postage stamp mounting

I'm going to try to make a two-piece mount on a CNC router. The biggest obstacle will be the air-intake clearance when mounted.. and using a strong-lightweight material. I'm thinking about making "twin"; either PVC or acrylic "ears" that mount to the backplate spacing holes; either side of the NVA/air-intake and extend outward approx. 5/16" with mounting-holes to accept #3-4 wood screws. The 2-56 crankcase screws may have to be custom-cut to length using a Dremel-tool.. but this will depend on how deep I countersink the holes for the screw-heads. I'll try to design the mounts to accept standard-length Cox screws if I can.

I recently got called back to work at the company I worked at several years ago; where I was able to design several custom Cox engine mounting-plate configurations; as well as some nice precision parts custom made for my models. They even have all my old CAD design files saved on their server.

Thanks guys.. It's good to be back to work.. especially in a familiar surrounding with people I worked with several years ago. I just took some measurements off a 290 backplate that I have.. and am trying to figure out the best way to design a simple lightweight mount. I have sheet PVC in several thicknesses, and  I think I can make up a couple of mounts in different configurations to try. The Bee's crankcase screw pattern is 17/32" square on center. I'm shooting for a one-piece mount drilled with that hole pattern, that "keys" into the backplate with a central bore for the venturi protrusion, a slot for the spraybar.. and "ears" for mounting radially to a firewall. Maybe I'll put a radius-cut on the backside for breathing. The 10mil (3/8" nominal) thickness material should work. We'll see.

Well.. here's my 1st attempts at a one-piece 10mil PVC mount for the Cox 290 back-plate. It will be refined further.. but I wanted some opinions. I'm using a CNC table router to cut the material.. and all the work that you see in these photos is on the front side of the mount. I will make a jig-block that the mount will fit snugly in, for counter-boring the case holes on the backside.. as well as a radiussed slot for the air intake between the firewall.






The outer "firewall" mounting hole counter-bores will be opened up a little more.. probably to clear the head of a #4 socket-head wood screw.


This version (below) has a .500" through-hole that would allow for breathing through the slot for the NVA/fuel-line.. rather than a radiussed breather slot on the backside.


I did this today on my lunch hour. Later versions will be cleaned-up a lot better with rounded "inlay" corners. The finished parts will also be sanded to remove the "fuzz" that you see here.

I'm also going to try a 2-piece mount.. which might provide for more breathing; without having to mill the backside. So far; this mount design caters to running the NVA/fuel-line horizontally on the airplane, which could have a fairly narrow firewall of only .750" wide by 1.250" vertically.

I will be making some cool new wheels too.

getback wrote:Nice work Roddie , is the idea to counter sink the holes in the mount and use longer screws to sandwich it all to the engine so it will have a mount on a mount as to say   ?If so I would consider the length of the 8cc tank bee style screws that way they will be ready available.  Are you going to produce some of these for sell ? Getback  

Thank you Eric, I have both 8cc and 5cc tank screws in my parts stock. I'm thinking that the 5cc length screws will do the job. I'll check a couple of cases (for variances) by bottoming-out the screws through the postage-stamp.. and take some measurements to see how deep to counter-bore my plate.. and add maybe 3/32" for clearance. I also may source some custom-length socket-head 2-56 machine screws for this application. If all goes together well.. I'll either sell some with the custom screws.. or specify the Cox OEM ones to use.

Andy, I'm working on making the mount as light as possible.. while still having it be reasonably strong. I really like the idea behind the "290 special's" 2-piece metal mount. This would be VERY easy for me to make. I would design it differently though. I would form a perpendicular "Z" which could be mounted to a conventional plywood firewall. If I make the plates from .040" aluminum.. they'll be super lightweight. This may actually be the better way to go, because it would allow for very easy fuel-line changes and ample breathing. I already have the case-hole spacing measurements saved in a CAD file.. and just need to design a 4-hole plate with scores for bending... and space the firewall screw-holes to be easily accessible with a screwdriver.. with any cylinder orientation. Four standard Cox "prod." back-plate screws would secure the 2 mounts through the back-plate to the case... and mounting to the firewall would be the builder's-choice for whatever #4 screws they wish to use.

