Calibrated airspeed?

Discussion in 'Flight Following' started by bflynn, May 28, 2021.

  1. bflynn

    bflynn Final Approach

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    Question - can someone give a simple and clear explanation of calibrated airspeed and why it is important? I see references from time to time about it and people seem passionate about its importance, but for all my time, I’ve never crashed for not knowing. What am I missing?
     
  2. EdFred

    EdFred Taxi to Parking

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    Because at varying angles of attack you will get a different IAS. Stall speeds are generally based on CAS.
     
  3. dmspilot

    dmspilot En-Route

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    Calibrated airspeed is indicated airspeed corrected for instrument and position error. The largest error occurs at high angle of attack when air enters the pitot tube at an excessive angle.
     
  4. nauga

    nauga Administrator Management Council Member PoA Supporter

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    It's important because it's the easiest to obtain fundamental indication of speed through the air that doesn't have some presumed error. It's the basis for stall and other performance speeds, dynamic pressure, etc. Most engineering and flight test literature attributes a majority of the error to the static source, which is why you sometimes see it called 'static source position error' and why an external static source like a trailing cone or bomb is an accepted test method.

    Nauga,
    trailing bombs
     
    Last edited: May 28, 2021
  5. Velocity173

    Velocity173 Touchdown! Greaser!

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    Yep. Anytime you get a high AoA or disturbances in airflow or pressures around the static port, you’re indicated and calibrated will be different. Look at test aircraft with their long air data booms. Keeps the instruments away from the fuse to reduce errors and get the most accurate IAS possible. They’ve got different pitot tubes (Kiel) to align the airflow as well.

    Helicopters are an extreme example of indicated vs calibrated in that the rotor down wash has a pronounced effect. High power climbs for some aircraft can produce a large increase in IAS.
     
  6. eman1200

    eman1200 Touchdown! Greaser!

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    I think the largest documented difference in my PoH is 2mph so I don’t lose too much sleep over it.
     
  7. MauleSkinner

    MauleSkinner Final Approach

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    Hitting the earth, apparently.

    If your airplane is already certificated, and you don’t have test flying to do, indicated airspeeds are your reference.
     
  8. sarangan

    sarangan Cleared for Takeoff

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    Exactly. Indicated airspeed is all you need for flying.

    Calibrated airspeed becomes important if you are trying to figure out stall margin, or conversion to true airspeed. For instance, 1.3 * Vso is not indicated airspeed multiplied by 1.3. You have to convert IAS to CAS, multiply by 1.3 and then convert CAS back to IAS.
     
    Last edited: May 29, 2021
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  9. luvflyin

    luvflyin Touchdown! Greaser! PoA Supporter

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    They locked the door when they were writing the POH, but the engineers broke in. So a compromise was reached. They published both IAS and CAS. IAS is the color of the sky in the world in which most pilots live.

     
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  10. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    If your plane is old enough, you may find the "speeds" listed in TIAS ("True Indicated") which is the old term for calibrated.

    Calibrated diverges from indicated at high angles of attack. This is why you need to be real careful on things like the 1.3 Vso rule of thumb for approach speed to make sure you're using the calibrated airspeed and then converting back to what the indicator should read.
     
  11. bflynn

    bflynn Final Approach

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    Hmmm, in 10 years I have yet to do that. I guess I should try harder?
     
  12. MauleSkinner

    MauleSkinner Final Approach

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    It’s a big target...:D
     
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  13. denverpilot

    denverpilot Tied Down PoA Supporter

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    It varies significant between aircraft.
    So... You actually need to know both. Lol

    Let your POH be your guide... Or buy an AOA indicator. Ha.
     
  14. Velocity173

    Velocity173 Touchdown! Greaser!

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    If your POH has a cruise chart with standard conditions (sea level 15 C) with IAS and TAS, then you can figure out your CAS.

    An example of even without the effects of rotor wash, in helicopters the changes in pitch angle with IAS can create large differences in CAS. Not quite standard conditions but you can see this pitot / static is designed so that the only time instrument error doesn’t exist is around 100 IAS.

    AD27068B-7F52-43D5-AAB0-6E25853C4327.jpeg
     
  15. Ryan F.

    Ryan F. Cleared for Takeoff

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    This is a consistent area of weakness at the private and commercial certificate levels.

