Why is IAS used instead of TAS?

[guitarxe]

So from reading manuals of several aircraft in DCS, it seems like all of them are capable of showing the true indicated airspeed by taking into account air density and humidity changes with altitude and all that.

 

However on cockpit instruments, it is the indicated airspeed that is more prominent and on the HUD you see IAS only. So how come? Why does it seem like IAS is a more important indicator for pilots than the actual true airspeed?

[Flagrum]

So from reading manuals of several aircraft in DCS, it seems like all of them are capable of showing the true indicated airspeed by taking into account air density and humidity changes with altitude and all that.

 

However on cockpit instruments, it is the indicated airspeed that is more prominent and on the HUD you see IAS only. So how come? Why does it seem like IAS is a more important indicator for pilots than the actual true airspeed?

The performance of an air craft depends a lot on the density of the air. For example the stall speed. The thinner the air is, the earlier you get into stall. Therefore IAS is much more relevant. My (layman) understanding is, that the stall speed is about the same in IAS, independent of the altitude.

[ARM505]

IAS is more relevant to actually flying the aircraft (consider IAS to be the airflow that the aircraft actually 'feels'), TAS is more relevant for navigation.

[roadrabbit]

This is a fundamental thing: think of IAS is the effect of the air on the aircraft. TAS is the speed at which air particles are passing the aircraft in flight. As you go higher air density drops and in space is zero. However you can imagine being at 40,000 feet, where air density is very low. To maintain flight you have to travel very fast to achieve the required lift for level flight. Thus your TAS would be high. However your IAS would be very low in comparison, and could (in the earlier days of flight) reduce to your aircraft's stall speed.

Think of lift being produced by the number of air particles passing over the wing - TAS is the speed of their passing.

If you want to follow this further, google "coffin corner" which is relevant.

[kontiuka]

And then there is ground speed which is your real speed over the ground. This is usually determined by radar or GPS.

[sebbomann]

This is a fundamental thing: think of IAS is the effect of the air on the aircraft. TAS is the speed at which air particles are passing the aircraft in flight. As you go higher air density drops and in space is zero. However you can imagine being at 40,000 feet, where air density is very low. To maintain flight you have to travel very fast to achieve the required lift for level flight. Thus your TAS would be high. However your IAS would be very low in comparison, and could (in the earlier days of flight) reduce to your aircraft's stall speed.

Think of lift being produced by the number of air particles passing over the wing - TAS is the speed of their passing.

If you want to follow this further, google "coffin corner" which is relevant.

 

Really good explanation! :thumbup:

[Home Fries]

TAS is really only useful for determining winds, which is often automatically done by inertial or GPS navigation systems. IAS tells you how you are doing in your air mass, and Ground Speed (GS) tells you how long it will take to get someplace.

[roadrabbit]

TAS is really only useful for determining winds, which is often automatically done by inertial or GPS navigation systems. IAS tells you how you are doing in your air mass, and Ground Speed (GS) tells you how long it will take to get someplace.

 

Quite right! I shall show my age now :D, but way back when we had to navigate before INS or GPS were around, we used TAS with forecast winds to provide a provisional course to steer (heading) to maintain a desired direction (which we in the UK called 'track'). This calculation also gave us ground speed, so we could estimate our ETA at the next navigation point. When we got our next fix (no, Brian, not that kind!) we then found out how good or bad the forecast had been, and adjusted our next heading to steer.

[peter]

IAS=what the airplane is feeling

TAS=what the aircraft is doing

GS(ground speed)=how fast the airplane is going from point A to point B

[Essah]

Good informative post. something I've been wondering about too.

 

now we're at it, can someone go over CAS (Calibrated Airspeed)?

[ViFF]

Calibrated Airspeed (CAS) is most of the time just a small correction to your IAS, taking into account the measuring error of the instruments and some small adjustments for relative humidity, side wind and during transition to supersonic speed where the shock wave can affect the precision of the measuring instrument and the way the air enters the pitot tube.

 

This is all in wikipedia:

http://en.wikipedia.org/wiki/Calibrated_airspeed

 

Cheers

[Home Fries]

Quite right! I shall show my age now :D, but way back when we had to navigate before INS or GPS were around, we used TAS with forecast winds to provide a provisional course to steer (heading) to maintain a desired direction (which we in the UK called 'track'). This calculation also gave us ground speed, so we could estimate our ETA at the next navigation point. When we got our next fix (no, Brian, not that kind!) we then found out how good or bad the forecast had been, and adjusted our next heading to steer.

 

You can't be that old :D. Despite having INS/GPS, we still had to calculate TAS from IAS, spin our winds to come up with a drift angle, DR using groundspeed calculated via TAS+winds, etc. I still have my MB-4A!

[WildBillKelsoe]

A very good thread with a heap of useful information.