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Dumas 1/2A Atlas van lines build

Old 10-21-2009, 05:42 PM
  #51  
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Default RE: Dumas 1/2A Atlas van lines build

I love the intake swirl left on top of the piston on that one.[8D]

I think Cox knew what they were doing with their porting. You don't see that kind of swirl happening in too many engines.

Each one is a little tornado.
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Old 10-22-2009, 09:52 PM
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Default RE: Dumas 1/2A Atlas van lines build


Heres the lastest pics of my 18" Dumas Atlas vanlines U-71.

What I've done so far was to make some cutouts in the bulkheads towards the transom in an attempt to lighten her up. Theres another bulkhead thats 1/4 inch in front of the transome that still needs to be cutout and also in the sponson stringer next to the engine well.

If anyone really wants to make a hobby out of this I would probably go ahead and agree with Mr Hydronuts reconmendation of replacing all of the woodwith a wood that is more functional. The mahagony wood that comes with these kits is not bad however it is very porus and dry which may be typhical of mahagony I'm not sure. My only concern is adding uneccessary weight when sealing the hull. I can only imagine the weight I've saved by making my cutouts as seen in the pics.

If you scratch build, I would say 3/32 balsa which is close to the same thickness of the main bulkheads and stringers in the kit.I would use 3/32 for the skins as well. The boat would be very light and surprisingly strong.....I would reconmend 1/32 ply laminated on the transom (outside) andrunning surfaces of the sponsons.....Ace brand or similar airplane mount for a Tee-Dee or Medallion .049....

Also, in the pics you may notice that I added a epoxy mixed w/micro balloons along the sponson bottom and along the sponson transoms, Those area are high stress areas and thats my way of ensuring no cracks form.....In addition I added some epoxy along the joint where the hull bottom and sponson stringers meet torwards the rear bulkheads since theres really nothing for the bottom to hang on to.
I'm waiting for my turnfin and rudder to arrive before I put the decks on because I want to install mini blind nuts for them....
Enjoy!
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Old 10-22-2009, 10:31 PM
  #53  
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Default RE: Dumas 1/2A Atlas van lines build

Hate to say it, but microballoons mixed with epoxy isn't a structural filler, it's a cosmetic filler. If you want to strengthen that area, you would be better served with straight epoxy, epoxy with silica fibers or epoxy and shredded fiberglass. I wouldn't build the boat out of balsa either. That will take just as much to seal as the mahogany and probably wont' hold up unless you glass the outside of the boat.
Old 10-22-2009, 11:15 PM
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Default RE: Dumas 1/2A Atlas van lines build

This boat was built out of 3/32 balsa, the transom and running surfaces of the sponson bottoms are laminated with 1/32 ply. It was built in spring of 2006 and ran8 cells with an Astroflight .05 swinging a X632
as its in the pics now it doesnt weigh much more than the 18" Atlas as it stands now....and its 22 inches X 10".

Now, if someone wishes to compete or if you want a stronger hull, then its a mere few ounces more to glass or laminate the entire craft with 1/32 ply...

Point I'm trying to make is you must have a balance of weight, strength and longevitiy....I want my boats to last more than a few race seasons so I build them in a certain way using material and techniques that worked for me before...
Balsa is not expensive. Its easily repairable...Easy to work with....
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Old 10-23-2009, 01:53 PM
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Default RE: Dumas 1/2A Atlas van lines build

RJ where did you get the Al's Hobby Shop sticker?? ...I buy everything there...best hobby shop in Chicagoland..probably in the Midwest
Old 10-23-2009, 02:20 PM
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Default RE: Dumas 1/2A Atlas van lines build

Hi!

