Alternate air device on vertical induction

I had a single backfire that blew the alternate air device off the bottom of the air cleaner housing, taking some of the fiberglass with it. So I installed an .032 aluminum plate on the bottom of the air cleaner using tank sealant, and it covers an area that is about .15" larger than the outside of the air filter, which also covers the hole where the alternate air device used to be. (I think it will provide a better seal at the bottom of the air filter than bare fiberglass (as designed) because the aluminum plate is perfectly flat.)

I noticed that the alternate air device was almost worn out after only 90 hours of engine time. The hole that the cover plate pivots around is elongated maybe 025" and the screw that the control cable pulls was worn about 1/3 the way thru where the control cable touches it. The control cable end was wrapped twice around that screw, and one turn of the wrap had broken off. This seems to be one of the weakest designs that Vans has done, and now I am wondering if I really want it, since it is so prone to failure.

Has anyone had any problems with the Vans vertical induction alternate air device?

Has anyone significantly improved this alternate air device? Like maybe putting a beaing in the pivot, or putting some sort of sturdy pivoting linkage for the attachment of the cable end for the control?

Has anyone ever USED the alternate air device on a vertical induction injected engine?. It seems that that only way that this device would be needed is if you fly thru precip and you were very close to freezing, such that evaporative cooling caused freezing of the moisture on the air filter, in which case a "carb heat" type of control would provide enough heat to thaw the air cleaner and restore full air supply to the engine. The advantage of the "carb heat" apparoach is that you can shut the control when you no longer need it, whereas the alternate air device requires a reset on the ground after landing.

Not yet -- but where would I post pictures?

jim skala said:

Not yet -- but where would I post pictures?

Click to expand...

See "insert pics" on the upper left of this web page.

I use a sliding "door"

Just last weekend I was going to install the alternate air on my FAB. After fiddling around with it a bit, I decided it was c**p and made a little sliding door instead. I have read posts about this being a very poor design and I would hate to find bits inside a cylinder that should be at the bottom of the FAB!

My little sliding door is only 20 gr. heavier than the Vans alternate air thingy. It is not installed permanently yet, but it slides really well and I gues that it will also close in flight. It will definitaly be more reliable than the pivoting thing from Vans.

I may post a few pictures when I have it finished.

Regards, Tonny.

This is it:.....

It's finished and I like it!

Inside view:



In the "Open" position:



In the "normal" (closed) position:



The FAB "brace":



It slides well enough to be able to open and close it from the cockpit. (It may be more difficult to push it back in, when the engine is sucking on it, though). I added a backing plate on the inside, for the door as well as for the nutplate that holds the cable clamp, because those rivets will otherwise just pull through the polyester.

Also installed a brace, because lots of people complained about a cracked top plate, due to vibrations.

Regards, Tonny.

Tonny, I think your approach has real merit. It's got to be better than the Vans design which is truly a disgrace (and it's their second attempt).

Can I ask a couple of questions.

1. It looks to me as if you have used three layers of aluminium under the FAB floor (and another reinforcement layer above the FAB floor). Presumably you want the sliding door to slide between layers of aluminium. Did you consider letting the sliding door slide between aluminium and fibreglass and hence reducing one layer of aluminium.

2. What thickness of aluminium did you use.

3. What clearance do you have between the left side of the fibreglass FAB (mixture side) and the lower cowl. Some people with vertical induction report that they have to mod the FAB to get it as far right as possible to provide even minimal clearance. What did you do.

For those who are interested in this topic I recommend the following thread for extra reading.
http://www.vansairforce.com/community/showthread.php?t=45860&highlight=FAB

Captain Avgas said:

Tonny, I think your approach has real merit. It's got to be better than the Vans design which is truly a disgrace (and it's their second attempt).

Can I ask a couple of questions.

1. It looks to me as if you have used three layers of aluminium under the FAB floor (and another reinforcement layer above the FAB floor). Presumably you want the sliding door to slide between layers of aluminium. Did you consider letting the sliding door slide between aluminium and fibreglass and hence reducing one layer of aluminium.

2. What thickness of aluminium did you use.

3. What clearance do you have between the left side of the fibreglass FAB (mixture side) and the lower cowl. Some people with vertical induction report that they have to mod the FAB to get it as far right as possible to provide even minimal clearance. What did you do.

For those who are interested in this topic I recommend the following thread for extra reading.
http://www.vansairforce.com/community/showthread.php?t=45860&highlight=FAB

Click to expand...

Bob,

1. Indeed I used three layers of aluminium, with the middle layer the same thickness as the sliding door. The middle layer is actually a u-shape that got painted with a few extra coats of primer to give the clearance so the door can slide. You need a good flat surface and also you need the layers to make it stiff enough so it remains perfectly straight, otherwise the door will not slide. The whole things weighs 20 gr. more than Van's pivoting door, so the additional weight is marginal.

2. I am not sure what the thicknesses are, I used what I had laying around and felt stiff enough to do the job. The door should be stiff enough to not bent and get sucked inside!

3. The clearance between the FAB and the cowl, on the LH side is approx. 20 mm. I did not modify the FAB to get this clearance. If this is not enough, a lot of other things (exhaust, etc.) are going to hit for example the engine mount. I had a problem though with the mixture arm on the throtle body. It would hit the radius of the scoop so I had to bend it slightly inboard, to get clearance when it is in the forward position. In the last picture you can see that I also filed of a bit from the corner of the arm. I hope it will be enough not to hit the cowl.

Regards, Tonny.