Sleepy Gomez · Jul 10, 2008 02:37 PM
#0 sourceReasons for using or not using pressurized fuel tanks? Muffler pressure or crankcase pressure? With uniflow style or standard vent tanks? Just trying to get a handle on fuel delivery. Thanks SLEEPY
pressure tanks?
Stuka Stunt Main Forum · 6 of 6 known posts recovered
>Reasons for using or not using pressurized fuel tanks?
>Muffler pressure or crankcase pressure? With uniflow style or
>standard vent tanks? Just trying to get a handle on fuel
>delivery. Thanks SLEEPY
Uniflow would be my choice as it is for most set-ups. Muffler pressure or not, it is still uniflow. Crankcase pressure is seldom if ever used for stunt. We aren't seeking maximum power, but good power delivery.
Muffler pressure helps smooth things out a little, and, again, it depends on your set-up. Some with "normal" engines use it. Some don't. Most using pipe set ups I think have drifted away from it. But it also has the advantage of keeping the wind out of the fuel delivery (it can do funny things sometime, depending on how your vents are located.) Disadvantage: One more thing to go wrong. We always used it with the non-piped set-ups, but not with the pipe. Still, it is your choice.
and it is an easy one to make. Try it either way and see which way works best for you.
>Muffler pressure or crankcase pressure? With uniflow style or
>standard vent tanks? Just trying to get a handle on fuel
>delivery. Thanks SLEEPY
Uniflow would be my choice as it is for most set-ups. Muffler pressure or not, it is still uniflow. Crankcase pressure is seldom if ever used for stunt. We aren't seeking maximum power, but good power delivery.
Muffler pressure helps smooth things out a little, and, again, it depends on your set-up. Some with "normal" engines use it. Some don't. Most using pipe set ups I think have drifted away from it. But it also has the advantage of keeping the wind out of the fuel delivery (it can do funny things sometime, depending on how your vents are located.) Disadvantage: One more thing to go wrong. We always used it with the non-piped set-ups, but not with the pipe. Still, it is your choice.
and it is an easy one to make. Try it either way and see which way works best for you.
Leonard Neumann
Indianapolis, Indiana, USA
Indianapolis, Indiana, USA
Jim Thomerson · Jul 10, 2008 04:17 PM
#2 sourceUniflow is the way to go. I run crankcase pressure off the backplate with TD 049 and 09, with a oneway valve in the line. Muffler pressure on larger engines if I have it, no oneway valve (tried the valve and didn't like it.) On open vented uniflow tanks, I put a oneway valve on the uniflow to stop syphoning and shield from the wind.
Sleepy,
There's more to this than you may think.
Fox 35 engines that I've used, as an example, don't seem to do anything better with even muffler pressure. Straight suction feed is plenty.
Uniflow tanks, as both Leonard and Jim said, are excellent - but they have one little problem. Because they establish a very precise relationship between the tank and the engine, they MUST be in the right place to do well. Whatever model you are considering, it may be a good idea to allow some room to move the tank up or down until the engine run is identical upright and inverted.
Other engines are a bit less critical to tank (relative) height, but still do very well with a uniflow tank at the right height.
What's involved is that the place where a uniflow vent allows air to enter the tank is the "reference" tank height the engine sees. If it is too high upright, it will be too low inverted. In terms of run, upright will be getting gravity feed - fuel will flow downhill to the spraybar. As a result, when inverted, the engine wil have to draw fuel uphill. So, upright will - in effect - be richer, and inverted leaner.
Because "traditional" (over and under open vents) lean out over the duration of the flight, and uniflow venting stays the same, it is worth taking some time to get the uniflow tank height 'just right.'
Now, with pressure - say, muffler pressure - and the typical uniflow tank, which has the vent outlet inside the tank pretty near the end of the fuel pickup tube, it is a toss-up between putting the pressure to the uniflow tube or the overflow tube. It CAN happen that the vent air bubbles are so close to the "pickup point" that they interfere with fuel delivery to the spraybar... In that case, it's worth trying the pressure line on the overflow tube. (Many of my models work better that way. In fact, I try it that way first. If it works reliably, I may try pressure to the uniflow vent as a double-check, and go from there.)
More important than whether or not to use muffler pressure, is that the tank be in the best position relative to the engine, whatever the venting system might be.
And that requires several practice flights, with close attention to how close to identical upright/inside flight is to inverted/outside flight. EVERY practice flight teaches something, if we do it right, so that's not wasted time...
Once it is dialed-in, you can fill, fire up, fly, time after time and just work on YOUR skill... not on tweaking this or that.
There's more to this than you may think.
Fox 35 engines that I've used, as an example, don't seem to do anything better with even muffler pressure. Straight suction feed is plenty.
Uniflow tanks, as both Leonard and Jim said, are excellent - but they have one little problem. Because they establish a very precise relationship between the tank and the engine, they MUST be in the right place to do well. Whatever model you are considering, it may be a good idea to allow some room to move the tank up or down until the engine run is identical upright and inverted.
Other engines are a bit less critical to tank (relative) height, but still do very well with a uniflow tank at the right height.
What's involved is that the place where a uniflow vent allows air to enter the tank is the "reference" tank height the engine sees. If it is too high upright, it will be too low inverted. In terms of run, upright will be getting gravity feed - fuel will flow downhill to the spraybar. As a result, when inverted, the engine wil have to draw fuel uphill. So, upright will - in effect - be richer, and inverted leaner.
