The series of speeds associated with an airplane’s takeoff

Once you’re lucky enough to find your seat, stow your bag, pull away from the gate and be cleared for take-off, you may not care too much about exactly how fast the plane is moving when it finally leaves the ground. This is understandable considering how slow commercial air travel can feel sometimes. It’s may also be hard to keep track of, since technically the exact speed a plane needs to be going to leave the ground changes with every flight based on weather, the location, and the plane itself. There are even cases where the plane can leave the ground while parked, although that’s probably not an option any pilot would purposely pick.

Air wafting over your wings

Before getting into the exact circumstances that can lead to liftoff without a plane heading down a runway, it’s probably helpful to quickly review how planes get off the ground under normal circumstances. For a plane to fly, it needs to be lifted off the ground by the air around it. Most of the time, air molecules aren’t lifting heavy objects off the ground, but if a plane can get more air pressure pushing up from below than is pushing down from above, it can be lifted into the sky. To get this upward push, or lift, airplanes rely on what’s called the Bernoulli Principle.

The Bernoulli Principle describes how the movement of liquid or air can influence the amount of pressure that substance exerts on the things around it. In the case of an airplane, the curve of the top of the wing compels air to move more quickly over the top of the wing than the air underneath. So all that’s needed for liftoff is enough air rushing past the plane’s wings in the right direction, which can sometimes happen even if the plane was just sitting on a tarmac. Once a plane is airborne, it’s likely to keep moving just enough to glide around for a bit, although even if powered on these low speeds would make the plane very difficult to actually control.

Three speeds for one take-off

Obviously, a scheduled take-off can’t rely on perfectly-timed gusts of air to leave an airport, even if it sometimes feels like that would be faster. Before an airplane approaches the runway, pilots need to calculate three speeds to coordinate their departure. These take the plane’s weight, atmospheric conditions, runway length other local variances into consideration. The first speed, called V1, is the point when the plane is moving too fast to safely abort the take-off, as there isn’t enough runway left. This is followed by VR, which is the speed when the nose of the plane can start to rotate upwards. Finally, V2 is the speed at which the plane can fly and clear local obstacles, even if an engine fails. As most large jets take off around 160 miles-per-hour, these speeds are calculated largely to ensure the safety of the plane and surrounding area, not to understand how to get a plane off the ground.

Soaring with the Earth’s spin

Now, to get picky about take-off speeds, there is also the fact that the air around your parked plane is actually moving at around 700 to 1000 miles-per-hour. This is due to the rotational speed of the Earth, and unfortunately it doesn’t really help us take off any faster. Because the plane, the air, and the ground all they’re “sitting” on are all moving at the same velocity, there’s no change in air pressure above or below your plane’s wings. Even worse, that lack of relative movement even prevents the Earth’s rotation from directly helping you reach your destination any faster, even if you’re flying west. The Earth’s rotation does help create tailwinds though, at least your flight might get a bit of a boost once it’s finally in the air.

Source: How Airplanes and Aircraft Fly, Avjobs.com