Hi all, I had an interesting find last Friday. In Belgium we had afternoon temperatures of around 30+ degrees Celsius.
The fuel in the fuel jug was heating up from around 15 degrees (fresh in the garage) to 30 degrees at the end of the flying session.
I made about 8 flights and I kept on closing the needle valve after each flight to cope with the decreasing viscosity of the fuel due to the fuel warming up.

I found out that an increase of 15 degrees Celsius (or Kelvin) requires about a full turn on the needle valve of my Saito 72.

I never was aware of this problem with my 2 strokes. Or I did not understand what was going on..

I have a rather long, bent feed tube in my tank of about 7 inches (1/8" outside diameter) and the flow is about 4 inches per second.

Now I have 2 options: I can take into account the fuel temperature and adjust the needle accordingly before each flight or I can try to keep the fuel temperature constant in an temperature isolated jug. What is the best practice?

Quote from: Paul Van Dort on May 24, 2026, 01:43:22 PMHi all, I had an interesting find last Friday. In Belgium we had afternoon temperatures of around 30+ degrees Celsius.
The fuel in the fuel jug was heating up from around 15 degrees (fresh in the garage) to 30 degrees at the end of the flying session.
I made about 8 flights and I kept on closing the needle valve after each flight to cope with the decreasing viscosity of the fuel due to the fuel warming up.

I found out that an increase of 15 degrees Celsius (or Kelvin) requires about a full turn on the needle valve of my Saito 72.

I never was aware of this problem with my 2 strokes. Or I did not understand what was going on..

I have a rather long, bent feed tube in my tank of about 7 inches (1/8" outside diameter) and the flow is about 4 inches per second.

Now I have 2 options: I can take into account the fuel temperature and adjust the needle accordingly before each flight or I can try to keep the fuel temperature constant in an temperature isolated jug. What is the best practice?

It's not just fuel temperature. But also outside air temperature which probably has a bigger effect. Where I normally fly it can be 15C in the early morning and go to 32C by early afternoon.

Ambient air gets less dense as it gets hotter. Therefore, there is less oxygen in the air to burn the fuel. If you don't adjust the needle the engine fuel mixture will get richer and richer as the outside air warms up.

Since I know the prop rpm needed for a particular model/engine combination, I routinely tach the engine before each launch. Usually have to tweak the needle leaner bit by bit as the day progresses warmer to obtain the rpm I want.

And it also applies to 2-strokes, not just FS's.

Colin

Quote from: Colin McRae on May 24, 2026, 03:27:39 PMAmbient air gets less dense as it gets hotter. Therefore, there is less oxygen in the air to burn the fuel. If you don't adjust the needle the engine fuel mixture will get richer and richer as the outside air warms up.

So is it fair to say as air warms up and gets less dense it can hold more moisture (grains of moisture/lb of air)? And as the amount of water vapor in the volume of air increases the oxygen is displaced reducing the combustion performance.
 

Quote from: Steve Dwyer on May 24, 2026, 06:26:40 PMSo is it fair to say as air warms up and gets less dense it can hold more moisture (grains of moisture/lb of air)? And as the amount of water vapor in the volume of air increases the oxygen is displaced reducing the combustion performance.
 

Absolutely. To cope with this I install a larger air inlet. 0.1 or 0.2 mm larger in diameter. E.g from 4.7 to 4.8 dia.

It's mostly the viscosity change because of temperature, but of course a small change in air density during the day (less density = rich).
You can also see it during one flight when the heat radiation from engine/muffler heats up the fuel in tank. With 4-stroke   and muffler outside fuselage it's less of a problem, but it never hurts to insulate the tank from heat radiation.
You can do quite a lot just by keeping the fuel in shadow and keeping a wet rug over model nose at the pits. And fill the tank just before flight.
I also use a small usb-fan to cool the engine/model nose between flights. It's great especially on hot & calm days, engine is ready to go in 5 minutes instead of 20 without fan.
All this sounds like a lot of extra hassle, but when you make it a routine, it's not a big deal.
Just remember that in Australia the shadow mover to the other way.
The Chinese always kept their fuel in thermos bottles. I haven't gone that far yet. L

Thanks, Lauri. You confirm my observations. I measure the temperature of the fuel in the jug. Keeping the nose cool with a damp rag is a splendid idea. I like a good routine before each flight. Like going over a checklist in real aviation. In the mean time I am a big fan of my Saito 72. Happy and predictable flying

Quote from: Paul Van Dort on May 24, 2026, 11:42:10 PMTo cope with this I install a larger air inlet. 0.1 or 0.2 mm larger in diameter. E.g from 4.7 to 4.8 dia.

