Author Topic: Engine getting richer as flight progresses (Read 3261 times)

Paul Van Dort · « **on:** February 17, 2022, 01:07:24 AM »

Hi all, just wanted to share an experience that has puzzled me for a long time. Some extensive reading on this forum gave me some beter insights.

I am using a modified ST 51 with a clunk tank. Fixed uniflow tube.

What is the issue?

After take off, the engine behaves correctly the first figures and then slowly it gets richer. It is not gettng blubbery rich, but enough to notice and to make the square eigth more difficult to complete than with the normal (take off) setting.

I don't know if people here have noticed the same behaviour. Usually getting leaner appears to be the issue

I have been doing experiments with the tank in the kitchen to verify whether the flow was increasing when the level dropped, but the flow remained the same. Strange and somehow disappointing.
Not understanding the cause, I moved the uniflow outlet more to the inside of the tank to get rid of the uniflow effect and to have a leaning out effect in the 2nd half of the flight. This workaround did function well, but the issue kept on bothering my mind.

One thing I did not mention on purpose: I am using muffler pressure.

By reading more on the effects of muffler pressure on this forum, I understood that the hot exhaust gas entering the tank might be heathing up the fuel gradually.
The flow in a tube depends on pressure differences (my only focus so far) but also on the viscosity of the fluid. Bingo!
The hotter fuel wil have a lower viscosity, enhancing the flow and maing the mixture richer.

I still need to verify this hypothesis. But Storm Dudley is making any outdoor flying impossible these days in Belgium.

What do you think?

Kind regards,
Paul

Mark wood · « **Reply #1 on:** February 17, 2022, 08:18:52 AM »

That's curious problem. It is round tank or a square tank? While the temperature is an interesting theory I don't the think the amount of heat transfer is going to be enough to significantly raise the temperature of the fuel and then the temperature increase of the fuel would have to be significant in order to change the viscosity appreciatively. If this were happening it would be seen in the RC world as a global problem which is it isn't. In order to confirm it, a simple bench test can be performed by running a tank and an engine then monitoring the fuel temperature.

I'm going to go with some experience with the pressure taps on the muffler and on the pulse jets. The pressure tap is a restrictor which regulates the tank pressure. The tanks pressure is a function of the volume, flow out and the pressure - flow in. If that balance isn't correct, the result is the tank either gaining pressure in time or loosing pressure in time. In this case, replace the restrictor with one having a smaller diameter hole. The current one could be soldered closed and drilled out to a number drill or two smaller than the existing one.

Back in the day, Norm Drazy had a crazy cool pulse jet he built which had a very interesting pressure regulator in it to take the engine pressure and push the fuel to the engine. That airplane was the first one to break 200 MPH in the late 60' early 70's. In later years we learned that a simple pressure tap worked. My best top speeds were set with suction feed though.

Dan McEntee · « **Reply #2 on:** February 17, 2022, 09:16:13 AM »

Quote from: Paul Van Dort on February 17, 2022, 01:07:24 AM

Hi all, just wanted to share an experience that has puzzled me for a long time. Some extensive reading on this forum gave me some beter insights.

I am using a modified ST 51 with a clunk tank. Fixed uniflow tube.

What is the issue?

After take off, the engine behaves correctly the first figures and then slowly it gets richer. It is not gettng blubbery rich, but enough to notice and to make the square eigth more difficult to complete than with the normal (take off) setting.

I don't know if people here have noticed the same behaviour. Usually getting leaner appears to be the issue

I have been doing experiments with the tank in the kitchen to verify whether the flow was increasing when the level dropped, but the flow remained the same. Strange and somehow disappointing.
Not understanding the cause, I moved the uniflow outlet more to the inside of the tank to get rid of the uniflow effect and to have a leaning out effect in the 2nd half of the flight. This workaround did function well, but the issue kept on bothering my mind.

One thing I did not mention on purpose: I am using muffler pressure.

By reading more on the effects of muffler pressure on this forum, I understood that the hot exhaust gas entering the tank might be heathing up the fuel gradually.
The flow in a tube depends on pressure differences (my only focus so far) but also on the viscosity of the fluid. Bingo!
The hotter fuel wil have a lower viscosity, enhancing the flow and maing the mixture richer.

I still need to verify this hypothesis. But Storm Dudley is making any outdoor flying impossible these days in Belgium.

What do you think?

Kind regards,
Paul

You can over pressurize a tank. Try some flights with no muffler pressure, even if you don't fly the pattern. Fly some inverted flight laps about half way through and see how it responds. If you get good results and you like it, just leave it that way. If you wish to continue using pressure, make some restrictors to put in the pressure line, Telescope some lengths of tubing down to the smallest size they make and put a section of that in your pressure line and fly again. The location of the uniflow tube could be the cause like you are thinking. If it is uncovered as the run proceeds it will go rich. This will all be a trial and error exercise. If you are using plastic tanks and can make up a spare tank or two with different tube locations it may help the process.
Type at you later,
Dan McEntee

Brett Buck · « **Reply #3 on:** February 17, 2022, 10:12:14 AM »

Quote from: Paul Van Dort on February 17, 2022, 01:07:24 AM

By reading more on the effects of muffler pressure on this forum, I understood that the hot exhaust gas entering the tank might be heathing up the fuel gradually.
The flow in a tube depends on pressure differences (my only focus so far) but also on the viscosity of the fluid. Bingo!
The hotter fuel wil have a lower viscosity, enhancing the flow and maing the mixture richer.

Very possible, and it heats faster the lower the level (same or very slightly increasing heat input, progressively less volume to heat), so you have two things happening at once.