Well.. I liked the idea behind the "290 Special's" 2-piece mount shown in the drawings earlier in this post.. so I cut some at work today. They're a little "crude".. but are really easy to make. The important things are the hole-spacing, plate width/length and a "bending jig" (which I haven't made yet..) which sets the thrust angle.

In case you don't know.. the 2-56 screws that mount this "postage stamp" back-plate to the crankcase are .300" (or 7.6mm) in length, measured from under the head. These mounts shown, are made from .040" (1mm) sheet aluminum.




I had to use a rat-tail file for venturi clearance.. but will add a radius-cut in the CAD file. Incidentally.. this back-plate's venturi measures .173" (4.4mm) in diameter.




A pair of these weighs 2 grams... a lot lighter than the airplanes that the "290" was mounted in...

Next week at work.. I will make a bending jig from a PVC block with a "slot" cut to the correct depth. This will make the 90 degree "Z" bends in the two plates uniform in length. The plates are also much longer overall, than they need to be... and I will shorten them up so that after they're bent; the venturi will "just clear" the firewall.

I'll post more photos of my progress next week. I'm sure that there are plenty of you who have the "290" backplate (that haven't cannibalized it for it's fine-thread NVA)  ... that would enjoy running it on a model.

Design consideration/question; Do you feel it would be advantageous to have the mount's hole pattern at the firewall, "match" the pattern of a "Cast" .049 backplate (mounted vertically/"needle-up") That spacing is .750" wide.. and 1.050" vertically (on center). Some kits come (or came) with plywood firewall stock that was pre-punched with that pattern for the Cox Bee's. The postage-stamp backplate, I am orienting "horizontally" which is the way I would think most people would want to run it for using an external fuel tank or bladder.

I often design my own airplanes, so I will be making "some" of these mounts similar to what's shown on the "290 Special" drawing that is pictured earlier in this thread, with a hardwood block that accepts screws from the top and bottom. (i.e. only a single bend in the mounts, at the backplate) This makes for an extra streamlined frontal area.

JPvelo wrote:Roddie,
Ideally you could make two versions, one that uses the tanked bee type mount pattern and one that uses the horseshoe pattern. If your only going with one I would say make the bee type since it is the more common pattern.

Jim

Yea.. I agree Jim. If I were making these for myself.. I wouldn't change much of what I already have.. but other people may be interested.. and it may make mounting on a model that previously had another engine difficult. I'll have to re-design the plates "trapezoidal" to get the .750" hole spacing clearance on the firewall side.

sort of like this

It's not necessary that the 290 backplate be cut .750" square with a razor saw as shown on this drawing, to use the mounts shown. It would be a lot of work, and the metal-plate mounts will fit in-between the molded "ears" on the backplate if they're cut to the correct width. Whereas this model is a "racer".. it's done for maximum streamlining. The wheels shown here would induce much more drag than a stock un-modified backplate ever would.

I shortened-up the 2-piece mount plates for the 290 backplate by .375" and once they're bent-up; they should locate the backplate snugly against a firewall, with enough clearance for breathing and access to R & R a fuel line if needed. I should have modified the CAD program to cut-in the radiuses to clear the protruding venturi.. but that can be done with subsequent runs.. if enough modelers have interest in these. A few swipes with a rat-tail file works fine for the ones shown here.  




These "shorter" plates don't weigh much...



and a "290" backplate weighs half of what the "horseshoe" does...




Still less weight than the "horseshoe" with both plates.



sometimes "every gram" makes a difference.. when balancing/trimming a competition model.

Think about this when you consider removing that nice fine-thread NVA.. from the "290".. for use in the heavier "horseshoe".