Prototype MKII

Normally, when I get one of my silly ideas I will throw together a quick proof of concept of whatever it was I had thought up.  Once I prove my idea works, I usually just throw it on the shelf and move on to the next hair-brained idea that I thought up in the shower.

However, when I took my inline twin proof-of-concept out to the field to test fly it, one of the club members went gaga over it.  That was about six months ago, and he can’t stop talking about it.  So, since I was building seven Skyray kits for the club fly-in, I figured I’d build a second prototype, trying out some new ideas, and put it on a plane for him.

Since the first inline was built using a hen’s tooth Avion reed valve assembly that I’ll probably never find again, I knew I’d have to find a way to make it with parts I have on hand.  So, I started with a 290 engine with the red postage stamp backplate.  First, since it has the same piston/cylinder/crankcase as a Black Widow, b) since it would be the easiest to modify, and 3: because it has a fine needle valve so I could use a bladder with it, as the club member also wants to try a bladder fuel system.  I started with the beat up backplate, and cut away the housing until there was just a thin wall to sandwich between cases.  







The first test case used a Carl Goldberg postage stamp mount, but I wanted to see if I could cut down on the length, so I got a piece of 1/8” aluminum to use for a backplate.  I also wanted to try something on the air intake.  The normal tanked Bee backplate has that slot on the bottom outside for air to get to the screened intake.  I thought I’d do the same, but not on the bottom where it is most likely to get dirt sucked up.  I figured the cleanest area to suck air would be to the inside of the circle.  I also wanted to be able to mount this in the normal mounting holes for a tanked Bee engine, so that’s what I used for the dimensions.




After marking it all off, I used a Dremel tool, hand drill, and needle files to make the backplate.  As you can see, a couple of the holes are not drilled too accurate.  I really need to set up my work bench so I can use my drill press and scroll saw.





After measuring the assembly front to back, it worked out that a 1” countersunk 2-56 machine screw would hold everything together.  A quick trip to Ace Hardware for 8 screws (so I could backfit my first test) and it was time to put it all together.




A good clean and oiling and it all went together with a new crankcase gasket and  mylar reed.






And mounted on the Skyray that I built for it.







The only thing left is to test run it to see where the needle setting needs to be before I can complete the engine.  I’m going to take it out to the field tomorrow and see if I can get it running.  And to show it to the club member and let him go nuts over his own Cox twin engine.

The Repeat Mark