Engine basics > Engine set up tips
Engine seemingly going richer throughout flight?
Lauri Malila:
--- Quote from: Dan McEntee on March 19, 2024, 10:36:06 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
--- End quote ---
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
Motorman:
Engine needs more load and more nitro. Put a draggy prop on it and build some heat.
Dave_Trible:
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.
Dave
Dan McEntee:
--- Quote from: Lauri Malila on March 19, 2024, 11:30:05 AM ---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
--- End quote ---
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
Brett Buck:
--- Quote from: Dan McEntee on March 20, 2024, 06:42:50 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.
--- End quote ---
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
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