Alternate air idea

[Larry DeCamp]

I have read the many threads with differing priorities and hardware designs. One design with cable operated sliding plate on the bottom of the fab addressed the hardware failure since pieces would not go in the engine.

Rocket Bob referenced a passive spring loaded design shamelessly copied from a Bonanza.

I envision a passive actuation ( no pilot or cable ) using gravity instead of a spring to close. This could be a machined disc covering holes in the bottom of the fab. I have experimented in my RV3 that somewhere around 1200 to 1400 RPM keep the plane flying indefinitely. So. The holes could be ?tuned? on the ground to supply the desired RPM. The diameter and weight of the ?poppet valve? depends on intake suction. As I recall from messing with auto engines, the vacuum is about 18 inches.

Does anyone know of a good starting point (inches of suction) for calculations on the Lycoming ? Suggestions & critique are welcome .

[rv7charlie]

If the intake is plugged, the throttle butterfly still operates normally to control MAP, as long as there's an alternate source for air. Suction is still going to change with throttle butterfly setting.

If depending on gravity, the alt air plate will tend to open with every bump in turbulence. Is that acceptable?

[cajunwings]

A good starting point might be to look at some production planes. Many of the Pipers in the Cherokee tribe have a spring loaded alt air door incorporated into air filter lid. My IO-320-B1E came off a Twin Comanche, it had a spring loaded door in the duct between the air filter and servo. I will transplant that assembly onto the bottom of my FAB and possibly add a push/pull control as well.

Don Broussard
RV9 Rebuild in Progress
57 Pacer

[Carl Froehlich]

Be very careful of spring loaded doors and such. I?ve seen more than one on the bottom of a FAB that the hardware ?disappeared? - the obvious result of being sucked into the engine.

The FAB is fragile and fasteners will vibrate out. I found that adding the much needed piece of 0.032? aluminum on the bottom of the FAB prevented the always present ablation of the bottom fiberglass caused by the air filter. It also provides metal to sandwich in the bottom FAB fiberglass between this plate and the standard Van?s bypass door aluminum ring. Add some pro-seal to the rivets if you want to make double sure nothing gets sucked in.

Carl

[Larry DeCamp]

Charlie, assuming the intake is occluded, the holes that supply air to push open the valve would be the equivalent of a throttle butterfly. That said, my oversight is that there is no need to limit the air supplied. The challenge is simply balanced forces to keep it shut normally but open when needed.

Obviously a geometry can be conceived to stay shut normally but stay open when the intake is compromised as evidenced by the spring designs.

Gravity dependent components could bounce, as suggested, so I will think about that.

Keeping parts out of the engine is a fascinating FMEA activity 👍