At what speed does an airplane take off?

On a passenger jet, takeoff is calculated before every flight using three speeds that the pilot notes down and the system calls out during the roll. V1 is the decision speed: below it the takeoff can be aborted and stopped on the remaining runway; above it, the takeoff continues even if an engine fails. VR is the rotation speed, when the pilot pulls back on the controls and the nose lifts. V2 is the safety speed, the one that guarantees a climb with a single engine. For an Airbus A320 or a Boeing 737 with a typical load, VR falls between 135 and 155 knots (250-290 km/h); for a loaded Boeing 747 or A380, between 155 and 180 knots (290-330 km/h). The aircraft leaves the ground a few seconds after rotation, slightly above VR.

Lift grows with the square of speed and with air density, so the heavier the aircraft, the faster it must go, and the less dense the air — because of heat or airport altitude — the longer and faster the run. A takeoff in Madrid in August, at 600 m elevation and 38 °C, needs more runway and more speed than one in Amsterdam in January. Takeoff flaps increase the wing's lift to reduce these speeds at the cost of some drag; a setting with more flap allows a slower takeoff on short runways, while one with less flap suits situations with plenty of runway where quick altitude gain is wanted. At airports with long runways it's also common to take off with reduced thrust to extend engine life, which likewise lengthens the roll.

A Cessna 172 rotates at around 55 knots (100 km/h) and climbs at 70-75 knots; a regional turboprop like the ATR 72 takes off at around 110-115 knots (205-215 km/h). Fighters depend on their load: a clean F-16 becomes airborne at about 150 knots, but with tanks and weapons it may need 170 or more. At the other extreme, mountain STOL aircraft and ultralights take off below 40 knots (75 km/h), barely faster than a bicycle. A towed glider leaves the ground at around 45 knots. All these figures are indicated airspeeds, measured by the pitot tube from dynamic pressure; at a high, hot airport the groundspeed is higher even though the indicator reads the same, which is why the roll needs more runway.