Stuka Stunt Main Forum · 17 of 17 known posts recovered
I find it much more useful to route the uniflow to a place perhaps close behind the cylinder on a profile or other spot where there is a constant flow.
Curt
Bigiron
If it works, do it!
I tried it by placing a short piece of fuel tubing over the end of the U vent, then inserting a 1/4" piece of 1/8 tubing that had been soldered shut--then drilled through with a 1/32" dia drill.
The other thing I had tried and it seemed to work ok at reducing the variation in speed in wind was the old foam microphone wind screen. It looked kinda funny, but stuffing a bit of pourous foam over the vent seems to work. Same stuff that is used in the venturi filters.
Curt
Jim
On my Fox powered Mustang, I route the vent back into the fuse. I believe this allows the tank to draw vent air from a relatively 'dead' air space (certainly minimal pressure changes).
Comment on Marvin's engine runs....NICE!
Fred
I tried the restriction on several planes and never noticed a difference. It may depend on the engine, how you have it set up, maybe a bunch of other factors. Don't know. One thing you can do is put a short piece of tubing in line between the uniflow vent outlet and the tank itself and then squeeze this down a bit of a time to see what effect it has. If it helps, keep it. If not, don't. This is one of those things that (probably) doesn't hurt and might help.
Matt of late has been putting the uniflow inlet in an air scoop at the top of the fuselage right behind the prop. It is directly in the prop blast this way where it can have minimal change due to wind. It also seems to help. Oh, and the location makes it easy to fuel, too. The scoop brings in air to cool the bottom of the case, and lets a reverse flow of air cool down the engine while sitting on the tarmac.
I reduce the tip by soldering in sleeving diameters down to an eventual 1/32 ID. I also file the tip bullet-like, sorta like a pitot head. There may be some 'leading edge' flow effect going over the front of the tube...
As one post mentioned, pressure is pressure... Force, however, is pressure times area. The same air pressure on a 3/32 ID makes 9 times the force that a 1/32 makes. (Circle's area varies as square of radius or diameter.)Whether that makes a difference or not doesn't so much matter as that the tips reduced upstream area seems to reduce headwind/tailwind influences for me.
In stronger winds, the model's kinetic energy (momentum) briefly maintains slightly higher airspeed coming into the wind. (That is, following the tailwind segment acceleration.)Pressure reflects airspeed, so we sometimes hear a 2/4 engine sound richer in that zone.
Past the upwind point, the engine can also sound like it is going towards leaner -- inertia delays the tailwind acceleration, and the air is moving more or less in the same direction as the model -- reducing air pressure at the vent tip.
It might all be mumbo-jumbo... Much else that we do because we believe -- or think -- it works, *might* have questionable *science* behind it, or not. What matters is our confidence in it,whatever it is. We can practice, push, and demand of the model and flier, BECAUSE we trust our setup.
Now what MAY happen is that the small hole restricts the RATE OF FLOW of air into the tank, and dampens out the response to the change in velocity. Tiny hole, slow flow, large dampening effect on pressure change in the tank. Obviously we are not talking about a steady state condition.
If you sped up for a long period the engine response should be the same as with a big hole. While holding your running model, try driving upwind then down at 60 mph for a prolonged period and see if you get the change. (I'm kidding, don't try this for real, but think about what the result would be).
Jim
I ran quite a few bench tests, testing a few NVAs as a vent air valves, back in the 1970's. Wrote it up for an early Stunt News. In short, NVAs do not meter air *finely* enough to make this a useful idea. I expect that remains true.
IIR, Wiley combat engines used a long, small-ID restrictor in the fuel supply line -- a coil of ?1/16" OD? tube stuffed into the backplate well -- either to tame needling for bladder tank pressure or to do away with the NVA. Anyone recall more specifically? I only saw one or two mentions in magazines, never saw an actual setup.
...Anyone else ever notice that a uniflow tank, filled and set wedge down, either doesn't drip or drips very little? Pull the overflow cap and it flows freely; same occurs at the end of the fill, when the in-tank end of the uniflow line starts to uncover.
Thinking pressures, this suggests little or no net head pressure at the outlet of a uniflow tank, whether set up as above or under in-flight conditions. It may be merely standpipe effect, where fuel drawn out to the engine 'pulls' fuel down the vent line (standpipe) to allow air to replace its volume, or almost...
Remember, the g-load picture in level and many "uniform" flight conditions is about 3 g outward, 1 g downward, plus some portion of maneuvering g. -- This is for relatively uniform flight: long straights in any direction, smooth rounds. ...Conditions where rapid, drag-caused accelerations don't appear -- Those g loads act on the fuel in the main tank volume and in the standpipe as well. They stay pretty well balanced against each other...