Isn't this counterproductive because it further reduces the velocity pressure drop inside the venturi drawing less fuel and reducing the atomization? While having little effect on the still increased volume of moisture impacting engine performance.
 

Quote from: Steve Dwyer on May 25, 2026, 07:32:38 AMIsn't this counterproductive because it further reduces the velocity pressure drop inside the venturi drawing less fuel and reducing the atomization? While having little effect on the still increased volume of moisture impacting engine performance.
 

The idea is to increase the a amount of oxygen to bring it to the same level as in colder circumstances. I use the same trick when flying in higher sites. I usually fly at sea level.
The pressure drop in the venturi is very small and suction (about minus 10 inch of water column) is still plenty

This is getting a little towards homeopathy.
Of course, I don't know about 4-strokes, but with my .77 I *very* rarely need to go up with venturi size, and even then I play with +/-0,05mm from the nominal ø5mm venturi. And that's between cool sea level and very hot Ciriè. Often there the density altitude is equivalent to about 2000m.
Because I use no nitro, I've made the venturi change very easy, it only takes a few seconds.
Beringers use the venturi cross area adjustment screw in their Saito's, that sounds handy. I just have a big collection of venturies.
In general, people play too much with engine parameters. To me that most often tells that something is badly off with the basic settings, or engine is of poor quality. L

Quote from: Lauri Malila on May 25, 2026, 07:47:50 AMThis is getting a little towards homeopathy.
Of course, I don't know about 4-strokes, but with my .77 I *very* rarely need to go up with venturi size, and even then I play with +/-0,05mm from the nominal ø5mm venturi. And that's between cool sea level and very hot Ciriè. Often there the density altitude is equivalent to about 2000m.
Because I use no nitro, I've made the venturi change very easy, it only takes a few seconds.
Beringers use the venturi cross area adjustment screw in their Saito's, that sounds handy. I just have a big collection of venturies.
In general, people play too much with engine parameters. To me that most often tells that something is badly off with the basic settings, or engine is of poor quality. L

Same here: big collection of venturies :-)

Quote from: Paul Van Dort on May 25, 2026, 07:43:53 AMThe idea is to increase the a amount of oxygen to bring it to the same level as in colder circumstances.

Humm...But the molecular relationship of oxygen to water remains the same no matter how the volume changes.

Of course do the beforementioned things and your life will be easier, but maybe you should also go up with nitro %. I don't know how many % you have it now but with 4-strokes it seems to be the key to stability.
With 2-strokes there are some ways to manage without nitro, like higher compression ratio and the combustion chamber design, they are not so easy changes with a 4-stroke. L

Quote from: Lauri Malila on May 25, 2026, 01:31:14 PMOf course do the beforementioned things and your life will be easier, but maybe you should also go up with nitro %. I don't know how many % you have it now but with 4-strokes it seems to be the key to stability.
With 2-strokes there are some ways to manage without nitro, like higher compression ratio and the combustion chamber design, they are not so easy changes with a 4-stroke. L

The topic was the effect of the viscosity of the fuel on the needle setting. The decrease in power because of higher temperatures is a completely separate issue. I use 10% nitro and the Saito is happy with it. The engine has too much power available, but I reduce the power by using smaller venturi inserts. So power is manageable and therefore not an issue for me. Many threads are covering power issues, but very little can be read about fuel temerature and the effect of changing viscosity. While the effect is real and is affecting our engine runs.

Has anybody ever actually measured the fuel viscosity and documented accurately if/how much temperature actually affects it?

I wonder if the change in atmospheric conditions affecting the operation of the engine as temperature increases might be the culprit that we're trying to maybe assign to fuel viscosity.

