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Engine basics => Engine set up tips => Topic started by: Dennis Toth on July 27, 2015, 05:19:19 PM
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Guys,
I posted this question here to see if more flyers could add to the conversation. I am looking to build a ship that I intend to fly fast on long lines. I plan on running a 2-2-2 setup. I have done searches on pressure fuel systems for stunt and besides bladders not much information. So being around this hobby for some 40 plus years I remember using crankcase pressure into hard tanks for combat ships which need strong fuel supply. From what I've read the problem is that you still get a rich to lean run although not a bad as open venting. Seems that once pacifiers came along they provided even better fuel supply for combat and that kinda ended the crankcase pressure development.
Well, then we start using uniflow tanks with muffler pressure and that works pretty good. It still gets a little rich-lean in the stronger winds (not as bad as open uniflow or open vents) but I was looking for a little stronger system and thought since with uniflow it doesn't care how much pressure it only knows the pressure reference point so why not crankcase pressure into the uniflow. This would give higher pressure fuel delivery with the convenience of a hard tank and the steadiness of uniflow.
Having said this it all sounds logical but the proof is in the air. Things in stunt don't change much since if things work we don't have a lot of time to try different approaches. Not wanting to reinvent the wheel I put this out to the forum to see if others have tried this, how did it work and what would you advise or do different.
Best, DennisT
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Guys,
I posted this question here to see if more flyers could add to the conversation. I am looking to build a ship that I intend to fly fast on long lines. I plan on running a 2-2-2 setup. I have done searches on pressure fuel systems for stunt and besides bladders not much information. So being around this hobby for some 40 plus years I remember using crankcase pressure into hard tanks for combat ships which need strong fuel supply. From what I've read the problem is that you still get a rich to lean run although not a bad as open venting. Seems that once pacifiers came along they provided even better fuel supply for combat and that kinda ended the crankcase pressure development.
Well, then we start using uniflow tanks with muffler pressure and that works pretty good. It still gets a little rich-lean in the stronger winds (not as bad as open uniflow or open vents) but I was looking for a little stronger system and thought since with uniflow it doesn't care how much pressure it only knows the pressure reference point so why not crankcase pressure into the uniflow. This would give higher pressure fuel delivery with the convenience of a hard tank and the steadiness of uniflow.
Having said this it all sounds logical but the proof is in the air. Things in stunt don't change much since if things work we don't have a lot of time to try different approaches. Not wanting to reinvent the wheel I put this out to the forum to see if others have tried this, how did it work and what would you advise or do different.
I would test to make sure the pressure actually went up with RPM instead of down, because if not, it is unstable. I seem to recall that Howard found out that the pressure was not in the expected direction.
Brett
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I have used the pressure tap on a TD 049. It worked well. I don't see why it wouldn't work on a uniflow tank. The TD tap is timed so you don't need a check valve. I suspect you will need a check valve for a crank case tap. The other problem with a pressure set up is hard starting. I used a starter motor on my TD.
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Dennis,
How long do you expect the lines will be?
C
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Brett,
I did fine the post by Howard and the reverse pressure results. Also found that when they used this in combat they did some work with small (~0.018" diameter) pressure tap holes to add stability. Also they just ran the pressure line to the fill vent, uniflow might just make it a little smoother. Unfortunately, just as they started to make some progress everyone switched to bladders and it seems development stopped.
Question is if you go too small do you wind up with less pressure then muffler pressure? I do believe that it would be less sensitive to up wind / down wind variation, unless even in a 2-2-2 the engine rpm changes cause significant pressure blips that cause a lean - leaner run.
I have tried the check valve with muffler pressure. I thought the system would stabilize but the pressure just kept increasing and not really usable. The starting is simple, just need a U loop in and out of the fuse with a piece of tubing that you can pinch off during starting.
Best, DennisT
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Dennis, could you post a link to Howard's thread?
If the pressure truly does drop off as RPM goes up then, unless you find a positive solution, you just aren't going to get there from here. It could be a consequence of engine timing, so it may not happen with a stunt motor. It could be that a TD timed tap would fix the problem because it only grabs pressure when pressure is there -- but can you count on that?
I'd be tempted to consider a Perry pump, if such gizmos still exist. Or, just run a big enough "standard" stunt motor to do what you want.
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Guys,
I posted this question here to see if more flyers could add to the conversation. I am looking to build a ship that I intend to fly fast on long lines. I plan on running a 2-2-2 setup. I have done searches on pressure fuel systems for stunt and besides bladders not much information. So being around this hobby for some 40 plus years I remember using crankcase pressure into hard tanks for combat ships which need strong fuel supply. From what I've read the problem is that you still get a rich to lean run although not a bad as open venting. Seems that once pacifiers came along they provided even better fuel supply for combat and that kinda ended the crankcase pressure development.
Well, then we start using uniflow tanks with muffler pressure and that works pretty good. It still gets a little rich-lean in the stronger winds (not as bad as open uniflow or open vents) but I was looking for a little stronger system and thought since with uniflow it doesn't care how much pressure it only knows the pressure reference point so why not crankcase pressure into the uniflow. This would give higher pressure fuel delivery with the convenience of a hard tank and the steadiness of uniflow.