I'm originally from Downers Grove. I started off back in the mid 80's with the Timberlake Amp eaters Fast electric boat club and I've always shopped at Al's....They've always been a good sponser of our IMPBA races. Theres that Old world barber shop a few stores down, best haircuts in town!
Elmhurst is a beautiful town! I miss it very much.
But your right, Al's is the best I've seen period...And i hope they're still doing well. I bought so much boat stuff from them over the years.
Moved to south Texas a few yrs back....
Not sure if they have any more decals, but I grabbed a handful back around '06....
Old 10-23-2009, 06:55 PM
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Default RE: Dumas 1/2A Atlas van lines build

I wonder about using silkspan and epoxy instead of glass with epoxy. You can get it in three weights. Probably would not add as much strength but still could add some, either way it would definetly help level out any coarse grain just as if you were using it building model airplanes that its designed for. It could probably be put down with a glue stick and left to shrink, then epoxied over. It would go straight through and seal. It could be a weight saver by doing this to prevent over saturation into the wood adding weight letting the silkspan act as the blotter.
Old 10-23-2009, 06:56 PM
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Default RE: Dumas 1/2A Atlas van lines build

I can agree with the strength/weight/longevity and balance, just not a fan of balsa on the water. You strengthened yours considerably with the 1/32 ply. I'd probably build it out of 1/32 ply with 1/8 ply transoms and 1/16 ply sponson insides, tunnel bottom and runners. The big difference is I run nitro in everything and you run electric in the boat you showed, meaning less vibration and stress. If it works for you, it's all good
Old 10-23-2009, 08:29 PM
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Default RE: Dumas 1/2A Atlas van lines build




Gotta remember, The boats I'm refering to are the Dumas 18" boats that are designed to run a Cox .049.

Obviously dont mount the engine to a balsa bulkhead, but 1/8 ply or some hardwood rail....

The only experience I have regarding using epoxy to strengthen areas is mixing with micro ballons, andusing light strips of fiberglass in high stress areas...Using fiberglass cloth in joints for example I learned when I built several Carl Goldberg R/C planes. Actually applying Ca over the cloth strips was stated in the instructions...Although in the case of a boat I would just use epoxy, thinking that it would flow into cracks creating a better bond...

I think my next build will be a BALSA version of the Dumas Miss Thriftway....Templates almost done!

Old 10-24-2009, 03:10 PM
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Default RE: Dumas 1/2A Atlas van lines build

My Cox micro servos arrived today. Man these things are tiny! They weigh only .06 grams and produce 9.9 oz. torque.

I went to work on my Atlas today and this is what she looks like so far. I had to sand the 1/4 x 3/8 x 3 1/2" balsa that forms the front of the fork down to match the bulkheads.
I wasnt crazy about how the sponson decks went on so I cemented 1/8 balsa strips along the stringers between the bulkheads to give the deck something to hang on to...

I'm still going to glue on some more 1/8 balsa stringers along the rear deck area. Now the rear deck of the boat is applied in three sections, top deck and two triangular strips that go along the sides of the deck that form that somewhat rounded look....The top deck is first, then the side pieces last, The side pieces may provide enough area for the deck, but we'll see...

As you can see in the pic, I made a mini sanding block made from 3/32 x 3/8 balsa strip....works great to sharpen up the sponsons edges and in that little tricky area towards the nose and pickle fork....

Now I'm setting the boat up for right hand turns, so the turn fin is going to be mounted on the right sponson, I'm guessing an angle of about4-6 drgrees pointing in, instead of mounting the fin vertically. And the rudder will be mounted on the right side of the transom...The servo will most likely be mounted standing up. Most likely I'l glue some scrap 1/16 ply accross the rails to mount the servo...The reciever will be stuck in a balloon (waterproof!) and I'l put a drop or so of silicone adhesive to secure it in place...I'd like to find a short base loaded anteana if I can find one...

I chose not to go for water cooling at this time....









Old 10-24-2009, 04:42 PM
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Default RE: Dumas 1/2A Atlas van lines build

R.J.,

The O.K. Cub engines are going to be the weakest of the engines you have lined up. On average they but out about half of the rated power for their size i.e. the OK Cub .049 has the power output of an .020 sized engine. Also is that a standard glow plug conversion head for a Cox engine I see in the one picture? While the thought is nice to be able to use a standard glow plug in the Cox engine they are real power robbers....you are looking at taking 1,500-2,000rpm out of the engine by doing so.

LAter,
Tim
Old 10-24-2009, 04:53 PM
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Default RE: Dumas 1/2A Atlas van lines build

[link=http://www.merlinglowplugs.biz/Aircraft.html]Merlin Glow Plugs[/link] to the rescue.