Bill,

Not directly the viscosity index, but the fuel temperature in tank during flight has been measured, as well as in-flight rpm. I have a nice wireless gadget for doing that.
The best proof of this theory (or is it common sense) is the immediate improvement after the tank heat shields are in place.
Also the needle valve mounting makes a measurable difference, I mean remote valve vs a valve bolted to crankcase. Remote valve is much more logical during the first 30sec. or so as the Carter heats up.
Of course the air density changes during the day, but it has a smaller effect.

Paul,
This is not the first time this subject comes up, I'm sure you'll find something if you use the search. L

Quote from: BillLee on May 25, 2026, 03:06:40 PMHas anybody ever actually measured the fuel viscosity and documented accurately if/how much temperature actually affects it?

I wonder if the change in atmospheric conditions affecting the operation of the engine as temperature increases might be the culprit that we're trying to maybe assign to fuel viscosity.

The mix of methanol and oil is really affected by temperature. AI gave me the calculation and the flow through the feedpipe was increased with 30% with the fluids temperature rise from 10 deg celsius to 30 degrees.

Quote from: BillLee on May 25, 2026, 03:06:40 PMHas anybody ever actually measured the fuel viscosity and documented accurately if/how much temperature actually affects it?

I wonder if the change in atmospheric conditions affecting the operation of the engine as temperature increases might be the culprit that we're trying to maybe assign to fuel viscosity.

  In this case it is unclear because the range of temperatures is so small. 59 degrees to 86 degrees is a pretty small range and we have alway attributed the sort of effect to air density. Both effects are going at the same time (fuel warming up and air getting thinner) so hard to say. It may well be that 4-strokes are more sensitive to it because the metering orfice is tiny compared to a 2-stroke since it is using so little fuel.

   All the cases we found where the fuel temperature affected the viscosity were with much larger temperature changes, like the exhaust header heating up the tank, or doing it intentionally with a fuel pre-heater. These were *drastic* effects that were undeniable.

     I never ran a 4-stroke myself but many of my cohorts did, one thing we found was it tended to be extremely sensitive to adjust, and that adjusting the mixture had very weak effects on the speed and many times unfortunate effects on the way it ran. People were just cluing into the Berringer-style throttle - for which Bob Reeves found an excellent implementaion that anyone could do - when it became obvious that electric was the right way to go.

    But your idea is intriguing, maybe we should make some more careful measurements, and it's not *that* hard to do. Painters use "viscosity cups" to decide ho much thinner to use, basically a spoon with a hole in the bottom, and they measure how long it takes to run out .

   Brett

 p.s. again, I am no 4-stroke expert (or in fact engine expert) but changing venturis in .008" increments (.1 "mm") for a very minor temperature change seem like a lot. On a 4-stroke I think that is more viable than trying to needle it.

These days we have 35 degrees Celsius here, compared to 12 last week. Fuel heats up from 15 degrees Celsius (garage, car , first flight) to 33 degrees 2 hours later. The tubing in my tank is 1/8 outside diameter and about 6 inches long. The decreasing viscosity certainly plays a role in the decreasing flow resistance in the tube. My rule of thumb for my Saito: 15 degrees temperature increase of the fuel requires the needle to be closed a quarter turn. 1 minute per degree :-). The cylinder is outside the fuselage (Sidewinder) and the tank is partially outside the fuselage on the other side in the breeze. So engine heath is nothing to worry about in my case.

Then make a simple test. Get a thermos bottle to keep the fuel in constant temperature, and see what happens during the day. If it still goes richer, then it's the air density to blame. And if it's that, the easies way is to add a little nitro in hot weather.
And it's kind of asking for trouble to store the fuel in garage temperature +15 and expect it to work the same all day.
At least let the fuel warm up to room temperature before flying.

Quote from: Paul Van Dort on May 26, 2026, 12:10:24 PMThese days we have 35 degrees Celsius here, compared to 12 last week. Fuel heats up from 15 degrees Celsius (garage, car , first flight) to 33 degrees 2 hours later.

   That's not much of a range, and, I don't see how a substantial amount of fuel stays in equilibrium when it goes up 30+ degrees (conventional) in 2 hours. I just carry fuel around in the back of the car, it goes from the mid-40's to maybe 130 closed up in a hot car,  I can't really tell much difference in how it runs independent of the air density changing. As Laurie suggests, put it in a styrofoam cooler (maybe with ice) and see if it makes any difference.

