IAS vs. CAS

[uwochris]

Hey guys,

Like everyone else, I have been taught that stall speed (IAS) is the same regardless of altitude.

However, is it not more proper to say that the stall speed in CAS is the same regardless of altitude, and not the IAS? I realize that IAS and CAS are often very close, so perhaps the authors of all the flying books just make an assumption that IAS=CAS for simplicity??

 

[Singlecoil]

There error between CAS and IAS doesn't change with altitude. You apply the correction by taking a given IAS and applying the correction from the table. Since you already know the plane will stall at the same CAS, regardless of altitude, you know that it will stall at the same IAS, regardless of altitude, since the difference between the two doesn't change with altitude.
Generally, designers seek to make CAS and IAS the same at low speeds, when accuracy is the most critical; i.e., on approach. At cruise you will see a knot or two error in the average light trainer.
However, some after-market mods, especially STOL kits can create a large (ten knot) difference between IAS and CAS at low speeds due to the change in airflow over the leading edge of the wing and therefore the pitot tube.
Hope that helps.