Brett

Paul Van Dort · « **Reply #4 on:** February 17, 2022, 10:18:16 AM »

Quote from: Mark wood on February 17, 2022, 08:18:52 AM

That's curious problem. It is round tank or a square tank? While the temperature is an interesting theory I don't the think the amount of heat transfer is going to be enough to significantly raise the temperature of the fuel and then the temperature increase of the fuel would have to be significant in order to change the viscosity appreciatively. If this were happening it would be seen in the RC world as a global problem which is it isn't. In order to confirm it, a simple bench test can be performed by running a tank and an engine then monitoring the fuel temperature.

I'm going to go with some experience with the pressure taps on the muffler and on the pulse jets. The pressure tap is a restrictor which regulates the tank pressure. The tanks pressure is a function of the volume, flow out and the pressure - flow in. If that balance isn't correct, the result is the tank either gaining pressure in time or loosing pressure in time. In this case, replace the restrictor with one having a smaller diameter hole. The current one could be soldered closed and drilled out to a number drill or two smaller than the existing one.

Back in the day, Norm Drazy had a crazy cool pulse jet he built which had a very interesting pressure regulator in it to take the engine pressure and push the fuel to the engine. That airplane was the first one to break 200 MPH in the late 60' early 70's. In later years we learned that a simple pressure tap worked. My best top speeds were set with suction feed though.

When using muffler pressure, at the end of the flight, all the fuel will be replaced by exhaust fumes and some oil. The tank is not particulary cooled very well and the exhaust gasses are hot. (e.g. 150-200 C) . A minor temperature rise of the fuel can affect the viscosity enough to change the flow rate in the NVA. Our engines are very sensitive to flow changes. I will change to suction and keep an eye on the issue. Suction will give me less needle sensitivity as well I reckon. The experiment you suggest will take place in the near futur. Thanks .

Brett Buck · « **Reply #5 on:** February 17, 2022, 10:23:59 AM »

Quote from: Paul Van Dort on February 17, 2022, 10:18:16 AM

When using muffler pressure, at the end of the flight, all the fuel will be replaced by exhaust fumes and some oil. The tank is not particulary cooled very well and the exhaust gasses are hot. (e.g. 150-200 C) . A minor temperature rise of the fuel can affect the viscosity enough to change the flow rate in the NVA. Our engines are very sensitive to flow changes. I will change to suction and keep an eye on the issue. Suction will give me less needle sensitivity as well I reckon. The experiment you suggest will take place in the near futur. Thanks .

Two ways to check - take your IR thermometer and aim at the tank right after the end of the flight, see if it is noticeably warmer. Alternately, warm up the fuel (a little bit, not boiling) right before you put it in, and start it quickly.

Brett

Mark wood · « **Reply #6 on:** February 17, 2022, 10:59:06 AM »

Quote from: Brett Buck on February 17, 2022, 10:23:59 AM

Two ways to check - take your IR thermometer and aim at the tank right after the end of the flight, see if it is noticeably warmer. Alternately, warm up the fuel (a little bit, not boiling) right before you put it in, and start it quickly.

Brett

Yup, that would work.

Interesting details if you work out the thermodynamics which creates the heat transfer. The primary exhaust product is water vapor so the density and specific heat of water vapor can be used to calculate the amount of heat transfer taking place. Both the specific gravity and heat for oil and alcohol are close enough to call them similar enough to use in the magnitude of the heat transfer. For water vapor the specific heat is 1.8 Joule/gram*K and the specific density is 0.76 gram per liter. For the fuel side the specific heat is 2.51 Joule / gram*K and the specific gravity 812 gram / liter. What this means is that it would take either a huge amount of exhaust vapor to be bought in to the tank or a very long time to increase the temperature of the fuel inside of the tank. I'll let you guys work out the math which isn't that difficult.

Brett Buck · « **Reply #7 on:** February 17, 2022, 11:23:00 AM »

Quote from: Mark wood on February 17, 2022, 10:59:06 AM

Yup, that would work.

Interesting details if you work out the thermodynamics which creates the heat transfer. The primary exhaust product is water vapor so the density and specific heat of water vapor can be used to calculate the amount of heat transfer taking place. Both the specific gravity and heat for oil and alcohol are close enough to call them similar enough to use in the magnitude of the heat transfer. For water vapor the specific heat is 1.8 Joule/gram*K and the specific density is 0.76 gram per liter. For the fuel side the specific heat is 2.51 Joule / gram*K and the specific gravity 812 gram / liter. What this means is that it would take either a huge amount of exhaust vapor to be bought in to the tank or a very long time to increase the temperature of the fuel inside of the tank. I'll let you guys work out the math which isn't that difficult.

The kicker is that as you go on, it takes less and less heat, because the volume goes down. We have had several cases of stuff like this, both from the exhaust, and direct heating from the header (or muffle in one case). The plastic tank probably helps with the header/muffler but makes the situation with exhaust gas worse.

I haven't used pipe pressure on my regular airplanes for years. I get a small version of what Paul is talking about on the 20FP, barely detectable. Paul is also probably using FAI fuel, meaning really small needle openings AND pressure, which also makes the metering hole smaller, meaning it is much more subject to viscous effects. David has no problem with pipe pressure from this issue - and he is also running 8.5 ounces of fuel!

Brett

Paul Van Dort · « **Reply #8 on:** February 17, 2022, 12:53:06 PM »

Thanks for your feedback, gentlemen.

