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Author Topic: Engine seemingly going richer throughout flight?  (Read 582 times)

Offline Curare

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Engine seemingly going richer throughout flight?
« on: March 19, 2024, 01:40:10 AM »
HI guys, I've got a bit of a head scratcher at the moment. I've been flying a refurbished ship with a new-ish Moki 51, and its doing a puzzling thing at the moment, which is seemingly going fat, just as I start entering vertical territory.

I'll get through the vertical 8 OK, but the hourglass is hit and miss, sometime's it'll stay on song, and other times it'll start 8-stroking on the lap leading up to it, which means I ususally end up floundering at the third corner for lack of airspeed.

I've tried new plugs, adding and removing head shims, cleaning the NVA, cleaning the tank. The issue is nothing if not consistent.

Any other things I should be looking for?

Oh, the fuel mix is 10% castor 10%Synth and 5% Nitro. Plug is a Thunderbolt RC long, and the prop is a Xoar 12x6.

Cheers
Greg
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Online Lauri Malila

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Re: Engine seemingly going richer throughout flight?
« Reply #1 on: March 19, 2024, 04:48:42 AM »
Is it a profile or is the fuselage built-up?
Of course, the fuel tank may be faulty, but if we assume it and the engine itself are ok, then I'd look at the thermal insulation of fuel tank. L
« Last Edit: March 19, 2024, 11:12:02 AM by Lauri Malila »

Offline Dan McEntee

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Re: Engine seemingly going richer throughout flight?
« Reply #2 on: March 19, 2024, 10:36:06 AM »
Is it a profile or is the fuselage built-up.
Of course, the fuel tank may be faulty, but if we assume it and the engine itself are ok, then I'd look at the thermal insulation of fuel tank. L

      Can you expand a little bit on this? I have had experiences in the past where I wondered if the engine was heating the fuel tank and such or not. Some designs make things pretty snug up front. What happens and what do you use for the insulation?

  Thanks in advance,
  Dan McEntee
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Offline Brett Buck

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Re: Engine seemingly going richer throughout flight?
« Reply #3 on: March 19, 2024, 11:07:39 AM »
Is it a profile or is the fuselage built-up.
Of course, the fuel tank may be faulty, but if we assume it and the engine itself are ok, then I'd look at the thermal insulation of fuel tank. L

   That's a pretty good guess, but the few times I have seen that happen were with fairly extraordinary arrangements of fuel tank and muffler or header. I think a picture would help immensely.

     Brett

Offline Brett Buck

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Re: Engine seemingly going richer throughout flight?
« Reply #4 on: March 19, 2024, 11:10:56 AM »
      Can you expand a little bit on this? I have had experiences in the past where I wondered if the engine was heating the fuel tank and such or not. Some designs make things pretty snug up front. What happens and what do you use for the insulation?

   Last few times, a 1/16 balsa sheet between the muffler and the tank worked pretty well.

    Brett

Online Lauri Malila

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Re: Engine seemingly going richer throughout flight?
« Reply #5 on: March 19, 2024, 11:30:05 AM »
      Can you expand a little bit on this? I have had experiences in the past where I wondered if the engine was heating the fuel tank and such or not. Some designs make things pretty snug up front. What happens and what do you use for the insulation?

  Thanks in advance,
  Dan McEntee

When fuel gets warmed by heat from engine/muffler, its viscosity goes down and it flows easier through the needle valve, and engine gets richer.
I cover my tanks with 2-3mm balsa & aluminised mylar, and try to get as much fresh air flowing between tank and silencer as possible.
Ideally you should have separate engine- and tank compartments with separate cooling air canalisation.
a very small richening, that seems to be inherent for my uniflow tanks however I do them, can be compensated by tilting the tank a little, by moving the outside rear corner about 3...4mm more out. L

Offline Motorman

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Re: Engine seemingly going richer throughout flight?
« Reply #6 on: March 19, 2024, 10:21:36 PM »
Engine needs more load and more nitro. Put a draggy prop on it and build some heat.

Offline Dave_Trible

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Re: Engine seemingly going richer throughout flight?
« Reply #7 on: March 20, 2024, 01:57:17 PM »
If you are using a uniflow tank and it is working well you COULD be building up a little more air head pressure than you need as the flight continues.  You might try restricting the airflow just a bit going in by sleeving down the inlet tube a size or two.  It is similar to the effect of flying into the wind and the engine goes rich,  then speeding up downwind.  Might not be the issue but is simple to try.

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Offline Dan McEntee

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Re: Engine seemingly going richer throughout flight?
« Reply #8 on: March 20, 2024, 06:42:50 PM »
When fuel gets warmed by heat from engine/muffler, its viscosity goes down and it flows easier through the needle valve, and engine gets richer.
I cover my tanks with 2-3mm balsa & aluminised mylar, and try to get as much fresh air flowing between tank and silencer as possible.
Ideally you should have separate engine- and tank compartments with separate cooling air canalisation.
a very small richening, that seems to be inherent for my uniflow tanks however I do them, can be compensated by tilting the tank a little, by moving the outside rear corner about 3...4mm more out. L

