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Glassed Filtered Air Box Alternate Air Door

CharlieWaffles

CharlieWaffles

Well Known Member
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.

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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.

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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.

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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.

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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.
 
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.
 
I did roughly the same thing, emphasis on the roughly.
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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.
 
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.
 
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.
 
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
 
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.
 
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Wayne, you are right, I was thinking of my manifold pressures, not the airbox.
But in full throttle flight my MP tends to be very slightly higher than outside, so I think the airbox may be slightly pressurized. And as you say, for sure at part throttle. OTOH it must be below atmospheric at full throttle takeoff before you are moving appreciably. So you still want it to seal both ways.

Frankly, I think this is a nice solution to a non-existant problem. The stock alternate door can be mostly closed from the cockpit - it's just the last 1/4" of travel that might be hard, as the door is forced under the lips that give it the tight seal when closed. Once on the ground there is no need to pull the cowl - either long arms or a yardstick will do to close it the last 1/4".

But, if you do go stock, you do want to add an aluminum backing ring inside the box, where the door frame rivets on. The fiberglass isn't strong enough by itself, the rivets will pull out. But they shouldn't go anywhere, they're captured in the door frame.
 
I'm bangin' with Tom Martin on this. Also, I've seen countless busted aluminum airboxes. Welded, patched like a quilt. Patched like a quilt and welded... Those busted bits all went into engines. I've seen fried out shook apart FAB boxes, but not much fiberglass missing. Usually its rivets work through or the stupid snout hinge sheds a chunk. I like this door idea. I won't put the stock alternate on as it comes. I'd rather go without.
 
DanH said:
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?
Click to expand...


I did a slide valve in fiberglass similar to this last year and had the same concern about being able to open the alt. air door when really needed. My solution was to have a thin (1/32") piece of UHMW fixed between the fiberglass airbox and the aluminum slide plate. Virtually no friction, yet still seals nicely.
 
Ahhh, UHMW. We used to call it "poor man's Teflon." It has a lot of similar properties to Teflon(c). [UHMW stands for Ultra High Molecular Weight polyethylene.]
 
over engineered; KISS principle.

Hey Fellas:

Why not use a square (or circle) of silicone baffle material, laid over a reasonably-sized hole? Put it on the inside so it opens with any vacuum - automatically; align it so the expected NORMAL airflow will help keep it shut. If you are also worried about the filter clogging, put a second bypass in the filter top.

Any silicone parts going thru the engine are probably benign. I am not going to test this theory!

Remember the bypass does not need to provide full power - only enough to keep you in the air. The mixture might need some twiddling, but I prefer the idiot-proof approach to a bypass (keep the pilot out of the loop, as a vernatherm does).

Then again, nothing is idiot-proof because idiots are so clever!:eek:

Carry on!
Mark
 
Trouble with adding UHMW to the design is the requirement to fixate it somehow....parts count, loose hardware, etc.

Practical notes..the coefficient of static friction is unlikely to be more than 1, and certainly won't be if you simply grease the sliding surfaces. At 1, worst case you have to be able to pull 75 lbs, which a meat servo can probably do when excited ;). The important detail is making sure the cable anchor points will handle 75 lbs. Along those lines, the anchor point gets stronger and the static friction coefficient gets lower if the plate is steel rather than aluminum (assuming equal thickness). Better fatigue performance too.

Mark and I think the same about alternate air. With trees in my face and power going to zero I'm not smart enough/quick enough to diagnose the reason as intake blockage and find a knob. My own airbox incorporates a spring loaded door which automatically pulls open at 3" H20. There will be a slight power loss, but I think I can (literally) live with it.

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The baffle flap probably won't stay closed. I tried an aluminum magnet door with really good magnets. It took a good thumb push to open it and it would pop open in flight. Indication was the MAP swing by about a half inch. If Dan is using a spring in the same location it must be pretty stout.
 
aerhed said:
The baffle flap probably won't stay closed. I tried an aluminum magnet door with really good magnets. It took a good thumb push to open it and it would pop open in flight. Indication was the MAP swing by about a half inch. If Dan is using a spring in the same location it must be pretty stout.
Click to expand...

http://www.vansairforce.com/community/showpost.php?p=364799&postcount=15

As eventually installed in the airplane, the door has a microswitch connected to a flashing LED in the cockpit...it doesn't open in flight.
 
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That's a good idea Dan. I couldn't believe mine was opening because it took so much force to pop the magnets. I slapped a piece of fuel cell tape over the bottom and it stopped. Then I put in a cable. I was pretty disappointed the magnets didn't work. Probably blew ten hours on it. Not as bad as the twenty hours I blew on a hidden oil door latch that didn't work.
 
