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having trouble understanding the importance of certain speed references


fitness88

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I've read many articles but still come up short on understanding the following:

 

 

I understand the purposes of knowing ground speed for navigation and IAS for understanding how your plane can expect to handle given current altitude and temperature.

I'm not sure what the purpose of TAS and Mach is for.

Also does the F-15 use INS/dead reckoning to know its ground speed?

 

 

Thank you.

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TAS itself is usually not very useful to the pilot, at least not in the F-15 (some aircraft have reference speeds given in TAS). But it is needed to calculate wind from ground speed.

 

Mach is very useful as the aircraft behavior and drag changes significantly around the Mach 1. This is your primary speed indicator when you are going near the speed of sound or faster. Also at higher altitudes cruise speeds are given in mach number, because it's a better, more stable reference point. Your IAS at given Mach number varies depending on altitude and air temperature.

 

Yes, INS is needed to know its ground speed. It can probably also use the radar for that.

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Also does the F-15 use INS/dead reckoning to know its ground speed?

 

Yes, and it may be corrected by the radar though I'm not sure. I don't recall if the two are connected or not - that is, I know you can update the INS with he radar for accuracy, but I'm not sure if it transfers in the GS measurement.

 

The F-15 radar takes a GS calibration sample every 7 minutes IIRC.

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TAS, is this the movement of the plane relative to the air around the airframe?

 

Yes.

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Headwind has no effect.

 

TAS is your actual speed moving through the air - not IAS/CAS/EAS ... all of which are modified by air pressure, special computations etc. so in some way those are airspeeds that your airframe 'feels' ... but TAS is the actual speed through the air.

 

TAS, is this the movement of the plane relative to the air around the airframe? I keep thinking its like you're flying 400kts with a head wind of 50kts, then your TAS is 350kts...but this is ground speed.

[sIGPIC][/sIGPIC]

Reminder: SAM = Speed Bump :D

I used to play flight sims like you, but then I took a slammer to the knee - Yoda

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Headwind has no effect.

 

TAS is your actual speed moving through the air - not IAS/CAS/EAS ... all of which are modified by air pressure, special computations etc. so in some way those are airspeeds that your airframe 'feels' ... but TAS is the actual speed through the air.

 

 

 

Thanks, I think I got this...it's the peed of the airframe through the air molecules surrounding it. I hear passenger planes use this for calculating their arrival.

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TAS itself is usually not very useful to the pilot, at least not in the F-15 (some aircraft have reference speeds given in TAS). But it is needed to calculate wind from ground speed.

 

Mach is very useful as the aircraft behavior and drag changes significantly around the Mach 1. This is your primary speed indicator when you are going near the speed of sound or faster. Also at higher altitudes cruise speeds are given in mach number, because it's a better, more stable reference point. Your IAS at given Mach number varies depending on altitude and air temperature.

 

Yes, INS is needed to know its ground speed. It can probably also use the radar for that.

 

 

 

The thing about Mach is that at different alt. you have different Mach # for the same given ground speed. So 1 plane at 20000 ft. flying Mach 0.8 and another plane at 35000 ft flying at the same Mach # will have different ground speeds. So for this to have any value you would have to say your alt. and temp...etc. and do calculations for someone to know how fast your moving over the ground?? I know I have to be wrong here.doh.gif

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cmon man u should be flying with no hesitation, are you a pilot or no.

For example i had 0 hours practice in f15 before i got DCS..and yet i am flying like champ regardless of no practice in real f15..be brave :))

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The thing about Mach is that at different alt. you have different Mach # for the same given ground speed. So 1 plane at 20000 ft. flying Mach 0.8 and another plane at 35000 ft flying at the same Mach # will have different ground speeds. So for this to have any value you would have to say your alt. and temp...etc. and do calculations for someone to know how fast your moving over the ground?? I know I have to be wrong here.doh.gif

:) You're over thinking this. You use Mach when measuring high speeds because it's entirely related to the speed of sound. So 0.9 Mach is 90% the speed of sound. And you're concerned with the speed of sound because the airflow around your aircraft (as already noted by Some1) changes near, at, and beyond that number. Drag numbers changes. Fuel efficiencies change. Structural limitations come into play, etc. So at high speeds you are concerned with Mach. At slow speeds you are not. But the real concern is the speed of sound.

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:) You're over thinking this. You use Mach when measuring high speeds because it's entirely related to the speed of sound. So 0.9 Mach is 90% the speed of sound. And you're concerned with the speed of sound because the airflow around your aircraft (as already noted by Some1) changes near, at, and beyond that number. Drag numbers changes. Fuel efficiencies change. Structural limitations come into play, etc. So at high speeds you are concerned with Mach. At slow speeds you are not. But the real concern is the speed of sound.

 

 

 

It's information not relevant to the other...thanks.

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