Some extra info.
Rather cold weather: 3 a 5 degrees C.
No Nitro in the fuel. 24% oil 50/50 castor/ synthetic
Graupner "Kunstflugtank" 150ml
7 cm silicone tube between muffler and entering the fuselage to the tank.
Flight: First 2 minutes a rich 2 stroke (5.1 laptime) , evolving to a full 4 stroke (5.6 s laptime)

Dan McEntee · « **Reply #9 on:** February 17, 2022, 02:01:17 PM »

Quote from: Lauri Malila on February 17, 2022, 01:03:05 PM

..why that much oil? L

ST.51 is a ringed engine, I guess you are aware. I have run these a lot over the last 20 years or so, but I don't use that much oil once it's broken in and ring seated. Some believe that much oil is necessary for a ringed engine. I think any where between 20 and 24 % should work as long as it's half synthetic. I run mostly SIG Champion which is 20% total oil, half castor and half Klotz, and after a flight or two castor is dripping off the bottom of the bottom of the fuselage marking it's territory! I still have the first two engines I bought back in 1994 when they were introduced still in service and on the original rings yet. One could use a new ring and will gat to that as soon as I get around to it! I have never had any troubles with engines getting too sticky and gummy running what I run. and if the rings lasted this long, they must be lubricated enough for my style and use.
Type at you later,
Dan McEntee

Mark wood · « **Reply #10 on:** February 17, 2022, 02:13:52 PM »

Quote from: Brett Buck on February 17, 2022, 11:23:00 AM

The kicker is that as you go on, it takes less and less heat, because the volume goes down. We have had several cases of stuff like this, both from the exhaust, and direct heating from the header (or muffle in one case). The plastic tank probably helps with the header/muffler but makes the situation with exhaust gas worse.

I haven't used pipe pressure on my regular airplanes for years. I get a small version of what Paul is talking about on the 20FP, barely detectable. Paul is also probably using FAI fuel, meaning really small needle openings AND pressure, which also makes the metering hole smaller, meaning it is much more subject to viscous effects. David has no problem with pipe pressure from this issue - and he is also running 8.5 ounces of fuel!

Brett

The thing is the exhaust is dead headed in the tank and is cooled by the mass of the fuel. The incoming vapor is simply filling the volume, there is no continuous constant high temperature flow of vapor which is how the conceptual process would work. The only transport of heat is via the amount mas the vapor has that enters the tank which is equal to the volume of the fuel that is exiting. If the fuel were not moving out, the exhaust vapor could not transport any heat in to the tank and the entire heat transfer would be via conduction through the fuel line connection which is virtually zero since the silicon is a very good insulator.

The result is that the incoming exhaust vapor replacing the outgoing fuel is cooled by the remaining fuel and is diluted with the already cooled vapor. By the time the tank is empty the vapor temperature in the tank will, comparative to the initial temperature, be much lower. This means the heat difference is really just the mass of the initial volume of fuel vs the final mass of the vapor that displaced the fuel and the corresponding temperature change can be calculated by the heat transfer which occurred within the volume of the tank. That is a heat ratio of pretty close 1,000:1 which means the temperature increase is very minimal despite the exhaust vapor temperature being very high. The final fuel temperature would be then a function of the ratio of the heat contained in the fuel vs the heat of the contained in the vapor displacing it. Given the significant difference between the mass being displaced and the mass replacing it, the amount of heat exchanged is quite minimal and I'd expect the temperature increase to be less than 1 degree Fahrenheit. I'll let you work the math to see how close my guess is.

Dennis Nunes · « **Reply #11 on:** February 17, 2022, 02:50:44 PM »

Sometimes it's the most simplest things that can contribute to a strange engine run. I haven't seen this mentioned yet but have you tried a new or different glow plug? I had a ST v60 run strangely and I did everything, different needle valve assembly, muffler pressure, no muffler pressure, relocated the uniflow vent both in and outside the tank, replaced all fuel lines, fuel filter and even made a new tank to no avail.

Changed the glow plug ---- presto-chango the problem solved.

Dennis

Brett Buck · « **Reply #12 on:** February 17, 2022, 02:57:57 PM »

Quote from: Mark wood on February 17, 2022, 02:13:52 PM

The thing is the exhaust is dead headed in the tank and is cooled by the mass of the fuel. The incoming vapor is simply filling the volume, there is no continuous constant high temperature flow of vapor which is how the conceptual process would work. The only transport of heat is via the amount mas the vapor has that enters the tank which is equal to the volume of the fuel that is exiting. If the fuel were not moving out, the exhaust vapor could not transport any heat in to the tank and the entire heat transfer would be via conduction through the fuel line connection which is virtually zero since the silicon is a very good insulator.

The result is that the incoming exhaust vapor replacing the outgoing fuel is cooled by the remaining fuel and is diluted with the already cooled vapor. By the time the tank is empty the vapor temperature in the tank will, comparative to the initial temperature, be much lower. This means the heat difference is really just the mass of the initial volume of fuel vs the final mass of the vapor that displaced the fuel and the corresponding temperature change can be calculated by the heat transfer which occurred within the volume of the tank. That is a heat ratio of pretty close 1,000:1 which means the temperature increase is very minimal despite the exhaust vapor temperature being very high. The final fuel temperature would be then a function of the ratio of the heat contained in the fuel vs the heat of the contained in the vapor displacing it. Given the significant difference between the mass being displaced and the mass replacing it, the amount of heat exchanged is quite minimal and I'd expect the temperature increase to be less than 1 degree Fahrenheit. I'll let you work the math to see how close my guess is.

Ground Truth > Analysis.

Brett

Mark wood · « **Reply #13 on:** February 17, 2022, 03:00:21 PM »

Quote from: Brett Buck on February 17, 2022, 02:57:57 PM

Ground Truth > Analysis.

Brett

Without actual data it's myth not truth.

Ken Culbertson · « **Reply #14 on:** February 17, 2022, 03:44:49 PM »

I doubt that the muffler pressure is heating the fuel. It is not a constant airflow into the tank it is simple pressure being exerted on the fuel mass. The total amount of air actually pumped into the tank is about equal the the volume of the fuel. This assumes that you are capping the fill tube.

Before mufflers, I used crankcase pressure on my McCoy's. Worked quite well and that air is cool.
I can see a metal tank warming some on a profile from the proximity of the cylinder. But if it had this effect it would be a common problem, and it isn't. Somehow I don't think heated fuel is the right tree to be barking up. I would be looking more engine temperature. Does it have compression when it lands? If you restart it right away does it have the same pattern? And with that you have exhausted my IC knowledge base!