QuoteThe tubing in my tank is 1/8 outside diameter and about 6 inches long. The decreasing viscosity certainly plays a role in the decreasing flow resistance in the tube.

   The big change is at the needle, at the point where the metering occurs. We noticed absolutely *no* effects, even with 2x flow rate, from the supply tubing. It only seemed to make a difference when it got to 8.5 ounces in 5:30. The big effect was at the spraybar, specifically, the PA spraybar were the inlet side was very small, and counterboring it made a HUGE difference.

     But for sure the big effect is at the metering, that is by definition the smallest passage and the place most affected by viscous flow. 4-strokes probably *are* more prone to viscosity effects because they have pretty decent suction and require tiny amounts of fuel, so the metering are has got to be extremely small, so much more prone to viscous effects.

    Brett

Bill--yes.

Quote from: Dave Hull on May 29, 2026, 07:07:47 PMBill--yes.

And the answer is .....

Bill,

I had to go back and find out where I put the data. It was not labeled as a stand-alone test, it was folded in with other fuel spreadsheets and notes....

In general, there is a noticeable increase in viscosity of the fuel I was concerned with (5N/18Klotz Original Techniplate/77Alky) over a reasonable user ambient temperature range. I went from 60F to 130F as measured by a kitchen mercury thermometer. I used an ice bath and a hot water bath to get there. You can see the pot I used to the far right in the picture. I used a 5 oz. sample size, hoping for better accuracy. There is no attempt to correlate to any viscosity scale; this is more akin to using a cup to measure paint viscosity. I had an issue with a specific airplane and wanted to see what might be a possible influence, sorting out big things from little things. Along those lines, I tested one or two runs with an in-line fuel filter, which had a MUCH larger effect on fuel flow than the temperature range that I investigated.

Data points 1 and 5 overlay each other and appear as "one" dot to the left in the graph. The repeatability was good for this 60F test. This probably had a lot to do with the ambient air temp in the shop being closer to 60F and I didn't have to rush the test execution? The data scatter to the right seems large, but is only around a time/reading error of 2 seconds or so. I would have needed some real test equipment to do significantly better. I didn't bother filling in the middle because the plane I was debugging had an internal tank and I wondered if I was getting significant in-cowl heating. I did calculations prior to this on the energy transfer to the fuel from the muffler pressure and it was negligible.

The scatter in my data was large at the "hot" end because I needed four hands, not two. Still, you can see the trend line (I highlighted in yellow for this post) if you squint just right. I recall taking more data but not sure where it went or why I didn't continue plotting it. Maybe because I wanted to fly, and the quickest way to see if fuel draw was my problem based on these tests was to simply take out the fuel filter and fly again.

Certainly more could be done here. I thought I had data for some Fox fuel (10N/29Castor/61Alky) but I don't see it now. I did find viscosity data for castor oil alone which I plotted up. Not surprisingly, it REALLY turns to molasses at colder temps. That one is measured in centistokes, so it is more than just a relative comparison. For example, 1000cS at 20C and 300ish cS at 40C. But again, it is largely diluted by the alcohol and nitro. And since the Fox Stunts and Max-S's all have pretty good fuel draw, it would seem less of an issue.

I hope this answers some of your questions.

Dave

Quote from: Dave Hull on May 30, 2026, 07:42:15 PMBill,

I had to go back and find out where I put the data. It was not labeled as a stand-alone test, it was folded in with other fuel spreadsheets and notes....

In general, there is a noticeable increase in viscosity of the fuel I was concerned with (5N/18Klotz Original Techniplate/77Alky) over a reasonable user ambient temperature range. I went from 60F to 130F as measured by a kitchen mercury thermometer. I used an ice bath and a hot water bath to get there. You can see the pot I used to the far right in the picture. I used a 5 oz. sample size, hoping for better accuracy. There is no attempt to correlate to any viscosity scale; this is more akin to using a cup to measure paint viscosity. I had an issue with a specific airplane and wanted to see what might be a possible influence, sorting out big things from little things. Along those lines, I tested one or two runs with an in-line fuel filter, which had a MUCH larger effect on fuel flow than the temperature range that I investigated.