       There have been some threads over the last few years on the subject of fuel and oil viscosity that I read with interest. That lead to curiosity about engine heat and exhaust affecting the fuel in the tank. I don't think I have ever had any problems of this nature on a full fuselage model. I have never run any kind of rear exhaust engine to have to worry about a pipe or muffler heating up a tank. On profile models, I used to think that being out in the open air it was just a problem you never have too deal with, but then would encounter some one with bad runs and one of the fixes was stuffing something between the engine and the tank. A soda can sheet metal deflector o or a scrap of balsa at least 1/8" thick. it hasn't happened enough to really remember details. Mark Hughes built an Imitation with a regular sidewinder profile fuselage and powered it with an LA.46 purchased from a notable engine person, and that set up gave him fits for months. The nose was a bit short and the tank was right on the back of the engine, and the problem was he would get one to two good runs and then it would run away. I suspected engine heating the tank, and put in heat shields but that didn't help. In the end, we found that the cylinder liner was installed off line, and the alignment pin left a witness mark on the bottom of the lip at the top pf the liner, and tightening the head at assembly tweaked the top pf the bore out of round right at TDC. We replaced the liner with a good used one, and he hasn't had a bad run since over hundred of flights. heat shields have been removed for a long time.  I have a Ringmaster with a Fox .35 that does the slight richening thing over a tank full, and it picks up 2 to 3 tenths of a second over the length of the run. Nothing else and cured this and I was wondering if the opposite of what you say might be happening, that is once in the air, the tank get cooled from air flow and as the fuel load runs down, It has been incredibly consistent even with engine changes and all the other usual remedies, but I don't think I thought to try a heat blocker between the tank and engine. If the weather ever cooperates and we get to fly again in my life time, I'll test that out. I'll also try reducing the uniflow vent to also as Dave suggested. It's just a foo foo fun fly airplane but I love flying it and I think it's the best flying Ringmaster I have out of three. I don't have a muffler on it, and as it sits, the tank may be in line with the exhaust as it exits the stack and any hot oil in the exhaust could be sticking to the tank and heating it up.
   Thanks for the insight,
    Dan McEntee
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Offline Brett Buck

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Re: Engine seemingly going richer throughout flight?
« Reply #9 on: March 20, 2024, 10:20:46 PM »
       There have been some threads over the last few years on the subject of fuel and oil viscosity that I read with interest. That lead to curiosity about engine heat and exhaust affecting the fuel in the tank. I don't think I have ever had any problems of this nature on a full fuselage model. I have never run any kind of rear exhaust engine to have to worry about a pipe or muffler heating up a tank.
 

   It's not all that common. I have seen maybe 3-4 obvious cases of it over the years. Ted's Tucker Special (yes, that one) has a Rustler-Merco 40 Metamorph, which is a generally excellent engine with a switchable exhaust. In rear-exhaust mode, the front part of the muffler does not clear a 1" tank, and the rest of it is just a little higher than 1". In Ted's installation, he relieved the front of the tank to clear the head end of the muffler, but otherwise the rest of it ran maybe 1/8" away with nothing in-between. It would take off and run for a while but at some point it would start getting richer and richer, to the point of becoming incapable of continuing the flight.

    I would also note that, like a lot of the "highly developed" baffle-piston engines, it is extremely fuel-efficient and runs very very hot (which is why it is so efficient). So the muffler would get extraordinarly hot, even just 4-stroking, and of course the low fuel flow also led to a tiny metering setting on the needle.  We noticed that the tank was also getting very hot, and it *may* have  started to melt the nylon 1/4-20 screw holding the tank down. So, I got some 1/16 balsa out of my repair box, and we cut it to fit, cut a hole in it, and held it in with the screw. Next flight, and all subsequent fiights, no problem, dead steady.

   Under normal circumstances with normal engines, even running the tank into the header on a TP engine, there is rarely a problem. I have a tank floor, so I have an insulating layer of 3/32 balsa.

    After this all happened, it dawned on me that if you could heat the fuel repeatably and keep it at a constant temperature, you could use that to get more power out of it, since the thinner fuel is easier to draw, so you could would either use a bigger venturi without hurting the run quality, or, get a smoother run without losing power. So, I made a crude fuel pre-heater that wrapped some copper tube around the header, the fuel goes in the back of the coil, gets heated by the exhaust heat, then out the other end and into the spraybar. This actually worked surprisingly good, greately smoothing out the already smooth run, like running a smaller venturi without the loss of power.

    The problem with it is that it got *too* hot, too close to the decomposition temperature of nitromethane by itself, and the copper doing who-knows-what catalytically. So, I switched back, and pursued the same effect with different (and less) oil, and fuel system flow improvements instead of heat. It wasn't really that big of a deal on the Jett 61, because it was already extremely steady to begin with. Similar things on the PA75 were much more obvious, cleaning up a lot of little problems that David had just been living with.
 
      Brett
       

Offline Steve Helmick

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Re: Engine seemingly going richer throughout flight?
« Reply #10 on: April 02, 2024, 05:17:27 PM »
Is a new-ish Moki .51 ringed, AAC or ABC? It runs a 4-2-4 run, with the 6" pitch prop? I'd try different fuels, perhaps different oils or oil ratios. Borrow a cuppa fuel from your mite... #^
 
I'd buy into the heating of the fuel being the likely problem, but won't go so far as to say "Eureka" yet. Faulty fuel tanks can do some weird stuff, like a perforated or cracked uniflow tube. Can you 'splain what the tank is? Tin, plastic, production or home brew? I'd like to hear what further testing has shown.  H^^ Steve



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Offline Dan McEntee

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Re: Engine seemingly going richer throughout flight?
« Reply #11 on: April 03, 2024, 11:15:51 AM »
   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
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