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 :-)