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Steve,
I think you hit the nail on the head - the "ram air" system is more of a filter bypass than a ram air, since the filter is in the same general location as the ram air inlet and the valve only opens up the alternate path and doesn't close the filter. If the ram air actually had more pressure, flow through the filter would reverse and prevent the full pressure from reaching the engine. Old Mooney owners (oops, owners of old Mooneys) have found out that dirt builds up in the back side of the filter, only to be ingested the next time the valve is moved. Not a big issue since supposed you only use the bypass when the air is clean. The real question is, is it advisable to go without an alternate air source? You make a good point, but I think mostly because the filter, being round is pulling air from all directions and it is unlikely that all surfaces will get plugged with water or ice. If it did plug though, I don't think the K&N filter is designed to support a large pressure drop and it might collapse into the intake. That's why I used a flat panel aircraft "certified" filter, but I built a spring-loaded blow-in door. I like your design - simplicity has its own rewards.
Gary Casey
ES #157
On the subject of Legacy Induction, here's my swing at it and why. I don't see how the Ram Tube can add manifold pressure when the K & N Filter is vented to cowl pressure between the Ram Air Inlet and Engine Intake. The manifold pressure increase when the butterfly is open to ram air would seem to be due to the small K & N Filter causing a restriction when alternate air is selected.
The K & N I used has several times larger area than K & N's formula requires. If pressure in the upper cowl is uniform (based on piccolo tube tests an engineer friend ran on his RV Plenum I believe it is) then I should get close to the same manifold pressure as Lancair's ram air with my design, maybe better.
Potential problems? I do not have an alternate air provision but the proximity of the back of the K & N to the cylinder should cause radiant heat to keep that part of the filter free of ice or snow (neither of which I intend to fly in longer than it takes to reverse course). I plan to put a temp probe on both sides of the filter to check. Ok, if I have enough heat to prevent ice, will that heat induction air enough to significantly reduce MP? Tests will tell the story. Modifications may change the design completely. Stay tuned for results, the plane is in the Paint Shop.
Steve Colwell Legacy N15SC
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