Building Tips and technical articles. > 1/2 A building.

Half A at Brodak's - Rules discussion

<< < (5/5)

Chuck Matheny:
The most powerful Cox .049 of any type that I have ever  handled was a Killer Bee.
Some people think that the ability of the reed to flutter at 500 beats per second and to process  as much air/fuel  as a TD was the limiting factor....but it's not.

Dan McEntee:

--- Quote from: Chuck Matheny on September 08, 2024, 12:57:19 AM ---The most powerful Cox .049 of any type that I have ever  handled was a Killer Bee.
Some people think that the ability of the reed to flutter at 500 beats per second and to process  as much air/fuel  as a TD was the limiting factor....but it's not.

--- End quote ---

   Hi Again Chuck!!
      In addition to the Doodlebug model with the Norvel .061, I have been playing with a Brodak Baby Clown ARF that I started out with a Medallion .049 on the nose. I thought it was too much engine for the really light baby Clown and no room for a bigger tank to be able to do a complete AMA pattern, so dropped down to a Cox product engine. These are the last ones that they produced and were dumping on the market years ago with the choke tube and gray plastic back plate at 10 buck or so each. The cylinders are the same as the Killer Bee or TD, I think, and produce some pretty good power. I had the first one running between 14 and 15,000 RPM really easy on 15% nitro fuel, but that only lasted for 5 or 6 flights, then the run got very erratic. I pulled the  engine apart and the rod/piston ball joint was so loose that I thought it was almost ready to come apart. I swaged the piston cup back down to a nice fit, just a but of end play and turned easy, but when installed, it kind of bumped through bottom dead center and would not run very well at all. Thinking that given the engine was from a batch being dumped on the market that there may be defective parts in them,  I grabbed another one and gave it a more thorough break in program, and then installed it on the airplane, and this engine went a few more runs than the first one before the run went south. I pulled this engine apart and the rod/piston joint was OK, so I looked at the reed, which was the stainless reed, and it was visibly deformed like it was shaping itself to the seating ring of the venturi. These are only .002" thick, so I tried a beryllium copper reed. This only ran twice, and not up to par, so i pulled it down again and it was deformed the same as the stainless reed. next in line was a Teflon reed, and it ran for 5 or 6 runs as I was increasing the RPM each time and when I went over 15,000 RPM it started to run rough. I pulled it down, and the reed wasn't damaged in any way, but seemed fuel soaked, or transparent if that makes sense to you. I could literally see the venturi through it, and it is .010" thick! I had a reed I made from some plastic shim stock I have from my days as a maintenance tech at a printing plant, so i installed that. That material is some pretty tough stuff, is color coded for thickness, is made form a directional polyester material,  and comes on .001" increments. I made one that is .005" thick as that may be the optimal thickness but have not tried it out yet. I had to rebuild the nose of the fuselage, as it was starting to get fuel soaked and mushy, so I did that and have the model ready to fly again, just waiting for the opportunity. I have heard about "reed float" or maybe what you call "flutter" but I don't think I have ever experienced it before, but maybe I have now?? It seems that issues come about after I hit the 15,000 RPM range, and reed thickness and material may be critical here above that. I am using the standard glow head, and 15% nitro fuel that is more or less like the SIG 20% total oil Champion mixes. I will be continuing my testing, and may even try an aluminum reed. I don't think any one has tried that before. Soda can stock is usually about .004" thick and is a stiff, harder aluminum. I also have some printing plate aluminum that is .004", .005"  and .008" thickness, just have to sit down and cut the reeds. In my mind, I'm not looking for the same speed as a mouse racer or anything speed related, just a strong, steady RPM to be able to fly the full AMA pattern, and I'm close, but the reed seems to be the limiting factor at this point. I would just like to be able to fly the model for 20 flights or more, like a normal C/L stunt model, before anything had to be done on the engine maintenance wise. Flying in some 1/2A stunt contests is the goal, and for it to be a reliable sport flying model also, just lie any other stunt model I own and fly.   What do you think about the aluminum reed possibility? I think I have read where in some of the tach races that the CEF guys have run, they are going way more RPM than I am running, like in the 18,000 to 20,000 range, don't they??

   Type at you later,
  Dan McEntee

Navigation

[0] Message Index

[*] Previous page