Ken

Brett Buck · « **Reply #15 on:** February 17, 2022, 06:40:41 PM »

Quote from: Mark wood on February 17, 2022, 03:00:21 PM

Without actual data it's myth not truth.

I note neither I, nor anyone else, is on the hook to "prove" anything to you.

Brett

Mark wood · « **Reply #16 on:** February 17, 2022, 06:45:46 PM »

Quote from: Brett Buck on February 17, 2022, 06:40:41 PM

I note neither I, nor anyone else, is on the hook to "prove" anything to you.

Brett

Well said. You are not that is for certain. Good physics trump hypothetical truth.

Paul Van Dort · « **Reply #17 on:** February 18, 2022, 01:16:31 AM »

I like this technical discussion and indeed the thermodynamics of the gas entering the fueltank cannot have a big effect on the temperature. The mass difference fuel/ gas is huge.
When looking at the fuel flow to the engine, only 4 factors are affecting it: Pressure, Viscosity, tube diameter and tube length.

Let's assume the tubes remain pretty much unaltered, that leaves me with Pressure and viscosity.
Perhaps the muffler pressure is increasing for some reason. But I could not detect any leaks or so.

I ran the engine statically in the model yesterday, without muffler presure. The needle (enya) had to come out about 1.5 turns.

This also indicates that the needle setting is much more critical with pressure than without. It also means that any disturbance in my 4 factors of above has a huge effect on the run.

As soon as possible I will try flying without muffler pressure and hopefuly my issue is solved.

Experimenting with compression and glowplugs is also ongoing, but it is not solving the richening.

Thanks for all feedback, gentlemen!

Dan McEntee · « **Reply #18 on:** February 18, 2022, 03:15:06 PM »

Quote from: Paul Van Dort on February 18, 2022, 01:16:31 AM

I like this technical discussion and indeed the thermodynamics of the gas entering the fueltank cannot have a big effect on the temperature. The mass difference fuel/ gas is huge.
When looking at the fuel flow to the engine, only 4 factors are affecting it: Pressure, Viscosity, tube diameter and tube length.

Let's assume the tubes remain pretty much unaltered, that leaves me with Pressure and viscosity.
Perhaps the muffler pressure is increasing for some reason. But I could not detect any leaks or so.

I ran the engine statically in the model yesterday, without muffler presure. The needle (enya) had to come out about 1.5 turns.

This also indicates that the needle setting is much more critical with pressure than without. It also means that any disturbance in my 4 factors of above has a huge effect on the run.

As soon as possible I will try flying without muffler pressure and hopefuly my issue is solved.

Experimenting with compression and glowplugs is also ongoing, but it is not solving the richening.

Thanks for all feedback, gentlemen!

After reading this last post, I'll say that this is a matter of over pressurizing the tank. If you ran the engine static and had to open it 1.5 turns, that's not very far. The Enya needle must be pretty blunt and the threads pretty coarse. I have run ST.51's quite a bit, and the stock ST needle valve usually has to be opened about 3 turns for starting and even under muffler pressure, a typical setting is some where between 1.5 and 2 turns. I would guess that your setting under pressure is less than 1 turn. What I think is happening is this: The ST.51 is a relatively big bore engine, and when running between 9000 and 10,000 RPM is pumping out a lot of exhaust. If you have a needle valve that is that far closed, it is almost shut and if the over flow is capped, it is not unconceivable to me that you are filling the tank with exhaust faster than fuel is being consumed, so you will get an increase in pressure. I have seen this before, and it does not need to be very much increase is pressure. I think that if you leave everything else the same, and try a few flight with out pressure, and you can get a satisfactory needle setting, you will see a difference. If you are using an Enya NVA, and the stock venturi, you may be getting some mixed effects from that. If you have or can get a stock ST needle valve, give that a try also. The only issues I have found with the ST needle valve assembly over a long period of time is that vibration will make the taper on the needle get a little fish mouthed where it beats against the seat in the spray bar. This tends to make getting that perfect setting a little hard some times, and you either need to chick it up into something to polish it back to being straight or replace it. These days finding replacements is getting to be another difficulty but I have had them last two or three seasons before needing replacement and that is with a lot of flying. If all you have is Enva NVAs, you may try making one of those needles a longer, sharper taper. I have done it with the recent last batches of OS needles by just chucking them up in a drill and spin them while dressing them down with a fine file, then polish with fine sand paper or a stone. In short, I run exhaust pressure on most of my ST.51 and .60s and have never had this problem, and I doubt that you are heating up the fuel. Your problem involves simple flow problems that may have crept in slowly over time, if you never had this issue with your set up before.
Type at you later,
Dan McEntee

Paul Van Dort · « **Reply #19 on:** February 18, 2022, 05:36:11 PM »

Quote from: Dan McEntee on February 18, 2022, 03:15:06 PM

After reading this last post, I'll say that this is a matter of over pressurizing the tank. If you ran the engine static and had to open it 1.5 turns, that's not very far. The Enya needle must be pretty blunt and the threads pretty coarse.

I meant opening 1.5 turns more then with muffler pressure.

Dan McEntee · « **Reply #20 on:** February 18, 2022, 05:50:09 PM »

Quote from: Paul Van Dort on February 18, 2022, 05:36:11 PM

Quote from: Dan McEntee on February 18, 2022, 03:15:06 PM
After reading this last post, I'll say that this is a matter of over pressurizing the tank. If you ran the engine static and had to open it 1.5 turns, that's not very far. The Enya needle must be pretty blunt and the threads pretty coarse.

I meant opening 1.5 turns more then with muffler pressure.