    I decided to write something called "Untangling the Knots" in an attempt to put a little positive spin on the ball, so to speak... out there in practical test land.

    https://www.ryanfergusondpe.com/untangling-the-knots/
     
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  16. Matthew Rogers

    Matthew Rogers Cleared for Takeoff

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    The difference between IAS and CAS at stall for the common C150 is up to 11 mph. Stall speeds are listed as CAS in the POH and you have to convert them to IAS to be useful.

    Power off full flaps stall is 48 CAS and 39 IAS.
     
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  17. David Megginson

    David Megginson Pattern Altitude

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    For trip planning, you need your groundspeed. To get your groundspeed, you take the forecast winds and your true airspeed. To get your true airspeed, you take your calibrated airspeed and adjust by density altitude.

    All of that probably happens automatically in your app now, but if you were doing it by hand with an E6B, that's where CAS would come in.
     
  18. brcase

    brcase Pattern Altitude

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    Indicated Airspeed is the indication you get when you use the pitot probe that is mounted on your airplane

    Calibrated Airspeed is the number you would show if your pitot probe looked something like the one attached and was mounted 6 to 12 feet in front of the airplane or wing.

    Was the best photo of one I could find, could probably use a simpler one that simply had the vane to correct for Angle of Attack would get you very close.
     

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  19. bflynn

    bflynn Final Approach

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    I’m still not getting a sense of what CAS really is. For example, IAS is a measurement of the pressure of the air molecules pressing against the sensor in the pitot tube. TAS is the airplane’s speed through the air, relative to the air.

    The description of how CAS is calculated doesn’t help with understanding. Is CAS describing an “aerodynamic” speed, what you would see if you model the speed/density of the air going past the wings? And then IAS is the inaccurate measurement of CAS?
     
  20. nauga

    nauga Administrator Management Council Member PoA Supporter

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    Essentially. Dynamic pressure (q) is the difference between total pressure and ambient pressure (Pt-Pa). A pitot tube measures total pressure Pt (with some small error at high angles of attack) and a static port measures ambient pressure Pa with larger errors due to airflow entering the port and introducing some dynamic pressure component.

    Dynamic pressure is also equal to the familiar 1/2*rho*V^2, where rho is density and V is speed, but if you use calibrated airspeed Vc the equation becomes:
    q = 1/2*rho_ssl*Vc^2, where rho_ssl is the density of air at sea level standard day conditions, or Vc = sqrt(2q/rho_ssl). The nice thing about this relationship is you don't need to know any local atmospheric properties to get airspeed from dynamic pressure. Since the pitot-static system has errors (see above) it doesn't really give you this exact quantity, the resulting dynamic pressure measurement and airspeed calculation gives you indicated dynamic pressure (qci) and indicated airspeed Vi. Since it's all referred by standard day sea level conditions you can test to find the corrections to airspeed as a function of airspeed and not have to worry about (e.g.) temp or pressure when testing.

    In terms of true airspeed (Vt) and the same dynamic pressure,
    q = 1/2*rho_a*Vt^2, where rho_a is atmospheric density at the point where the measurement was taken, and Vt is true airspeed. With a little math you can also show that Vt = sqrt(T_ssl/T_a) *Vc (for low subsonic speeds) where T_ssl = temp at sea level standard day and T_a = ambient temp where the measurement is taken. Watch units, with no conversion factors temps must be absolute (R or K) and speeds ft/s.

    Also note that the static errors also result in an altimeter error since your static port isn't reading the exact ambient pressure. A good source error correction test will get the corrections for both indicated airspeed and indicated altitude (there are requirements for both). Also note that if the errors were due solely to pitot errors there would be no altitude correction required. Hmmm...

    The Naval Test Pilot School fixed wing flight test performance manual (search for USNTPS FTM-108) has a really good writeup on all of this if you want gory details including different test techniques.

    Nauga,
    pneumatically
     
    Last edited: May 30, 2021
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  21. brcase

    brcase Pattern Altitude

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    Most of the difference comes from the difference in angle of attack and pitot location. This is why accurate pitot tubes have a vane on them so that the pitot is always directly into the airflow. Or some/many use a hemispherical probe to accomplish the same thing, it is simpler to build and maintain but more complex to explain. The 2nd and perhaps largest error comes from the fact that the pitot is mounted near the airframe, in most (Many?) cases on the bottom of the wing, the high pressure builds up under the wing, especially during high angles of attack causing the Airspeed to read higher than it would if the pitot were out away from the wing/airframe.

