Real Newb Question

[Cmano]

Ok I just downloaded AH, and I am currently enjoying it.  I definitely intend to re-up my membership after my 2 weeks are up.  Here is my first dumb question, probably with many more to follow in the future.   What is the difference between airspeed (white needle) and true airspeed (red needle)?  How fast am I really going?   Thanks for your help.

[Furious]

True airspeed is the indicated airspeed corrected for the effects that temperature and pressure have on the airspeed indicator.

[J_A_B]

The little red bar is your True airspeed.  This is how fast your plane is actually moving.  

The white needle is Indicated airspeed.  This shows your speed, but adjusted for altitude.

Think of it this way--air pressure decreases as altitude increases.  Now think of 2 planes moving at 300 MPH--one is at sea level, the other is at 20,000 feet.  While they're moving at the same real speed, the plane at sea level is developing more lift and can do more maneuvering than the one at 20,000 feet.  This is what Indicated airspeed accounts for--200 MPH Indicated will give your plane roughly equal performance at 20,000 feet as it would at sea level.

As you might have guessed, Indicated airspeed is far more useful for A2A combat.

J_A_B

[Furious]

Originally posted by J_A_B
The little red bar is your True airspeed.  This is how fast your plane is actually moving.  

The white needle is Indicated airspeed.  This shows your speed, but adjusted for altitude.
J_A_B

Check that.

Indicated airspeed (IAS) is the direct reading obtained from the airspeed indicator, uncorrected for variations in atmospheric density.

[rshubert]

Why we have both:

True airspeed tells you how fast you'll get there, wind considerations not withstanding.  TAS was mostly a navigation tool, and is useful for bragging rights as to which plane really is faster.

The plane flies, and all the numbers like stall speed, never exceed speed, flap extension speeds, etc.  are based on INDICATED air speed, since that is a better measure of the forces on the airframe.  It is affected slightly by temperature, and mostly by air density and humidity.

We don't have Corrected (or calibrated)  Air Speed, which is a measure of the accuracy of the Indicated Air Speed at a particular speed reading.  That is mostly affected by the angle of attack for a plane flying straight and level at a particular airspeed.

[BigMax]

TAS is a key consideration for BUFFs.  It is your actual speed over the ground and is always repeatable.  If you were to calibrate to IAS & change altitudes, at the same indicated speed, your eggs would be WAY off...

The reduction in IAS is due to the fact that at higher altitudes atmospheric density is less, therefore, there are physically less molecules of air flowing past the speed sensor and subsequently your sensed speed will be less.

[ccvi]

Originally posted by BigMax
TAS is a key consideration for BUFFs.  It is your actual speed over the ground and is always repeatable.

Speed over the ground is GS (ground speed), which is TAS corrected by wind.