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We compensate for the variable "gravity" with tank position. If
the fuel tank is positioned so that its horizontal midline is located
3/8 to 1/2 inch below the engine's fuel inlet, usually the needle valve,
the engine will need to draw fuel against the force of the earth's
gravity while on the ground. In effect, the fuel will have to flow
"uphill" to get into the engine.
Once in flight, many common maneuvers can only serve to "richen" the
mixture. In level, inverted flight or rolls, the earth's gravity tends
to pull the fuel "downhill" into the engine, resulting in a slightly
richer mixture. When the aircraft's motion pulls fuel away from the
inlet, as in the tight turns and outside loops mentioned previously, the
"mixture leaning" effect is reduced since the engine has already been
set to pull fuel "uphill."
A photo shows the best tank position in relation to the engine's needle
valve. Tank distance from the engine is also critical. For .25-.65
engines without fuel pumps—most trainer engines—the fuel tank should be
a maximum of 4-5 inches behind the engine. The closer, the better.
Remember that the engine must draw the fuel over this distance as well
as fight gravity.
Why can't we just put a 16-ounce tank behind a .25 cu. in. engine and
fly for an hour? Because of something called "head pressure," which is
the second force pushing fuel into the engine.
The weight of the fuel itself is acting to push it through the small
opening, into the engine. The larger the tank size, the heavier the fuel
is and the greater the force pushing it out of the tank. In the
.25-engine scenario, the needle valves would have to be set extremely
"lean" to compensate for the full tank's high head pressure.
But as the tank empties during flight, the head pressure drops.
Approximately halfway into the flight, the pressure gets so low that the
mixture settings, made with a full tank, are too lean. The engine dies
in the next vertical climb or high-gravity ("high-G") maneuver. The
initial mixtures could be set extra rich to compensate, but then the
first half of the flight would be underpowered, if the aircraft could
even take off, and not much fun at all.
But isn't muffler pressure—
the third force acting on fuel
flow—supposed to compensate for varying head pressure? It is and it
does. But remember that the engine is pumping pressure into that large,
full tank while you are setting the mixtures on the ground.
In flight, the muffler pressure remains constant—well, relatively
constant based on the engine's speeds. As the head pressure drops, the
flow forces still decline since the engine does not apply more pressure
just because the fuel level is getting lower.
Muffler pressure is far more effective in helping to keep flow rates
constant during steep climbs and high-G maneuvers, which momentarily
reduce fuel flow, than in compensating for long-term flow reductions.
Still, muffler pressure does help somewhat to reduce head pressure's
detrimental effects. This is why there is a range of tank sizes rather
than one best size for each engine displacement.
In addition, today's engine designers include muffler pressure's effects
when they design the carburetor. Since muffler pressure increases fuel
pressure, designers can increase the size of the carburetor's air inlet
for additional power, and believe me, they do.
Therefore, much of the muffler pressure is already being "used" to feed
additional fuel into a carburetor that would otherwise be drawing too
much air and not enough fuel. There is not much left over to compensate
for tank size and maneuvers.
Click on photo to view large image with caption
The "dummy fuselage" photos are almost self-explanatory, but some
parts are worth mentioning. Notice that the tank's fuel-outlet line is
roughly the same height as the engine's fuel inlet. Try to run the fuel
line directly to the inlet, without going far downhill, and then way
back up. If there is too much "uphill," the engine could quit lean as
fuel levels reach the last few ounces and head pressure vanishes. If
your engine always quits before the tank is empty, check for this
roller-coaster condition.
Also note that the fuel tank is not centered behind the engine. The
tank's fuel outlet is positioned slightly more toward the side with the
needle valve. This reduces the uphill/downhill effect no matter which
direction the aircraft banks or rolls. Fuel flow remains almost
constant. If possible, mount the tank inside a thin foam layer to reduce
possible "bubbling" from the engine's vibration, as shown.
If the engine is tightly cowled, or the fuel line to the engine cannot
easily be disconnected for refueling, you will need a third line to the
fuel tank. This "fill line" must be blocked off after filling to prevent
muffler-pressure loss during operation. The photo shows a "fuel dot"
used for this purpose, which is simple and popular. Little can go wrong
unless you somehow lose the dot while refueling.
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