    Definition
    Calibrated airspeed (CAS) is indicated airspeed corrected for instrument errors and position error (due to incorrect pressure at the static port caused by airflow disruption).

    Good point that the static port affects this as well which is why accurate (for flight testing) pitot tubes have a static port built into them and of a vane or similar apparatus with multiple ports. It is also mounted way ahead of the aircraft for accurate readings.

    I am not sure why they say CAS corrects for instrument errors. this would imply that the error is the same in all production airspeed instruments, I don’t see how a manufacturer can correct for instrument errors as a standard airspeed indicator that is working properly should be quite accurate. I guess if the manufacturer knows that the 50kt mark is painted 2Kts in the wrong location they could include that in the IAS vs TAS chart. But more likely the would just have that batch of Airspeed indicators fixed.

    The calculations nauga mentioned are for True airspeed. I don’t think you can really apply a formula to to get CAS. One could do some complex fluid dynamics modeling to try to estimate it but, would probably still get it wrong.

    Brian
     
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  22. Matthew Rogers

    Matthew Rogers Cleared for Takeoff

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    Except on my Cessna 150 the Indicated Airspeed values are all lower than my CAS in the POH airspeed correction table (by up to 11 mph). So my IAS under the wing reads lower, not higher, than a probe theoretically would out in free air.
     
  23. Velocity173

    Velocity173 Touchdown! Greaser!

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    In simplistic forms, just think of IAS as being a sometimes unreliable instrument indication because of the way the pitot static system is mounted.

    The helicopter chart above is the best way of understanding it because of the extremes involved. 30 KiAS on that chart is 38 KCAS and in turn, 38 KTAS (almost standard conditions). The aircraft is going through the air with a slight nose up attitude. Because of that, the airflow into the pitot tube isn’t flowing parallel and isn’t as high of a velocity as it’s read on the instrument. If the pitot tube was on a gimbal and tilted to keep it pointed directly into the oncoming air, then 30 KIAS would equal 30 KCAS. It’s a flaw in the system that can’t be corrected for all conditions but like I said, at 100 KIAS, those flaws are non existent. Go above or below 100 KIAS and those flaws (instrument errors) get worse the farther you get from 100 KIAS.
     
  24. FastEddieB

    FastEddieB Touchdown! Greaser!

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    I think it was some time in the mid-70’s that the FAA mandated some standardization to Owner Handbooks/Flight Manuals and instrument markings. Prior to that, I think the Vspeed markings on airspeed indicators were CAS, requiring conversion to IAS. Now I believe the instrument marking are required to be in IAS.

    Am I remembering correctly?
     
  25. MauleSkinner

    MauleSkinner Final Approach

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    Yes. 1974, I believe.
     
    Last edited: May 30, 2021
  26. Ryan F.

    Ryan F. Cleared for Takeoff

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    You're at 70 knots, flaps up in a Cessna 172S. At this airspeed, the IAS and CAS are the same due to the AOA and "design" of the pitot tube.

    CAS = 70 knots
    IAS = 70 knots

    You drop the flaps to 30 degrees and slow down to 50 knots indicated airspeed. But the airspeed is actually higher -- due to the physical installation of the pitot tube you're indicating an airspeed which is 6 knots lower than your actual airspeed.

    CAS = 56 knots
    IAS = 50 knots

    The CAS is the "real" airspeed at which the airplane thinks it's flying, but it's not indicated on your airspeed indicator. Your airspeed indicator is not in agreement with the airplane, reading 6 knots low.

    You speed up to Vy, retract the flaps, and then entire a screaming dive from a safe altitude. You reach an indicated airspeed of 140 knots. But again, because the pitot tube is only "accurate" at a certain range of airspeeds, it'll be wrong again.

    Now,

    CAS = 137 knots
    IAS = 140 knots

    Since we accept that the instrument will have errors, we look for a calibration chart somewhere in the aircraft AFM/POH. This chart provides the pilot with the range of errors at various airspeeds and flap settings, if so equipped, so that the pilot is able to account for that delta.

    Hope that helped?
     
  27. David Megginson

    David Megginson Pattern Altitude

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    Yes, that's right. CAS is the actual difference between dynamic and static pressure, while IAS is what you get with the measurement errors.

