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Building Tips and technical articles. => 1/2 A building. => Topic started by: Larry Renger on June 19, 2009, 08:44:13 AM
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I got some numbers here. I thought I'd calculate the velocity through the venturi and then the available pressure drop at the fuel orifice.
20,000 rev/minute
.061 cubic inches
.01 square inch passage area (Ap)
.038 sqare inch at venturi entry (Ae)
Volume = .061 x 20,000 = 1220 cubic inches/minute (assuming total efficiency as a pump...probably noticeably less in reality)
Velocity (at passage) Vp = 1220 / .010 = 122000 inches/minute x 1/12 ft/in x 1/60 minute/second = 169 ft/second
Similarly, Veloctity (at entry) = Ve = 44.2 ft/second
Benoulli tells us that pressure drop = 1/2 x .00238 x Ve^2 x ( [Ae/Ap]^2 -1)
This calculates out to .21 psi. That isn't much to draw in the fuel! No wonder muffler pressure and crank pressure make such a difference.
There is more to it than that, of course. The obstruction creates a pressure drop, which is why a spraybar with the hole at the bottom still works. I need to do a bit more research to figure that one out.
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I made a low pressure test setup once using a u-tube manometer made from fuel line, 1/4" tygon tubing, and a scale on a vertical stand. Used it to measure exhaust pressure on a 40 size engine. I was surprised at the very low readings even in inches of water.
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On 45' lines at 3.8 second laps, I calculate that ram air pressure is about .05 psi. That is a 25% improvement over the suction I calculated above. Point those tank vents into the wind, dudes. y1
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Engine reviews sometimes gave a suction figure; how much the tank could be lowered and the engine still kept running. As I recall, figures around 12 inches were thought good. My crude calculation from Larry's figures suggest that .21 psi = @9 inches of water.
I disagree with Larry on one point, that of using ram air on tank vents. Airspeed changes and thus ram air pressure changes in maneuvers. I use a one way valve on my open air uniflow tubes to help keep the pressure in the tank constant.
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One fix for the varying pressure is to solder up the vent, and drill a very small hole. The pressure can then change only very slowly, slower than a half lap time, so the internal pressure is more constant. This could be a restrictor in a piece of fuel hose slipped over the vent, so that fueling is not slowed down.