A swept-wing airplane enters a sideslip with the right wing forward into the relative wind. Instructors describe this producing a rolling moment away from the sideslip, similar to dihedral effect. What is the correct explanation?
- AWing sweep eliminates dihedral effect entirely, so geometric dihedral must fully compensate for directional stability
- BSweepback decreases directional stability by shortening the vertical tail's effective moment arm
- CThe trailing wing generates more lift because its effective span exposed to the airflow is longer than the leading wing's
- DThe forward (leading) wing presents a reduced effective sweep angle to the airflow, increasing its effective lift compared to the trailing wing
Show answer & explanationAnswer & explanation
Correct answer: D. The forward (leading) wing presents a reduced effective sweep angle to the airflow, increasing its effective lift compared to the trailing wing
In a sideslip, the wing that is 'leading' into the relative wind sees a reduced effective sweep angle (airflow strikes it more perpendicular to the leading edge), increasing its effective lift, while the trailing wing has an increased effective sweep angle and produces less lift. This asymmetric lift creates a rolling moment away from the sideslip, contributing to positive dihedral effect, independent of geometric dihedral.
Why the other options are wrong
- A. Wrong — sweep adds to dihedral effect rather than eliminating it
- B. Wrong — this describes an unrelated effect on the vertical tail, not the wing sweep mechanism
- C. Wrong — this reverses the actual relationship; the trailing wing has more effective sweep and less lift
Wing Sweep and Dihedral Effect
Wing sweep contributes to positive dihedral effect because, in a sideslip, the leading wing has reduced effective sweep and greater lift than the trailing wing, producing a rolling moment away from the slip.
- Adds to dihedral effect independently of geometric dihedral
- Leading wing = less effective sweep = more lift
- Excessive total dihedral effect can contribute to Dutch roll tendency
Memory trick: Sweep back your arms in a sideslip — the forward arm feels the wind straighter and lifts more