Cranking up a reed valve cox on bladder pressure.

NIce job, JP. Is it easy enough to dial it in a little rich but still keep it running while you go out to the lines for takeoff? When I do my reedy experiments I might try sharpening a needle(without shortening it) to get less sensitivity. I have an extra one I can mangle while trying without ruining my day. They only cost $5 at TT anyway. You might find every NVA has a different setting.

On My Tee Dees it shuts off at the end of a run before the last little bit of fuel is out, so it's still pumping it in as you land. This usually makes the next crank prime-free, as it comes home a little flooded already. Don't put so much fuel in it you get tired of flying. I remember my first bladder run, I put two ounces in and thought it was never going to run out. I usually run about 3/4 to 1 ounce for a good flight time.

I won't be able to fly it until tomorrow morning so I can't answer your first question yet. Hopefully I can set just right rich but the needle is super sensitive. like it says the needle is only open 1/4 turn.
Jim

Jim,

How much fuel did you put in for your test run? Reason I ask is more fuel=more pressure. That could be one reason why anything more than 1/4 turn floods it out. Just a thought, I am by no means an expert.

Ron

cribbs74 wrote:Jim,

How much fuel did you put in for your test run? Reason I ask is more fuel=more pressure. That could be one reason why anything more than 1/4 turn floods it out. Just a thought, I am by no means an expert.

Ron

3/4 of on ounce

Well, that blows that theory.........

Being a Combat pilot, bladders are my default fuel tank. I've run all sorts of Cox engines on bladders - Tee Dees of course (use the Kirn or Kustom K. fine thread NV, which I suspect is a Postage Stamp NVA grafted onto a housing) but also a Killer Bee (a real one Larry Renger sent me) and also various product engines. I never did have to go through changing needle valves on the product engines, it works well enough, it seems. I think it's the same fine thread needle valve as I got on the Killer Bee.

The T-junction and check valve would remove one major hassle when it comes to the product engine (or Killer Bee) - the fact that the fuel nipple is REALLY HARD TO GET TO. Ordinarily, you'd fill the bladder, clip off the fuel line then reconnect it to the needle valve. With the product engine, the nipple is recessed, so this is impractical. I used a separate in-line connector halfway down the fuel line to fuel up the model, while leaving a piece of tubing permanently attached to the nipple. What I've also seen done is to connect the check valve to the other (normally closed) end of the bladder, this makes more sense if the bladder isn't encased in the wing.

One thing you must absolutely do when running any sort of 1/2-A on a bladder is keep the fuel absolutely clean. Any dirt, or oil flocculation, will get caught in the needle valve. Particles that will normally pass through a needle valve when you're running on suction will clog it up when running on a bladder. With 1/2-As it's also much worse compared to running bladders on bigger engines. So filter the fuel, use a fine in-line filter between the syringe and bladder. Some put a filter between the tank and the needle.

Fuel consumption should be the same, given the same prop and fuel. Unless you drill out the venturi, of course (the main reason people went to pressure tanks was to run wide-open venturis). The benefits are obvious - the tanks are dirt cheap, you can fly ANY maneuver without starving the engine, and you can increase the power by opening up the venturi.

Iskandar

cribbs74 wrote:Jim,

How much fuel did you put in ... more fuel=more pressure.

Ron

No. Pressure bladders would be useless if that were true. The pressure is exactly the same from start to finish(unless you overfill or constrict it) or the needle setting would need constantly adjusting. It wouldn't work at all.

I guess I am confused then.

Seems to me the more you force the bladder to expand the more pressure it would exert on the fuel inside.

Although, more to your point it wouldn't keep the needle setting if that was the case.

Ron

where would I get a few of these fine needle valves and do I need them on tee dee's? Do they look different, could I look at them to know if I already have them?

I share Ron's confusion on the above point.

Need help understanding please.

duke.johnson wrote:where would I get a few of these fine needle valves ................

texastimers.com

Yep, that's where I get them, as well as the bladder tubing fittings that make using them more convenient. The Tee Dee must have a fine thread NV. You can look at the needles side by side and see that the 128 thread per inch needle is noticeably finer threaded.

cribbs74 wrote:I guess I am confused then.

Seems to me the more you force the bladder to expand the more pressure it would exert on the fuel inside.

Although, more to your point it wouldn't keep the needle setting if that was the case.

Ron

The bladder only expands to a fixed diameter. At this point the pressure on its walls is fixed. As you pump more fuel into it, it does not expand any more, it only elongates, so the PSI on the wall is constant no matter how much fuel you add after it reached its maximum diameter. Do the calculus!

