Scratch build BRM - a first

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

1/2-3/4 oz wedge?

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!

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.

Yes.. I remember now. Good deal.. and either one should haul that model no problem! The only "older" horseshoe backplate in my collection is a white one with an aluminum needle..




I have no idea where it came from.. probably was thrown-in when I bought some other hobby stuff years ago.