate bubbles and let it cure.

When it's cured, remove the box sides and split the mold. If the vasoline did it's job then the mold will split and the plug can be removed. Wash off the vasoline with detergent and water. You should let the mold cure until the vinegar smell is gone; epoxy won't cure properly if there's any acetic acid around (vinegar is effective at removing uncured epoxy from your skin). I was curious to see how well the mold turned out so I went ahead and cast some 5 minute epoxy anyway.

The mold worked well. There is a little flash at the seam of the two halves but not bad. The surface looks rough because the epoxy didn't cure properly because of the out gassing vinegar. In a few days the mold should be entirely cured and the vinegar smell will disappear. Then it will be ready to cast a several sets of pipes.
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Old Aug 03, 2011, 12:00 AM
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slick work!
david
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Old Aug 03, 2011, 03:55 AM
Wilde Sau
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Nice!!!

Quote:
Originally Posted by pmullen503 View Post
There is a little flash at the seam of the two halves but not bad.
No issue at all, both the exhaust pipes and turbocharger inlet scoops on the real thing had flash on the seems of the two halves from the welding...

However, the exhaust pipes were shorter/less protuding and had a distinguished oval shape, just like on the Bf109 for example.
I can scan a pic if you wanna change that, but no biggie anyway!
edit: on a second look, it seems you got the oval shape and then it also looks right with the overall shape and lenght/angle. The dark color made it hard to see!

Hope my Do335 will get in soon...
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Old Aug 03, 2011, 10:58 AM
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Originally Posted by Beltpilot View Post
Nice!!!



No issue at all, both the exhaust pipes and turbocharger inlet scoops on the real thing had flash on the seems of the two halves from the welding...

However, the exhaust pipes were shorter/less protuding and had a distinguished oval shape, just like on the Bf109 for example.
I can scan a pic if you wanna change that, but no biggie anyway!
edit: on a second look, it seems you got the oval shape and then it also looks right with the overall shape and lenght/angle. The dark color made it hard to see!

Hope my Do335 will get in soon...
The pipes look long because they have to protrude through the 1/4" fuselage wall (the flange will be glued to the inside surface) and I plan to cut off about 1/8" off the ends to open them up after casting. I tried to make the cross section a rectangle with rounded corners transitioning from horizontal at the engine to vertical at the outlet. There are plenty of photos of the engine on a stand in museums so I think I got a pretty good idea of the actual shape.

Pat
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Old Aug 03, 2011, 05:02 PM
Wilde Sau
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Germany
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Sounds reasonable!
Can't wait to see the finished result...

What did you use to cover your wooden template before you fabricated the mould? That brown color, I guess it is some sort of release agent?

Cheers,
Bernhard
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Old Aug 03, 2011, 05:31 PM
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Quote:
Originally Posted by Beltpilot View Post
Sounds reasonable!
Can't wait to see the finished result...

What did you use to cover your wooden template before you fabricated the mould? That brown color, I guess it is some sort of release agent?

Cheers,
Bernhard
The brown is just some primer to help me judge the shape and look for flaws. The plug also has several coats of clear acrylic to seal the wood and fill the grain. You do need to fill the wood grain or it will show in the finished product. Even my corn starch and caulk mix can reproduce very fine details (or flaws!).

You don't need any release agent on the plug itself. The silicone will not stick to it.

You need to use a release agent to prevent the silicone from sticking to itself when you make a 2 part mold.

Pat
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Old Aug 05, 2011, 04:07 PM
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To solve the nose heavy CG problem I added a second battery compartment behind the cockpit, above the CG. I had to rewire the power system and move the servos but it wasn't too bad. I also moved the ESC for the rear motor cutting about 6" off of it's run. The batteries are wired in parallel. While less convenient to change batteries, the two compartment solution does give me a lot of flexibility in terms of battery options. For version 2, I'll make single battery compartment under the cockpit.

Hope to fly it it today with the new configuration. With a pair of 2200 mAh batteries I'll have plenty of flight time and no worries about over taxing a single 2200 mAh battery.
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Old Aug 07, 2011, 11:41 AM
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Here's the first set of pipes. I taped the set to a piece of FFF to check how they'll look when mounted. I'm happy with the shape and spacing.

I tried to cast them hollow but enough epoxy pooled in the tips so that even though I cut about 1/8" off the ends it was still solid. I ran into problems with the epoxy beading up from surface tension on the mold surface. I had to coax the partially cured epoxy to coat the surface with a toothpick. Even if the tips were open, I'm sure the wall thickness would be uneven. From a few feet away it still looks pretty convincing. A bigger problem will be the thick fuselage wall visible through the cutout.

I got to fly the plane with a pair of batteries and really test it out. It flies great! It looks good and sounds cool! The rear motor was running hot so I stepped down from a 10x6 to a 9x6. With the extra 6 oz. of battery and smaller rear prop it no longer will go vertical, but it will hover, which is pretty amusing to see. The front motor and ESC come down cold so I'll try a larger prop up front next.

I am bummed out about losing the front hatch. I was diving to do some high speed runs and the hatch blew off. I guess there was enough pressure inside the fuselage to pop it off. I watched helplessly as it floated down into the alfalfa about 75 yards away. I spent a little while looking for but I knew it was hopeless. The alfalfa is about 18" high and if it fell down through the plants to the ground I couldn't see it even if I were standing over it. Undaunted, I taped a piece cardboard over the hatch and continued flight tests. I bent the rear motor shaft at one point but I happened to have a plane with the same motor (with a prop saver) with me so I swapped it out and continued flying. Without the rear spinner and just the upper fin it looks a lot like the proposed Do-435 (with a turbojet replacing the rear motor). V.3 might have be a fantasy scale Do-435.
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