[Jamie]

Last night I pulled my cowl to replace my flywheel (see this thread). While the cowl was off, I started checking everything out FWF as I usually do. This is when I noticed my airbox having problems. My filter started pushing through the bottom of the fiberglass. I also noticed that the mounting plate that attaches to the FI servo was broken.

These are two very common problems on these airplanes. A search here on VAF reveals that to be true.

I walked next door to recruit a hand to help me pull my prop (for the flywheel change) and my hangar neighbor had the cowl off of his Decathlon. His plane has a very similar engine setup to mine -- parallel valve 360 with vertical induction FI. His airplane has a nice, small little airbox on the bottom of the servo with alternate air trap door and scat tube on the front leading into a square filter. When I saw it I thought -- wow..why are these not used on RVs? It looks like it would be a simple gizmo to build up. The scat tube could run right up to the scoop on the bottom of the cowl.

Here are the advantages I see of this setup:

1) Smaller, thus no cowl interference issues
2) The fiberglass gets wet with oil and avgas which while wet weakens the structure. I believe this is why the filters have a tendency to push through the fiberglass. With an aluminum setup this is not possible.
3) It has a better alternate air setup than the Van's FAB which is notorious for cracking.
4) It can be built up using just aluminum, eliminating one fiberglass task from the list.

So, what is the advantage of the Van's FAB over this setup?

 

[vic syracuse]

Jaime, I haven't seen the airbox you mention, but I have never used the Van's supplied adapter plate. They are too thin and prone to cracking. I've always cut it out of thicker aluminum, usually .060 or thicker. No cracking.

You can also add a little support bracket from one of the engine bolts to the front of the adapter plate.

The carb heat is another whole topic.

Vic

 

[Special Delivery]

Efficiency!

So, what is the advantage of the Van's FAB over this setup?

Van's FAB is a VERY efficient airbox. It's greatest advantage over other boxes is its efficiency in controlling intake air turbulence. More knots on less fuel! Van got it right... but that's no surprise.

 

[lostpilot28]

Van's FAB is a VERY efficient airbox. It's greatest advantage over other boxes is its efficiency in controlling intake air turbulence. More knots on less fuel! Van got it right... but that's no surprise.

Maybe this is a question for another thread, but what makes it efficient? And, more importantly, how does the variation in build play on that efficiency?

 

[noelf]

In one of the past RVator issues from Van's, Van wrote a several page article on the air box design, pros vs cons vs design style. This article is also reprinted in the xxYears of the RVator.

 

[Mike S]

As was stated above, make a thicker plate for the top of the box, and build some diagonal braces to keep it from flexing.

As to the filter wearing out the bottom of the fiberglass part----think about it for a minute. The filter captures -------dirt!! Now add a small bit of oil, and what do you have???? Valve grinding compound, or at least in this case, fiberglass grinding compound

A simple fix is to make an aluminum plate to fit in the bottom of the FAB, size it to lay where the filter makes contact. A pop rivet or two and the plate will stay in place, but can be easily replaced when it wears through, eventually.

Good luck.

 

[kentb]

Hmmmm....

As to the filter wearing out the bottom of the fiberglass part----think about it for a minute. The filter captures -------dirt!! Now add a small bit of oil, and what do you have???? Valve grinding compound, or at least in this case, fiberglass grinding compound

A simple fix is to make an aluminum plate to fit in the bottom of the FAB, size it to lay where the filter makes contact. A pop rivet or two and the plate will stay in place, but can be easily replaced when it wears through, eventually.

Good luck.

So does them mean that I have a choice of either having fine particles of fiberglass or fine particles of aluminum going through my induction as it wears?

Maybe I need to add another filter between the FAB and my induction.

Kent

 

[pierre smith]

The FAB.......

.....is designed to slow the fast air down and does so by widening and air enters the filter all the way around and gets "straightened" as it turns upwards, with little or no turbulence.

Regards,