As you can see in this link (Roskam, Airplane Aerodynamics and Performance, Chapter 2.5.3 Airspeed corrections) ...

... in the first and second line of page 30 Roskam tells that, in hypothesis of incompressible flow $$ \frac{\Delta p}{\overline q}=\frac {\frac{1}{2}\rho_0V_c^2-\frac{1}{2}\rho_0V_I^2}{\frac{1}{2}\rho_0V_c^2} $$ " depends on factor such as weight, flap deflection and other airplane parameters ". Why does it depend on weight, flap deflection?

Which "other airplane parameters" do affect $\frac{\Delta p}{\overline q}$ ?

Note: for completeness of definitions, above in the same link, page 27 Roskam is defining:

"[...] $V_I$ (IAS) is the indicated airspeed corrected for instrument error, $\Delta V_i$, only:

$V_I=V_i+\Delta V_i$ ________________$(2.54)$

$V_c$ (CAS) is the calibrated airspeed. It is equal to the indicator reading corrected for position error due to incorrect static port location $(\Delta V_p)$ and also corrected for instrument error:

$V_C=V_I+\Delta V_p$ ________________$(2.55)$ [...]"

  • $\begingroup$ Sorry, but the linked page will only show by chance - in my case, "La pagina non è disponibile". Without that context I would need to guess what the question is about. $\endgroup$ Jun 4 '17 at 7:01

The formulas deal with the relationshop between calibrated airspeed and indicated airspeed. The difference is due to two factors:

  • instrument errors: these errors occur within the airspeed indicator instrument when transforming the pitot pressure to a speed indication.

  • position errors: these errors are caused by the position of the pitot tube on the airframe causing the pitot pressure from being different than the freestream pitot pressure.

Since the pitot tube is usually close to the fuselage of the aircraft, the local airflow around the pitot tube is influenced by the aerodynamic properties of the aircraft itselfs. The pitot tube's location is chosen such the the local flow characteristics deviate as little as possible from the freestream characteristics but there will be a measurable difference in pressure between the freestream total pressure and the pitot pressure. The pressure differences depends on airflow around the aircraft which is infuenced by the angle of attack (and thus weight) flaps, slats, gear, turn rate, side slip angle etc.

In order to analyse the position errors during a flight test programme, test aircraft are often fitted with additional pitot probes protruding ahead the airframe and/or trailing cones (for static pressure measurement) far behind the aircraft.

  • $\begingroup$ Thank you. Does someone know how is it affected by weight, too? $\endgroup$ May 4 '17 at 22:21
  • $\begingroup$ The weight affects the angle of attack $\endgroup$
    – DeltaLima
    May 5 '17 at 5:21

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