If the 'magic' of uniflow venting is this low or no outlet pressure thing, we have to look for pressure changes elsewhere. Where? There's a pressure drop across the NVA jet(s) from air acceleration past it(them.) There's ram air pressure on the forward uniflow tip. There's an 'uphill' distance the fuel must be raised in the direction of 'local gravity,' and the fuel *weighs* more (by the g load in that direction) than when at rest on the ground. How far is the 'uphill' distance?
How can it be more than the distance from the tank outlet, or possibly uniflow vent's in-tank end, to the jet -- and again, in the 'local gravity' direction. What's that, then, an inch more or less? Except for that uphill distance, other dynamic forces and accelerations act across the fuel flow - not significant!
Fuel is much more viscous than air, so it's easier to *meter* with the NVA. Uniflow tanks are 'balanced' neatly. That may be why they are more noticeably sensitive to tank 'height' relative to the engine.
>If the 'magic' of uniflow venting is this low or no outlet
>pressure thing, we have to look for pressure changes
>elsewhere. Where? There's a pressure drop across the NVA
>jet(s) from air acceleration past it(them.) There's ram air
>pressure on the forward uniflow tip. There's an 'uphill'
>distance the fuel must be raised in the direction of 'local
>gravity,' and the fuel *weighs* more (by the g load in that
>direction) than when at rest on the ground. How far is the
>'uphill' distance?
SNIP------------------------------------
One additional pressure change would be the effect of ram air into the opening of the venturi. This would have an effect of reducing the pressure differential in the tank (over the fuel head) versus the air pressure at the inlet. It goes from being the same to an increase of pressure at the venturi which would reduce draw at a given needle setting. Poorly design cowls which pack air will have an adverse effect on fuel draw. Seems to me that the effect will be more pronounced for engines with a restrictive venturi where the resistance to flow is greater.
Actually, this effect is going to be quite negligible. Neither ram air nor muffler pressure adds much pressure to the inlet of the tank at all, and certianly does not affect the working of the uniflow tank. I tested for muffler pressure on a couple of our engines by attaching a piece of tubing to the pressure tap and sticking the other end in a jar of water. Somewhere between 4 to 6 inches down the bubbles quit coming out of the tube. If normal air pressure will hold a column of water thirty some feet high with a perfect bacuum above it, 6 inches would only give us about a 1 1/2 % differential over outside air. Now try that with a tube facing into the prop blast right behind the propeller. It won't bubble far into the water at all.
I do not understand the physics of uniflow. Can someone explain with equations and translation from the original Latin?
Jim
If you think about the standpipe effect, it may help you grasp the way I see it. The tank shell is closed except for the outlet pipe and the vent tube. For fuel to leave without collapsing the tank, air enters to replace the fuel taken out.
Liquids seek their own level -- as in a clear vinyl tube level, where distance from one end to the other isn't relevant. Put water in, and the surface at both ends is the same height, right?
In a uniflow tank, fuel tries to reach the same level in the vent tube as in the rest of the tank shell volume. (Remember that flight loads change both the direction and quantity of the 'local gravity'. Len, I think, has mentioned that ~3g, outward and down about 20ยบ pretty well matches level flight conditions. I agree.)
For the vent air to reach the tank interior, it has to push past pressure in the vent tube, which the fuel creates trying to reach the same level there, as in the rest of the tank.
Regardless of actual g loads, these pressures stay balanced with each other. It's the same fuel inside and outside the vent tube, and under the same loads. Only the DIFFERENCE between them matters. As I see it, the tank interior settles to a slightly lower pressure than atmospheric, unless something else is acting -- like ram air pressure at the forward facing vent tube tip.
As mentioned, these tanks drip very little, if at all, in static conditions when put wedge down. That remains true until the venting changes -- pulling the overflow cap, or fuel level uncovering the in-tank end of the vent tube.
What's the pressure through the venturii's minimum area zone? We start with atmospheric pressure, about 14.7 lb/sq in at sea level, right? The pressure DROP through the min area, according to some figuring I've done, may possibly reach -3 to -4 PSI(gauge). Intake airflow maximum velocity is very high -- several hundred feet per second...
So, the pressure across the jet(s) may be around 11 to 12 PSI, which fits your numbers. Actual pressure, there, tells me less than its *difference* from atmospheric, static pressure. THAT difference is what sucks fuel out of the tank...
Dick F. mentioned air into or over the venturii opening. Sure there's an effect. But if we're flying at 88 FPS, prop slipstream is only in the low-90s FPS, and airflow inside the cowl, behind a spinner, or actually AT the venturii entry is probably less than either of those values.
If peak airflow velocity through the min choke area reaches roughly 470 FPS, flow over the venturii mouth likely has less effect than one might think. (A Fox Stunt 35 with stock NVA and choke diameter, in crudely simplified idealized flow conditions, might have a peak flow velocity of about 475 PFS at 10,000 RPM.)
Does any of this make sense?