Air Speed and Surface Speed differ with no wind?

[bonyetty]

I just found that when right clicking on radial air intakes that they have an Air Speed reading. I do not understand why it differs from Surface readings when Kerban has no wind. I found this while flying west from KSC at about 20000m, Surface 1829m/s, Air Speed 1933m/s, Orbit 1636m/s. Note it is hard to keep straight and level at these altitudes so readings taken several second apart. Orbit is different because of Kerbans rotation but why is there a 100m/s difference between surface and air speed?

I'm sure I will be face palming soon

[Renegrade]

Uhh air speed? What air speed? You must be reading that from a mod. I would imagine that the airspeed is basically 'surface at my current altitude' or somesuch, whereas surface is probably relative to the surface at the reference altitude (0)...?

[bonyetty]

Uhh air speed? What air speed? You must be reading that from a mod. I would imagine that the airspeed is basically 'surface at my current altitude' or somesuch, whereas surface is probably relative to the surface at the reference altitude (0)...?

Mechjeb and Alarm clock are the only mods so maybe one of them. Right click on the radial air intakes. Not sure how altitude would make a difference not that I'm saying that it wouldn't I just dont understand how.

[Vanamonde]

I am told that the engines need a certain minimum airspeed to function, and so a boost has been arbitrarily added to the airflow at the intake to prevent the engine from stalling. Think of it as the action of compressor turbines.

[Bill Phil]

Ground relative velocity takes into account the rotation of Kerbin, and Air Speed is relative to the surrounding air. That's why it's about 100m/s faster.

[RocketPilot573]

Uhh air speed? What air speed? You must be reading that from a mod. I would imagine that the airspeed is basically 'surface at my current altitude' or somesuch, whereas surface is probably relative to the surface at the reference altitude (0)...?

OP is talking about the readouts on the air intakes. It says "air speed," though I have no idea what that is measuring.

[cpast]

Shouldn't the air be rotating with Kerbin?

[Bill Phil]

Shouldn't the air be rotating with Kerbin?

Air moves around in complicated ways. But it does rotate with Kerbin a bit. It's going North and South and East and West all the time though (assuming invisible weather).

[Crzyrndm]

OP is talking about the readouts on the air intakes. It says "air speed," though I have no idea what that is measuring.

It's always current surface velocity + 100m/s

[Kami_K220]

Coming from a pilot it maybe looking at indicated airspeed rather than True airspeed as you have been discussing. I haven't used FAR in a while but I know that it can differentiate between the two. Pressure and temperature have a big play on IAS

[bonyetty]

I am told that the engines need a certain minimum airspeed to function, and so a boost has been arbitrarily added to the airflow at the intake to prevent the engine from stalling. Think of it as the action of compressor turbines.

Ok that would make sense. Thanks Vanamonde. Just makes me think how big the 1.0 release will be to balance everything.

[Renegrade]

OP is talking about the readouts on the air intakes. It says "air speed," though I have no idea what that is measuring.

Oh, THAT airspeed. Vanamonde's explanation covers it. Basically the intakespeed would be zero on the runway initially otherwise and airbreathers would stall out. You'd need to ullage them without that

Basically, a rude hack.

[aleis]

This would also be relevent if say you're climbing vertically, as your surface surface would be zero ,but hopefully you have air speed.

[Crzyrndm]

No, ground speed (would be zero in a climb. Velocity vector projected onto the ground plane relative to a point on the ground) is different to surface relative speed (velocity relative to a point on the ground ), which is different to orbital speed (velocity relative to the center of the planet)

Intakes use surface speed (what is shown on the navball in surface mode), not ground speed

Edited March 2, 2015 by Crzyrndm

[cantab]

It couldn't be down to horizontal flight at altitude being round a larger circle than on the surface of Kerbin could it? Since Kerbin's so small, at 20 km your true airspeed would be 20/600 = 3% higher than your ground speed. Although, the OP is reporting more like a 5-6% increase.