Lou Crane
Posts: 362
Score: 100
Joined: 5/11/2006
Last Login: 3/14/2010
From: Sierra Vista, AZ, USA
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Jim, and much obliged Downunder, that fit right in...
A uniflow plumbed tank is much more sensitive to "relative tank height." I mentioned the advantages of getting it right, but not the reasoning.
But, first, yes - the forward facing vent tube end is exposed to some ram air pressure as the model flies. Not much... And, whether the vent air provided into the tank interior is below local atmospheric pressure, at that pressure, or slightly above, the reference point that the engine 'senses' is the inside tube end where the bubbles come out. For simpler handling of a hard (metal) tank, I usually take pains to get that uniflow vent tube at exactly half the height of the tank shell. That way I know where it is, if flight experience shows it isn't quite right.
This location is key, as several who use converted plastic bottle RC tanks 'trim' the run by twisting the uniflow vent tube to a different height inside the plastic bottle. Nothing else needs to be shifted.
Say the engine runs richer inverted and outsides than upright and in insides... The tank is too "low" when the model is upright. The engine has to draw fuel uphill, even if only a small distance. That same distance is doubled when the model flies the other way. If we can call the "perfect" tank height a sweet spot, in this example, the vent end in the tank is some distance below the sweet spot in upright/inside conditions. Inverted and outsides, it is that same distance above the sweet spot...
Since I have the uniflow outlet at a known height inside the tank, I can shim the whole tank, in this case up with the model upright, and see if it needs more or less to get it the same both ways...
And uniflow tank venting can be VERY sensitive to this, particularly for richer-running 4/2/4 engines. With muffler pressure, the difference is less than with the forward facing external tube open to the airstream. With more recently designed engines, almost invariably ABC/AAC or ABN's designed for RC then converted for CL use, their most natural running mode is somewhere in their quite wide range of 2-cycle RPM. ...And most often they run on muffler pressure, so the problem is quite a bit less. Even so, a careful ear, and a sensitive hand can feel the unavoidable variations in power when a uniflow tank is not just right.
I've developed a routine to check tank height before first flight on a ukie. Shortest description:
Hold the model with the fuselage centerline level at all times when checking...
Start the engine and rotate the wings to vertical, outboard tip down (re-read line above...)
Set engine slightly rich, so you can hear - or better, tach - RPM changes. (Fuselage level, right?)
Rotate the wings to about 45° from vertical, say cockpit-side up for the starter. Note RPM by ear, or better, by tach. Rotate wings to the same angle, cockpit-side down, and recheck RPM. (Keeping fuselage centerline level.) Repeat a few times to be sure any response is consistent, and that you can recall which way it went.
Now, you can relax that First Commandment about level fuselage centerline... Turn model nose-down, inboard-tip down to uncover the fuel pickup in the tank. If it doesn't shut off quick enough that way, you can cap an open uniflow external tube with a finger...
Your test wig-wagging crudely represents the angles of the "gravity vectors" the model feels in flight. NOT their intensity, just their direction. And, about 45° is both far enough to cause a response difference if things aren't just-so, and easy enough to approximate on both sides of vertical, outboard-down.
RPM drops from richening on the side where the tank is too high. Conversely, RPM rises when the tank is too low in that attitude. Adjust the height accordingly, and run thru the same sequence to see if that was enough or too much. After two or three times through this process, you should be close enough that, at least, you won't flame out or starve out on a first flight.
There is no hard and fast rule based on the layout of the engine type for the relative height of spraybar and inner uniflow vent end. Most individual engines of of a mass-produced type, e.g., Fox 35, or OS 40FP, or 40LA, share the same tank preference, but I wouldn't guarantee that the solution for an LA40 works with an FP40. I built a test bench which allows me to check unfamiliar engines for tank height preferences and sensitivities.The commercial engine mount (J'Tech) mounts on a board to hold the tank (adjustable as to height in the relevant direction, sidewinder is different from upright or inverted). The board can be rotated, roughly about the crankshaft centerline, while the engine is running. Testing is similar to the wig-wag process above. Inspiration for the wig-wag and the bench followed an experience with an engine that needed the reference height more than 3/8" different from usual practice. ...caused a few crashes... But when the engine was sorted out, it was excellent!
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Lou Crane, Sierra Vista, AZ
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All Forums >> Radios, Batteries, Clubhouse and more >> Control Lines >> Tanks 
Tanks - 
) but the end of the Uniflow line can be pretty much anywhere along the outside wall of the tank because in flight (recalling Lou's ~3G centrifugal force) the outside wall of the tank effectively is the "bottom" of the tank. This outside wall is of course where the fuel pickup will be even if it's a clunk tank. As Jim said earlier, he's used the Uniflow end anywhere from the front of the tank to nearly at the pickup. It's best to keep the end of the Uniflow line no closer than about 1/4" from the pickup to avoid the chance of air bubbles getting into the fuel pickup.