I know this forum is "officially" about 1/2a building, but I still think it should be a good place for some 1/2 a tips.

I think some of you know that last weekend we had our annual "This is only a hobby" 1/2 a event (Jim Renkar CD), and one of the events we have is a 1/2A "Coxy Hazel" 100 lap race. I think Fred K. posted a link to the flyer on our rules, but briefly the event is limited to the Sig 1/2A Skyray and a Cox reed valve engine with the integral tank like the one that came with the Black Widow engine. This is a 100 lap race with required 2 pits. We use Sig 15% Champion fuel (the 50-50 mix of castor and synthetic). We fly with contest supplied 35 foot 0.012 lines (for ruggedness).

The winner was Bill Smith who posts here, who by far and away, had the best performance of anyone. What really impressed me was that he was both fast and was getting about 40-44 laps per pit. I think he was flying with a Tornado white 5-4 rubber ducky prop (Bill if you are there, correct me if I a wrong). His flight time was about 5m 40s, about a minute 20s faster than the next pilot. Oh I forgot to mention, you are forced to hand prop the engine to start it--no springs or electric starters

So my question for Cox experts is what do you think contributes to such performance? The engine is obviously singing, but also is getting pretty good efficiency from the fuel load to get the 44 laps. I am sure the plane is pretty straight too. My own miserable performance gives less than 3 laps per tank.

I accept opinions too!   Thanks..

          I wonder what the venturi size is on these engines. They could all be different on every plane. Different reeds will also contribute to endurance and performance issues. You would also be quite surprised how many of these engines leak fuel through the screws in the backplate. I've had metal tank backs leak as well as plastic ones. The tank to tank back could be leaking a little as well. This can cause performance to lack and shortened runs. The other possibility for those that are getting shortened runs are that the pick up tube isn't in the optimum place and there is still a bit of fuel left in the tank. Some piston fits are also better than others which causes some engines to run really well while others are so so. Keep in mind if the venturi o-ring is not in so good of shape it will cause poor running characteristics. Many many factors are all possibilities here. I discovered all of these same problems while building a mouse racer. Ken

Also, the little black gasket that seals the venturi to the backplate is very important.  If you can't get a fresh one, they can be cut from fuel hose.

Paul Giabault puts a piece of fine thread inside the groove in the tank, and oils it before assembling the backplate.  He says that this is a cure for leaks there.

If you are only getting 3 laps, I bet your fuel pickup is in the wrong location, probably on the inside rather than the outside of the circle.  Of course there are a gazillion of other possible problems.  Tell us what prop, fuel, engine configuration, glowhead you are using, and we can certainly get you up to 4 laps!   

I meant to say I was getting somewhere between 20-28 laps (I missed  the "2" key when I typed!).

I have the pickup on the correct side, but I still wonder if it is in the best position to suck every last drop up. I am guessing somewhere near the forward location on the tank where it begins to neck in. Also a little on the low center side---this is because I assume the plane is probably slightly angled to outside of circle, and gravity is ~20-30% of the "centrifugal" force (need to actually recalculate that for 1/2a parametes.
Another issue is that you can get fuel foaming and that will cause fuel loss out the vents. Also fuel out the front bearing if it is too loose.

Also have all the gasket stuff ok. I did try a pressure check of the tank---closed off the needle valve and saw that the tank would hold some positive pressure for at least a short time.


Actually you can use any of the reed valve variants, but you do want something that will pit well, and at a minimum give you 34-45 laps per tank as well as being fast. You need a little more than the bare minimum to account for the backwards starts and racing "realities".

Keep the suggestions coming. Sorry for the dumb typo  .