Question

Consider the four basic symmetrical flight loading conditions corresponding to the corners of a typical \(V\!-\!n\) diagram. For one condition it is observed that:
(i) the compressive bending stresses are maximum in the bottom aft region of the wing cross-section; and
(ii) the tensile bending stresses are maximum in the upper forward region of the wing cross-section (see figure).
Which flight loading condition corresponds to these observations? \begin{center} \includegraphics[width=0.5\textwidth]{06.jpeg} \end{center}

• A. Positive high angle of attack
• B. Positive low angle of attack
• C. Negative high angle of attack
• D. Negative low angle of attack

Hint:

Wing cross-section stresses are the sum of bending (sets top-in-compression, bottom-in-tension) and torsion. A strong nose-down torsion—typical at high speed and low \(\alpha\)—shifts maxima to upper forward (tension) and bottom aft (compression).

Answer 1

The Correct Option is B

Step 1: Separate bending and torsion effects on a wing box.
- Upward bending (from positive lift) tends to put the upper surface in compression and the lower surface in tension uniformly across the chord.
- Nose-down torsion (clockwise moment about the quarter-chord for a conventional airfoil with \(C_m<0\)) produces tension on the upper forward (leading-edge top) and compression on the bottom aft (trailing-edge bottom).

Step 2: Interpret the observed maxima.
The given pattern—tension at upper forward and compression at bottom aft—matches a strong nose-down torsional loading superposed on the usual upward bending. This occurs when the aerodynamic \(|C_m|\)–effect is pronounced.

Step 3: Link to \(V\!-\!n\) corners.
At positive low angle of attack but high airspeed (right–upper corner of the \(V\!-\!n\) envelope), dynamic pressure is large. Even with modest \(\alpha\), the aerodynamic center pitching moment (typically nose-down for transport airfoils) produces a large torsional moment, dominating the chordwise stress distribution and giving exactly the observed tension/compression locations.
At positive high \(\alpha\) (near-stall corner), bending dominates (upper-surface compression, lower-surface tension across the chord), which does not yield the specific forward/aft maxima stated. Negative-\(\alpha\) corners would reverse signs.

Final Answer:
\[ \boxed{\text{Positive low angle of attack}} \]