Semi Monocoque, Mono-what?

Just about every pilot’s operating handbook and airplane flying manual makes mention of the same property: “The fuselage is of a conventional semi monocoque construction…” And that’s the last mention of semi monocoque construction anywhere in the book. I’m willing to bet that your training manuals make little to no mention of it either.

Airplane Fuselages, the Old Fashioned Way

The Vickers Warwick featured a geodesic airframe.

The Vickers Warwick's geodesic skeleton

Much like the foundation of a house, the fuselage is the foundation of an airplane. It is the central attachment point for the wings, tail and engines. As such, it needs to be strong enough to support the loads imposed by the weight of the airplane in maneuvering flight.

Locked into the paradigm of their time, aviation pioneers constructed their fuselage much the same as a building or bridge. They used sturdy materials to construct heavy internal skeletons that would bear the full load of flight. It was not until 1913 that a Swiss man challenged the traditional wisdom of fuselage design.

Monocoque Construction

Note the lack of an internal skeleton in the Deperdussin monocoque

Notice the lack of any internal bracing in the Deperdussin monocoque racer

Eugene Ruchonnet thought outside of the box. Rather than mounting panels onto a solid load-bearing skeleton, Ruchonnet’s idea was to let the airplane’s skin carry the load. He did this by forming the fuselage out of multiple layers of wood. The layers were glued together with their grains running in different directions to strengthen the skin so much that no internal skeleton was needed! He called the new technique monocoque construction, or single-shell construction.

Monocoque construction works fine with tiny airplanes, but there is a point at which the weight of the monocoque fuselage is greater than the weight of an internal skeleton. Furthermore, monocoque construction is unforgiving of even the slightest structural failure: think of how easily an aluminum can is crushed once a single dimple forms in it’s skin and you’ll see the inherent problem.

Semi Monocoque Construction

Semi monocoque construction uses stressed skin reinforced by a partial skeleton

Bulkheads and supporting beams (stringers) reinforce stressed skin in semi monocoque airplanes

In order to build bigger and stronger airplanes, a hybrid of the two construction techniques was put forward and remains in use today. The idea behind semi monocoque construction is quite simple. Instead of building a full internal skeleton, aircraft designers chose to build a partial skeleton to reinforce the skin in critical areas. A semi monocoque airplane’s skin supports much of the load, with some internal bracing and bulkheads in place to maintain structural integrity. This design works surprisingly well, and remains in place on most modern aircraft from single engine pistons to the brand new Boeing 787 Dreamliner.


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About the author
Pat Flannigan is a professional pilot and aviation blogger. He has been flying for fifteen years and is currently working as an airline pilot in the United States.

5 Replies to Semi Monocoque, Mono-what?

  1. Julien says:

    Thanks Patrick, I had always wondered about what semi monocoque actually meant but never looked it up, now I no longer have to! Another structural question I have is the following: how are wings attached to the airframe in typical GA airplanes? Is it just a matter of two holes, one on each longeron, lining up and then some big metal ping going through them? Or is there more to it? Thanks for the posts!

    • Well Julien, with that question you gave me a great idea for a future article!

      I’ll have to do some research, and maybe look at some kit-planes under construction, but I do have some understanding on the subject.

      Most modern airplanes feature cantilevered wings, which is a fancy way of saying that they dont’ require external bracing such as struts or wires. In this case, one or two central wing spars (much like steel girders on a building) run from the fuselage out to the wingtips. These spars act as a foundation for the basic airfoil supports to be attached.

      The spar usually goes through the fuselage and either continues to span the other wing, or bolts onto the other wing’s spar.

      Hope this helps!

  2. Jeffrey says:

    Patrick,

    Terrific! I really enjoyed your post. How can you not when you love airplanes? It is great when you can learn these little things that give more meaning to flying and you can benefit from those that have paved the way before us!

    Regards,

    Jeffrey
    FlyCRJ.com

    • Thanks Jeffrey. It does give you a different perspective when you realize the history and insights that went into building your airplane. It’s also amazing to see how much (and how little) things have changed from the barnstormer days to the modern airliners we fly now.

      Many thanks to the Wrights, Curtiss, Junkers, and so many others!

  3. Sylvia says:

    *nudge*

    Where did you go?

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