In that event, that sounds about correct. You will have to open the needle more without muffler pressure. Applying muffler pressure is usually one way of taming an erratic needle setting. If it ran the tank out in about the same amount of time with no variation in speed, the only things missing then are the pressure and centrifugal force. If everything n your fuel system is correct and tight, you can expect the engine to richen up a bit after the first lap from the effects of CF. Just don't change anything else until you fly it like it is.
Type at you later,
Dan McEntee

Paul Van Dort · « **Reply #21 on:** February 19, 2022, 04:15:22 AM »

Quote from: Lauri Malila on February 18, 2022, 04:45:36 PM

[quote author=Paul Van Dort

Experimenting with compression and glowplugs is also ongoing, but it is not solving the richening.

Thanks for all feedback, gentlemen!

Paul,
Is that what you believe or have you got practical experience of it?
Because, the key to stability is the efficiency and stability of burning process. IF you are running the engine under-compressed, it will greatly increase the instability and make top
Of power curve narrower. It also means that all kind of variations, like the fuel viscosity index issue in your case, will have greatly bigger effect in the engine run.
If you set the compression correct, you may well get the richening to acceptable level, or to a level you can totally compensate away by fiddling with fuel tank position.
Again, lowering compression, and increasing oil persentage of a healthy engine are not the ways to increase stability, quite opposite. L

I have experience with compression and cylinder head shapes. Stable running is not the issue. I can tune the uniflie tank for correct behaviour in inside and outside maneovres. 24% oil is not exaggerated as other experts will confirm. The strange is the slow richening as the flight progresses. I will test without pressure and will share my results. Thanks for the feedback.

Dennis Toth · « **Reply #22 on:** February 19, 2022, 10:05:42 AM »

If it is very sensitive to the needle perhaps the pressure nipple is expanding as it is heated from the engine exhaust allowing the pressure to increase very slightly and richening the mixture. Or there is residue in the pressure fitting that expands with the exhaust heat causing the same. Could flush out the fitting might eliminate this. Other option would be to oversize the pressure fitting then us a small piece of tubing to restrict the pressure downstream in the pressure line. This would give the gas some time to cool and possibly not cause the restrictor to expand significantly.

Best, DennisT

john e. holliday · « **Reply #23 on:** February 19, 2022, 10:22:34 AM »

I read through this again and did I miss him trying without muffler pressure? I have two planes almost the same size and both powered with LA 25. They both started with muffler pressure. Oh almost identical tanks. One plane now flying on suction. Both tanks are uni-flo.

Paul Van Dort · « **Reply #24 on:** February 19, 2022, 11:49:13 AM »

Quote from: john e. holliday on February 19, 2022, 10:22:34 AM

I read through this again and did I miss him trying without muffler pressure? I have two planes almost the same size and both powered with LA 25. They both started with muffler pressure. Oh almost identical tanks. One plane now flying on suction. Both tanks are uni-flo.

You can address me directly. No problem. I will try without muffler pressure as soon as the weather permits.

Al Ferraro · « **Reply #25 on:** February 20, 2022, 09:17:50 AM »

My Fox 35 powered Nobler did this. I insulated the tank with a dense Styrofoam bulkhead 3/16 thick back behind the engine top to bottom, cured the problem. I am using a uniflow tank.
Al

Tim Wescott · « **Reply #26 on:** February 20, 2022, 12:44:51 PM »

Quote from: Al Ferraro on February 20, 2022, 09:17:50 AM

My Fox 35 powered Nobler did this. I insulated the tank with a dense Styrofoam bulkhead 3/16 thick back behind the engine top to bottom, cured the problem. I am using a uniflow tank.
Al

All castor fuel? Castor's viscosity is way more sensitive to temperature than synthetic, and you'd want to use a lot of it in a Fox. It certainly makes theoretical sense to me.

Dan McEntee · « **Reply #27 on:** February 20, 2022, 03:35:17 PM »

Quote from: Al Ferraro on February 20, 2022, 09:17:50 AM

My Fox 35 powered Nobler did this. I insulated the tank with a dense Styrofoam bulkhead 3/16 thick back behind the engine top to bottom, cured the problem. I am using a uniflow tank.
Al

This just insulated the tank from radiating engine heat. Even a balsa bulkhead would have helped. If the tank was close enough, it would have caused problems no matter what fuel or oil you run. If there was not adequate cooling air flow that would have aggravated the problem.
Type at you later,
Dan McEntee

Reptoid · « **Reply #28 on:** February 21, 2022, 12:25:52 PM »

This problem is likely not a single issue but a combination of very suttle things effecting an overly cold set-up to begin with. An ST 46 is a fairly low compression engine. Zero nitro fuel with 24% oil is a very cold running fuel. 3 degrees C is around 37 degrees F which isn't far from freezing. No Nitro fuel reduces the needle range (movement from full rich to over lean) for correct mixture considerably from fuel with even 5% nitro. This exacerbates minor mixture changes caused by things listed above. It is even possible if the humidity is high you could have ice forming on or in the venturi, which of course would disappear almost immediately when engine shuts down. The other contributing factor and maybe the most likely is the location of your uniflo line inside the tank. A well sealed uniflo tank will run consistently UNTIL the uniflo vent is uncovered by lower fuel level. When the line is uncovered it will actually cause the mixture to richen until the tank is empty. Assuming the normal location of against the outer wall next to but ahead of the fuel pick up, you will get richening for a few laps then lean out and quit. The amount of change depends on how broad a mixture range your overall set-up has, and because of the no nitro and cold weather your's will be a larger change than say: 5-10% nitro and more moderate temperatures.

Paul Van Dort · « **Reply #29 on:** February 21, 2022, 01:30:00 PM »

Quote from: Reptoid on February 21, 2022, 12:25:52 PM

A well sealed uniflo tank will run consistently UNTIL the uniflo vent is uncovered by lower fuel level. When the line is uncovered it will actually cause the mixture to richen until the tank is empty. Assuming the normal location of against the outer wall next to but ahead of the fuel pick up, you will get richening for a few laps then lean out and quit.

