Forum: aviation.stackexchange.com
Tags: <takeoff><airspeed>
Title: Why is indicated airspeed rather than ground speed used during the takeoff roll?
Question: My instructor asked me this question during my lesson and I couldn't come up with any answers. He asked why do I need to look at the indicated airspeed rather than ground speed when in takeoff roll or as soon as we touchdown?   It would be great if you could give me the reference to back it up.

Answers:
sents: text: Because wings work on air moving past them, not ground moving below them.
text: Heck, in a 35 knot headwind, the Antonov-2 could be rolling backwards at 2 knots and still take off!
answer_details: author: aviation.stackexchange.com/users/22864/Harper - Reinstate Monica
score: 81
sents: text: Because what determines the amount of lift generated is the indicated airspeed, not the ground speed.
text: As usual, it is always easier to think about an extreme case.
text: If you have an aircraft with V R (speed at rotation for takeoff) of 90 knots, and there is an 80 knots head wind, in theory it will rotate with ground speed of 10 knots even though the indicated airspeed will be 90 knots.
answer_details: author: aviation.stackexchange.com/users/3363/DeepSpace
score: 43
sents: text: Airspeed is always important as it determines lift which keeps you in the air.
text: At the moment of take-off or landing, airspeed is critical because it is the point at which you transition to or from flight; you need to be going fast enough but not too fast so that the transition is positive without being abrupt or overly stressful on aircraft components.
text: This is why there are specified take-off and approach speeds.
text: Ground speed matters in navigation because it determines flight time which affects fuel required to get from A to B. Ground speed is a consideration for take-off and landing (ideally kept to a minimum), which is why it's always preferable to do so into the wind, but ultimately it is airspeed that matters and ground speed is what it is at the moment of take-off or landing.
answer_details: author: aviation.stackexchange.com/users/3218/Anthony X
score: 11
sents: text: The Wikipedia article on indicated airspeed has a good description.
text: An airspeed indicator is actually more of a "dynamic-pressure" meter, with the dynamic pressure converted to airspeed.
text: Dynamic pressure is $q=\frac{1}{2}\rho V^2$ where $\rho$ is density and $V$ is airspeed.
text: The Wikipedia article on the lift coefficient explains that lift is proportional to dynamic pressure, the area of the wings, and the lift coefficient, which in simplified terms can be considered a function of the angle of attack.
text: $L=\frac{1}{2}\rho V^2 S C_L(\alpha) =
text: qS C_L(\alpha)
text: $
text: The takeoff speed is the speed at which you will have enough lift to get the airplane off the ground at the angle of attack that the airplane will have post-rotation.
text: So, for a given aircraft at a given weight, wing area, post-rotation angle-of-attack, and lift curve, you will be able to take off at a particular dynamic pressure.
text: In fact, all the aerodynamic forces on the aircraft are proportional to the dynamic pressure.
text: So that's why stall speed, never-exceed-speed (above which the aerodynamic forces could start damaging the aircraft structure), etc.
text: are all given in indicated airspeed - because it's a proxy for dynamic pressure.
text: True airspeed is the speed of the aircraft relative to the air mass it's flying through and can be calculated from indicated airspeed by correcting for density and temperature.
text: Your ground speed is then the true airspeed added to the wind speed.
answer_details: author: aviation.stackexchange.com/users/29021/moink
score: 5
sents: text: The IAS includes factors such as Wind Component (Tail, Head or Cross), Pressure and Temperature.
text: All these have an influence on your takeoff.
text: The takeoff Speed Vr is calculated as Indicated Airspeed.
text: Imagine you have a Headwind Component of +50.
text: You Groundspeed would be way lower than your Airspeed.
text: What counts is the amount of air that flows over the wing in order to takeoff, not the speed relative to the ground.
answer_details: author: aviation.stackexchange.com/users/38007/Windshear
score: 3

Summary of the answers:
At the moment of take-off or landing, airspeed is critical because it is the point at which you transition to or from flight; you need to be going fast enough but not too fast so that the transition is positive without being abrupt or overly stressful on aircraft components.
The lift is ultimately determined by the amount of air moving past the wings, not the speed of ground moving below the aircraft. Therefore, airspeed is more important because it will determine how smooth the lift will be. Ground speed determines flight time and the amount of required fuel, and whileit myst be taken into consideration, it is airspeed that matters for the take-off.