    (Warning — the rest of this is uninformed speculation) I don't think it's possible using a pitot tube to get an accurate reading at all angles of attack, because the air's not always going straight into the tube; so they usually point the tube to get a reading close to CAS at a normal cruise angle of attack.

    That means the ASI will under-read at high AoA/low speed. CAS matters mainly for flight planning at cruise speeds, though, so that's not a big problem there, and they just bake the IAS errors into the V-speeds for low-speed maneuvers.
     
  28. Maxnr

    Maxnr Pre-takeoff checklist

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    Just remember that the pitot is only fixed in one direction. We mere mortals let the nose wander all over and call it good.
     
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  29. Larry in TN

    Larry in TN Pattern Altitude

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    CAS is what IAS should be, but isn't.

    If the pitot tube isn't perfectly aligned with the relative wind it will not capture all of the dynamic pressure and IAS will be somewhat lower than it should be. CAS is corrected for this error.
     
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  30. AKBill

    AKBill En-Route

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    So the IAS is off when slipping, climbing, turning and such..
     
  31. Larry in TN

    Larry in TN Pattern Altitude

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    IAS is off at anything but the exact AoA that matches the mounting of the pitot tube.

    Look at the CAS/IAS chart of a relatively modern airplane and you'll see that it's accurate at one airspeed and the error increases the farther you go from that airspeed.

    The marking on the airspeed indicator are in CAS. That's why you can fly slow-flight below the bottom of the white arc without stalling. Your IAS is lower than your CAS.

    In jets, the air-data computer takes the pitot, static, and AoA inputs and uses the AoA to correct for installation error.
     
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  32. dmspilot

    dmspilot En-Route

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    Only for older aircraft. The year 1974 was mentioned above but not sure if that is exactly correct, it was certainly sometime in the 70s.
     
  33. David Megginson

    David Megginson Pattern Altitude

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    On my Piper PA-28, at least, the airspeed indicator shows IAS — the POH has a correction table to get from that to CAS. When my ASI shows 44 KIAS, I'm really flying at 50 KCAS.

    The reason you often stall below the Vs0 and Vs markings on the ASI is that Vs is weight-sensitive, and is usually given for maximum gross weight. Unless you're loaded right up to max and haven't burned off any fuel yet, your actual (unaccelerated, wings-level) stall speed should be at least a couple of knots lower.
     
    Last edited: May 30, 2021
  34. David Megginson

    David Megginson Pattern Altitude

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    6 kt (7 mph) difference for my Piper PA-28-161 at Vs0. I don't lose sleep over it either, but it's worth knowing about.
     
  35. Larry in TN

    Larry in TN Pattern Altitude

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    Yes, that's the definition of IAS. The markings, white arc, green arc, etc., are CAS.
     
  36. dmspilot

    dmspilot En-Route

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    Not in aircraft produced in the past ~45 years or so.
     
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  37. David Megginson

    David Megginson Pattern Altitude

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    My ASI and arcs are both calibrated in IAS. For example, my PA-28 has a Vs0 of 44 KIAS (50 KCAS), and the bottom of white arc is at 44 on my ASI, not 50. That way, there's no mental arithmetic required.

    As I mentioned earlier, the reason I can maintain slow flight below the arcs in the practice area without stalling isn't that the arcs are in CAS (mine aren't), but simply that I'm well below max gross weight.
     
    Last edited: May 30, 2021
  38. nauga

    nauga Administrator Management Council Member PoA Supporter

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    The main source of indicated airspeed error is the *static source*, not the pitot, unless your pitot tube is really messed up. Pitot tubes are far less sensitive to alignment errors than static ports. There's a reason some of the accepted methods of calibration measure altitude only :)

    Nauga,
    and old wives' tails
     
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  39. Maxnr

    Maxnr Pre-takeoff checklist

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    [QUOTE="
    In jets, the air-data computer takes the pitot, static, and AoA inputs and uses the AoA to correct for installation error.[/QUOTE]

    I like to go as fast as possible. I know, A major personality defect on my part. I found that if I select co-pilots air data computer to my side, I can get another 3KTS before the airspeed tape turns red. Not sure exactly what goes to the FDR. I don't want any incriminating evidence hidden there.
     
  40. luvflyin

    luvflyin Touchdown! Greaser! PoA Supporter

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    Maybe the same year that Knots became the standard instead of MPH? The voices in my head are saying 1976.