I tend to disagree.
In my setup the uniflow tube is about 1/2 inch from the outside (of the circle) tank wall. As soon as the uniflow tube becomes uncovered, the uniflow effect disappears and the mixture will slowly become leaner due to decreasing fuel head. This effect lasts until the tank runs dry (or the pick up tube becomes uncovered) . Then you will notice a short richening (1 -2 seconds) because the emptying fuel tube is decreasing the flow resistance. It is as if the fuel tube becomes shorter. And we know that flow resistance is proportional to the tube length. When the fuel line is finally empty, the engine leans out an quits.

Reptoid · « **Reply #30 on:** February 21, 2022, 05:46:08 PM »

Quote from: Paul Van Dort on February 21, 2022, 01:30:00 PM

I tend to disagree.
In my setup the uniflow tube is about 1/2 inch from the outside (of the circle) tank wall. As soon as the uniflow tube becomes uncovered, the uniflow effect disappears and the mixture will slowly become leaner due to decreasing fuel head. This effect lasts until the tank runs dry (or the pick up tube becomes uncovered) . Then you will notice a short richening (1 -2 seconds) because the emptying fuel tube is decreasing the flow resistance. It is as if the fuel tube becomes shorter. And we know that flow resistance is proportional to the tube length. When the fuel line is finally empty, the engine leans out an quits.

You can tend to disagree all you want. That's your prerogative, but it won't change how a uniflow tank works and it won't change the fact that you setting up your tank with the uniflo outlet a 1/2" inboard of the pick up makes it not a real uniflo tank. It will get richer when it uncovers and then act like a non-uniflo tank on muffler pressure.
Here's an explanation that will help you (and others on here) The concept was invented in the 17th century by a French physicist.
https://web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/36.07.html

Mark wood · « **Reply #31 on:** February 21, 2022, 06:01:44 PM »

Quote from: Reptoid on February 21, 2022, 05:46:08 PM

You can tend to disagree all you want. That's your prerogative, but it won't change how a uniflow tank works and it won't change the fact that you setting up your tank with the uniflo outlet a 1/2" inboard of the pick up makes it not a real uniflo tank. It will get richer when it uncovers and then act like a non-uniflo tank on muffler pressure.
Here's an explanation that will help you (and others on here) The concept was invented in the 17th century by a French physicist.
https://web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/36.07.html

Trouble is that Paul is correct. You interpretation of the French experiment is flawed in the affect of the uniflow tank after the vent un-ports. The vent being lowered in to the column recues the head from the overall height of H, the distance from the outlet to the top of the liquid column, to h the distance from the pick up to the vent. At the moment the vent un ports the head is h and from then until the liquid reaches the outlet the head decreases as the distance from the outlet to the liquid surface. What this means is in terms of fuel mixture is the the fuel pressure will decrease as direct function of the head which means it will be leaning out. It's simple physics.

Reptoid · « **Reply #32 on:** February 21, 2022, 09:58:56 PM »

Quote from: Mark wood on February 21, 2022, 06:01:44 PM

Trouble is that Paul is correct. You interpretation of the French experiment is flawed in the affect of the uniflow tank after the vent un-ports. The vent being lowered in to the column recues the head from the overall height of H, the distance from the outlet to the top of the liquid column, to h the distance from the pick up to the vent. At the moment the vent un ports the head is h and from then until the liquid reaches the outlet the head decreases as the distance from the outlet to the liquid surface. What this means is in terms of fuel mixture is the the fuel pressure will decrease as direct function of the head which means it will be leaning out. It's simple physics.

You and Paul are correct, but only from the point of uncovering the vent; When the vent uncovers the head will be "H" The Point you are both missing is that in a uniflow tank you don't want that to happen until the very end of the flight. That's why the uniflow vent should be on the outboard side of the tank ; not 1/2" inboard. The pressure prior to the vent being uncovered is NOT "H" but essentially "nill" since the inlet and outlet are the same distance (H=0) That's why a uniflow runs the same throughout the tank. At the instant of uncovering the outlet pressure goes from H=0 to whatever H actually is in the tank so the outlet pressure goes up slightly (determined by how far inboard the uniflow exit is from the pick up tube; in Paul's case H=1/2", SB less than 1/8"). . In Paul's case the pressure change from H=0 to H=1/2" will be far greater and it WILL go rich, then gradually lean as it goes to empty. In a properly constructed tank with uniflow outlet 1/8" or less inboard of fuel pick up the engine will go slightly rich at vent uncovering (H=0 to H= 1/8"), run about two laps and lean out and die. Exactly how all of my uniflow tanks run and have for 50+ years including dozens of tanks I have built for others

Brett Buck · « **Reply #33 on:** February 21, 2022, 11:00:34 PM »

Quote from: Reptoid on February 21, 2022, 05:46:08 PM

You can tend to disagree all you want. That's your prerogative, but it won't change how a uniflow tank works and it won't change the fact that you setting up your tank with the uniflo outlet a 1/2" inboard of the pick up makes it not a real uniflo tank. It will get richer when it uncovers and then act like a non-uniflo tank on muffler pressure.

As much as I hate to put my useless real-life experience up against everyone's thought experiments, what Paul is describing was a relatively well-known and pretty ancient (like, Air Trails in 1952/3 ancient) way of adjusting the time response of engines. In fact, you can build a tank as he describes and it will run at a constant head pressure until the vent uncovers, then lean out from there.

It won't abruptly go richer at that instant, because the "negative" ullage pressure approaches the vent pressure (i.e. getting less "vacuum" in the ullage) as the fuel level drops and will exactly match the external pressure as the point the fuel level gets to the vent.

