Induction Air System
By Tony Spicer
CAUTION: This document is in no way a publication of Sonex, Ltd. or any other corporation. All products mentioned are not necessarily recommended for use, but are included for informational purposes only. Builders tips and instructions are not meant to replace the plans and instructions from Sonex, Ltd.. All Builder's tips and instructions are presented only as a source of information and a forum for exchange and the sharing of ideas and construction methods. NO responsibility or liability is assumed, expressed, or implied as to the suitability, accuracy, safety, or approval thereof. Any party using the suggestions, ideas, instructions or examples on these pages, does so at their own risk and discretion and without recourse against anyone.
Original setup with filter mounted in upper left corner of firewall.
Original 2" NACA inlet dumps air on front edge of filter. Circle on cowl is filter location. Lower NACA is 2 1/2" that feeds oil cooler.
2 1/2" OD thin wall alum tube supported by 1/16" alum brackets.
Filter enclosure riveted in place. Make top and bottom sides wide enough to extend beyond side of fuselage, then with box clecoed in place, lay straightedge on side of fuselage and mark trim lines on box. Calculate distance from trim line to inside of cowl and draw a second line. Trim box on the second line and cleco in place. Install cowl with top fasteners locked, and side and bottom pins in place. Go in thru the open side with a flexible 12" metal scale and check the space between the edge of the box and the side of the cowl. If you did it correctly, the cowl should just touch the box all the way around. Mark the location of the front edge of the box on the inside of the cowl. With the cowl off, bisect the line with a second line parallel to the top longeron. A 30/60/90 triangle will come in handy. That's the centerline for the NACA inlet. Transfer the shape of the inlet to the front edge of the box, then trim 1/4" outside the line.
Decide what you want to use to seal the sides of the box to the cowl. I used a 3/4" wide, 1/2" high adhesive-backed bulb seal from McMaster-Carr, pn 93085K68. Trim another 3/8" from the edges, and you'll get a 1/8" squeeze of the seal. A bit more squeeze would probably be ok, but mine seals fine with the 1/8". Bend up some 3/4"x3/4" aluminum angles and rivet to the edges. I used 1/16" rubber and a clamping plate to seal the front edge. The 1" hole is for screwdriver access to the hose clamp holding the filter in place. Get a snap-in plug at Home Depot or Lowes.
Filter box with K&N filter installed.To determine how many square inches of filter area your engine requires, go here: http://www.knfilters.com/facts.htm#SELECT To identify a specific filter, go here: http://www.knfilters.com/search/univround.aspx Note cap in place over 1" hole. Rubber on front was cut 1/16" undersize to seal tightly around NACA inlet.
When cutting the front opening for the NACA inlet, I left an extra 1/4" across the top, then bent it back 90 deg to act as a stiffener. Rivets in lower right corner of box required bent stems to pull.
Back of box showing how flange was notched.
NACA inlet glassed in place. Note centerline on cowl. Black line on inlet shows location of rubber seal. Blob in background was former location of inlet feeding oil cooler which had to be moved to bottom of cowl.
New inlet. Barely visible on bottom is relocated inlet for oil cooler.
Balsa plug used to make NACA inlet.
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Subject to Caution Notice on first page of this document.