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Dean Pichon

Well Known Member
I'm trying to improve the connection between the lower cowl of my -4 and the airbox. The current configuration is basically per Van's plans and has a couple of shortcomings. First, there is only about 1" clearance between the airbox and the cowl. I believe this short gap allows cowl movement to transmit load to the airbox which ultimately causes cracking of the airbox mounting plate. Further, this limited clearance between airbox and cowl makes the removal of the lower cowl difficult. Finally, the wrapped fabric approach may leak some air and reduce, to some extent, the manifold pressure.

To improve this situation, I plan to cut back the airbox to increase the gap from 1" to 1-1/2" or 2". I would like to change the interface seal from silicone sheet rubber to SCAT tubing. Has anyone tried this? Has anyone else tried something different that they could recommend?

Thanks in advance for the help.
 
Yes...

....there is a way that Vans recommends and that is to extend the cowl inlet back an inch or so by using foam in the intake and glassing around that. This allows the cowl to drop straight down as it clears the front of the airbox.

Then pop rivet baffling material around the FAB,

Regards,
 
pierre smith said:
....there is a way that Vans recommends and that is to extend the cowl inlet back an inch or so by using foam in the intake and glassing around that. This allows the cowl to drop straight down as it clears the front of the airbox.

Then pop rivet baffling material around the FAB,

Regards,
Click to expand...

Hi Pierre,

Thanks for the reply. I recall reading Van's procedure for extending the inlet aft. I don't see how this helps. It would reduce the gap between the cowl and FAB. Also, I don't see why extending the inlet would allow the cowl "to drop straight down". I seem to be missing something....
 
Re-think how the rubber sheets work...

Put the rubber sheeting around the sides and bottom, attached to the cowl. Then attach the piece which seals the top to the FAB snout. You will need to add some sort of flexible stiffener to this top piece, like spring steel tabs or something. This way, you can drop the cowl straight down without having to reef the seal fabric out of the way. It may take some playing around with stiffeners, but I've had this arrangement for almost 1000 hours with no problems.
 
Wow! 1" to 2" gap? I have 3/8"" to 1/2" gap without any problems for over 700 hrs. I'd expect a 2" gap would be hard to seal, but if you could get a SCAT tube on that would be the best.

Kevin Belue
RV-6A
RV-10
 
Wow a 3/8" gap...

I'm surprised you haven't cracked FAB mounting plates. When I spoke with Don at Airflow Performance, he suggested the current gap in my installation (about 1") is too small and that I should consider increasing it. Did you build an extension to the air inlet on the lower cowl? Is your lower cowl difficult to remove? Thanks for the info.
 
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