Question

A general aviation airplane is gliding with a speed \( V_g \) at minimum glide angle. Which of the following statements is/are true?

• A. \( V_g \) is equal to the speed corresponding to the maximum lift to drag ratio of the airplane.
• B. \( V_g \) increases with decreasing wing loading when all other parameters remain constant.
• C. \( V_g \) increases with decreasing altitude when all other parameters remain constant.
• D. \( V_g \) increases with increasing altitude when all other parameters remain constant.

Hint:

For gliding flight, the glide speed \( V_g \) is equal to the speed corresponding to the maximum lift-to-drag ratio. \( V_g \) increases with altitude because the air density decreases, requiring a higher speed for the same aerodynamic performance.

Answer 1

The Correct Option is A, D

The minimum glide angle occurs at the speed corresponding to the maximum lift-to-drag ratio (\( L/D_{{max}} \)). Step 1: \( V_g \) is equal to the speed corresponding to the maximum lift to drag ratio.
This is the defining condition for the minimum glide angle. At this speed, the airplane is able to maintain the most efficient flight path in terms of energy expenditure. Therefore, statement (A) is true. Step 2: \( V_g \) increases with decreasing wing loading.
Wing loading is defined as the weight of the aircraft divided by the wing area. When wing loading decreases, the required speed to achieve the maximum lift-to-drag ratio also decreases. Hence, statement (B) is false because \( V_g \) actually decreases with decreasing wing loading. Step 3: \( V_g \) increases with decreasing altitude.
At lower altitudes, the air density is higher, which results in a higher aerodynamic force for the same airspeed. Since \( V_g \) corresponds to the speed that achieves the maximum lift-to-drag ratio, the airplane can glide more efficiently at a lower speed at lower altitudes. Hence, statement (C) is false. Step 4: \( V_g \) increases with increasing altitude.
As altitude increases, the air density decreases, which requires a higher speed to maintain the same aerodynamic performance. Therefore, \( V_g \) increases with increasing altitude. This makes statement (D) true. Thus, the correct answers are (A) and (D).