There's also other high performance current replacements for Cox glow heads such as Galbreath which takes a turbo plug.
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Old 10-24-2009, 05:29 PM
  #63  
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Default RE: Dumas 1/2A Atlas van lines build

Jetpack,

I love Merlin Glow Plugs.....that's why I am a dealer for them!!!! The Galbreath head is a great choice if not the #1 choice for an upgrade for a Cox head, note that the Galbreath head uses the the Nelson plug not the turbo style plug. I myself make the 1/2a TurboHead that takes the turbo style plug.

LAter,
Tim
Old 10-24-2009, 05:43 PM
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Default RE: Dumas 1/2A Atlas van lines build

i dont know why the pics comes out like that, so I deleted them and reposted them in an attempt to keep the thread clean...
Old 10-24-2009, 05:48 PM
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Default RE: Dumas 1/2A Atlas van lines build

I dont know why the pics come out so big.....
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Old 10-24-2009, 07:05 PM
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Default RE: Dumas 1/2A Atlas van lines build

Engine crazy has a good point. I dont use the plugs washer on the after market heads that use standard plugs. This drops the glow element closer to the piston head  keeping the volume down like the the 3 fined high compression heads cox sells. I ran a micro 25 watt rigger today about the output of a tee dee 010 im guessing.  I would love to swap out the brushless motor and use the cox 010 or the 020 would be a treat.
Old 10-24-2009, 08:55 PM
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Default RE: Dumas 1/2A Atlas van lines build

I really appreciate the info guys, thanks.

I thought I'd try the std glow plug head and experiment w/different plugs in an attempt to get more power/RPM's. The merlin site is pretty cool and probably the best choice for Cox.

The o.k. cub .049 was mentioned in the Miss Thriftway plans and I figured to give one a shot, hey it comes with a pull starter....

The engines I have in my collection are:
1. Cox Babe bee .049
3. medallion .049's
1.Tee-dee .051
and of course, 1 o.k. cub .049......

Is there some way to coax 30,000 RPM outa a Cox engine? mini turbo chargers maybe? Nitros Oxide? Estes model rocket engines for that last second dash for the finish line?




Old 10-24-2009, 09:46 PM
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Default RE: Dumas 1/2A Atlas van lines build

R.J.,

Sadly asking for 30,000rpm out of a Cox engine redardless of which model is just asking to much. For that kind of power you are looking at the Fora's and Cyclone 1/2a's. I get 23,000rpm out of my highly modified Skyline F/X .049's and that is about the limit for a Cox reed valve engine.
The problem with a standard glow plug in a 1/2a sized engine is the lose of compression ratio. With a standard plug you have to take into account the gain in chamber volume due to the larger element hole in the plug itself and because the plug seals at the top the volume you gain due to the threads on the plug this all take AWAY from the output of the engine. The Nelson and turbo style plugs have a tapered seat and seal at the bottom of the plug and depending on the brand have a small element hole which increases your compression ratio which of course equals more power. I have never seen a standard plug head that even breaks even with a regular low compression Cox glowhead.

LAter,
Tim
Old 10-25-2009, 06:49 PM
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Default RE: Dumas 1/2A Atlas van lines build

Today I glued on a 1/8 square balsa strip along bulkhead # 8 along the center to allow the deck some more surface area, other than that I mixed up a 1/2 ounce 30 min epoxy with 1/2 oz. 91% alcohol. I used a small modeling brush and brushed the insides and it was enough for the outside of the hull as well.

yeah the wood is somewhat dry however it did not soak thru to the other side, probably to the middle of the wood which is fine by me cause I guess I wont have to worry much about the wood being damaged by water.....

The 91% alcohol mixed well with the epoxy, it just turned into a watery syrup. With the alcohol, it allows the epoxy to soak into the wood/get into cracks, then it evaporates...So I added at least 1/2 ounces weight to the hull. But imagine if I didnt make those cutouts...

I'm still waiting for my running gear to arrive, apparently its coming straight from Hong Kong...One of them is the mini turn fin which is the reason why I havent installed the sponson decks yet cause I want to use mini blind nuts however I'm not sure of the thread size if the machine screws that come with the gear...so I gotta wait.

Heres some suggestions I reconmend for this boat.

