# [Advanced GUIDE] Relationship between Airspeed and Groundspeed

## On ground

When an aircraft is stationary on ground while the mass of air is moving around it, this is perceived as wind. If this mass of air is moving straight towards you at a 100 kts (headwind) the aircraft would perceive this as flying at 100 kts through the air. For an airfoil there is absolutely no difference between moving through the air at 100 kts or being stationary while the air is moving around it at 100 kts (hence the principle of a wind tunnel), aerodynamically this is exactly the same thing. In this example, although the airspeed indicator would indicate 100 kts and the aircraft performance being the same as flying through the air at 100 kts, the ground speed would be zero!

Using a less extreme example. If your aircraft needs 60 kts indicated airspeed to rotate, and there would not be any wind at all, the aircraft needs to accelerate 60 kts. In standard atmosphere and at sea level we can directly relate indicated airspeed (technically the calibrated airspeed) to groundspeed. So for the example above both airspeed and groundspeed at time of rotation would read the same (60 kts).

With a 20 kt headwind, otherwise same conditions, the groundspeed at rotation would be 40 kts. With a 20 kt tailwind the aircraft needs to accelerate to 80 kts groundspeed. Accelerating to 80 kts obviously requires more runway, in addition the aircraft would also experience more wheel drag at 80 kts compared to 60 or 40 kts. In all the above conditions the indicated airspeed would be 60 kts and unless looking outside and seeing the end of the runway approaching faster during the take-off roll you wouldnâ€™t notice the difference, the aircraft in all three conditions would feel and perform the same way.

## In-flight

It is important to understand is that an aircraft in flight is moving in relation to the air around it (hence airspeed) and not in relation to the ground anymore. In flight an aircraft does technically not experience wind as the aircraft is flying within a mass of air taking it somewhere, although we generally keep calling it headwind if the mass of air with you inside it is moving in opposite direction, vice versa for tailwind or moving sideways along the ground is called crosswind, Iâ€™m also using these terms in my examples below but technically not correct. Compare this to a log of wood floating in a river, the log of wood itself does not move in relation to the water, the mass of water with the log of wood floating on top is going somewhere.

There might be sudden changes of direction and / or speed within this airmass which we perceived as turbulence or even windshear. We consider a smooth, turbulent and shear free air mass for all the examples below. We also consider sea level at standard atmosphere so we can directly relate indicated airspeed to groundspeed without having to correct for density (TAS) first.

When airborne the aircraft flies in relation to the air around it. In other words if the aircraft is climbing in a 100 kt headwind the aircraft is not gonna â€śfeelâ€ť the difference. It takes the same power, the rate of climb is the same etc. If climbing at 100 kts airspeed with a 100 kt headwind the ground speed is gonna be 0 kts and the aircraft is climbing straight up like a helicopter according to an observer on the ground. Not that this is very practical in real life as a 100 kt wind is probably gonna be turbulent. The pilot would not notice any difference however, the aircraft responds and performs the same as if there were no wind. Only when he looks outside the window he would notice that he is not moving forward.

I have flown backwards this way in small single engine piston aircraft. Having an airspeed of lets say 50 kts while the mass of air with your airplane inside is moving in the opposite direction with > 50 kts, in relation to the ground the aircraft is flying backwards.

To explain it completely the mickey mouse way. Imagine a fish in a fishing bowl swimming from one end to the other end of the fishing bowl. If you pick-up the bowl in the mean time and move it somewhere else, this fish is not gonna notice anything, does not have to swim faster. The fish would not notice anything at all unless looking outside and noticing moving in a different direction in relation to the table, in relation to the water in the fishing bowl nothing has changed.

Wind does not have any affect on an aircraft performance in relation to the air. In relation to the ground it does have an effect, an observer on the ground will see an aircraft climb-out steeper with a headwind and more shallow with a tailwind, when flying from A to B with a headwind it takes longer so more fuel is required. When there is X-wind you have to apply a wind correction angle to fly the correct ground track.

## Examples

I created a little example below, the square is a mass of air, 1000 ft high. The blue arrow represents an aircraft flying from one end of the airmass (point A) to the other end (point B) in one minute (1000 ft / min rate of climb) with 100 kts indicated airspeed. If there is no wind the blue line is both the air vector and the ground vector, it is the path the aircraft travels in relation to both the air and the ground.

If we now assume the airmass itself with the aircraft inside it is moving in the opposite direction with 100 kts while the aircraft is flying from point A to point B with 100 kts, we will have the situation as illustrated below.

• Blue is the air vector (the path the aircraft travels through the air),
• Red is the wind vector (the direction and speed the air mass is moving),
• Green is the ground vector (the aircraft movement in relation to the ground).

In other words the aircraft does exactly the same in relation to the surrounding air, no matter the wind, same airspeed, same rate of climb etc. the pilot would not notice anything as long as he doesnâ€™t look outside the window. the aircraft behaves the same and performs the same. An observer on the ground would see the aircraft climb vertically like a helicopter. The way to look at this is as follows: the aircraft is flying from A to B, but whilst it is doing that, point B is shifting in opposite direction.

If we take this example even further and assume the windspeed is higher than the aircraft speed through the air we can even fly backwards. Backwards in relation to the ground to be clear, in relation to the air we are still going 100 kts:

To add to this when you are on the runway on the brakes with lets say 20 kts of headwind, this is 20 kts of airspeed you have to accelerate less. The groundspeed at which you rotate is 20 kts lower so the overall ground distance is shorter. Same is true when landing. Still in relation to the air nothing has changed, the aircraft rotates at the same airspeed as if there was no wind. Only different is the reduced drag from the wheels due to the lower ground speed.

Another fun example: as you know a glider has no engine and to overcome the drag and keep flying it needs to descent continuously. This way part of the weight is acting as thrust (like rolling a shopping cart down a ramp). As soon as a glider pulls the nose up in order to climb this weight acting as thrust reduces and does not compensate for the drag anymore, the airspeed reduces and the plane eventually stalls.

Q: So if a glider has to descent continuously in order to keep flying, how does a glider stay in the air?

A: As soon as the air mass with the glider inside is climbing faster than the glider is descending, the glider climbs in relation to the ground. A glider is however ALWAYS descending in relation to the air:

If you would take the examples above into the horizontal plane (top down view) and draw the same triangle you can figure out what will happen with the ground track. If the mass of air with the aircraft inside is moving to the left (crosswind) the aircraft will need to correct for that by flying to the right within the airmass to cancel out the â€świndâ€ť and fly the correct ground track. Looking outside you will notice the aircraft flying over the ground in a â€ścrabâ€ť, i.e. the nose not pointing in the direction the aircraft is traveling over the ground.

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