Cowl Modifications

Here are some pictures of the cowl modifications that I have made to help keep the engine cool.

Cooling Part One

The first item is a pair of masks that are bolted to the front of the radiators. These masks block off a portion of the radiator inlet so air does not escape around the cowl inlet openings. Notice in the picture that this radiator shroud also has a hole cut for a scat tube. This tube goes to an enclosure around the fuel pumps mounted on the firewall.

Radiator Mask

The next item is to seal off the opening around the oil cooler shroud. I made some metal strips that seal the gap between the metal shroud and the fiberglass lower cowl to make up for the difference in angle.

Oil Cooler

The final item is an enlargement to the cowl exit area. I found some nice plastic vents at West Marine that were about 11″ X 3″. The left one is fairly close to the exhaust pipes on the inside, so I made a simple heat shield to avoid melting. After over 45 hours the plastic vents are still in excellent shape with no sign of heat damage.

Exit Vent

Cooling Part Deux

Upgrading to the Gen-3 in January 2008 caused some cooling problems due to the increased engine output that made some further changes necessary.

First, I took some baseline measurements of airflow using a Magnahelic gauge to get the air pressure behind the radiators and oil cooler. Then, I took my die cutter and started whacking off pieces of the cowl. I enlarged the intake openings to more closely match the size of the radiator shroud, and removed the “masks” shown above. I used some duct tape to seal the shroud to the cowl and flew again to see if there was any difference in airflow.

New Intake
New Intake with Tape

There was very marginal difference. The next step was to enlarge the exit area, so I clipped the corners off of the rear center section of the cowl.

Enlarged exit

To my dismay, this had little to no effect. I was hoping to avoid having to design and build a cowl flap, but that is eventually what I did. I first repaired the corners that I had cut off, then cut the entire bottom middle section out of the cowl. I built two aluminum sides and used some leftover piano hinge at the front edge to hold the flap on to the cowl. The flap was then set at a fixed position and I used some angle reinforcements to stiffen up the weakened areas a bit.

Fixed cowl flap

While the flap is not currently moveable in flight, I basically have a couple of options. It is hinged, so during the winter, I can opt to close it up a little and reduce drag. Or, I can spend more time and money on an actuator system to control it from the cockpit. For now, it is staying fixed. I have found that dropping the center section four inches is a good compromise between cooling and drag.

All I need to do now is finish fixing up the fiberglass. I have roughly glassed over the new intakes in order to make them more permanently airworthy,
but they are far from complete.

Reglass intakes

Cooling Part Drei

Cowl v2

It is now May 2009, and I think I can finally close this chapter of the cooling mods saga.

I have finished up the fiberglass and cowl flap, electing to stay with the fixed opening for now. I also decided to clean up the installation of the plastic vents that I had originally installed by glassing them in flush with the outer surface of the cowl. This really made the vents look like they belonged there, not like they were bolted on as an afterthough.

Bottom vents v2
Bottom vents v2

The bottom cowl and cowl flap had all of the major structure built up with several layers of glass and West Marine epoxy, same as usual. After knocking down the rough edges and filling in big holes with flox or micro as needed, a thin layer of Bondo body filler was applied. This filled in most of the remaining pinholes and depressions. Interestingly, about 98% of the Bondo ended up being sanded off anyway.

The next step was to apply glazing putty, which filled in everything else that I could see or feel. Again, most of this was sanded off, and what remained were little spots of putty where low spots and pinholes were. Good deal! Finally, the entire cowl was sanded to rough up the surface for new paint.

So I took a long leap away from my usual senses; whether they are good are bad only time will tell. A few years ago I toyed with the idea of using common hardware store paint, such as Rustoleum, to finish the plane. Given the lousy results of the specialized airplane paint I used from AFS (not entirely their fault), I figured I would not be any worse off than I was. Hell, every other major part of the airplane is already a different shade of white anyway.

Of course it is now May, with record high temperatures and high winds expected. If you have been reading this journal carefully, you know it is time to break out the paint gun. Believe me, I don’t plan for it; it just is what it is.

Amazingly, the paint came out quite beautifully. I thinned the Rustoleum with mineral spirits just enough so it would flow well, then shot it with a new Harbor Freight HVLP gravity fed $19 paint gun. I sprayed several very fine mist coats, then layed on a couple of good wet coats. There was a little orange peel at first, but it ended up flowing out very nicely. The worst part is that now it makes the rest of the paint look worse. I may end up having to repaint the whole airplane now!

Sadly, transporting the cowl back to the airport and subsequent reassembly of the cowl flap ended up scraping some of the nice new paint. Ah, well. It is what it is.

Cowl reinstalled