Data points 1 and 5 overlay each other and appear as "one" dot to the left in the graph. The repeatability was good for this 60F test. This probably had a lot to do with the ambient air temp in the shop being closer to 60F and I didn't have to rush the test execution? The data scatter to the right seems large, but is only around a time/reading error of 2 seconds or so. I would have needed some real test equipment to do significantly better. I didn't bother filling in the middle because the plane I was debugging had an internal tank and I wondered if I was getting significant in-cowl heating. I did calculations prior to this on the energy transfer to the fuel from the muffler pressure and it was negligible.

The scatter in my data was large at the "hot" end because I needed four hands, not two. Still, you can see the trend line (I highlighted in yellow for this post) if you squint just right. I recall taking more data but not sure where it went or why I didn't continue plotting it. Maybe because I wanted to fly, and the quickest way to see if fuel draw was my problem based on these tests was to simply take out the fuel filter and fly again.

Certainly more could be done here. I thought I had data for some Fox fuel (10N/29Castor/61Alky) but I don't see it now. I did find viscosity data for castor oil alone which I plotted up. Not surprisingly, it REALLY turns to molasses at colder temps. That one is measured in centistokes, so it is more than just a relative comparison. For example, 1000cS at 20C and 300ish cS at 40C. But again, it is largely diluted by the alcohol and nitro. And since the Fox Stunts and Max-S's all have pretty good fuel draw, it would seem less of an issue.

I hope this answers some of your questions.

Dave

Good data. Real measurements illustrating a real physical characteristic, taken under controlled and repeatable conditions. A testing methodology even a caveman could repeat.

Thanks, Dave.

Now begs the question: how significant is it for the operation of our engines in conjunction with the air density changes due to temperature?

Perhaps that is the question this thread is addressing.

Bill

Quote from: Dave Hull on May 30, 2026, 07:42:15 PMBill,

I had to go back and find out where I put the data. It was not labeled as a stand-alone test, it was folded in with other fuel spreadsheets and notes....

In general, there is a noticeable increase in viscosity of the fuel I was concerned with (5N/18Klotz Original Techniplate/77Alky) over a reasonable user ambient temperature range. I went from 60F to 130F as measured by a kitchen mercury thermometer. I used an ice bath and a hot water bath to get there. You can see the pot I used to the far right in the picture. I used a 5 oz. sample size, hoping for better accuracy. There is no attempt to correlate to any viscosity scale; this is more akin to using a cup to measure paint viscosity. I had an issue with a specific airplane and wanted to see what might be a possible influence, sorting out big things from little things. Along those lines, I tested one or two runs with an in-line fuel filter, which had a MUCH larger effect on fuel flow than the temperature range that I investigated.

Data points 1 and 5 overlay each other and appear as "one" dot to the left in the graph. The repeatability was good for this 60F test. This probably had a lot to do with the ambient air temp in the shop being closer to 60F and I didn't have to rush the test execution? The data scatter to the right seems large, but is only around a time/reading error of 2 seconds or so. I would have needed some real test equipment to do significantly better. I didn't bother filling in the middle because the plane I was debugging had an internal tank and I wondered if I was getting significant in-cowl heating. I did calculations prior to this on the energy transfer to the fuel from the muffler pressure and it was negligible.

The scatter in my data was large at the "hot" end because I needed four hands, not two. Still, you can see the trend line (I highlighted in yellow for this post) if you squint just right. I recall taking more data but not sure where it went or why I didn't continue plotting it. Maybe because I wanted to fly, and the quickest way to see if fuel draw was my problem based on these tests was to simply take out the fuel filter and fly again.

Certainly more could be done here. I thought I had data for some Fox fuel (10N/29Castor/61Alky) but I don't see it now. I did find viscosity data for castor oil alone which I plotted up. Not surprisingly, it REALLY turns to molasses at colder temps. That one is measured in centistokes, so it is more than just a relative comparison. For example, 1000cS at 20C and 300ish cS at 40C. But again, it is largely diluted by the alcohol and nitro. And since the Fox Stunts and Max-S's all have pretty good fuel draw, it would seem less of an issue.

I hope this answers some of your questions.

