Cooling a rear mounted engine is a bit trickier than a front mounted engine. With the traditional front mounted engine, you put a couple of openings at the front of the cowling and let the propeller push the air into the engine compartment. Then the air is forced down through the cylinders and exits out the bottom of the cowling through cowl flaps.

The Velocity uses a pair NACA Submerged Inlets (or ducts) on the top of the fuselage to provide airflow into the engine compartment.  Unfortunately, the ducts supplied by the factory aren’t true NACA’s. They have some minor deviations which reduce their efficiency. But they should be good enough. If I had to do it again, I would take the Andy Millin approach and build them myself from scratch.

The air from the NACA’s is ducted into a plenum. This plenum is basically a fiberglass box that is fitted to the engine.

This is a picture of Rich Guerra’s finished plenum and ducts. Rich is running a different engine, but you get the idea.

I have four problems with this: 1) Every other piston airplane I’ve seen doesn’t have a plenum. They use the engine cowling to create the plenum. 2) When you have to service the engine, you have to remove this plenum. 3) With the plenum so close to the engine, it will be very difficult to distribute the airflow evenly over all the cylinders and 4) I’ve already had to modify the cowling to give the engine enough clearance in three places. This means that I’ll now have to modify the plenum too.

So after a lot of discussion with Malcolm and Lynn, I decided to wander way out into left field (or come in from left field) and go a more traditional route and use the cowling to create the plenum.This will require a sheet metal baffle that will seal against the cowl.

I looked into buying a set of baffles from Cirrus or Lancair (both use the same engine I have) and then modify it for the Velocity. But that was WAY too expensive. So I bought a couple of sheets of aluminum and some construction paper and got to work.

Here’s after building up the copilot side out of construction paper. The box with the “X” is the approximate location that the duct will penetrate the forward baffle.

The area below the oil cooler was a real challenge (not much room).

The pilot side was more of the same. snip a little, position, snip, position, snip, position, etc.Once I had all the paper templates, I transferred them to the aluminum and cut it out. After they were cut, Lynn let me use his brake so I could bend them where necessary.

Here’s the piece that goes between the oil cooler and the #1 cylinder.

The right front.

Right shelf.

Right rear.

Then I started putting all pieces in position (and trim some more) until they fit together and to the engine. Once I had everything fitting nicely, I began drilling and preparing to install the nutplates.

Here’s the final result. I don’t have all the attachment holes drilled just yet.

Next comes the brackets that attach the whole thing to the engine.