Question 1

What are the three principal axes used to describe an aircraft's orientation?

Accepted Answer

Aircraft orientation is defined by three principal axes originating at the center of gravity. The yaw axis is perpendicular to the wings and describes side-to-side motion. The pitch axis runs parallel to the wing and controls up-and-down nose movement. The roll axis runs the aircraft's length and describes vertical wing movement.

Question 2

How do engineers use lift and drag coefficients to evaluate aircraft performance?

Accepted Answer

Lift and drag coefficients are dimensionless values that enable engineers to model complex effects of shape and airflow on aircraft forces. These coefficients are calculated using measured lift and drag, reference area, dynamic pressure, and free stream velocity. They allow standardized comparison of aerodynamic performance across different aircraft designs and scales.

Question 3

What does a positive stability derivative indicate about aircraft stability?

Accepted Answer

A positive stability derivative indicates aircraft instability. If the angle of attack increases due to a wind gust and the pitching moment coefficient continues increasing, the aircraft cannot return to its original position. This means the aircraft will become increasingly unstable rather than self-correcting to level flight.

Question 4

Why is the aircraft tail essential for directional stability?

Accepted Answer

The tail provides directional stability by generating a yaw moment that opposes changes in yaw angle. Without the tail, the yaw moment coefficient decreases as the yaw angle increases, preventing the aircraft from returning to zero sideslip. The tail ensures the aircraft naturally corrects directional deviations, maintaining stable flight.

Question 5

How does removing the tail affect the pitching moment coefficient of an aircraft?

Accepted Answer

Without the tail, the pitching moment coefficient increases as the pitch angle increases, creating longitudinal instability. With the tail installed, the pitching moment coefficient decreases with increasing pitch angle, providing stability. This demonstrates that the tail is critical for preventing the aircraft from entering uncontrolled pitch oscillations.

Question 6

What corrections must be applied to wind tunnel measurements before calculating aerodynamic coefficients?

Accepted Answer

Three corrections are applied sequentially: first, subtract strut forces measured without the model to remove strut effects; second, subtract model weight forces measured with zero wind speed; third, subtract the adjusted strut forces from adjusted model forces. These corrections isolate true aerodynamic forces from structural and balance contributions.

Question 7

How does a wind tunnel force balance measure aircraft aerodynamic characteristics?

Accepted Answer

A six-component external aerodynamic force balance measures lift, drag, and moment forces on a scale model at controlled pitch and yaw angles. The balance records forces with and without airflow, at different angles, and with different configurations. This method is widely used in aerospace industries and research labs for aircraft and rocket development.

Performances aérodynamiques d'un modèle réduit d'avion : Le DC-6B

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