Dave

   Interesting test. I would have thought that once oil is diluted in methanol, it might not be affected by temperature at all, at least not the swing from a morning temp of say 70F degrees  to 90F degrees in the afternoon. One of the first things I learned about operating engines in stunt 40 some odd years or more ago, was that as the temperature warmed up, it was necessary to lean the needle setting to get the same RPM I wanted, because the air was thinner, and had less oxygen, so the engine needed less fuel, or as we fondly referred to it as "chasing the needle." And to  put a little less fuel on board if I was close to over running in the first round of a contest. Then as time went on, I watched as some guys would tend to leave the needle alone, and just add the required proper amount of nitro to their fuel mix to get the same needle and power setting? That alone would confirm that the change was due to an increase in density altitude? And certainly not limited to four strokes, but given that thumpers use less fuel to operate than a comparable two stroke, they might be more sensitive to it? My experience with four strokes is limited to the Saito .56 in a Top Flite Score that I have. It got fitted with a screw that gets turned in or out with major temp changes, in  for when it got significantly cooler, and out when it got warmer, and if I didn't turn it in to decrease the size of the venturi opening as the temps got down to and below 60F degrees, the 4 ounce fuel tank would be enough to complete the pattern. And is this change in viscosity also affect the methanol? Alcohol has a much lower boiling point than water, doesn't it? Where does that fit into the results?

    This leads into another question, then, I think. If you use muffler pressure, does the temperature of the exhaust discharge heat the fuel at all? It might not in the first minute or so of running, but given the exhaust temps will be well above 350F degrees, that exhaust being pumped into a tank has to be well above ambient air temperature. I often see smoke coming from a tank if you pull the pressure line as soon as possible after landing. I usually plug the overflow on my Score, and direct muffler pressure to the fill/vent tube, which has the other end immersed in fuel for almost the entire flight. Is that heating up the fuel load any? One thing I have noticed about this four stroke set up I run is that the engine runs dead nuts the same throughout the run, and only increases slightly at about the hourglass, and I take that as typical of a standard vented tank, and would expect that on a Fox .35 also. It's pretty subtle, and is seen mostly in the lap time, and increase of a tenth, maybe two tenths of second at most, and at that point in the pattern, is usually helpful. As some questions get answered, more questions come up.

  Type at you later,
  Dan McEntee

Dan,

AS I noted in an earlier post, I calculated the temperature rise of the fuel due to the use of muffler pressure. It is negligible. The easiest way to understand this is to simplify the statement of the problem down. You can only put in exactly 5 volumetric oz. of exhaust air into a 5 fluid oz. tank. It does not cycle thru the entire flight. The heat content of that volume of exhaust is actually not that large. The thermal conductivity of the tubing and a column of exhaust air inside is negligible as well. So if the air is barely moving and it is also a poor conductor, then...that question is answered.

On the other hand, running a rear exhaust racing motor on a Clown and having it right up against your metal fuel tank provides a dramatic change in the run. A simple exhaust deflector solves that problem. Note that in this example, no tank pressure was used (it is not allowed by rule), so ALL of the change came from the fuel being at a much hotter temperature.

Dave

My last reply seems to have disappeared. Let me try again....

As I noted in a prior post, the heat content from the exhaust when using muffler pressure is negligible. This becomes more obvious when you consider that the maximum amount of exhaust that can go into your 5 oz. tank is one tank-volume-worth. Unless you have an air leak--which means the setup is already screwed up. The fuel line and the almost stagnant air in the muffler pressure line are very poor conductors of heat, so conduction is REALLY negligible.

That's not to say that heating up the tank cannot have a dramatic effect on the engine run. As one EXTREME example, if you run a Clown racer with a rear exhaust engine open-faced, and that exhaust impinges DIRECTLY on a 1-oz. metal tank, there are all kinds of bad things happening. Install an exhaust deflector and everything becomes predictable and normal.

