I finally got the opportunity to get some flights in last week to test some things. Mother Nature has really been mad at us around here!! Always cold and windy!! We got a one day reprieve last week so took advantage of it. The gap between the back of the engine and the tank is about an inch, so I just couldn't see how the engine might be heating up the tank on this airplane. With the engine running, I can put my finger between the cylinder and tank and not sense any excessive heat. Got two flights as it was, just to get things set for the day, and still getting the exact same variation in the run that I had been getting. These two flight I paid closer attention to really listening to the engine run and when doing the inverted laps, it sounded just a tad more rich that upright, so I raised the tank a bit more. This changed the ground setting for the needle and on the next flight there was great improvement!! What I had barely noticed in who knows how many flights before and made a tiny adjustment for made a big difference on this airplane and this engine. Several more flights confirmed things. Then I tried the restrictor in the uniflow vent. It was just a short length of 1/8" copper tube that I crimped one end down until the resulting opening was between 1/16" and maybe 3/32". I had never, ever had the need to try this on any airplane I have ever flown in my entire life, so I was very surprised when I started the engine and the RPM was lower than it had been and required a new needle setting. The resulting run during the rest of the day's flying was improved a bit more again! This made me wonder if the uniflow vent inside the tank might be a bit too close to the fuel feed pick up? The tank is a GRW 4 ounce uniflow tank that they called a "snub nose" or "stubby" tank, I believe. I have used these a lot and like them enough to have made scratch built copies of them in other capacities and always had great results. I have had this tank apart looking to solve this funny run symptom and added some more solder to the uniflow/pick up tube joint while I had the back cover off just because it looked a little thin. but didn't ever find anything at fault, and when flown some more, had not changed anything. I am suspecting that what I have been experiencing with this airplane/engine combination is that this engine might be really sensitive to tank location adjustments and uniflow vent placement. The center line of the tank is a bit more that the usual 1/2" higher than centerline that I usually set them for a Fox .35. I have had other models with other engines that didn't seem to notice the tank slipping a bit because some rubber bands had let go, but this one is different for some reasons. I may have to attach some brass brackets to the tank and secure it at it's current location. The airplane is just an old, Craig's List rescued Ringmaster, original S-1 kit with original 2" bell crank, built with Ambroid, been rehabbed itself with taller control horn, repaired and recovered during my knee rehab, weighs 32 ounces, but I just love to fly it !! It was the first airplane I tried Spectra lines on , and that helped the airplane a lot also. So, there are some other reasons why an airplane can go rich in flight, I guess. If my tank was too low when sitting upright on the ground, and the needle set for that position, it would actually be a little lean for flight and it got the extra fuel head pressure from centrifugal force. The extra air in the uniflow vent was just confusing the issue until it got in the air and up to speed also. And some engine/tank combinations can be more or less sensitive to adjustment than others?? This will make me take a closer look at the rest of my fleet next time I fly them. Who knows when that will be? Mother Nature continues to be pissed off at us here!!
Type at you later,
Dan McEntee
Dan I don't think I'd worry about the proximity of the uniflow outlet to the fuel inlet. I solder them together within 3/16" of each other in my tanks and never fear air jumping over to the fuel pickup-ain't happanin'. The air coming out the line makes a B-Line to the air pocket instantly without thinking about crawling over laterally into the already full pick up. I do bevel the edge of the airflow tube away from the fuel pickup just in case. I keep them this close to get a clean run up and cutoff at the end of the run. When the uniflow line is suddenly exposed to the open air in the tank the engine goes lean for however many laps until the fuel is exhausted rather than a few second run up. The trick to a uniflow is keeping the end of the uniflow submerged and keeping the air in the tank at slightly greater pressure than open static air pressure. If you don't then you are just running a suction tank from another mother.......
Dave
Just thought-if anyone doubts this do this experiment. Tape two soda straws together. Submerge one end of straws in a glass of water. Blow into ONE straw and watch where the bubbles go.......and if you want have someone else suck the water through the other straw at the same time.
Recent Posts
