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Scratch build BRM - a first

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Scratch build BRM - a first

Post  wha-tah-hey on Thu Feb 16, 2017 10:50 pm

I built a Bamboozle in '60, a Li'l Satan in '68 and the Nats Memorial Flite Streak in '97.
I decided I had to start somewhere if I was ever going to build planes, so for my first scratch build, I've taken elements from a couple of online plans for the Baby Ringmaster to come up with my own, 23.5" ws.
The components (basswood LE and center):


The sub-assemblies glued up, shaping done:


Tomorrow, dope and tissue base coats.

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Re: Scratch build BRM - a first

Post  rsv1cox on Fri Feb 17, 2017 7:19 am

Looking great Terry, your own design. Postage stamp Cox, where are you going to locate the fuel tank? Smile

Bob

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Re: Scratch build BRM - a first

Post  wha-tah-hey on Fri Feb 17, 2017 8:19 am

Fuel tank will go under the OB wing, Bob.

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Re: Scratch build BRM - a first

Post  rsv1cox on Fri Feb 17, 2017 8:23 am

wha-tah-hey wrote:Fuel tank will go under the OB wing, Bob.

1/2-3/4 oz wedge?

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Re: Scratch build BRM - a first

Post  wha-tah-hey on Fri Feb 17, 2017 8:47 am

Old wedgie, Bob, not sure the size. Needs cleaning, then I can see.

Edit: turns out it's ~5cc

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Re: Scratch build BRM - a first

Post  Cribbs74 on Fri Feb 17, 2017 9:51 am

Looks good. That tank is only going to last about a minute...

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Re: Scratch build BRM - a first

Post  wha-tah-hey on Fri Feb 17, 2017 10:12 am

Yeah, I thought it might do 'til I measured it, Ron.
I found a 1/2 oz wedgie in my parts box that'll work.

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Re: Scratch build BRM - a first

Post  getback on Fri Feb 17, 2017 11:57 am

Looking GOOD Terry !! Coming together quickly , bee watching the progress .. Popcorn

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Re: Scratch build BRM - a first

Post  wha-tah-hey on Fri Feb 17, 2017 12:40 pm

Thanks, Eric.
Got the LG, wheels (spare Kiel Kraft) worked out.
Still hafta make up some nose blocks.
Raw component wt, everything except LO & pushrod is 4.6 oz.


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Re: Scratch build BRM - a first

Post  roddie on Fri Feb 17, 2017 11:18 pm

I've got a couple of those little tanks... cute little buggers!



Balloon-tanks are something you might want to try.. Not a bladder.. but an airtight "collapsing" vessel that needs no vent.. and isn't particularly sensitive to location (elevation). The pick-up tube usually has multiple "notches" cut into it, to assure flow if a passage becomes blocked by the collapsing balloon.




I like the looks of the model. My 24" span designs fall close to that weight Terry. About 4.5/5oz. Keep us posted on the progress!

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Re: Scratch build BRM - a first

Post  GallopingGhostler on Sat Feb 18, 2017 5:41 am

A 1/8 oz. tank is 3.7 cc, a little less than 3/4 of a Babe Bee's 5 cc tank capacity. Might be a choice for free flight or .020's. A 1/4 oz. tank is 7.4 cc, a touch less than a Black Widow's 8 cc tank. 1/3 oz. is 9.9 cc. 1/4 or 1/3 oz. would be more suitable.

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Re: Scratch build BRM - a first

Post  wha-tah-hey on Sat Feb 25, 2017 11:30 pm

I have the initial nitrate coats and tissue on, waiting for Sig Lite-Cote to get here.
I bought decal paper and printed some miniature BOTR decals for the rudder.


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Re: Scratch build BRM - a first

Post  Oldenginerod Yesterday at 1:28 am

Terry. You refer to this as a Baby Ringmaster, but I'm pretty certain the "Baby" has a built up wing and the slab wing version is known as a "Beginners" Ringmaster. Anyway, it's scratch built so I suppose you can call it whatever you like.

Rod.

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Re: Scratch build BRM - a first

Post  wha-tah-hey Yesterday at 8:04 am

Right you are, Rod.
I call this one the BRM Special.

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Re: Scratch build BRM - a first

Post  NEW222 Yesterday at 4:17 pm

Looking good. Should be a good flier. I really like the decals you produced.

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Re: Scratch build BRM - a first

Post  Canso07 Yesterday at 8:34 pm

Yes those are really nice decals! I tried about a week ago to joing there group but never heard anything back from them.

Canso.

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Re: Scratch build BRM - a first

Post  wha-tah-hey Yesterday at 8:45 pm

Thanks, guys.

Canso - seems it took a while to hear from them when I joined a few months back.
Not sure, but I may have had to contact admin.

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Re: Scratch build BRM - a first

Post  roddie Yesterday at 8:53 pm

Hi Terry! Those decals look GREAT! They'll add a nice touch to your custom-model! I have a question regarding your current engine configuration. Do you know the origins of that red horseshoe-style backplate? I'm assuming that it's a stock/ARF product-engine type backplate. They run just fine with an external-tank of your choice. The earlier Cox designs used a circlip-style reed-retainer (same as the tanked-Bee engines).. and in later years; was redesigned without the wire circlip.. for a "cap-style" retainer with a thin flange that mated with a case-gasket. You may want to experiment with different reeds for proper function. There's a design "cross-over" point; where Cox switched from their standard .001" copper-reed.. to the .005" Mylar. Cox did this before they redesigned the retainer-system. This is "my own" observation.. and subject to speculation.. but test your reed-function prior to test-running your engine. If your backplate uses a wire-retainer.. it may require the .001" Copper "star-shape" reed. I have a backplate that had this arrangement. It's my assumption.. that Cox switched to a lower-grade plastic at some point.. which would not stand-up to the pressure of the circlip bearing on the thin molded area which retained it.

Take a look at this backplate.. I have a couple more with the same condition.



Your backplate may not exhibit this issue. The very early Cox 190/290 "Postage-Stamp" backplates are made from a very dense-plastic that doesn't seem to degrade this way. I have custom-mounts that I made for those.. just in case you have any that you'd like to resurrect! The "posties" have a fine-thread (128 TPI) needle valve.. which is very nice! If you ever want to experiment with a pressure-bladder feed.. you'll want to use one of those backplates.



Needle comparison.. 80TPI-left (Cox later standard-issue) 128TPI-right (early Postage-stamp issue)



Just a little food for thought.

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Re: Scratch build BRM - a first

Post  wha-tah-hey Yesterday at 9:27 pm

The backplate came on one of a "twofer" on eBay - $22.50 total, Roddie, both with the fine thread, mylar reed/wire clip - hadda thin the clip a bit to get it to seal properly.
It turns a 5-4 TD @ 17.3K on Cox 15%, which I think isn't too shabby.
The 2nd has a slit cylinder and it's down ~1.5K, same prop/fuel.

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