Something else to note about the test I ran: I used the largest calibrated volume I could (the top line in a 5 oz. syringe) to try to improve accuracy. Of course, this is a gravity-fed experiment. The conditions in my garage are pretty close to 1-G. Which we can assume is 32.174 ft/s2. If you look at a "typical" stunter with the fuel tank flying at 63' from the center of rotation (including your arm and some Temu Spring Kitty Klips) and you are hypertweaked to 5.3 seconds per lap, your tank is experiencing right around 2.75 lateral G's. Add that to the gravity at your flying site and you pretty much have 3 G's acting on your fuel. That means that the free flow of my 1-G test even with the fuel filter drains the tank much faster than the SuperBansheek Mk IV in my theoretical example, since you are probably going to want six minutes or more of run time. This is really great news because it means that your needle valve is actual restricting the flow rate, giving you control of the mixture--just what we want.

This also points out that "tuning" your fuel tank is a real thing. That having a solid run on the ground is no proof that it won't shift--sometime dramatically--once in the air. Perhaps at a ratio of up to 3:1 on the initial pressure head. Profiles suffer from this constraint, but full-fuse planes much less so. The time-proven formula of a 2" wide tank that is centered in the fuse, and the engine that is also centered in the fuse, and the pee-hole in the spraybar is ALSO on the fuse centerline means that all of the math can be forgotten and just substitute the universal tribal knowledge (and the parts from Brodak). It works, so we use it.

(I see my "lost" post has magically appeared....  Now I can check to see if I said the same thing!)

Thank you all for contributing to my original post. The scientific approach of testing the viscosity in function of temperature was great.
I am using an oil content of 7% Castor and 15% Klotz. This would result in a bit more viscosity than in the tests that were carried out.
Additionally my feedpipe is also longer than average because of my particular tank design (9 inches length). Therefore flow losses in the feedpipe are
more substantial than usual. Off course the element that is restricting the flow most is the NVA, but all elements add up for the total flow resistance. Flow rate, Pipe length, diameter (2mm), viscosity, curves, diameter changes, restrictions, Reynolds number. The flow rate in the feedpipe is about 4 inches per second for the Saito 72. It uses about 130cc per 6 minutes, 30 seconds.

[quote author=Dave Hull
This also points out that "tuning" your fuel tank is a real thing. That having a solid run on the ground is no proof that it won't shift--sometime dramatically--once in the air. Perhaps at a ratio of up to 3:1 on the initial pressure head. Profiles suffer from this constraint, but full-fuse planes much less so. The time-proven formula of a 2" wide tank that is centered in the fuse, and the engine that is also centered in the fuse, and the pee-hole in the spraybar is ALSO on the fuse centerline means that all of the math can be forgotten and just substitute the universal tribal knowledge (and the parts from Brodak). It works, so we use it.
[/quote]

The "centered" tank position is not ideal, it's more a result of the obsession with narrow fuselages & limited tank room. But it seems that the huge fuel consumption you seem to like hides most of the errors it causes in engine run.
You can improve the run a lot with correct lateral placing of fuel tank, especially with non-piped setups.
The difference between ground- and level flight rpm's is just one thing, not the most important.
The important thing is to find the tank position that gives the correct leaner "boost" in overhead maneuvres, and also to richen and prevent speeding-up in wind as centrifugal force increases.
In practise, at least with my stuff, it means that pickup point in tank should be ~6...8mm "outside" the fuel spigot in venturi.
It's difficult to give accurate values because different models fly with different yaw angle, plus other differences. But experimenting with that is abdolutely worthwile. L

Quote from: Lauri Malila on June 03, 2026, 04:14:56 AMIn practise, at least with my stuff, it means that pickup point in tank should be ~6...8mm "outside" the fuel spigot in venturi.
It's difficult to give accurate values because different models fly with different yaw angle, plus other differences. But experimenting with that is abdolutely worthwile. L

Hi Lauri, I think the location of the uniflow outlet in the tank governs the pressure of the fuel at the spigot. Not the pickup. If both pickup and uniflow are on the same lateral distance e.g. ~6...8mm "outside" the fuel spigot , I fully agree with your finding. My uniflow ends not so deep in the tank. I let the tank act as non-uniflow for the last 2 minutes of the flight. I noticed a tendency for the engine (My Saito 72, but also my ST 51 in another model) to get slightly richer towards the end of the run using a tank with the uniflow ending at the outside tank wall. Now the enginerun remains stable. I never understood the richening effect from 75% of the flight onwards. But the cure works. The size of the bubbles out of the uniflow might have something to do with it. I created a static test setup a few years back, testing the uniflow behavior (Mariotte bottle) and the size and speed of the bubbles exiting the uniflow was not constant.
I read recently in an article not related to Control line, that it can help to "wedge" the exit of the uniflow tube in the tank to have a better "bubble behavior"

