CAS vs IAS

C

clestudentpilot

Well-Known Member
Question, not just for the CFI's, but for all pilot's out there. When flying, and observing certain speeds for certain things (approach speed, stall speeds, best rate of climb, etc.) am I correct in saying that the speed to be concerned of is the calibrated airspeed, such as 1.3*Vs0 would be a calibrated airspeed, right? I know I recently read something in a discussion on here about it, and I am a low time pilot, and there is so much that was discussed by my instructor that I guess I couldn't let sink in. I'm just trying to work on one of my flaws (as it is with many pilots), and that is the approach to landing. While I can land a plane just fine, I need to work on doing it better more consistently. I am just making sure that I have this correct, I know I need to look at the conversion from CAS to IAS, I just want to make sure that the speeds used are CAS Thanks in advance for the replies!
 
clestudentpilot said:
I correct in saying that the speed to be concerned of is the calibrated airspeed, such as 1.3*Vs0 would be a calibrated airspeed, right?
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Depends on what you're doing with the speeds. For relatively modern airplanes, most speeds are published in KIAS, rather than KCAS, but sometimes both. Since you can't read KCAS off the airspeed indicator (by definition), knowing the KCAS won't do you much good.

You really only need calibrated airspeeds when you're performing calculations with the number, such as increasing it by 30% (1.3Vso) or converting it to TAS.
 
CAS is IAS adjusted for instrument installation error and position error. It is not something you need to figure out for your approaches.
 
Brian Z said:
CAS is IAS adjusted for instrument installation error and position error. It is not something you need to figure out for your approaches.
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Right, but I'm flying older 172's, and Piper airplanes, and I'm just making sure I understand that for the V speeds along with approach speed, that I need to take the speed in the POH, and convert to IAS, so I know in my head what speed I need, or for best rate of climb (although I know that is getting close to where the two are the same). Well actually I need to convert MPH to knots first, then to IAS the POH is all in MPH.

Tgrayson, yeah, I know the CAS doesn't do me much good, that is why I am just trying to make sure that I am grasping this correct, that I need to convert them to IAS, so I know the correct speed to fly
 
Its good to see you're still learning and trying to improve, but don't overthink it too much. Remember the first rule of aviation - Fly the Airplane.

If you happen to forget a particular target airspeed, say approach speed, and you need to pull out the POH and look it up, OK. I wouldn't worry about finding the airspeed in the POH, then converting it (if needed) and then flying it. Fly what is published. Its enough to keep you safe. Also, older aircraft perform differently than they did when new. A 1966 C-150 will not perform today the same way it did in 1966. You may get a better Vy at an airspeed other than what is in the POH, but using the POH number will get you close enough.
 
Thanks for the replies. Yes, the aircraft I fly are older, the newest one is a 1974 172. It just has all of a sudden started making sense to me and I am just verifying that what I think is correct. I am not intending to start converting v speeds while in flight, and yes, I know fly the airplane first (I will always hear my instructors voice in my head "Fly the airplane!"). I made my own checklist for the airplane and I realized that the speeds I have on it (I have all the v speeds at the end of the checklist) are not correct, seeing as I have the calibrated ones on there, and those won't do me a whole lot of good in flight. I know speeds like Va or Vfe will be almost the same, maybe one knot off, but Vs and Vs0 might be close to 10 knots off, so it is a little more crucial for me to get a better grasp on those indicated speeds. So anyway, thanks for the replies, and confirming my thoughts
 
tgrayson said:
Only if your POH gives the speeds in CAS, as some older POHs do.
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Yes, I was able to find something in two of the POH's for planes I fly that states something to the effect of "all speeds are calibrated unless otherwise stated." The only one that doesn't is the oldest one, a 1963 Cherokee 180, in which half the v speeds aren't even in the POH, and the POH provides very little useful information
 
KIAS and KCAS are nearly identical for all practical purposes. For V-speeds, they fortunately stay constant with changes in temperature and pressure.

