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Factors Affecting Liftoff Speed
One factor that influences liftoff speed is airplane weight, which varies based on the aircraft. For commercial airplanes, this changes for every flight, depending on the number of passengers and the amount of luggage. Another factor is air density, which is calculated using the air temperature and the elevation of the airport. The last factor is the use and efficiency of lifting devices like wings, flaps and rudders. All these factors are taken into consideration when calculating the correct liftoff speed.
Other Things to Consider
Some of the factors used to calculate liftoff speed involve lift and velocity (the airplane's speed). The most basic piece of the equation is ensuring that there is enough lift to overcome the weight of the plane. However, pilots must also consider the possibility of engine failure and how that could affect their ability to control the plane should something go wrong. This is where other velocity measurements come into play (e.g., the minimum control speed with only one engine operating).
Calculating the Liftoff Speed
The official calculation for lift is as follows:
lift force = 1/2 x air density x aircraft velocity (airspeed) x reference area (surface area of the top of the wing, including where it passes through the body) x coefficient of lift (taken from a chart).
This is the most basic of the formulas; there are many more to ensure that no accidents occur, such as the tail scraping the ground on liftoff.
Example Speeds
Some pilots estimate the average takeoff speed to be around 160 to 180 miles per hour. For commercial planes like the Boeing 747, the liftoff speed (if the Boeing weighs around 800,000 pounds) could be about 180 mph.