Glassed Filtered Air Box Alternate Air Door

[CharlieWaffles]

There have been some threads I've read during my build discuss options for an alternate air door on the FAB other than the stock Van's door that can't be reliable reset without removing the lower cowl. Many builders have worked up a new door design that used hardware such as aluminum guides or delrin to provide a means to slide a door back and forth over an alternate air door. But these have all used additional hardware and weight. Dan Horton had commented an alternative would be to use fiberglass to provide these rails and eliminate the hardware. So I took that challenge.

I didn't like the round door provided by Vans as I wanted a door that could be opened and closed in flight. I was concerned that if I knew I had to remove the cowl to reset it, that I was unlikely to use it unless I had to. That meant it would be on the bottom of my list of engine troubleshooting. I wanted something that could be opened and closed and didn't present any additional items that could be ingested into the engine.

So I made up the fab with the standard approach and drilled a 2.5" hole in the bottom of the fab - towards the forward edge of the inside of the filter. This hole is slightly larger than the intake of the fuel injector body itself. I then made up a 3.5" wide by 4.5" long door out of .040" and a small joggle to be able to mount a push/pull cable end. I also made up a temporary 3.5" wide piece that was longer than the FAB.




I scuffed up the bottom of the FAB and waxed the long temporary aluminum piece and placed it over the alternate air hole in the FAB. I then laid up three layers of 9oz glass with a rough channel in the center.



Once cured, I removed the temporary aluminum piece and trimmed up the glass rails to be even and also ensure the alternate air door was unobstructed.

I fit the actual door in the channel created by the glass and verified the operation of the door. I used a #10 screw and an aluminum spacer to secure the push/pull cable at the same height as the cable nut.





Once verified, I painted the FAB out of vanity and mounted it and verified that the assembly worked as intended. I used the temporary long door to mask the channel from the paint so it didn't cause the channel to bind on the door after painting.

[Wayne Gillispie]

I used a red knob for this control only. Oil cooler damper and two heat controls are black. Never opened in 100+ hrs. Ice or bird injestion would probably be our only reason to pull it.

[CharlieWaffles]

Exactly what I did too

[DanH]

Mark,
Assume you have collected a big plastic shopping bag in the normal intake and 80% of the filter is obstructed. The engine is windmilling, throttle open, so there is a lot of pressure against the alternate air door. Given the resulting friction, will it slide?

It's possible to actually restrict the box intake while running, with obvious propeller risk. Or you could take the airbox off the plane, set it upside down on the bench, and stack weight on the door. The rough equivalent of sea level pressure with 100% filter obstruction would be opening area sq in x 15, or about 73 lbs.

[crabandy]

I did roughly the same thing, emphasis on the roughly.


I hope to fly this summer and for testing purposes I was going to try operating the door at various power settings. I figured either way the engine will be getting air in a test scenario and relatively easy to modify the airbox if it doesn't slide. My alternate air slides very easy (unweighted), I think if the cable is sheath is mounted securely it should have no problem overcoming the engine suction but its just a guess. Not sure how I would be able to stack that much weight on the little square to test it separately.
Then again this is my first rodeo and I usually miss the obvious.

[RV10inOz]

That is all great and dandy .........until the rest of the airbox disintegrates and bits go through your engine.

Fabricate a fully aluminium one and be done with it.

[J Twilbeck]

Looks great Mark. Nice simple design, I like it.
Thanks for sharing.

[BobTurner]

Are you sure you are getting a good seal with both positive and negative pressures? e.g., in cruise at wide open throttle the ram air coming in will have the airbox at slightly higher than ambient; if you have a leak out you'll loose some possible MP. At part throttle, of course, the engine will pull a vacuum on the airbox, and any leak will lead to dirt entering and by passing the filter.

[Tom Martin]

If there are no issues with opening this unit under pressure/vacuum as suggested by Dan Horton, then I believe that this approach is far superior to the stock set up. There is a lower parts count here then the stock unit and in the case of the OP there is a greatly reduced chance of aluminium bits ending up in the engine. There are no screws or fasteners that could come loose and get sucked into the engine. Personally I feel that the glass stands a much better chance of standing up to vibrations over time then does the original flimsy metal unit. In my opinion there will be less chance of air leakage with this slide mechanism then the rotating stock unit.
Experimenting with and without a small amount of lubricant on the slide would be worth considering. Good job

[Wayne Gillispie]

Downstream of filter where this plate is located is going to be below atmospheric pressure at full throttle, with or without ram air. If you close the throttle in flight then it may go above atmospheric pressure.