Even the tank shown on the Nobler plans has the effect to a small degree, staying ~constant feed pressure for about the first 1/3 of the tank, then leaning out from there - because the over/under vents are about 1/3 way across the tank.

It is relatively easy to make this adjustable, by soldering a short 5/32 ID through the inboard side of the tank, leaving enough to put some fuel tubing on, and then make your vent a long piece of 1/8" that slides through. That allows you to adjust the effect to your desires. \

Back in 4/2 days, this was used to speed the engine up for the last few maneuvers, but it is not necessary or desirable for modern engines and mine run constant until the final lean-out.

Brett

Reptoid · « **Reply #34 on:** February 22, 2022, 02:56:36 AM »

I'm not going to argue with anyone. If you do the bottle experiment as shown in the illustration you will easily find that with both tubes open no liquid runs from the feed line when they are at the same height no matter the level of liquid indicating equal pressure at both locations. When the liquid lowers to below the level of the vent or you raise the vent to be above the outlet or liquid level water will magically start to flow from the feed by itself obviously indicating a lower pressure at the feed line. This is exactly how an old fashion water cooler works.(uniflo tank) When you push the button it opens the spigot which contains the outlet spout and also the vent which is slightly above the feed so the water comes out at the same rate whether the bottle is full, half full, or nearly empty. If you poke a hole in the top (actually the bottom) of the 5 gallon jug and open the spout the water will come out very fast if it's full and slow gradually to a trickle as the bottle nears empty. (std tank) There will be a change in head pressure when the vent uncovers. How much it changes the mixture is dependent on many variables like how much fuel is left (H), viscosity, nitro content, prop loading, etc. This is also why saying adding muffler pressure to a uniflo tank is still uniflo; NOT Exactly because you added positive press at the inlet but not the final outlet. Try the bottle experiment with the vents even and see no water flowing; Now add 2 PSI to the inlet and the water will flow at a rate driven by the 2PSI because the outlet is still at 1 atmosphere. It effects the power boost in maneuvers because you are now running the engine on a higher positive pressure fuel feed while the lower than atmospheric pressure at the venturi/needle valve outlet stayed the same. If you want to exaggerate this to make the point, add a surgical tubing bladder filled with air to the uniflo input line (10+ PSI or more) I assure you that your engine tank combo now doesn't give a damn where the input vent tube is and will hold the needle setting throughout the tank. Why? because you have overwhelmed the paltry atmospheric pressure at the venturi with a pressure much higher that does not vary with fuel load.

Dan Berry · « **Reply #35 on:** February 22, 2022, 07:36:33 AM »

Is the tank plumbed with brass tubing?

Paul Van Dort · « **Reply #36 on:** February 22, 2022, 08:29:29 AM »

Quote from: Dan Berry on February 22, 2022, 07:36:33 AM

Is the tank plumbed with brass tubing?

My tank indeed uses brand new brass tubing

Mark wood · « **Reply #37 on:** February 22, 2022, 09:19:48 AM »

Quote from: Reptoid on February 22, 2022, 02:56:36 AM

I'm not going to argue with anyone.

Really?

I for one am very easy to convince when shown the correct math and physics. This problem is not complex and the bottle example is a physics 101 lab experiment. Both Paul and Brett are correct and your assessment is flawed. Arguing would be pointless without the tools necessary to understand. With the necessary tools to understand, you would see the flaw.

Dan McEntee · « **Reply #38 on:** February 22, 2022, 09:52:34 AM »

Quote from: Paul Van Dort on February 22, 2022, 08:29:29 AM

My tank indeed uses brand new brass tubing

In the future, switch to copper tubing. Fuel gets to the brass in a relatively short period of time and will just crack along it's length. Copper will eventually also, but it takes much longer. Put some samples of each in a small jar of fuel and watch what happens.
Type at you later,
Dan McEntee

Mark wood · « **Reply #39 on:** February 22, 2022, 10:08:40 AM »

Quote from: Dan McEntee on February 22, 2022, 09:52:34 AM

In the future, switch to copper tubing. Fuel gets to the brass in a relatively short period of time and will just crack along it's length. Copper will eventually also, but it takes much longer. Put some samples of each in a small jar of fuel and watch what happens.
Type at you later,
Dan McEntee

Interesting, hadn't ever realized that myself and figured it was fatigue. Is it the nitro or the alcohol that is doing it? I do know citric acid will leach the tin out of brass and other acids will too so I assume the nitro is cause. Time for a speriment in my shop.

Dan McEntee · « **Reply #40 on:** February 22, 2022, 11:06:14 AM »

Quote from: Mark wood on February 22, 2022, 10:08:40 AM

Interesting, hadn't ever realized that myself and figured it was fatigue. Is it the nitro or the alcohol that is doing it? I do know citric acid will leach the tin out of brass and other acids will too so I assume the nitro is cause. Time for a speriment in my shop.

The nitro is the prime offender and alcohol doesn't help when the nitro gets to it and has it etched. When nitro was made in the good old USA it wasn't as bad. Years ago, when I worked part time at a local hobby shop, we primarily sold K&B fuel. Red Max brand came along and while we didn't offer it, several shops in the area did. It was cheaper than K&B and that was the primary attraction. This was about the time that the first nitro shortage hit when one of the t6wo plants in the USA that produced it either shut down or blew up. The EPA would not let anyone else do a start up on new production. So, that was when Chinese nitro started to flow into the country. Chinese nitro was much more acidic. Guys using Red Max started having bearing failures and that prompted use if good after run oil to flush the excess fuel off the bearings. Word of this got around so we did the test live and in person at the check out counter. One small jar with K&B fuel and a 1" length of brass fuel line. Next to it was another jar with Red Max and the same 1" length of tubing. The jars were sealed with tape and dated. It didn't take long for the tube in the Red Max fuel to turn black. That was my first education on it. This was in the mid to late 1980's I think. After that I think I red in Stunt News where it was suggested to use copper tube in all fuel tanks. Fast forward to present day and all nitro comes from over seas. These days we are just happy to get consistent fuel. Glow Plugs will be the newest challenge. I'm stocked up pretty good but still keep my eyes peeled for good deals!
Type at you later,
Dan McEntee

Edit to add: There is the possibility that, if the tank Paul is using is old enough, this is what he is experiencing, and the only way to tell is to open the tank. I would still make some test flights without pressure first.

