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Structural Design Philosophy

The metal spar wing incorporates structural practices common in modern aircraft manufacturing that are not present in legacy wood-spar designs. The wing was engineered as a complete structural system, not simply a spar substitution. Ribs, drag truss components, tank supports, and attachments are designed to function together within a unified load path.

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Leading Edge Structural Integration

A key distinction is the leading-edge skin attachment.

In the metal spar wing, the leading edge skin wraps continuously from the top of the spar to the bottom and is attached along the span to both the ribs and the spar using formed attachment angles (commonly referred to as Z-brackets).

This differs from the wood spar design in two primary ways:

1. The leading edge skin does not fully wrap around the spar.

2. Attachment is accomplished using nails and spacer blocks rather than structural angles and riveted joints.

The wraparound leading-edge configuration significantly increases torsional rigidity by creating a partially closed structural section. In practical terms, this reduces twisting under aerodynamic load and improves overall structural stability.

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Torsional Rigidity & Aeroelastic Behavior

The metal spar wing exhibits substantially greater torsional stiffness compared to legacy wood-spar construction. Reduced torsional deflection minimizes aeroelastic effects — the interaction between aerodynamic forces and structural deformation.

When a wing twists under load, control effectiveness and aerodynamic loading change. Increased torsional rigidity helps maintain consistent aileron effectiveness, particularly at higher angles of attack, and supports predictable control response throughout the flight envelope.

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Bending Stiffness & Load Distribution

Bending rigidity is also significantly increased.

The Z-bracket leading-edge attachment allows the skin’s moment of inertia to contribute to bending resistance. This distributes loads more effectively across the structure and reduces spar deflection under aerodynamic load.

In legacy wood-spar designs, rib-to-spar attachment relies on bent flanges secured with nails. Because nails cannot be bucked or clamped in double shear, repeated loading can gradually loosen the joint as shear forces act on the fastener. Additionally, spar deflection under load can elongate fastener holes over time.

The metal spar wing addresses this through a double-gusset rib attachment system. Formed gussets overlap the rib and attach on both sides of the spar using solid rivets in double shear. This arrangement provides symmetrical load transfer and improved long-term durability.

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Aeroelastic Considerations

All aerodynamic structures deform under load. When a wing deforms, its geometry changes, which in turn alters the aerodynamic forces acting upon it. This interaction is known as aeroelasticity.

Improved torsional and bending stiffness reduces undesirable aeroelastic effects such as control reversal tendencies or performance degradation under load. Compliance with certification requirements addresses safety considerations; increased stiffness enhances consistency and control feel.

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In-Flight Characteristics

Increased structural rigidity contributes to:

• More consistent aileron effectiveness at higher angles of attack

• Improved roll response

• Steadier vertical performance

• Reduced control “softness” under load

While aerodynamic fundamentals such as wing area and airfoil remain unchanged, structural refinement can yield measurable performance improvements.

Representative outcomes (airframe dependent) may include:

• 2–4 mph increase in cruise speed

• Approximately 2 mph change in stall characteristics

• ~5% improvement in rate of climb

• ~10% increase in roll rate

• Improved vertical penetration in aerobatic flight

Performance gains vary by configuration and aircraft condition.