Glide Speed (5e Variant Rule)
From D&D Wiki
Some creatures or vehicles are powered simply by the aerodynamics of their shape, and possess no ability to maintain their altitude as they move through the air. These Creatures/Objects possess a Glide Speed. A Glide Speed assumes that the creature/vehicle falls by 5 feet every turn, and MUST move their full Glide Speed movement. The creature/pilot can choose to dive by converting horizontal movement into downward movement.
Example; A Gnomish Tinkerer is testing his new glider, by jumping from a 100 foot cliff. His Glider has a Glide Speed of 30 feet. Each turn he descends 5 feet, but moves forward 30 feet. 2 minutes later he lands, 600 feet away. On his second test as he glides he spots a Hippogriff swooping on him, and makes an emergency dive. He nose dives, avoiding the attack, but descending 35 feet that turn. 1 minute and 24 seconds after he starts he lands, 420 feet away.