I've heard that ground speed is higher than air speed when the plane is being pushed by wind wind. But how is this possible? Wouldn't your AirSpeed also increase since you're going faster?
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10$\begingroup$ If you walk towards the front of the train, your ground speed is greater than your TrainSpeed, right? Same thing for airplane in the atmosphere, when the atmosphere is moving across the ground. $\endgroup$– Charles BretanaMar 14, 2022 at 0:07
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4 Answers
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Have you ever watched a leaf floating along in a river? Waterspeed = zero, yet it is moving relative to you standing on the shore.
"Wind" is just a mass of air, (or river of air) moving along the surface of the earth.
Consider the same comparison but with a hot air balloon; the airspeed of the balloon is zero, but it moves along the ground at the same speed as the wind.
Now consider powered flight - airspeed will be whatever it is, but your speed and track over the ground will depend on which direction you are pointing relative to the direction the airmass is moving along the surface.
Does that make sense?
answered Mar 14, 2022 at 0:01
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Einstein was right: everything is relative.
Ground speed is speed relative to the ground, while airspeed is relative to the local air volume that the aircraft travels in. A spec of dust blowing in the wind has zero airspeed, but we see it flying past while we look at it with our feet on the ground.
Wouldn't your AirSpeed also increase since you're going faster?
Faster relative to the ground, yes. Not relative to the air.
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Planes fly through the air, not over the ground. The air itself is moving over the ground, which we call wind. It may be easier to grasp this if you first think in terms of water, since you’ve probably seen (or even experienced) how that works.
Imagine you are in a boat on a river, and you are paddling at a fixed speed of 10 knots through the water. However, the water itself is moving at 5 knots relative to the shore.
If you paddle upstream, your boat’s ground speed is 10-5=5 knots, but if you paddle downstream, your boat’s ground speed is 10+5=15 knots.
Is the boat really moving three times as fast? That depends on whether you’re measuring its speed relative to the water or relative to the shore.
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1$\begingroup$ Hah! all directly on point, except, to be picky, planes do fly over the ground... The point is that which frame of reference you choose to measure speed in is entirely arbitrary, and should be chosen based on your goal or the purpose you wish to use the "speed" for. You measure it with respect to the ground to determine things that are associated with the ground, like enroute or arrival time, etc. . You measure it with respect to the atmosphere for aerodynamic things, like stalls, or best climb rate, or gallons per hour fuel consumption, that are related to aerodynamics. $\endgroup$ Mar 14, 2022 at 16:02
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1$\begingroup$ When determining how much force (or pressure) to inject into the aircraft carrier catapult, they have to calculate what the jet's speed will be relative to the boat, when it achieves the necessary (safe) airspeed for flight. $\endgroup$ Mar 14, 2022 at 16:04
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1$\begingroup$ @CharlesBretana All true, but this is the simplest explanation I’ve found for people who don’t yet grasp wind as a moving air mass. Most default to a mental model of a car traveling at a fixed ground speed, with wind as a mysterious force pushing on the front or back of the car, and they won’t “get” the real answer until that mental model is replaced. $\endgroup$– StephenSMar 14, 2022 at 17:12
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1$\begingroup$ You have been here before I can see. Just a thought, for your consideration, but I think the hurdle for most people is the assumption that there is only one real frame of reference (the one they live in), or that the one they live in has and deserves some special privilege or preference over any others. This is the unrecognized or unconscious assumption they have to be educated out of. But planes DO fly over the ground... $\endgroup$ Mar 14, 2022 at 20:47
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1$\begingroup$ @CharlesBretana I still struggle with that myself when it comes to relativity! $\endgroup$– StephenSMar 14, 2022 at 20:50
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The plane doesn't get "pushed" by the wind. It flies inside of the wind.
It's as if the plane's flight dynamics take place inside of a gigantic balloon. You don't need to know whether the balloon is stationary over the ground in still air, or drifting over the ground at 150 mph in the jet stream. The plane's flight dynamics in relation to the local airmass-- the air inside the balloon-- remain exactly the same.
Think about it for a moment. Don't you believe that groundspeed can be lower than airspeed when the plane is flying into the wind? So why shouldn't the reverse be true when flying with the wind?
It's like if you were running on one of those conveyer belts that you find in airports with long walkways. Assuming for the moment that conveyer belt is also moving all the air above it along with it, so air resistance isn't a factor, if you want to cover 10 feet of belt per second, do your leg muscles have to work any harder if you are running one way on the belt, or the other way? No. So there you are, running along covering 10 feet of belt per second-- so let's say your "beltspeed" is 10 feet per second-- will your floorspeed or roomspeed-- the speed at which you are moving relative to the rest of the room-- be lower than 10 feet per second if you are running against the direction the belt is moving? Sure. And will your floorspeed or roomspeed be greater than 10 feet per second if you are running with the direction the belt is moving? Sure.
Now change the problem a bit to say "I'm running with the muscle effort it normally takes me to cover 10 feet per second when running over stationary ground." Your question is essentially "when I'm running with the direction the belt is moving, and am covering 20 feet through the room per second (because let's say the belt is moving at 10 feet per second relative to the rest of the room), does that boost in my "roomspeed" somehow also translate into a boost in my "beltspeed"? And isn't the boost in my "beltspeed" somehow such that my "beltspeed" somehow magically becomes exactly equal to my "roomspeed"?
(This is analogous to the idea that "airspeed" (beltspeed) should not be lower than "groundspeed" (roomspeed), i.e. that "groundspeed" (roomspeed) should not be higher than "airspeed" (beltspeed).)
How could that make any sense? And what number would you expect the "beltspeed" and the "roomspeed" to end up at, in a scenario like that? It sounds like we are setting up a feedback loop where increased "roomspeed" drives increased "beltspeed" (so that it becomes equal to the "roomspeed" again) which drives still more increased "roomspeed" which drives still more increased "beltspeed"-- hey, I think we've just invented the warp drive!
answered Mar 14, 2022 at 0:54