Speed Achieved vs. Speed Over Land vs. Surface Speed?

[illmatic]

Hey guys,

I've been playing KSP awhile and I'm embarrassed to say I never quite understood the differences between Highest Speed Achieved and Highest Speed Over Land within the F3 Flight Results. Also, how do those numbers compare to your Surface speed vs. Orbital speed.

Take this picture for instance:

I have different numbers for Achieved, Over Land, Surface and Orbital (which you can't see, but it's around 1,200m/s) - could anyone explain to me how these are measured and what they mean, exactly?

Mainly I'm concerned with which number would be used to calculate mach or flight speed records in real life. I can look those totals up online, but I want to know which number in KSP I should be using to compare them to... thanks in advance to anyone that can help me out!

Edited July 17, 2014 by illmatic

[Hodo]

It is just like real life. Air speed over land is different then your actual airspeed. This is because of math... plain and simple. I wish I could give you a more exact answer but I am not on my home computer. Real aircraft have to factor two speeds when flying, indicated air speed (IAS) which is the speed the plane is moving through the air. And true Air Speed (TAS) which is the adjusted air speed based on altitude above Sea Level.

[illmatic]

That... doesn't help me much. If I wanted to know how to calculate airspeed in real life, sure, you can look those formulas up online. What I'm asking is how IAS and TAS correspond to the numbers that KSP spits out, and for a better understanding of how Speed Achieved, Speed Over Land, and Surface speed are calculated and their differences within KSP, specifically. Finally, how can I use those numbers (without FAR) to make a best approximation comparison to the numbers used when recording airspeed records, such as those listed here: http://en.wikipedia.org/wiki/Flight_airspeed_record

[illmatic]

Over in the "Fastest Plane under 1,000m" thread they used KSP's surface speed as their indicator... and it seems as FAR correlated those numbers with its mach readout. So perhaps that is the answer to my question and the number I should be concerned with, yet, how does the low altitude of their tests factor in? Does KSP take altitude into account when determining surface speed? Should I just use 42 as my number and be done with it? So many questions...

Source thread: http://forum.kerbalspaceprogram.com/threads/65788-Fastest-Plane-under-1-000m-WE-HAVE-REACHED-MACH-5!-Tidus-Klein-at-1-714-m-s

[Hodo]

It depends on what you are doing. If you are going for orbital velocities then you should concern yourself with your orbital speed, but if you are just going for flight speed records then surface speed.

And yes it does factor your altitude. It isn't a huge difference in KSP because of the very short atmosphere height, but if you were to use Realism Overhaul and the Real Earth atmosphere scaling where space is 180km not 69.1km, you would find that it makes a bigger difference the higher you are.

[Claw]

Let's see if I can say this in a short but understandable manner...

If you understand vectors, this will probably make more sense.

"Surface" navball reading - Your speed relative to the planet's surface, but in any direction. It is relative to the surface because the planet's rotational speed is not included. So the reference frame is fixed to a point on the surface. You could be pointed straight up, or going tanget to the surface. On the launch pad, your surface speed is 0 m/s.

"Orbit" navball reading - Yor speed relative to the planet's core, in any direction. Works just like Surface except that planetary rotation is included in the initial speed. So on the launch pad, your orbital speed is 173 m/s.

"Highest Speed Achieved" is literally the highest "Surface" navball speed, in any direction that you achieved. It will be the highest readout you see in the "Surface" navball readout. However, it seems to max out at 750 m/s for reasons I don't understand.

"Highest Speed Over Land" is the highest speed tangential and relative to the surface. If you launch straight up, the speed over land is zero because you aren't traveling sideways.

These are all pretty different from Calibrated, Indicated, True, and Ground speeds.

Hope that helps.

-Claw

Edited July 12, 2014 by Claw

[Hodo]

These are all pretty different from Calibrated, Indicated, True, and Ground speeds.

Hope that helps.

-Claw

That helps me. I often get them confused. I only worry about my surface speed and my orbital speed nothing else matters.

[Claw]

I agree. The F3 menu is only helpful for record keeping. I think that's why so many challenges like to use the Highest Speed Over Land. Plus that one forces you to achieve the speed in relatively level flight (instead of a dive). Bot none of the F3 numbers are useful in flight, only maybe after the fact.

[illmatic]

Thanks guys, I have a better understanding now.

[Bryce Ring]

Nicely Explained.

Wiki will help with the other forms you mentioned

could you tell the meanings of IAS and TAS

I have a feeling Hodo got it wrong ?

[Leszek]

Nicely Explained.

Wiki will help with the other forms you mentioned

could you tell the meanings of IAS and TAS

I have a feeling Hodo got it wrong ?

IAS is the airspeed read right of the airspeed dial. TAS is the actual airspeed the craft is traveling through the air.

IAS is useful because the stall speed IAS doesn't change with altitude. (Err changes very little.) So if your aircraft will stall at 150 kts on the ground it will stall at 150 kts at 30,000 feet. Also speeds such as Vne (Never Exceed speed) can be in IAS. IAS changes as the atmosphere gets less dense. You can travel faster in less dense air before your wings rip off, but the IAS will show the same speed. Faster planes also have a Mach speed number as well. Many comercial planes then have a max IAS as well as a max mach. (Say 350 kts IAS or mach .82 whichever is lower.)

However you need to convert to TAS to navigate. TAS vector + Wind vector = path across the ground.

In KSP all speeds are TAS.

[Hodo]

Nicely Explained.

Wiki will help with the other forms you mentioned

could you tell the meanings of IAS and TAS

I have a feeling Hodo got it wrong ?

Wow necro much?

[Bryce Ring]

LOL that's a new word for me.

I have to look it up.

BrB

OK Looked it up. And I must confess, I don't know what you mean by that ...

Sorry, Now I feel stupid much.

Edited December 11, 2014 by Bryce Ring

[Bryce Ring]

Nicely Explained.

Thanks

Bryce.