Step 8.1, Firewall

The RV series of aircraft and most others constructed today, employ a construction  method called semi-monocoque.  Don't be put off just because the word sounds French.  Yes, the word monocoque is French.  But it is also Greek, and that is the part we can understand.  It turns out the French use the word coque when they mean to say shell, so monocoque simply means single shell.

To construct in truly monocoque fashion is to make an object like a soup can in which the exterior skin provides all of the structure for the object.  In contrast, the most commonly used alternative to monocoque is the internal truss method where a rigid frame is constructed and then covered with a non structural material such as fabric.  Monocoque construction is also known as stressed-skin construction because the outer shell carries all of the structural loads. The RV uses a semi-monocoque method which is a compromise between using an  internal trusswork like the fabric covered airplane and having to use a very rigid, and consequently heavy, material to produce a load bearing outer shell.

So semi-monocoque construction uses some internal structure for high load areas, but most of the forces are carried through the thin and very light aluminum skin.  It is this blending of methods that optimizes the strength/weight ratio and has made semi-monocoque the aircraft designer's method of choice for a strong and light airplane.

Now that the fuselage inventory is complete it's time to get going.  Van's build manual begins construction at the firewall which seems perfectly sensible to me.

The firewall is a large piece of stainless steel that represents the cross section of the airplane just behind the engine.  Despite being heavier stainless steel is used at this location to protect the occupants from a potential engine fire because its melting temperature much higher than that of aluminum.  In the picture above the first step is to debur the very sharp edges.

Fabrication begins with some flanges that are cut from 2" angle and then trimmed down on the bandsaw.

The final step is to smooth the rough edges with the belt sander and finish with the Scotch-brite wheel.  Then the firewall structural members are test fit and match drilled.  This view is from the back side, which is what you would see if you were to look under the control panel.

Once the parts are drilled, they are deburred on the top side and counter sunk on the bottom to mate with the firewall holes which are then dimpled.  The result is flush rivets on the inside of the engine compartment.

Roxy chews.  Trina sleeps.  Carol deburs.

After deburring, the aluminum parts go to the paint booth for priming.

While we wait for that to dry, the stainless steel part is measured for some of the holes that will allow engine control cables to go forward and cabin heat to come back.

With the parts primed, they are staged for final assembly.

This is the lower corner weldment that will tie the fuselage to the engine mount on the other side.  There are only 16 rivets here.  Hmmm.

And here it is.

Fire wall done.  Anything good in the fridge?