I highly suggest using 1/8 square balsa strips to help give the sponson decks more support along the stringers.

1/8 balsaeven w/ thetopsof the engine well and even w/ the top of bulkhead #8

sponson doubler on the inside of the r/h sponson transom for the turn fin. (left sideif you want to run counter clockwise)

use 1/8 ply for the transom

bead of epoxy along where the hull bottom and stringers meet...and along the seam where the sponson bottom and stringers meet.

If you drill any holes (this probably should apply to any wood hull) what I do is drill a hole alittle bigger than what I need, then fill the edges/hole with epoxy, allow it to set for a day or so, then redrill the hole to the correct size that you want.....why? because I've seen alot of water damage occur to hulls over the yrs because people make holes and not seal the edges of those holes....most comon areas.....rudders and turn fin mounting holes....

Back in the 80's while racing I noticed some guys would ruin or at least damage the transoms of the sponson or rear transoms of boats because they beached the boats or hit someone, whatever, because they either didnt have a kickup rudder or used metal screws. metal screws break wood guys.....

What I've done was to always use metal blind nuts, but instead of using metal machine screws, I use nylon screws from airplanes....
The nylon screws are strong enough for normal running/racing, but will snap off in the event an object is struck.....I would face replacing a turn fin, instead of a transom....try it...


Old 10-26-2009, 02:01 AM
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Default RE: Dumas 1/2A Atlas van lines build

You wouldn't want my recommendations for that boat, the cost would triple before you got it built[X(]
Old 10-26-2009, 12:29 PM
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Default RE: Dumas 1/2A Atlas van lines build

Thats we enjoy our hobby so much....

My reconmendations were intended for average joe that only wishes to build his model without investing in the tools and materials needed to basically build something entirely from scratch....

Besides, I noticed a pic of your what, four or five hulls that havent been decked up yet posing in front of your car...Looked nice and i admire your passion and skill, but C'mon...
lets see them run and how long will that last in the real world of racing....

Boats this small ( remember they are 18 inches long) can be built up, entirely out of BALSA, glassed over or laminated with 1/32 or 1/64 ply and be strong, last long ( in years) and cost under 40 bux for the wood at Hobby Lobby....
Balsa can be trimed with an Xacto knife by average joe kid without having to invest in a scroll saw....

Old 10-26-2009, 08:09 PM
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Default RE: Dumas 1/2A Atlas van lines build

Actually, I've decked one of those boats, made hardware for that one and one more, started the frame up of a Sport 40 Whiplash and started reworking a Steve Muck glass hull. Not to worry, I'll have some on the water for next season. As for how long they'll last, as long as your balsa boats will. Mine are made from all aircraft grade birch plywood with spruce stringers. Here is a not so good picture of the decked boat. If you like, I can throw in a couple of pictures of some of the hardware I've made as well. I went ahead and added some hardware shots for the fun of it
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Old 10-27-2009, 07:23 AM
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Default RE: Dumas 1/2A Atlas van lines build


ORIGINAL: R.J. West

I'm trying to locate a replacement piston & cylinder for it.
Cox engines are being made again here is a link to the Australian dist
http://www.wightsmodelaircraft.com.au/index.html
They are cheap as are the parts 12.95au for piston barrel and head
wish my OS parts where that cheap
cheers
Wozza

Old 10-27-2009, 12:33 PM
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Default RE: Dumas 1/2A Atlas van lines build

Wozza,

Cox engines are NOT being made again. Rather that Estes(who owned Cox) finally sold the remaing stock that was sitting in their warehouse and those are the parts you are seeing being sold now.

LAter,
Tim
Old 10-27-2009, 12:41 PM
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Default RE: Dumas 1/2A Atlas van lines build

ORIGINAL: Tim Wiltse-RCU

Wozza,

Cox engines are NOT being made again. Rather that Estes(who owned Cox) finally sold the remaing stock that was sitting in their warehouse and those are the parts you are seeing being sold now.

LAter,
Tim
Hi Tim
I was told they are being remade but the info could be wrong. If you have a look at the these http://www.wightsmodelaircraft.com.au/wmaBombers.html
a hot pink motor poor old Mr Cox would have a hart attack if he knew
Wozza

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