How to calculate the GS from IAS

I think he’s trolling, because there is no logical argument or case that he is presenting. The pictures that you posted, explain my argument clearly, so thank you. 👌🏼

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I asked for you to back up your claims with proof. Screenshots from a video game prove nothing.

I can backup what I’ve been saying with proof from NASA. I’d love to see where you pulled “groundspeed depends on air pressure/air density” from.

Oh and I just wanted to add in that my “theory” is not a theory, it’s a hypothetical statement. It is backed up by science. It would be a fact if I had a real 3D model to show you. Not really sure how you can’t comprehend a subject this simple.

Air pressure only effects airspeed when a pitot tube is used to measure airspeed. If that pitot tube becomes clogged and the air is trapped inside, as you ascend, your airspeed indicator will act as an altimeter and rise. As you descend, your airspeed indicator will decrease. This “DeerCrusher” guy claims to have 600 hours under his belt so if he doesn’t understand this simple concept, it should probably be reported to the FAA because either you didn’t pass private pilot ground or you’re deficient in your knowledge of the instruments.

Now even the NASA website proves what I’ve been saying, yet you’re probably still going to try to tell me that air pressure has something to do with groundspeed when it has nothing to do with it. Airspeed in some situations, yes. Groundspeed, no. Airspeed plus headwind component equals groundspeed. There is nothing in this extremely simple and first grade math equation that uses air pressure or air density.

Again, I aced a college level advanced aerodynamics. I know what I’m talking about.

Haha this is classic. Im not going to keep trying to explain it. You are completely missing the point.

Mods, please close this, it’s gotten out of hand and isn’t solving anything

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The farther object would have to maintain a faster airspeed to keep up with the closer object. If their airspeed remains constant, the closer object will have a faster airspeed than the farther object. The less dense air that the farther object is traveling through creates less drag on the aircraft and flying at the same airspeed is going to keep you at the same airspeed. Flying at the same power setting is a whole different ballgame and is what you are referring to, which has nothing at all to do with groundspeed when calculating groundspeed.

Closing a thread because there’s a debate on how to calculate groundspeed correctly? What’s the point in asking for help if you can’t get help. I would think this is more educational than just assuming the correct answer.

Hey, if you just showed proof once and backed up your claims, I’d back off and tell you I was wrong. In this case though, I’m not wrong. I even presented evidence from NASA, the world leader in all things that go through the air and you still try to tell me I’m wrong? Hey, good luck if you’re going to become a pilot. I’d stick to flight attendant if I were you though.

I now see your confusion… the difference between true airspeed and indicated airspeed.

What you are saying in your diagram is basically correct. You even referred to the less air density (which you originally refused to do).

As you gain that altitude, the air density is less. As a result, to maintain the same INDICATED AIRSPEED, your TRUE AIRSPEED (which in short is groundspeed plus/minus wind) must increase! Hence, your ground speed increases with it.

As a pilot, you do not fly to target a groundspeed. You are not sitting at a high altitude trying to hit a certain groundspeed. The thinner air is creating less drag (as you stated) and as a result, you’re indicated airspeed slowly drops. This theory is also shown in how a commercial airliner sets an N1 percentage, and holds a climb speed by adjusting the VS. they decrease it whilst maintaining a power level and it then maintains the indicated airspeed.

Look on flightradar - pick aircraft that are cruising at 35,000 feet. They will likely have a groundspeed of 600 knots… they aren’t flying at 600 knots indicated airspeed! They’re indicated airspeed would be probably 320 knots IAS (or there about) however they would be utilising Mach speeds at this altitude.

I can’t keep having this conversation with you. If you are too stubborn then it’s your problem.

Please go and study the following:
Equivalent airspeed
True airspeed
Indicated airspeed
Dynamic pressure
Altitude air density
International standard atmosphere

I won’t be responding again.

So what does true airspeed have to do with ground speed? I’m not the one that is confused. I only responded when I saw someone compare “air pressure/air density” to groundspeed. I’m still on that same exact topic.

CPKJ, you are wrong. Well, you might be right. The fact that object further away from revolution center has to travel faster then object closer to center od revolution (in order to keep up with it) is just a fact. That is no proof. It is just simple logic. I hope that with your pictures you are not trying to set example with objects in space… or on an orbit as then you need to learn Keppler’s laws and they quite say the opposite. Object on higher orbit travels slower.

Back to GS.
Wing going through air generates lift. More speed = more lift. I hope you still follow. Higher you go less air there is so if you keep GS the same less lift you gain (less air over wing) and you are more likely to stall. In order to keep in the air you have to keep the air speed the same. And because the air density is smaller higher up, relating to ground you have to travel faster in order to keep same amount of air going around your wing. Therefore your GS is higher.

It has a lot to do with air density.


I found this article which explains TAS IAS and ground speed in easily readable terms… Aircraft speed - FlightGear wiki

Check out the “Expressing Speed” section.

Right, but what you just said is about airspeed. I think it’s been established that airspeed is related to air density, but the original discussion was about groundspeed. Air density doesn’t change your groundspeed as it is not a factor in calculating it.


No, directly no. I thought that was what you were arguing about… I am sorry for getting involved so ;)

Groundspeed can be approximated using airspeed, temperature, atmospheric pressure (air density), the location of the pitot tube, rate of climb… The real measure of ground speed can be calculated by taking the horizontal speed of the aircraft relative to a fixed object at that altitude, and translating it ground level.

You can prove to yourself that groundspeed is not directly related to air density, by considering the following case. Assume you are flying at FL200, with a certain airspeed and groundspeed. If you all of a sudden enter a space where pressure (and air density) drops, your airspeed will drop immediately, but your groundspeed will remain the same.

I hope that doesn’t muddy the water too much. 😬

Yes that is right, which goes into my point the entire time… the low air density is what allows you to have higher ground speeds:

We’ll assume during that scenario you were flying at 320 knots IAS (and that was the maximum rated indicated airspeed for that aircraft). Let’s also say your groundspeed was 500knots. Once you hit this area of low density, your airspeed would drop to, as an example, 300 knots IAS. Taking out any factor of set cruise power, economical cruise power etc, you can theoretically accelerate again 20 knots IAS faster, therefore increasing your groundspeed to well over 500 knots.

Your example above is also the same as you climb. The scenario doesn’t have to just be suddenly hitting a low pressure air pocket whilst at a cruise flight level. As you climb, you are moving into thinner air.

My entire point is that the air density is related, as the lower air density allows your TRUE airspeed to increase, whilst maintaining the same INDICATED airspeed (in fact you can increase your INDICATED airspeed as you climb to thinner air with less drag on the airframe - therefore it’s almost ecponentional increases in true airspeed). As a result of this increasing true airspeed as you climb, it directly increases your groundspeed.

I can’t help but feel we are all somewhat actually arguing and agreeing on the same point, but are disagreeing on the term ‘directly related’… to me, my above explanation shows how both airspeed and groundspeed are related to the air density. In short, the lower air density ‘allows’ the greater airspeed.

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Agreed! We’re saying the same thing with a slightly different perspective! Nuff said. 😂

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