Having said this it all sounds logical but the proof is in the air. Things in stunt don't change much since if things work we don't have a lot of time to try different approaches. Not wanting to reinvent the wheel I put this out to the forum to see if others have tried this, how did it work and what would you advise or do different.
Best, DennisT
Crankcase pressure will work just fine with uniflow tanks.
It's still used locally with Super Tigre G15's in Vintage Combat models (.15's only) mostly for sports flying these days. The tanks have standard uniflow piping and the run is remarkably smooth and constant through the entire flight.
The backplate mounted pressure nipples (Brass K&S) are modified so that the hole is about .012". This seems to cure any of the ill effects noted above. Normally a piece of .012" c/l wire coated with carbon, paint etc is inserted into the standard pressure nipple along with some solder flux. A dob of electrical (60/40) solder is melted into the hole and the wire is pulled out before it can solidify. The same can be done with JB Weld replacing the solder.
Otherwise you can use the standard K&S pressure nipple and just crush down one end of a short length of 1/8" diameter aluminium, brass or copper tubing and insert it into the crankcase pressure (fuel tubing) line.
Some care is needed with refilling the tank. Most remove the pressure line from the tank and only reconnect it when ready to start. Otherwise crimping the pressure line prevents flooding through the backplate. I've never had much luck with "one-way-valves", they always seem to block up at the wrong time.
The older Enya glows had a pressure take of point opposite the inlet port of the shaft. It was for "timed pressure". The needle setting always seemed to be very sensitive to me when using it. Never tried closing the hole down to .012". Might have tamed the beast.
Good luck....
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Brett,
I did fine the post by Howard and the reverse pressure results. Also found that when they used this in combat they did some work with small (~0.018" diameter) pressure tap holes to add stability. Also they just ran the pressure line to the fill vent, uniflow might just make it a little smoother. Unfortunately, just as they started to make some progress everyone switched to bladders and it seems development stopped.
Question is if you go too small do you wind up with less pressure then muffler pressure? I do believe that it would be less sensitive to up wind / down wind variation, unless even in a 2-2-2 the engine rpm changes cause significant pressure blips that cause a lean - leaner run.
I have nearly zero actual experience but the size of the pressure tap isn't likely to matter very much, just the net average pressure. The smaller inlet (really small) will reduce the peak pressure but the ullage volume of the tank will act as an accumulator and very effectively smooth out the pressure at the fuel pickup. Point being, if the average pressure goes down as the RPM goes up, it will be unstable, the size of the pickup will not make it any better.
If you are concerned about excessive loading/unloading in the wind, even with pressure, that's an engine setup issue, not a tank vent or pressure issue. I ran open inlet ram air into uniflow during my Friday qualifying flights at the NATS this year, with the wind near the putative 20 mph limit (legitimate 20, not "stunt flier 20"/real 8 mph) and I had no issues whatsoever. It was my revenge for 2003, and gives some indication what might have happened had I been using my regular engine instead of the backup (which was much more prone to lean/rich issues).
Brett
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The pressure tap on TD engines is a timed pressure port. The pressure tap on Enya Model 5224 engines, the ones with the square venturi insert, is a timed pressure port. Timed crankcase pressure ports provide much higher pressure than pressure ports that are just into the crankcase. Normal crankcase pressure ports provide pressure that is the differential between the crankcase pressure at incoming charge compression pressure and crankcase pressure during charge intake into the crankcase. That is a low pressure. I ran an Enya 29 IV Model 5224 on the timed port crankcase pressure. I ran the engine on a Rat racer and a Combat wing using the crankcase pressure. I was running a Rev-Up 7-11 on the Rat Racer, and a Tornado 9-8 on the Combat Wing. I was using a standard Perfect tank with the fill line plugged and the pressure connected to the overflow.I don't remember any noticeable mixture change during the run. The needle setting is sensitive when the fuel is pressurized.
Pat
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My experience with pressure was with a ST .35 (G21). It has an un-timed
pressure port on the crankcase.
I saw little difference between using a small dia orifice and none at all. But, on
occasion, it could go unstable; as I remember this happened when the needle
was too lean.
To me the big advantage of pressure was to get more power from the engine, since
you didn't need a venturi at all to draw fuel.
The actual pressure was measured at about 9" of water in a manometer connected to
a "T" in the pressure line. Based on that, it would indicate that uniflow or conventionally
vented would be about the same.
I think the bladders are better because they survive crashes with no damage and provide
a lot more pressure rise than 9" water. Since 30 ft of water is one atmosphere, the conversion
is .5 PSI per foot of water. Bladders provide a lot more pressure than that.
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Over the weekend I did a test on reverse uniflow plumbing hookup. I wanted to see if you hooked the pressure to the "normal" fuel pickup and the fuel to the uniflow if you would get a cleaner shut off with less lean laps at the end. Well the short answer is it didn't cut off the way I thought it would. I worked ok but actually had about 4 more lean laps then the conventional hookup. I don't know if adding greater separations of the ends of the tubes would make any difference. My guess is that with the bubbles coming out behind the fuel pickup it just started to lean a little sooner so the cut off wasn't clean. I may try a tank with more separation. I was thinking that if base pressure is used this might be a good way to go if the cut off was clean.
Best, DennisT