Because "traditional" (over and under open vents) lean out over the duration of the flight, and uniflow venting stays the same, it is worth taking some time to get the uniflow tank height 'just right.'
Now, with pressure - say, muffler pressure - and the typical uniflow tank, which has the vent outlet inside the tank pretty near the end of the fuel pickup tube, it is a toss-up between putting the pressure to the uniflow tube or the overflow tube. It CAN happen that the vent air bubbles are so close to the "pickup point" that they interfere with fuel delivery to the spraybar... In that case, it's worth trying the pressure line on the overflow tube. (Many of my models work better that way. In fact, I try it that way first. If it works reliably, I may try pressure to the uniflow vent as a double-check, and go from there.)
More important than whether or not to use muffler pressure, is that the tank be in the best position relative to the engine, whatever the venting system might be.
And that requires several practice flights, with close attention to how close to identical upright/inside flight is to inverted/outside flight. EVERY practice flight teaches something, if we do it right, so that's not wasted time...
Once it is dialed-in, you can fill, fire up, fly, time after time and just work on YOUR skill... not on tweaking this or that.
\BEST\LOU
Sleepy Gomez · Jul 10, 2008 07:12 PM
#4 sourceJim, Leonard, Lou, Thanks a bunch. I think you have answered all my questions about this. Thanks again, SLEEPY
(snip)
>Now, with pressure - say, muffler pressure - and the typical
>uniflow tank, which has the vent outlet inside the tank pretty
>near the end of the fuel pickup tube, it is a toss-up between
>putting the pressure to the uniflow tube or the overflow tube.
>It CAN happen that the vent air bubbles are so close to the
>"pickup point" that they interfere with fuel
>delivery to the spraybar... In that case, it's worth trying
>the pressure line on the overflow tube. (Many of my models
>work better that way. In fact, I try it that way first. If it
>works reliably, I may try pressure to the uniflow vent as a
>double-check, and go from there.)
> (snip)
If you apply pressure to the overflow, you no longer have a uniflow tank. With a uniflow tank you can try the run either way. You can cap the overflow and you have a uniflow tank. If you open the overflow (or apply pressure to the overflow tube) then you have a standard vented tank. Adding ressure or not makes no difference. It is where you add the pressure that makes the difference.
Now, as to the bubbles--this isn't really the problem that many people imagine. If you put a helium filled balloon in the back seat of your car and then accellerate the car forward, which way does that balloon go? (Try it). It will go to the front seat as far forward as it can. While the people are being pushed into the back of their seats, the balloon will move forward. The heavier parts move back, the lighter parts move forward.
If you rotate a uniflow tank so the "V" portion of the tank is down (the pick up side) and the flat side opposite is up, and then run it on your test stand that way, which way will the bubbles go? (Try it with a clear tank set up for uniflow). The bubbles will go "up". They will not follow the fuel pick up, even though it is "sucking" fuel. When our planes are in flight, the fuel "head" (the amount of fuel remaining) is "pushed" towards the outside of the tank (towards the "V" pickup side) with about three times the pressure of gravity. The air bubbles, being much lighter, move in, not towards the pickup. If you see bubbles in the line, it is probably because of vibration and foaming, not because it is getting air from the uniflow tube.
>Now, with pressure - say, muffler pressure - and the typical
>uniflow tank, which has the vent outlet inside the tank pretty
>near the end of the fuel pickup tube, it is a toss-up between
>putting the pressure to the uniflow tube or the overflow tube.
>It CAN happen that the vent air bubbles are so close to the
>"pickup point" that they interfere with fuel
>delivery to the spraybar... In that case, it's worth trying
>the pressure line on the overflow tube. (Many of my models
>work better that way. In fact, I try it that way first. If it
>works reliably, I may try pressure to the uniflow vent as a
>double-check, and go from there.)
> (snip)
If you apply pressure to the overflow, you no longer have a uniflow tank. With a uniflow tank you can try the run either way. You can cap the overflow and you have a uniflow tank. If you open the overflow (or apply pressure to the overflow tube) then you have a standard vented tank. Adding ressure or not makes no difference. It is where you add the pressure that makes the difference.
Now, as to the bubbles--this isn't really the problem that many people imagine. If you put a helium filled balloon in the back seat of your car and then accellerate the car forward, which way does that balloon go? (Try it). It will go to the front seat as far forward as it can. While the people are being pushed into the back of their seats, the balloon will move forward. The heavier parts move back, the lighter parts move forward.
If you rotate a uniflow tank so the "V" portion of the tank is down (the pick up side) and the flat side opposite is up, and then run it on your test stand that way, which way will the bubbles go? (Try it with a clear tank set up for uniflow). The bubbles will go "up". They will not follow the fuel pick up, even though it is "sucking" fuel. When our planes are in flight, the fuel "head" (the amount of fuel remaining) is "pushed" towards the outside of the tank (towards the "V" pickup side) with about three times the pressure of gravity. The air bubbles, being much lighter, move in, not towards the pickup. If you see bubbles in the line, it is probably because of vibration and foaming, not because it is getting air from the uniflow tube.
Leonard Neumann
Indianapolis, Indiana, USA
Indianapolis, Indiana, USA