What is the proper airspeed to use when flying between thermals on a cross country flight against a headwind?

• A. The best lift/drag speed unaffected by wind
• B. The best lift/drag speed increased by the wind velocity
• C. The best lift/drag speed increased by one half the estimated wind velocity
• D. The best climb speed

Answer: (C)

When gliding from one thermal to the next, you’re aiming to maximize your distance gained over the ground for a given amount of altitude lost. Wind changes how fast you cover ground for a given airspeed, so you don’t stay at the calm-air best glide speed. Instead, you increase your airspeed a little to compensate for the headwind. The practical rule is to add about half of the headwind component to your best lift/drag (best glide) speed. This keeps you moving forward efficiently without incurring the extra sink that comes with flying much faster. If you had a tailwind, you’d subtract roughly half the wind from your speed. The option to fly at the best climb speed isn’t right for gliding between thermals, and the other choices either ignore wind effects or imply too large a change. So the correct approach is to increase the best glide speed by about half the headwind velocity.