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Calibrated Airspeed Graphs.

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Mutley Eugenius created the topic: Calibrated Airspeed Graphs.

I just discovered Calibrated Airspeed Tables for a Cessna 172. Are these calculated at MTOW angles of attack? Just curious how they know what angle of attack you'd be at for each airspeed.

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  • John.Heddles
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John.Heddles replied the topic: Calibrated Airspeed Graphs.

You would be aware that the pressure instruments take, variously, impact (pitot) and static (static source) inputs to work their magic and give you some useful numbers. The pitot input is not a major problem, generally, but every aircraft has a significant amount of work required to figure out where the static source is going to be located and what errors are associated with the location.

While preliminary calculations and wind tunnel work can provide a good indication of suitable locations on the aircraft where the static pressure measurements should be reasonably accurate, the proof of the pudding is in the eating, as the idiom goes. This requires a LOT of flight test work. A good summary of the story is in the following AC

A typical trailing static cone is shown in the following photo - normally we trail it from the top of the vertical stab as we need to get out of the aircraft wake

The numbers we get from the trailing source then can be compared to those we get from the aircraft test source position and, eventually, we find a suitable spot where we get acceptable data and that data ends up in the AFM/POH. The usual terms used are CAS, for the corrected post flight test IAS data, and PEC chart (position error correction) for the data to be used by the pilot. You will find this data in every POH you review. Separate charts (either graphs or tables) will be presented for the instrument systems and normal/alternate static source (to cover the icing scenario).

As to angle of attack, the PEC data will cover the operational envelope for the aircraft so a wide range of angle of attack to cover the speed ranges. We are not particularly worried about angle of attack, per se, as the aim is to provide flight instrument correction data for the pilot.

The aim of the certification work is to give the pilot a fairly complete story so that he/she can figure the corrected airspeed (CAS) from the system airspeed (IAS) and corrected height data. There are certification limits on height errors and this consideration then constrains the extent of airspeed errors, generally, to a few knots. Once you get near the stall regime, things can get a bit strange but that is a region we should be avoiding for all the right reasons. Back in the stall, it all goes a bit pear shaped - the majority of us have enjoyed the sight of Cessn ASIs with no speed on the clock - just says that the PEC is significant in that regime.

Engineering specialist in aircraft performance and weight control.

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Mutley Eugenius replied the topic: Calibrated Airspeed Graphs.

As always, an awesome answer. Thanks John.

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