THE FORMULA FOR FLYING

Want to fly? Then you must balance these four forces.

- Nick D'Alto

A 787 jumbo jet has a wingspan of 197 feet (60 meters), weighs about 502,000 pounds (227,700 kg), and flies at around 650 mph (1,050 km/h). A butterfly measures just inches across and weighs under an ounce (no more than a few grams). But to fly, they both must balance the same four basic forces. Here's how this important flying formula works.

1. Weight

Weight is the downward force from gravity that holds you down-whether you're flying or not. For an airplane, its "all-up" weight includes its own weight, the weight of fuel and supplies, and the weight of passengers and cargo, called the payload. Balloons are "lighter than air." They float when the air or gas inside of them weighs less than the air outside. So, a force called buoyancy lifts them upward. But heavier-than-air objects that fly must counteract weight using a second flying force.

2. Lift