Aerodynamics and Glider Performance Flashcards
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Read the first 6 Aerodynamics and Glider Performance flashcards as text
A glider pilot wants to maximize the time spent in the air to take advantage of a thermal. Which speed should the pilot aim to fly at?
Answer: Minimum sink speed
Minimum sink speed is the airspeed at which the glider has the lowest rate of descent, thus maximizing the time it can stay aloft. This is the ideal speed for circling in a thermal to gain altitude. Best L/D max speed maximizes distance covered, not time.
How does increasing the all-up weight of a glider (e.g., by adding water ballast) affect its performance, assuming the pilot flies at the correct corresponding airspeeds?
Answer: It has no effect on the best glide ratio but increases the best glide speed.
A glider's lift-to-drag ratio (L/D max), which determines its best glide angle, is an aerodynamic property of its design and is not affected by weight. However, to achieve that same L/D ratio at a higher weight, the glider must fly at a higher airspeed. The minimum sink rate will also increase, but the glide ratio for a given starting altitude remains the same.
A pilot is flying a glider with a published best glide ratio of 40:1 in still air. From an altitude of 5,280 feet above the ground, what is the maximum horizontal distance the glider can travel?
Answer: 211,200 feet
A glide ratio of 40:1 means the glider can travel 40 feet horizontally for every 1 foot of altitude lost. From 5,280 feet (1 statute mile) of altitude, the maximum horizontal distance is 40 times the altitude: 40 * 5,280 feet = 211,200 feet. This is equivalent to 40 statute miles.
Which of the following aerodynamic forces acts perpendicular to the relative wind?
Answer: Lift
By definition, lift is the aerodynamic force that acts perpendicular to the direction of the relative wind. Drag acts parallel to the relative wind, and weight acts vertically toward the center of the Earth. A glider has no thrust.
While on a cross-country flight, a glider pilot encounters a strong headwind when flying between thermals. To maximize the distance covered over the ground, the pilot should:
Answer: Fly faster than the best L/D speed.
When flying into a headwind, a pilot must increase their airspeed above the still-air best L/D speed to maximize ground distance. This increased speed reduces the time spent in the sinking air mass between thermals, covering more ground for the altitude lost. A common rule of thumb is to add about half the estimated headwind speed to the best L/D speed.
What is the primary reason high-performance gliders are designed with long, slender wings (a high aspect ratio)?
Answer: To reduce induced drag
A high aspect ratio (the ratio of wingspan to wing chord) reduces induced drag. Induced drag is a byproduct of lift and is most significant at lower airspeeds. By minimizing this type of drag, the glider achieves a higher lift-to-drag ratio, resulting in better glide performance.