There are so many erroneous airspeed videos out there it is great to see one by somebody who properly understands the subject. I do wonder why modern jet aircraft airspeed tapes still display CAS (not EAS). CAS was defined at a time (about 1926) when aircraft were beginning to fly at speeds where incompressible flow theory was no longer good enough, but before the invention of air data computers. Since the mechanical ASI measures only a single variable (impact pressure) CAS had to be defined as a function of impact pressure only. Today high speed / high altitude aircraft have air data computers with independent inputs of total pressure and static pressure. Since EAS is a function of these two variables any modern air data computer (given position error correction) could output EAS for the airspeed display thus eliminating compressibility error. The fact that CAS is still displayed on modern EFIS speed tapes seems to be just a matter of historical convention.
Thank you, Piers. Yes, I suspect you’re right. Displaying EAS would certainly make a lot of sense, particularly for checking structural limitations (max flap extension speed, etc). I wonder whether there are some exotic failure modes buried in the way some air data systems calculate EAS (at higher Mach numbers) that don’t affect CAS (perhaps TAT probe icing) and therefore it is deemed more robust? Also, I wouldn’t be surprised if some glass cockpit ASIs were actually showing EAS - I mean, they are often labelled ‘IAS’ when they almost certainly show at least CAS.
Thanks for the motivation to finally get python set up on my laptop! Worked as advertised once I figured out how to get all the modules, e.g. metar module, set up!
very thorough explanation and wonderful examples. This can be the most comprehensive airspeed video on youtube. I didn't know Python has library for this kind of problems. Will take a peek at first chance for sure. Thanks for all the effort. I know it's not easy and time consuming.
There are so many erroneous airspeed videos out there it is great to see one by somebody who properly understands the subject.
I do wonder why modern jet aircraft airspeed tapes still display CAS (not EAS).
CAS was defined at a time (about 1926) when aircraft were beginning to fly at speeds where incompressible flow theory was no longer good enough, but before the invention of air data computers. Since the mechanical ASI measures only a single variable (impact pressure) CAS had to be defined as a function of impact pressure only.
Today high speed / high altitude aircraft have air data computers with independent inputs of total pressure and static pressure. Since EAS is a function of these two variables any modern air data computer (given position error correction) could output EAS for the airspeed display thus eliminating compressibility error. The fact that CAS is still displayed on modern EFIS speed tapes seems to be just a matter of historical convention.
Thank you, Piers. Yes, I suspect you’re right. Displaying EAS would certainly make a lot of sense, particularly for checking structural limitations (max flap extension speed, etc). I wonder whether there are some exotic failure modes buried in the way some air data systems calculate EAS (at higher Mach numbers) that don’t affect CAS (perhaps TAT probe icing) and therefore it is deemed more robust? Also, I wouldn’t be surprised if some glass cockpit ASIs were actually showing EAS - I mean, they are often labelled ‘IAS’ when they almost certainly show at least CAS.
Thanks for the motivation to finally get python set up on my laptop!
Worked as advertised once I figured out how to get all the modules, e.g. metar module, set up!
very thorough explanation and wonderful examples. This can be the most comprehensive airspeed video on youtube. I didn't know Python has library for this kind of problems. Will take a peek at first chance for sure. Thanks for all the effort. I know it's not easy and time consuming.
Thank you! I plan to add more videos to the series in the near future :)