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A propeller creates forward thrust because the pressure on the leading side of the propeller blade is lower than at the trailing side.
The air around the propeller tries to equal out the pressure differences. The funnel avoids side access so the pressure difference on the blades remains higher.
Also, a slight taper on the funnel entry can harvest some forward thrust as the intake side does have low pressure, further enhancing efficiency.
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2011 Honda Insight + HID, LEDs, tiny PV panel, extra brake pad return springs, neutral wheel alignment, 44/42 PSI (air), PHEV light (inop), tightened wheel nut.
  lifetime FE over 0.2 Gigameter or 0.13 Megamile.
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