KTAS is what needs to be calculated, and does change substantially with altitude.
 
drunkenbeagle said:
KIAS and KCAS are nearly identical for all practical purposes. For V-speeds, they fortunately stay constant with changes in temperature and pressure.

KTAS is what needs to be calculated, and does change substantially with altitude.
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Correct me if I'm wrong but Vx increases and Vy decreases with altitude until the two speeds meet, aka the airplane's absolute ceiling.
 
Again, thanks for the replies! I think I have one last question on this matter. I am just rewriting the checklist I have for a 1973 172M, to reflect some of the changed in airspeed, since all I care about when I'm flying is IAS (I wrote my own checklist), and the POH (I do have a POH specific to the year and model 172) specifies certain speeds are CAS, and specifies some are IAS, what about the ones that are not specified what they are, should I assume that means it is IAS. Most of the time it is a speed that is not crucial whether it is one or another, maybe a 1 or 2 knot difference, but I still would like to know for my own personal knowledge. Again, I do realize that all I care about in the air is IAS, not CAS, but now it is just bugging me, so anyone who can help, I would appreciate it
 
drunkenbeagle said:
KIAS and KCAS are nearly identical for all practical purposes.
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When we discuss CAS, we need to understand what "installation errors" really mean. It's obviously not an installation error because the mechanic was drunk or couldn't read the instructions on how to install the Pitot tube.

The Pitot Tube works on ram air pressure, No? When do you think the "error" would be the greatest, at high or low airspeeds. I will answer that for you, the error is greatest at low airspeeds because the wings AOA (chord/relative wind) brings the Pitot up and out of direct air impact. So the "installation error" is the design charactroistic that puts the Pitot tube at a reduced AOA reletive to the wing chord. This then allows the Pitot tube to have a more direct impact on the reletive wind when the wing is at a higher AOA, (lower airspeeds)

So to expand on the above quote, the IAS and CAS is most closely to identical at the lower airspeeds, like on approach. So like an other poster said, fly the airplane, use 1.3 indicated, it will be just fine. ;)
 
EnRoute said:
It's obviously not an installation error because the mechanic was drunk or couldn't read the instructions on how to install the Pitot tube.
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I dunno buddy, it could be roger repairing your airplane...

Might also be my intro students fault, she grabbed onto the pitot tube to climb up and check the fuel tank...
 
drunkenbeagle said:
But isn't this mostly because of the loss of engine power, not a change in aerodynamics?
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You are correct that the aerodynamics don't change, but it's not so much the loss of thrust/power that causes it, but a change in airspeed at which maximum thrust/power occurs.

Take a flat thrust curve, for instance; the best angle of climb (Vx) would occur at the minimum drag airspeed; if you cut the thrust in half, Vx would occur at the same airspeed, but it would just be a shallower angle than with full thrust available. However, if you make the thrust available curve slope upwards to the right, this would shift Vx to the right.

(Hmmm, it just occurred to me that the maximum difference between the thrust available and thrust required curves probably occurs where the slopes of the two curves are the same. At slower speeds, the thrust available increases faster with airspeed than the thrust required increases, and at faster airspeeds, the thrust required increases faster than thrust available.)
 
tgrayson said:
Take a flat thrust curve, for instance
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For once tgray you have confused the crap out of me reading this. The thrust curve isn't sloped because of a turbo versus non turbo engine which it seems as though you might be implying or at least can read that way. Edit: As I am sure you know.

Drunken, the resultant change in Vx and Vy has to do with the clockwise rotation and upward shift of the power required curve. The power available curve will uniformly drop down, not shift left or right.

Here: http://www.nar-associates.com/technical-flying/altitude/part1/altitude_part1_wide_screen.pdf

Go to page 2.
 
I think I see my confusion, I hope I am not the only one here. A thrust curve can be flat, however a power available curve is not a measure of thrust in a propeller driven aircraft. Maybe my wording here is funky or partially incorrect but do you see where I am going with that tgray?
 
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