A Few Engineering Points of Interest in the Metal Spar Wing

From an engineering aspect, the metal spar wing takes many practices into account that the wood wing does not. All of which are industry standard which lends more to the statement "You don't see other manufacturers using wood" than just a different spar.

The most important difference is the leading edge skin attachment. Unlike the wood spar wing, the leading edge wraps all the way around the the leading edge from the top of the spar to the bottom of the spar. It is attached along the span to the ribs and  to the spar through attach angles (affectionately known as Z-brackets, in light blue, below). This arrangement differs from the wood spar wing, in two ways.  First, the wood spar wing design does not have the leading edge wrap completely around. Second, the leading edge to spar attachment is done with small nails and spacer blocks.

So what does this arrangement do? It is basically a half step towards a monocoque wing.  Picture two tubes side by side, one is whole, the other has thin slot cut down its length. Twisting these tubes yield grossly different results in stiffness. Yet, the amount of material is nearly the same.  Same situation in the metal spar wing, offering greater torsional rigidity.  Thus, deflections are reduced.   This leads to performance gains from reduced aeroelastic effects.

The metal spar wing has more than twice the rigidity of the wood spar wing in bending as well.  This is also due to the leading edge skin 'Z-brackets' which allow the leading edge skin moment of inertia to contribute to the bending stiffness.  This also aids in performance issues concerning aeroelastic effects.

The loads on the structure are primarily air loads, as inertial loads are very small in comparison.  These loads are supported by the ribs from the fabric.  The spar, of course, supports the ribs through the rib / spar attachment.   This is where the wood and metal spar wings differ greatly.  The wood spar wing translates these loads through a bent flange in the rib with small nails pounded into the spar.  Since you cannot 'buck' a nail, this is similar to putting a bolt through a hole without a nut on the back side.  As the wing is loaded and unloaded, the shear force slowly tugs the nail out.  Contributing to this is spar deflection under load causing the hole to become oblong during the loading period.  But wait, there is more.  The nails are pounded through the back side of the front spar (and front side of rear spar), effectively creating a single shear case for the rib to spar attachment and a localized twisting of the assembly from this non-symmetry.  The metal spar wing solves these problems with a special double gusset assembly.  Form fitting gussets overlap the rib and one another and are on both sides of the spar with a solid rivet through all three pieces allowing symmetrical loading of the spar.

So what is this aeroelastic mumbo jumbo?  Simply put, it is the relationship between areodynamic loads and aerodynamic structure.  All structures under load deform to some extent.  When an aerodynamic structure (such as a wing) deforms under aerodynamic load, it changes the load applied to the structure because of the new geometry.  Aeroelastic problems can range from the simple (like control system reversal) to the complex (like flutter).  Obviously this is both a safety issue and a performance issue.  Complying with the regulations usually takes care of the safety aspect.

By having more torsional rigidity, a pilot can expect to see more effective ailerons and better high angle of attack wing characteristics.  By having a rigid wing in bending, a pilot can expect to see better roll rates and vertical pulls.   So a metal spar wing takes off shorter, cruises faster, rolls faster, lands slower, is lighter on the controls, and has more vertical penetration.

How much extra performance will be seen?  Not as much as this article might have someone believe.  For as much as these changes help, the major aerodynamic contributors (like wing area and airfoil) are the same and still dominate the basis for aircraft performance.  None the less, a small amount of performance can be gained.  Expect things like: 200 extra feet of vertical penetration in a Super Decathlon; 2 to 4 mph rise in cruise speed; about 2 mph in stall performance; a 5% increase to rate of climb; and about 10% faster roll rate*

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