On my RV-6 I had the rev. 1 of Van's automatic alternate air door which used a galvanized steel door held in place with a magnet. It used a piece of baffle material for the hinge. I installed some additional magnets which I encapsulated on the bottom of the FAB with a pool of proseal, after seeing evidence that it was opening often. Simple and worked well.
 
I'm impressed by all the ingenuity shown in designing and making all these different air doors. What exactly is it that is wrong with the VANS design?

- OK, it's not the most elegant but it works
- You can't close it in flight but if your main intake is blocked, why do you want to? It's not as if it's something you want to be opening and closing on a regular basis
- as far as I can see, if any bits do come loose, they are going to fall into the cowl, not the engine. Having said that, I can see the pop rivets through he fiberglass are a weak point and I am going to glass over the inside. I also used RTV when I fitted the ring.

Although I am not flying yet, the alternate air door is fitted and, as far as I can see, works as advertised.
 
I used the stock kit provided door and I can open and close it when the engine is not running. It works every time. I have never tried it in flight. I assume the air pressure may not allow it to align and close. Ill have to try it next time I take a short flight.
Having said this, I may take a look at something like Dan has done. I like the lack of pilot intervention theory, and it would be one less control in the cockpit.
 
Bill.Peyton said:
I used the stock kit provided door and I can open and close it when the engine is not running. It works every time.
Click to expand...

Huge difference between "engine not running" and "engine windmilling with filter or primary inlet blocked". The first places no load on the door while the second loads it equal to a high percentage of atmospheric pressure x area.

I'm just sayin'....
 
Bill. I'm not sure how you open and close it. It doesn't have a hinge and when you pull it the door just dangles from the cable. Also, any "bits" that fall off don't go into the cowl. They go right into the engine like a big shop vac. I've replaced enough turbos and jugs from FOD on factory planes to know that "bits" are incompatible with high speed engine parts. The galvanized ring and rivets in the original setup will eventually work loose in the fiberglass. Proseal helps, but is not a great solution. Also, I like being able to check it in flight. If you suspect partial obstruction it's nice to open the alternate to confirm and shut it if it's not. There's also the case of "somebody" pulling the cable accidentally in flight. Most production planes also have an alternate air check on the checklist, unless they have an automatic door like a banana or a T210. BTW, the cessna magnet doors self destruct and ingest on a regular basis. I've never seen a spring door on a banana go bad.
 
.??? At least on the 10 the door doesn't dangle from the cable, it pivots around a screw. As I mentioned above, I can open and close mine in flight, but it's hard to get it closed the last 1/4" so it may be leaking slightly until I reach inside and push it closed the last little bit.

And it definitely needs a metal backing ring inside the airbox to keep the rivets from pulling thru the fiberglass.
 
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??? Now I'm remembering the screw, but some just had a little tab. Are there 2 kinds of Vans alternate door?
 
Still don't see how bits can get into the engine. Even if the whole door worked loose in the fiberglass, the rivets would still be held in the ring which physically cannot get through the hole. And if individual rivets somehow worked loose, they still can't get sucked in because of the c/s head - they would have to drop into the cowl. Unless the rivet head somehow breaks off - and what load is going to do that?

Having said all that, I think I will drill out my rivets and add a backing ring inside - and then glass over the lot....... Then I REALLY don't see a problem.
 
Paul, I have seen literally dozens of missing rivets on airboxes that went into the engine. No, the ring itself isn't going to suck in. I've seen the fiberglass bowl on a FAB rip right off the top plate from vibration. Airboxes get shook up. The rivet shanks get necked down and worn right through. Any looseness that develops in a joint starts to hum and eventually fails. The average spam can is 30 some years old now and there's tons of ratty airboxes out there because replacements cost their weight in gold. This has given me a good perspective on their design failings. A loose rivet in fiberglass will wallow out a countersink and neck down the rivet shank until it breaks. Bottom line: Keep parts, fasteners, and hardware to an absolute minimum downstream of the filter. Also, cracking aluminum (like the top plate) can shed little triangles and such where crack intersect. I've seen these go into engines too. As an aside, I think a lot of you RV builders just don't realize how crappy the spam can fleet is.
 
I fabricated a ring which reinforces the inside. I did not use pop rivets to attach the ring. The RV10 uses hardware to form a pivot for the door to open.
It is not likely that this arrangement would get ingested if properly inspected and maintained. I have not heard any reports of this issue. If anyone has feel free to share.
I do wholeheartedly agree that an automatic opening door would be an improvement from a safety perspective.