To further address the confusion regarding the pressure versus fuel quantity. As an example, it becomes more clear to think of an extreme case: if you cut a piece of bladder tubing a foot long and pumped a gallon of fuel into it, then unclip it, the fuel exerts the same pressure coming out when you first release it as it does when the last fraction of an ounce squirts out. If this were not the case, we could not use bladders because the needle setting would only be right for the instant you set it, then it would change as the fuel was depleted. If that were the case, pressure bladders would be of no use to us. They would not work.

Rusty, will the Texas needles fit the backplates I sent you?
Jim

In general, yes, the bladders produce the same pressure from beginning to end, BUT there is a richening at the very end of the run. It expands sideways like a sausage, as long as the diameter is maintained, the pressure is constant, but once you've reached the point that the diameter becomes less, the pressure goes up somewhat. Combat fliers in the past mitigated this by using pressure regulators, but no-one seems to be using them anymore. It's not that much of a problem, the rich phase only lasts for 3-5 seconds or so before the engine quits. Speed goes down a little during that time, but it also warns you the engine is about to quit.

Iskandar

Here's some bad video of me flying the pressure setup yesterday. You can't see the plane but you can hear the engine run. It leans out and runs great when I go inverted. What gives?

I watched it on Youtube at full screen, it's a good video. You can see the shadow zoom past pretty good. Your engine sounds like a mean one, it runs nice and fast. I see you've got that inverted thing figured out. The only short sputter I heard was probably just a random air bubble. Be sure and pull the air out before you push the fuel in. With the shape of that TT fitting it keeps air out of the line pretty well unless there is a lot in there. Don't you love that warning lap before it runs dry.

The TD needle will screw into it. I don't know how well it adjusts, but probably just fine.

RknRusty wrote:I watched it on Youtube at full screen, it's a good video. You can see the shadow zoom past pretty good. Your engine sounds like a mean one, it runs nice and fast. I see you've got that inverted thing figured out. The only short sputter I heard was probably just a random air bubble. Be sure and pull the air out before you push the fuel in. With the shape of that TT fitting it keeps air out of the line pretty well unless there is a lot in there. Don't you love that warning lap before it runs dry.

The TD needle will screw into it. I don't know how well it adjusts, but probably just fine.

I think I'm going to lose the check valve so I can draw air out of the badder as you suggested. It's also pretty exposed out there and developed a leak from landing in the grass. Love the slowdown laps at the end. I like to think of it as a victory lap after running out a whole tank/bladder without sticking it in the ground.

Ken Cook wrote: Jim, what I think your experiencing is fuel being thrown into the plug element. I notice just prior to going into the outsides, the engine went rich, it then went on it's back and went lean. You have a bit of a burp taking place there. This is quite inherent with the Fox.35 when mounted in the sidewinder position like on a profile. I've found on 1/2A profile models that rotating the cylinder inboard even though unorthodox, somewhat fixes the hiccup and burp problem. The problem with doing that on a Lil Satan is the Satan used the cylinder to the outboard side as tip weight. You mentioned running a Nelson setup. This will not only probably add an additional 1000 k rpm's but it may keep the burp to a minimum as well. Keep in mind that now your on bladder, you can also open the venturi for a bit more power as well. I'm just unsure of how much is too much. I recall opening my backplate on suction to 11/64 without a fuel draw issue. Ken

Thank you for the input Ken. I think I will try mounting the cylinder upright and see what I get. Was also planning on opening the venturi to help lean it out. Spent a frustrating morning yesterday because I was having trouble getting the needle lean enough. The flight in the video had the needle open less than 1/4 turn. I am hoping the larger Venturi will not only lean out the run but maybe widen the window for a proper needle setting just a hair.
Jim

The needle was just about closed in the video. What I find strange is that the engine only fully unloads during inverted laps, it would come alive and really rip but richen back up as soon as it re entered normal flight. I will try a larger Venturi before I fiddle with engine orientation. One change at a time. I am using the nelson plug. Needle valve is from a postage stamp backplate, not sure of the thread count but it is finer than the bee/product backplates.
Jim

If you have a screen on the intake, removing it may be some of the difference you need.

Maybe I need to bore the venturi on my Satan some more. I use a pretty conservative bore diameter. Its NV runs at only 1-2/3 turns, much less than the others, and it's not steady in the loops. I need to look up that old thread about bore sizes. I still use screens on the Tee Dees since it's right up front, and I like the protection from dirt if I plant it. But on the reedy it's not as much of a problem.

This is getting me interested in pressing my Killer Bee back into service. Right now it has the boring job of lugging the PT-19 around about twice a year. It is kind of funny to see it jump off the ground without rolling though.