A transport-category jet's fuselage cross-section is deliberately 'waisted' (narrowed) near the wing root compared to fore and aft sections. What aerodynamic principle explains this design feature?
- AThe area rule, which smooths the aircraft's total cross-sectional area distribution to reduce transonic wave drag
- BDutch roll damping achieved through fuselage shaping near the wing root
- CSweep theory, which reduces the wing's effective thickness-to-chord ratio
- DGround effect compensation to improve landing flare characteristics
Show answer & explanationAnswer & explanation
Correct answer: A. The area rule, which smooths the aircraft's total cross-sectional area distribution to reduce transonic wave drag
The Whitcomb area rule states that transonic wave drag is minimized when the total cross-sectional area of the airframe (fuselage plus wings) increases and decreases smoothly along the length of the aircraft. Narrowing the fuselage where the wing adds cross-sectional area prevents an abrupt area change that would otherwise increase wave drag near Mcrit.
Why the other options are wrong
- B. Dutch roll is a yaw-roll coupling issue addressed by yaw dampers and vertical stabilizer/dihedral design, not fuselage waisting.
- C. Sweep reduces effective thickness ratio of the wing itself, unrelated to fuselage shaping.
- D. Ground effect is unrelated to fuselage cross-sectional shaping at cruise altitude.
Area Rule (Whitcomb Rule)
A design principle stating that wave drag near transonic speeds is minimized when the aircraft's total cross-sectional area distribution changes smoothly along its length, often producing a 'coke bottle' shaped fuselage.
- Developed by Richard Whitcomb in the 1950s
- Reduces wave drag rise approaching Mcrit
- Visible as fuselage narrowing near the wing root on some transonic aircraft
Memory trick: Coke-bottle fuselage sips through the sound barrier smoothly