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aerhed said:
Paul, I have seen literally dozens of missing rivets on airboxes that went into the engine. No, the ring itself isn't going to suck in. I've seen the fiberglass bowl on a FAB rip right off the top plate from vibration. Airboxes get shook up. The rivet shanks get necked down and worn right through. Any looseness that develops in a joint starts to hum and eventually fails. The average spam can is 30 some years old now and there's tons of ratty airboxes out there because replacements cost their weight in gold. This has given me a good perspective on their design failings. A loose rivet in fiberglass will wallow out a countersink and neck down the rivet shank until it breaks. Bottom line: Keep parts, fasteners, and hardware to an absolute minimum downstream of the filter. Also, cracking aluminum (like the top plate) can shed little triangles and such where crack intersect. I've seen these go into engines too. As an aside, I think a lot of you RV builders just don't realize how crappy the spam can fleet is.
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Fair enough......

Reinforcing ring with glassfibre over the top it is then!
 
Paul, good idea. Bill, yeah that's the stock door thanks.The key words in your post are "maintained and inspected". Part of what we're doing here is to guard against a deficiency in those regards. Other guys, am I correctly remembering a version that just hooked under tabs and dangled or did I dream that?
 
Let's remember that the 50 hour check should involve cleaning and re-oiling the air filter so you should get a good look at the structure of the box........

Sorry, did I say I love this forum for highlighting potential problems like this so you can deal with them before they become an issue!
 
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Air box, redux.

Well, I don't know about the 10, but I've tried pretty much tried all of the above on my 1000 plus hour 6, and it now has no alternate air. I'm trying to stay in the southwest, where ice is harder to come by...
 
Engine Failure from Air box

Hey all, Thought I would share an issue I had just a week ago. Cruising along at 4500' and had an engine failure. Put down in a paddock and all ppl walked away.

Upon investigation the rivets holding the alternate air in place had let go and pulled through the fiberglass air box. The FCU air inlet was blocked by the alternate air panel and that's what had caused the failure.

Now fabricating a fully aluminum air box now....

Definitely worth the few $$ and replace your fiberglass box. It should almost be an AD. I know people have made their own and have seem some really nice fiberglass ones on here but 1 engine failure too many for me :)
 
Do you have some photos of the box that failed showing how it failed? Also your new design? Also how many hours?
Thanks
 
Wow! Glad you got down safely. How did the airplane fare? I know we are not supposed to care but as a builder, i do. ;)

The airbox is probably the most overlooked critical component on the plane. IMHO it is absolutely NOT good enough stock. The drain tube, the air seal, the filter, the alternate air door, and the alt air door control all go into this tiny little kit that is too easy to overlook. I have had to redo or tinker with almost every aspect of it in phase 2.
 
Dan,

You did a great job indeed. Not a scratch on man, machine or beasts! :)

For those interested, the following are pic's of the fabricated box we made a couple of years back.

This in my opinion is a SB needing to happen.

See ya tomorrow Dan! :)

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What exactly is it that is wrong with the VANS design?

- OK, it's not the most elegant but it works
- You can't close it in flight but if your main intake is blocked, why do you want to? It's not as if it's something you want to be opening and closing on a regular basis
- as far as I can see, if any bits do come loose, they are going to fall into the cowl, not the engine. Having said that, I can see the pop rivets through he fiberglass are a weak point and I am going to glass over the inside. I also used RTV when I fitted the ring.
Click to expand...

In my opinion, the original design was seriously flawed, if you're still running it, you shouldn't!

The failure experienced by dcpearso was from a temporary fix. The original design kept flapping on the initial test flights, I asked the chap that helped me build the aircraft if he had any ideas for a fix. Unfortunately he sealed the flap shut permanently. (non pilot, possibly unaware of the consequenses). I didn't check the fix.

Lesson 1: You as PIC and more importantly the builder MUST check these 'fixes'

I'm not a big 'what if' person. What happens.....happens. But the consequences of this incident could have been disastrous. The incident included a PAN-PAN call that escalated to a MAYDAY. ATC did a great job in support, dcpearso did a remarkable job putting it down with no injury or damage. The aircraft was flown out of the paddock the next day. Testament to the aircraft, there was NO damage!

Lesson 2: Maintenance Authority comes with a massive responsibility, it's a privilege.

Not that I didn't understand the gravity of the privilege, but there's more to it than just inspecting under the panels and changing the oil.
 
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