Gentlemen,

Upon close viewing photos of several Control Line models in Stunt News, it appears some of the Uniflow Vents are perpendicular to the aircraft flight direction.

I have always bent the Uniflow Vent line so it faces the direction flight.

These are setups where the Uniflow Vent is not connected to the muffler.

Are my eyes deceiving me about the Uniflow Vent perpendicular being to the flight direction?

If the Uniflow Vent is perpendicular to the flight direction, is there a siphon problem?

Thanks for your answers.


t michael jennings               
Knoxville, TN.

I also usually face the uniflow inlet vent to the wind. However, if using muffler pressure, it doesn't much matter.

Siphoning could be a problem, but the uniflow inlet tube is the only opening to the air outside the model, and then only if it is NOT getting muffler pressure. The uniflow system relies on drawing the fuel down the vent tube to the interior opening. Without the engine drawing fuel, the levels inside and outside that tube would be the same. The engine draws a slight negative (guage) pressure inside the tank shell.

With any kind of pressure fed into the shell it doesn't seem to me that uniflow or any other vent type makes much difference. Even with the very low pressure from a chip or tube-type muffler, siphoning would have to not only raise the fuel above the equal 'stand-pipe' height, but also against the muffler pressure provided.

A small thought: I've had better runs, more simply, by connecting muffler pressure to the overflow tube and capping the external uniflow tube. At least, the distance to the overflow tube is greater than from the pickup to the uniflow internal end. NO chance of bubbles entering the fuel supply tube. The overflow tube usually comes up to the inside,front top of the tank. The model spends more time upright than inverted, for one thing, and the fuel surface resulting from the combination of gravity and centrifugal loads moves away from the inside tank wall quite early in a flight. The pressure dumps into 'air' space, not out a tube end near the fuel pickup.

Now, diesel fuels can fizz up a lot more than glow fuels, and that can look like siphoning. That's another whole story, and not much relevant to the original question...

Hi Michael.  Not to doubt what you saw, but are you sure those aren't over flow vents? I have never seen uniflow vents perpendicular to fight direction unless they were intended to be attached to muffler pressure.

    Perpendicular is pretty common, and avoids any issues you might have with "ram air" richening up the engine as it comes around into the wind.

      Brett