The Plenum Project

Here's an obscure image that will immediately be recognized by anyone who has built a Velocity XL. Its a fiberglass cup made from laying up 3 layers of BID over a yogurt cup to make an access for the oil dipstick. You will note that this is a fine Acme product. Appropriately Wile E. Coyote-esque for a homebuilt aircraft.



This is going to be WAY too much information for anyone not building. However, I spent a substantial amount of time trying to figure out which half of the plenum forms went on which half of the engine, so....here goes and I hope some builders find it useful.



Here is the copilot side view of the plenum being joined with the runners and the flange being added to join the two halves.
This is a top view of the plenum and the runners being installed. Note the yogurt cup being bonded in place with micro to be trimmed and glassed in the next phase.
Here is the pilot side of the plenum illustrating how you make the flange to join the two halves and to join the runners to the plenum. Note that there are substantial gaps around the propeller end of the case. As you will see in later pictures, this gets fixed by putting duct tape on the engine and making a small flange to make a perfect fit.
Here are all the parts of the (finally) completed time sink - I mean plenum. The small black gadget in the middle is an appliance to hold the forward pilot side portion of the plenum tight against the engine. The plenum ended up consisting of 11 parts and LOTS of nutplates.
I used barbecue paint on the plenum on my last plane. This time I used it on the inside, but used gloss black high temp engine enamel from Pep Boys on the outside just because it looks better. Wait till you see what a difference painting made.
Here is a front view of the plenum after painting. You'll be surprised how much paint it soaks up and how porous the fiberglass is without filler. Both the inside and outside are now painted with high temp engine enamel. What a difference painting makes. My wife saw it and said "Now it looks like an airplane part!" I have always felt that with fiberglass airplane construction everything looks worse and worse until the finish is applied, when it miraculously looks beautiful.
Here's the pilot's side of the plenum. Firewall side is to the left.
Here is a view of the pilot's side with the inspection hole cover removed. Some builders prefer individual inspection openings and covers, but one large cover really stiffens up the whole plenum.
This is a view of the pilot's side of the plenum. The firewall would be on the left and the prop on the right. You can see the flange for the air line to the supplemental oil cooler. You will note that the access cover for the spark plug inspection openings is back in place.
Here is a view of the copilot's side of the plenum and the flange for the air intake to the fuel servo. Just to the left of the flange, you can see the inspection cover back in place over the spark plug access holes.
Here is a view of the firewall side of the plenum. You can see the parts of the runner that attach to the firewall at the bottom of the picture. They will attach with one sheet metal screw on each side of the runner and nutplates on the top that are captured by the cowling screws through the fuselage. Above them are the slip joints. When putting in the nutplates for them you have to decide whether to attach them to the short runners which are mounted on the firewall or the long ones attached to the plenum. By trial and error, I found that the nutplates need to go on the side with the long plenum or else the top of the long plenum will sag and create a leak. (more holes laboriously created and filled for the greater good)

Here is a top view of the finished product. There are still some imperfections in the finish, but I am trying to remember that it is an engine plenum that will be hidden by the cowl and not to get too crazy. I am satisfied with the look and fit. What's more, I don't have any skin left on my fingers after all the filling and sanding in the little corners.