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I understood from Why is stall speed listed in a POH? that the Cessna Skyhawk POH lists stall speeds for specific conditions due to a lack of AoA indicator. However, what's unclear to me is what the stall speeds actually are.

Page ii

In page ii of the Cessna 172 POH it lists 48 KCAS for the Flaps Down, Power Off-configuration:

Cessna 172 POH: Performance: Stall Speeds

Page 2-5

Then on page 2-5 it lists the white arc spanning 40 - 85 KIAS, with Lower limit is maximum weight Vs0 in landing configuration.:

Cessna 172 POH: Airspeed Indicator Markings

According the V Speeds Wikipedia page Vs0 is Stall speed or minimum flight speed in landing configuration. I understand that KCAS is not the same as KIAS, but this is quite a significant difference.

Question: what is the actual stall speed for a Cessna 172 while in landing condition and what do these two data points in the POH mean?

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  • $\begingroup$ More specifically: are they actually different speeds, and if so: what is the lowest speed you can fly/land before stalling? (Apologies if this doesn't make sense, I just started studying for my PPL :) $\endgroup$ – vincent.io Jun 20 '16 at 11:56
  • $\begingroup$ There is no exact stall speed listed because it's based on weight. $\endgroup$ – GdD Jun 20 '16 at 13:04
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    $\begingroup$ I was checking the CAS to IAS conversion for higher speeds and found only small differences. I incorrectly assumed this would translate the same way for low speeds as well. This is exactly what I was looking for. If you could leave a full answer I'll accept it. Just two sub-questions as follow-up: 1. Why isn't KIAS used in page ii as well? 2. Google returns this result for "Cessna 172 v speeds": wiki.flightgear.org/Cessna_172P#Airspeeds, which says Vs0 is 33kt (CAS). Is the Flightgear simulation based on a different Skyhawk model? $\endgroup$ – vincent.io Jun 20 '16 at 13:12
  • $\begingroup$ @vvanscherpenseel This is going to vary even in the same model due to variants and model years. This is a broad question. Maybe ask another local pilot to review the POH with you? The 172 I fly now is 33/44 KIAS. $\endgroup$ – Ryan Mortensen Jun 20 '16 at 15:23
  • $\begingroup$ @RyanMortensen: Thanks for your reply. 33/44 meaning flaps-down/flaps-up? $\endgroup$ – vincent.io Jun 20 '16 at 15:24
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Short answer

At low speed, near the stall, 40 KIAS (the low end of the white arc) and 48 KCAS (Vs0 in the POH) refer to the same actual speed. While there is a limited divergence (0 to 2%) between CAS and IAS at medium and high speed, this difference can be as much as 20% at low speed, when the aircraft pitch is large.


Details

In your question, you mention:

  • The POH states a Vs0 of 48 KCAS.
  • The low end of the airspeed indicator white arc is 40 KIAS.

The end of the white arc is also Vs0. That's a bit confusing indeed as two types of measures are used, but in fact they refer in practical to the same actual airspeed.

Indicated airspeed

Unless computations are done, airspeed values displayed to the pilot IAS (indicated airspeed).

enter image description here
(Source)

Errors affecting IAS

IAS reflects directly the difference between the total air pressure in the pitot tube, and the (mean) static air pressure at different static air pressure ports (on the side of the aircraft and in the pitot). The difference (the dynamic pressure) is subject to possible errors:

  • Total air pressure is wrong it the probe axis is not parallel to the airflow. Obviously the pitot tube angle changes between low and high speeds. For a given speed the pitot pitch also varies with mass and balance, and the status of drag devices such as flaps and landing gear.

  • Static air pressure at a pressure port is likely affected by pressure variations created by the aircraft fuselage. This error greatly depends on the port location and its design, but also on the yaw angle.

  • Airspeed indicator itself.

Obtaining CAS from IAS

All V-speeds are defined in term of "calibrated" airspeed (CAS). The aircraft manufacturer provides tables to find the CAS from the IAS read on the airspeed indicator. These tables defined for different drag devices status (e.g. flaps values) are valid at sea level and 15°C. They are included in the POH. I found this C172N one online:

enter image description here
(Source: POH C172N 1978)

For a 40° full flaps the table says that 40 KIAS are actually 47 KCAS. This is close to the 48 KCAS in your POH.

As you realized, the difference between KCAS and KIAS becomes larger as the air speed decreases, because the angle of attack and the errors in pressure measurement increase.

Multiple ways to express airspeed

TAS depends on parameters and need to be computed. However it can be approximated by the indicated airspeed. IAS is close to the TAS in favorable atmospheric conditions (sea level air density), and at low angles of attack.

The IAS is easy to display to the pilot using the an aneroid cell. For higher angles of attack, the calibrated airspeed (CAS) corrects for instrument and probe placement errors. CAS can be obtained using manufacturer tables. CAS, as IAS, is valid at sea level and 15°C.

When the air temperature or pressure are not close to the ideal ones (15° and 1013 hPa), then CAS differs from TAS.

The ASI usually has a window (see above) for reading the TAS after the outside temperature and altitude pressure have been selected.

(At speed higher than 200 KCAS or altitude higher than 10,000 ft, the compressibility of air becomes significant, and the equivalent airspeed (EAS) is better suited to describe the aerodynamic effects.)

Vs0 is determined assuming the maximum landing weight and the most unfavorable CG location. It is expressed in KCAS. If the Vs0 CAS were unknown, the pilot wouldn't be able to correct it to include the effect of weight and balance changes.

The ASI, because no correction is done, can display only IAS. So the IAS value for the lower speed of the white arc (Vs0) is in KIAS.

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  • $\begingroup$ Thanks for the extensive answer! Just one question related to the image you included: am I correct that the clock indicates TAS on the outside of the arc? Does it automatically correct for OAT, AoA and air density? $\endgroup$ – vincent.io Jun 20 '16 at 17:38
  • $\begingroup$ Yes, the 6 o'clock sector shows the TAS (there is a legend TAS close to the small ruler). It corrects for the OAT/Pressure combination the pilot selects in the 12 o'clock sector, labeled Temp°C and Press Alt. See a demo video. Density is assumed from pressure and standard atmosphere model. Only cruise speeds are converted, as this is where you are supposed to be in altitude and in need of TAS. $\endgroup$ – mins Jun 20 '16 at 17:47
  • $\begingroup$ An even more extreme example of this effect exists on the Cessna 172R. Vs0 is 33 KIAS = 47 KCAS. That's 14 kts difference - almost a whopping 50% difference. $\endgroup$ – Pugz May 27 '17 at 23:56
  • $\begingroup$ Vs0 is not determined at MLW but at MTOW. $\endgroup$ – mezzanaccio Jul 4 '18 at 12:38
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The answer for the POH you provided is on this page.

The page you are referencing is a revision made by someone who decided to use CAS for some reason.

Your stall speeds at max gross with max forward C.G. are:

  1. Vs0 = 40KIAS
  2. Vs1 = 48KIAS
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The design stall speed for a Cessna 172 at maximum weight with flaps fully extended is 40 knots indicated air speed.

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