Which of the following airplane configuration features contribute positively to dihedral effect ($C_{l_\beta} < 0$, restoring roll moment in positive sideslip)?

(A) Geometric wing dihedral angle (wing tips angled upward relative to the root)

(B) High-wing placement on the fuselage (wing mounted atop the fuselage)

(C) Wing sweepback in high-speed flight

(D) Geometric wing anhedral angle (wing tips drooped downward relative to the root)

Answer Key:

Verified in official key

### 1. Strategy & Theoretical Principles Dihedral effect refers to the rolling moment resulting from sideslip, quantified by stability derivative: $$C_{l_\beta} = \frac{\partial C_l}{\partial \beta}$$ By standard aerospace stability conventions, for a positive sideslip angle $\beta > 0$ (relative wind from the right), a stabilizing dihedral effect requires a negative rolling moment ($C_l < 0$, rolling to the left to lift the windward wing). Therefore, positive lateral static stability requires: $$C_{l_\beta} < 0$$ ### 2. Analytical Statement Evaluation - **Statement A:** **CORRECT.** Geometric dihedral angles wings upward, causing the windward wing to experience an increased effective angle of attack in sideslip, generating more lift and rolling the aircraft away from the sideslip ($C_{l\beta} < 0$). - **Statement B:** **CORRECT.** High-wing placement produces aerodynamic interference (fuselage cross-flow in sideslip pushes flow upward over the windward wing and downward over the leeward wing), acting like an effective dihedral of $1^\circ$ to $3^\circ$ ($C_{l\beta} < 0$). - **Statement C:** **CORRECT.** Wing sweepback causes the windward wing to have smaller effective sweep angle and larger normal velocity component ($V \cos(\Lambda - \beta)$), increasing its lift relative to the leeward wing, strongly contributing to $C_{l\beta} < 0$. - **Statement D:** **INCORRECT.** Geometric anhedral contributes *negatively* to dihedral effect ($C_{l\beta} > 0$), which is why anhedral is intentionally applied on high-wing swept transport aircraft (like the C-17 or An-124) to reduce excessive roll stability. ### 3. Exam Trap & Standard Reference - **Trap Alert:** Lateral static stability requires $C_{l_\beta} < 0$ (negative derivative), whereas directional static stability requires $C_{n_\beta} > 0$ (positive derivative). - **Standard Reference:** Nelson, R.C., *Flight Stability and Automatic Control*, Chapter 3.

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