Mark wood · « **Reply #41 on:** February 22, 2022, 11:32:53 AM »

Quote from: Dan McEntee on February 22, 2022, 11:06:14 AM

The nitro is the prime offender and alcohol doesn't help when the nitro gets to it and has it etched. When nitro was made in the good old USA it wasn't as bad. Years ago, when I worked part time at a local hobby shop, we primarily sold K&B fuel. Red Max brand came along and while we didn't offer it, several shops in the area did. It was cheaper than K&B and that was the primary attraction. This was about the time that the first nitro shortage hit when one of the t6wo plants in the USA that produced it either shut down or blew up. The EPA would not let anyone else do a start up on new production. So, that was when Chinese nitro started to flow into the country. Chinese nitro was much more acidic. Guys using Red Max started having bearing failures and that prompted use if good after run oil to flush the excess fuel off the bearings. Word of this got around so we did the test live and in person at the check out counter. One small jar with K&B fuel and a 1" length of brass fuel line. Next to it was another jar with Red Max and the same 1" length of tubing. The jars were sealed with tape and dated. It didn't take long for the tube in the Red Max fuel to turn black. That was my first education on it. This was in the mid to late 1980's I think. After that I think I red in Stunt News where it was suggested to use copper tube in all fuel tanks. Fast forward to present day and all nitro comes from over seas. These days we are just happy to get consistent fuel. Glow Plugs will be the newest challenge. I'm stocked up pretty good but still keep my eyes peeled for good deals!
Type at you later,
Dan McEntee

Edit to add: There is the possibility that, if the tank Paul is using is old enough, this is what he is experiencing, and the only way to tell is to open the tank. I would still make some test flights without pressure first.

I wouldn't expect a cracked fuel pickup to behave in that manor, but maybe. I'm going with muffler pressure.

Paul Van Dort · « **Reply #42 on:** February 22, 2022, 01:09:49 PM »

I am using brand new tubes and no nitro in the fuel. Checked the tubes. No cracks. I agree with the reactivity of brass to some fuels. Important feedback. Thx!

Reptoid · « **Reply #43 on:** February 22, 2022, 01:57:29 PM »

Quote from: Mark wood on February 22, 2022, 09:19:48 AM

Really?

I for one am very easy to convince when shown the correct math and physics. This problem is not complex and the bottle example is a physics 101 lab experiment. Both Paul and Brett are correct and your assessment is flawed. Arguing would be pointless without the tools necessary to understand. With the necessary tools to understand, you would see the flaw.

Yup, you're a genius and I don't have the "Tools" to understand.

Reptoid · « **Reply #44 on:** February 22, 2022, 02:04:07 PM »

Paul;
I don't know how hard it would be to modify your tank but after you try all of the "genius" fixes on here , if you still have the problem, you might want to try moving the uniflo outlet to the outboard wall of the tank about 1/2" shorter than the pick up which is where it should be anyway. You've got nothing to lose. Successful flyers all over the world have been building and flying with them built like that for decades.

Dan Berry · « **Reply #45 on:** February 22, 2022, 02:56:21 PM »

Quote from: Paul Van Dort on February 22, 2022, 08:29:29 AM

My tank indeed uses brand new brass tubing

I would redo it with copper. Current brass and current nitro seem to be incompatible

Mark wood · « **Reply #46 on:** February 22, 2022, 09:05:10 PM »

Quote from: Reptoid on February 22, 2022, 01:57:29 PM

Yup, you're a genius and I don't have the "Tools" to understand.

Some have said so but that is debatable outside of a narrow expertise band. I just have a physics degree along with an engineering degree which means I'm over educated on the subject and have the fundamental understanding. I do give you huge credit for trying and you do have a basic grasp of the concept which is much better than most.

Paul Van Dort · « **Reply #47 on:** February 23, 2022, 01:19:05 AM »

Quote from: Mark wood on February 22, 2022, 09:05:10 PM

Some have said so but that is debatable outside of a narrow expertise band. I just have a physics degree along with an engineering degree which means I'm over educated on the subject and have the fundamental understanding. I do give you huge credit for trying and you do have a basic grasp of the concept which is much better than most.

Damn, I just have an engineering degree :-)

Steve Helmick · « **Reply #48 on:** February 25, 2022, 02:10:01 PM »

I've run the Super Tigre G.51 a fair amount and found it to be a good stunt engine. I'm not sure whether I've run it on muffler pressure or not, but have run muffler pressure on Magnum (ASP) XLS .36 and OS .46LA with very good results. I would not blame the muffler pressure. If somebody wants to try muffler pressure, I would caution them to NOT try it IF they typically tend to have mufflers fall off and stuff come loose. Otherwise, I think it works great, but not sure it is needed on a G.51, if that is the ST .51 that you're struggling with. Super Tigre made at least 4 or 5 different .51's, so there is that.

Change to copper tubing in the tank, run 10% nitro to get the fuel consumption up, along with a decent sized venturi for the same objective. Glow engines don't like to be denied of plenty of fuel and thus, oil. If you must use zero nitro, then I'd suggest increasing the oil content in the fuel...it might help, or not. I'd wonder if the fuel filter is getting hot more than the fuel in the tank.

Steve

Dan Berry · « **Reply #49 on:** February 25, 2022, 05:22:46 PM »

Does it continue to go richer or go rich and stay put?