Engine cowl part 2



lower Foam blocked in place


The lower half of the foam shaping started by flipping over the fuselage.







I want to thank my Friend and neighbor Ed Gardner for all of his help on this one. He's a pretty big boy, airplane nut and an all around good guy!!
check out Eds project and Business: http://mmwauto.com/index.php/services.html

As shown in the upper cowl blog, the foam was fitted between the firewall and the 1/4" foam space that was attached to the rear of the spinner mold.

Again the rules for attaching the foam is to make sure the glue is well below the final contour and that internal cut outs are also below the final contour. Its not a disaster if they are, as you will see later, I ended up with a small hole.


Another view of the blocked in foam


After the foam is blocked in, I started the initial shaping. rough cuts were made with hand saw and final cuts were made using a sanding foam block with 36 grit paper. I have made other cowls before using higher density urethane foam, the result is similar, but the higher density just takes a lot longer to shape.




rough shaping with a hand saw.



I first hit on this idea of using floral foam when i built a cowl for an abandoned project you sometimes see hanging on my wall. I used to build a number of sail plane models and the nose cone was always carved out of a block of Balsa. The carving technique was always to profile the plan view and then the side view. after that the rest was sanded away. I found out about Telescope hobbyists using glass to grind glass and I realized that floral urethane foam would do the same. This really works quite well.

More rough cutting


You will notice in this picture that the center line template was exposed by sanding. I use 36 grit paper when I do this. There is no need to use any finer, it only slows down the process. The side templates will also be sanded to profile the plan view.






Final shaping


After the rough cuts were made, scrap pieces of the foam were used to finish the final sanding. as you can see there is a small hole in the foam, I'm not worried about this because the next step is to lay a layer of fiberglass and eventually fill and fair the surface with Bondo or Automotive body filler.

Another view of the final shape


I continued to 'tweek" and fine tune the shape for a little while after this. Once the basic shape of the main cowl is finished, I will start the process of adding the cheek cowls. These cowls will be assymetrical, The R/H cowl will be the fairing for the exhaust and the cooling air outlet. The L/H cowl will have an air inlet and also fair the carb and air filter.


Side profile before fiberglass is applied




Before fiberglass is added, the surface is prepped with a mixture of resin and Micro Balloon. The Micro is added to about 4 OZ of resin and mixed to the consistency of pancake batter. The purpose of this is to fill the open cells of the foam. If you do not fill the cells, the laminating resin will eventually wick into the cells and draw air into the laminate.


One layer of 8 OZ cloth added


Here is a picture of the cloth overlaid on the foam. I use the 8 oz Rutan style BID cloth. This is a 8 harness crowfoot weave style cloth. It is extremely drapable and will conform to almost any contour. I added layers of aluminum tape so I can have an excessive flange. This will be important later in the mold process. The tape on the side will be in the location of the eventual cheek cowl. A couple of plies of Peel ply were also laid at the lower edge of this lay up. They are removed before the upper surface is laid up. They allow a smooth transition and a bond able surface when peeled off.


Initial coats of Bondo applied

 The process was repeated for the upper surface and the first layers of Bondo were added. The idea on the bondo is to completely coat the fiberglass to hide and of the weave and to smooth out any small waves in the surface. I will not completely finish out the master at this stage, I still have to add the cheek cowls.




View looking aft

 This is a good view of the upper and lower surfaces. I didn't have to spend a great deal of time finishing the sides in the areas of the cheek cowls.

Ready for the Cheek Cowls.


The next step is preparation for the cheek cowls. I'm going to try something I have never done on this step. I will build up the cowls on the bench similar to the way I have done my wing tips. I will then remove the cowls from the backing board. I plan on temporally gluing some sand paper to the fuselage and then sand the contour into the cheek cowls.

That's the next blog entry.