Calibrated airspeed?

After all the interesting discussions in the thread, I think these are the essentials of why calibrated airspeed (CAS) matters to the average pilot:
  • You need to know CAS to calculate TAS for flight planning, and to calculate actual winds aloft when you're airborne (but it's usually a minimal difference from IAS at cruise speeds, so you can generally get away with just using IAS)
  • You need to know CAS if you want to calculate your own V-speeds, like 1.3×Vs for approach (but all published V-speeds are already converted to IAS for you)
Everything else falls into the "I'm a hopeless flight-theory nerd" department (you'll usually find me browsing the aisles there).
 
Slight detour here. My manual has no Vne, but states Maximum dive in smooth air is in CAS, which matches the redline on the ASI. It has no guidance on the difference between ias and cas. So, is it safe to fly up to redline even at altitude or not? Talk about the more you learn the less you know.
 
Salty said:
Slight detour here. My manual has no Vne, but states Maximum dive in smooth air is in CAS, which matches the redline on the ASI. It has no guidance on the difference between ias and cas. So, is it safe to fly up to redline even at altitude or not? Talk about the more you learn the less you know.
Click to expand...

What year? I think it makes a difference on what the red line on the airspeed indicator represents.
 
Salty said:
Slight detour here. My manual has no Vne, but states Maximum dive in smooth air is in CAS, which matches the redline on the ASI. It has no guidance on the difference between ias and cas. So, is it safe to fly up to redline even at altitude or not? Talk about the more you learn the less you know.
Click to expand...
Does your manual have the airspeed corrections to get to CAS from IAS? It's typically low to negligible at high CAS. "Even at altitude" is not a factor for CAS at the speeds most GA airplane fly at, as dynamic pressure is only a function of CAS at low subsonic speeds. If your Vne is set by flutter margin ( a la Van's) then it's really TAS and the red line on the ASI should be considered more as a reminder* unless it's variable.

*this does NOT mean it can be casually exceeded, it means the limiting TAS does not necessarily match the redline!

Nauga,
and his buzz kill
 
nauga said:
Does your manual have the airspeed corrections to get to CAS from IAS? It's typically low to negligible at high CAS. "Even at altitude" is not a factor for CAS at the speeds most GA airplane fly at, as dynamic pressure is only a function of CAS at low subsonic speeds. If your Vne is set by flutter margin ( a la Van's) then it's really TAS and the red line on the ASI should be considered more as a suggestion unless it's variable.

Nauga,
and his buzz kill
Click to expand...
Salty said:
It has no guidance on the difference between ias and cas
Click to expand...
 
Salty said:
It has no guidance on the difference between ias and cas
Click to expand...
If "it" is your POH then there's probably no one here who can answer that question with any certainty. Unless you have a really gooned-up static port(s) the correction is probably low at high speeds but what "low" means is subjective.

(ETA) OTOH, if the difference is very small it's also possible that your redline is marked at the IAS and the difference between that and the POH number is too small to discern.

Nauga,
who does the test himself if he really wants to know
 
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David Megginson said:
After all the interesting discussions in the thread, I think these are the essentials of why calibrated airspeed (CAS) matters to the average pilot:
  • You need to know CAS to calculate TAS for flight planning, and to calculate actual winds aloft when you're airborne (but it's usually a minimal difference from IAS at cruise speeds, so you can generally get away with just using IAS)
  • You need to know CAS if you want to calculate your own V-speeds, like 1.3×Vs for approach (but all published V-speeds are already converted to IAS for you)
Everything else falls into the "I'm a hopeless flight-theory nerd" department (you'll usually find me browsing the aisles there).
Click to expand...

Taking the Cessna 172N as an example, Vso=41 kias or 47 kcas. These are worst case numbers, based on max weight, CG at the forward limit and power off. If you want 1.3*Vso, the proper calculation gives 47*1.3=61 kcas. Then convert 61 kcas back to indicated, gives 60 kias. That is what you need to fly to get 30% margin above stall. Instead, if you simply mulitply the indicated stall speed by 1.3, that would give 1.3*Vso=1.3*41=51 kias. From the calibration chart, 51 kias is close to 53 kcas.
In other words, multiplying the indicated stall speed by 1.3 only gives you only 10% margin above stall, not 30%.
 
nauga said:
If "it" is your POH then there's probably no one here who can answer that question with any certainty. Unless you have a really gooned-up static port(s) the correction is probably low at high speeds but what "low" means is subjective.

(ETA) OTOH, if the difference is very small it's also possible that your redline is marked at the IAS and the difference between that and the POH number is too small to discern.

Nauga,
who does the test himself if he really wants to know
Click to expand...
If you're testing Vne yourself, let us know, so we can make sure we're not underneath you where the parts will be falling. ;)
 
Salty said:
Slight detour here. My manual has no Vne, but states Maximum dive in smooth air is in CAS, which matches the redline on the ASI. It has no guidance on the difference between ias and cas. So, is it safe to fly up to redline even at altitude or not? Talk about the more you learn the less you know.
Click to expand...
a) This is a valuable question, because it's always worth learning more about our planes, especially when the POH is vague/incomplete.

b) It's never safe to fly up to redline (much less close enough that the difference between CAS and IAS would matter). Once you're in the yellow arc, you're already essentially juggling nitroglycerin — any overly-abrupt control movement or sudden jolt of turbulence could cause catastrophic structural damage to your plane.
 
David Megginson said:
a) This is a valuable question, because it's always worth learning more about our planes, especially when the POH is vague/incomplete.

b) It's never safe to fly up to redline (much less close enough that the difference between CAS and IAS would matter). Once you're in the yellow arc, you're already essentially juggling nitroglycerin — any overly-abrupt control movement or sudden jolt of turbulence could cause catastrophic structural damage to your plane.
Click to expand...
The difference between cas and ias may be small, but the difference between ias and tas is quite large at 10,000 feet. Generally speaking, something larger than 20.
 
David Megginson said:
b) It's never safe to fly up to redline (much less close enough that the difference between CAS and IAS would matter). Once you're in the yellow arc, you're already essentially juggling nitroglycerin — any overly-abrupt control movement or sudden jolt of turbulence could cause catastrophic structural damage to your plane.
Click to expand...

Uh, no.
 
In aircraft certified before December 1978, the airspeed markings on the airspeed indicator were required to be marked in CAS. In these aircraft, the colored arcs and lines on the airspeed indicator may not correspond exactly to the airplane’s IAS. On airplanes with indicators marked in CAS, flying just at the edge of these markings may not be the best way to fly the airplane.

For instance, the start of the caution range in one common general aviation aircraft is 210 miles per hour (mph) calibrated airspeed. The caution range is marked by a yellow arc starting at 210 mph. If the pilot is flying 210 mph IAS in this particular airplane, though, the CAS is actually 214 mph, which means the pilot may be unknowingly exceeding the airplane’s limitations. The reason for understanding the difference between IAS and CAS is to avoid inadvertent operation at higher or lower speeds than desired, or operations that exceed the airplane’s published limitations, but no one ever told you this in ground school.
 
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