Tank location and the uniflow exit both define the change regime after launch, but also the change in load that the prop/ engine is experiencing

My recent tank is very short (5cm) and very wide (10cm) and 3,5cm deep. The pick-up point is  very close to the spigot. If this tank would have standard venting, the engine run would become very rich when airborne because the 10cm fuel column would create a higher pressure in the nva because of the extra G's from the centrifugal force. A uniflow tank helps controlling this effect to some extend.  I think it is a good idea to always do a test flight before a contest. I like setting the needle after a flight in stead of before. There the issue of the viscosity started to annoy me. A perfect run followed by a rich run 1 hour later, without touching the needle...This requires technical answers :-)
And as Brett pointed out, a four stroke engine can be very stable on RPM, while the mixture can be too rich or too lean, so adjusting this type of engine by RPM is very difficult. Chances are that the setting ends up at the wrong side of the power curve...

Quote from: Paul Van Dort on June 03, 2026, 12:00:04 PMHi Lauri, I think the location of the uniflow outlet in the tank governs the pressure of the fuel at the spigot. Not the pickup.

  ??  They are not independent of each other, but for sure, the reference pressure is *at the end of the vent*. Where it goes after that determines it at any other point. Since the path doesn't matter, only where it ends up, you can easily figure out what the pressure change from the vent outlet to the venturi inlet might be.

    Brett

Quote from: Brett Buck on June 03, 2026, 01:30:14 PM??  They are not independent of each other, but for sure, the reference pressure is *at the end of the vent*. Where it goes after that determines it at any other point. Since the path doesn't matter, only where it ends up, you can easily figure out what the pressure change from the vent outlet to the venturi inlet might be.

    Brett

I think the only requirement of the pickup tube is that it must be submerged, until you want to stop engine.. I don't understand the dependency with the uniflow end

Quote from: Paul Van Dort on June 03, 2026, 01:51:39 PMI think the only requirement of the pickup tube is that it must be submerged, until you want to stop engine.. I don't understand the dependency with the uniflow end

    I am not sure how to explain it any better. The end of the vent tube - that is, the end of the uniflow tube inside the tank-  provides a pressure reference (atmospheric pressure +-ram air pressure (miniscule)) for everything else. That's why it doesn't get leaner as the fuel runs out. The pickup can be anywhere as long as it doesn't suck air too early. So you can determine the fuel supply pressure at the spraybar by comparing the distance between the end of the vent tube and the metering point of the spraybar relative to the local acceleration field.

   If you need to shim the tank, but can't physically move it, you can open it up and move *just* the vent tube up or down, and leave everything else completely alone.

    Brett

Thanks Brett. If this principle was understood by the larger CL community many engine issues would be avoided.

Just a thought, but it may well be that your weird tank(100mm wide?? Plus the unnecessarily long plumbing) causes the sensitivity to fuel viscosity.
Just like if an engine is out of thermal balance, it becomes more sensitive to outside influence.
So, maybe you should try with a little more conventional tank before jumping into wrong conclusions. L

Quote from: Lauri Malila on June 04, 2026, 04:37:15 AMJust a thought, but it may well be that your weird tank(100mm wide?? Plus the unnecessarily long plumbing) causes the sensitivity to fuel viscosity.
Just like if an engine is out of thermal balance, it becomes more sensitive to outside influence.
So, maybe you should try with a little more conventional tank before jumping into wrong conclusions. L

As I indicated before, the long plumbing certainly is making things worse, but what wrong conclusions are you referring to?

I mean that a healthy system is much more tolerant.

Quote from: Lauri Malila on June 04, 2026, 05:10:20 AMI mean that a healthy system is much more tolerant.

Absolutely, but still happy with the great inputs on this topic of viscosity.

Quote from: Lauri Malila on June 04, 2026, 05:10:20 AMI mean that a healthy system is much more tolerant.

By the way David Liber and Luc Dessaucy were impressed by the engine run and flight, so my system appears to be healthy after all :-)