IAS accuacy

[Aviator168]

Anyone know how much I am off if flying very slow and with a fairly large AoA (3-6 degrees)? I know the pitot is not pointing at the direction of flight in this situation and IAS is reading lower than it really is.

[Mel]

You'll get pretty good accuracy until the AOA gets really steep. Remember the ASI works off of pressure, not air flow. 3 to 6 degrees is NOT extreme.

[Aviator168]

Thnx Mel. I thought the ram pressure would be proportionally lower since it is not pointing straight forward and it would produce a lower reading.

[Kevin Horton]

Originally Posted by Aviator168

Anyone know how much I am off if flying very slow and with a fairly large AoA (3-6 degrees)? I know the pitot is not pointing at the direction of flight in this situation and IAS is reading lower than it really is.

NACA studies showed that the pitot pressure should be very accurate as long as the airflow is aligned within 15 to 20 degrees of the pitot tube (see data from NACA TN 2331, reproduced in Figures 4.5 to 4.7 in NASA Reference Publication 1046, Measurement of Aircraft Speed and Altitude - 24 MB pdf file).

The pitot pressure should be accurate, except for conditions very close to the stall. Errors in the static pressure and ASI instrument error are more likely sources of airspeed system error at other conditions.

[Aviator168]

Thnx for the link Kevin. I am designing an attitude indicator that requires acceleration compensation. I was a bit concerned. Now I feel much more comfortable.