This morning I installed the two engine cooling NACA ducts and the fresh air duct.

http://www.velocity-xl.com/pictures/2008/IMG_5734.JPG

The installed ducts held in place with structural epoxy.

Inside view of the three ducts.

This morning, I started on engine cooling. In a conventional airplane (with the engine in front), there are openings at the front to allow ram air into the engine compartment to cool the engine. Since aircraft engines are air cooled, this is very important. Because the Velocity has a rear mounted engine, cooling is a challenge. The early Velocities used a pair of scoops on top of the fuselage just forward of the engine compartment. The scoops were similar to the hood scoops that you used to see on some high performance cars. The problem with scoops is that they aren’t very efficient and since they protrude into the airstream, they cause drag.

So instead of air scoops, we will use NACA Ducts. NACA (National Advisory Committee for Aeronautics) was the precursor of NASA. They discovered that if you create a recessed opening with the right shape, you can get more air with no drag.

So my first task was to cut out the two openings for the NACA ducts to the engine compartment and one for fresh air to the cabin.

From the back looking towards the front at the top of the fuselage. You can see where the two large ducts will be. There’s also a smaller fresh air duct in the middle. The tan colored area is the rear of the fuselage (also know as the firewall) where the engine will be mounted. I’ve also marked the openings in the firewall for the ducts.

Here, I’ve cut out the engine cooling ducts on top of the fuselage. At this point, I haven’t done the fresh air duct or the firewall openings (although they are marked).

After cutting the small NACA duct and the firewall openings, I prepared the surfaces mounted all three ducts with structural epoxy.

One of the things I’ve got be better at is taking pictures. When I get rolling on a task, I forget to pull the camera out and take pictures. This is why there are no pictures of the installed ducts and flight controls in the keel.

The NACA ducts provide cooling air to the engine. To obtain the maximum airflow (in my opinion) the path must be as smooth as possible. But I noticed a problem with this.

Here’s a picture looking up at the back of the pilot side NACA duct.

When the engine cowling is installed, there will be about an 1/8″ flange on the inside of the top of the duct.

To illustrate, here’s a couple of drawings

Here’s the “big picture”. Looking at the right side of the airplane at the top. To the right would be the front of the plane, left is the rear and the vertical line is where the engine cowling meets the aft end of the fuselage. The circle shows the area of interest.

This is the enlarged view showing the bottom of the duct (sloped) and the top/rear of the duct.

Here’s the same view but with the cowling installed (Red).

Now instead of a smooth surface along the top, there’s a “step”. The other problem is that the factory ducts are missing an important feature which is a large radius along that top lip.

And now my solution. I’m going to glue a strip (Blue) in front of the engine cowling which will be the same thickness as the flange.

This will accomplish 1) smooth surface on the inside 2) a thicker radius and 3) a more rigid surface across the top of the duct.

Here’s the same view as before with the strip installed.

Then it’s time for Malcolm to work his magic. Creating a proper, uniform radius is an artistic thing. And I’ve come to learn that when it comes to something like that, it’s best left to Malcolm.

It was hard to get a picture of the leading edge of the lip. This is the best I could do.

You may remember all the work that has been done to get the engine to fit: two of the intake tubes were replaced with custom built tubes to clear the cowling and then a hole was cut in the cowling for the main air intake.

Now it’s time to, sadly, modify the cowling so the air intake will fit.

Here’s a side, top and inside view:

I drew a shape to provide a symmetrical guide and rounded it out.  Then I made the cut and sanded the surrounding area.

To prevent an “bubble” where the hole was, I made relief cuts around the opening. Then I cut a piece of foam that was a little smaller than the relief cuts to go under the cowling. I large piece of aluminum sheet went under that. Then I made a “plug” out of triax that I covered with duct tape.

Then the area was covered with two progressively larger layers of BID.

Once the epoxy cured, I removed the cowling and flipped it over. A new triax “plug” was cut and epoxied into the hole which was then covered with 2 layers of BID and one layer of carbon fiber BID.

Once the inside work was completed, I put the cowling back on to see how much room I’ve got.

It’s hard to tell from the picture, but there’s about 1/2″ clearance between the top of the intake and the inside of the cowling. It’s tight, but it should be enough.

The top needed a bit of work though. It was a bit bumpy with a couple low spots. So after consulting with Malcom, I filled the low spots with micro and covered the whole thing with  one layer of fine BID.

A little bit (hopefully, very little) of filler and it should be ready to move on to the next step.