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spokes2112

For dynamics developers. IAS calibration.

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Hi all, ran across this one & thought it should be saved here to prevent loss.Posted in the FS9 Forum 24 March 2007 by Tom Goodrick here - http://forum.avsim.net/topic/226978-airspeed-confusion-in-fs/

The other day, while making a cruise performance table for the Cessna Skylane, I stumbled onto a problem with airspeed that I had never noticed before. I have been modifying flight dynamics files for many years to get better realism in FS aircraft flight performance. This was a shock. There is a line in the aircraft.cfg file for the Skylane 182S that determines how indicated airspeed is calculated in this sim. I was tabulating true airspeed and indicated airspeed for a fully-loaded Skylane at various altitudes, coming down from 8,000 ft which was where I matched a published specification. I was curious to see the variation of true airspeed with altitude at a constant power setting. I listed the indicated airspeed just for completion because that is what you should be looking at when flying the airplane. I was amazed to find the indicated airspeed nearly equal to the true airspeed when I got down to 5,000 ft. (I was using standard conditions having selected "Clear Weather" as I always do when testing aircraft.)After much consternation - I thought I knew the behavior of the Skylane since I have flown the FS version for several (~38) hours - I found the source of the problem. It is the line under the section heading [airspeed_indicators] in the aircraft.cfg file. the line says airspeed_indicator.0= 1.3, -24 as set by default by the folks at Microsoft. The first number is a scalar nd the second number is an offset. These are applied to the calibrated airspeed in the calculation of the indicated airspeed. To avoid a lot of confusion, I think these settings should be 1.0 and 0.0 as they are by default. This line is not even in most aircraft.cfg files. It should be removed from all. The line for the Skylane means there will always be a 30% error in the indicated airspeed!The problem I see is not so much that a calibration curve between calibrated airspeed (CAS) and indicated airspeed (IAS) is used but that it is WRONG! I don't have the real table for the Skylane but I have it in an old pilot's manual for the Skyhawk and for another aircraft I used to fly. This calibration curve is simple and linear while the real curves are more complicated and non-linear. The real data shows a big difference at the lowest speeds going to essentially no difference above 60 knots. The variation is mainly dependent on angle of attack rather than airspeed. Calibrated Airspeed is measured during certification tests for an aircraft using a long pitot-static probe that reaches far out ahead of the aircraft into the region where the flow is only slightly disturbed by the aircraft. The normal pitot tube used in the production aircraft is close to the wing where it sees flow that varies considerably with angle of attack and with flap deflection. The normal pitot tube gives us a direct measure of dynamic pressure from which airspeed is calculated using a calibration scale that assumes standard mean sea level density. This is "indicated" to us when we fly. We need this indicated airspeed so that we know at all times how much of a cushion we have above stall speed, regardless of altitude. While Cessna and many other manufacturers give stall speeds in CAS, pilots are taught to use a table that converts those speeds to IAS. Then we file the IAS values for stall speed in our brains for the transition to the aircraft (one for a clean aircraft and one for the landing configuration). As pilots we really have no need to know the CAS values. We will never measure them or experience anything directly related to them. We will see the indicated values every time we practice stalls.There is already enough confusion about airspeed in FS without adding this one. FS has always had the wrong concept about "truth" in airspeed. Every pilot is taught the primary res[onsibility of the pilot is to conduct a safe flight. Getting some particular place in a particular time is always secondary. You need IAS or KIAS (in knots) to fly safely. You can use TAS (or KTAS in knots) to estimate how long the trip will take (if you also know the wind speed). These days you glance at the Garmin to see how long it will take to get there.FS has always considered viewing true airspeed as needed for ultimate realism. You select display of IAS or TAS in the "Realism" menu. In FS9 they started showing the reference speeds in the aircraft.cfg file where stall speeds are to be listed in KTAS. Balderdash!!! List them in KIAS. If you use TAS, you'll have to add 10 knots by 5,000 ft and 200 knots by 40,000 ft. Using IAS the came number applies at all altitudes. (Technically, the stall speed also depends on weight. But using the values for max takeoff weight always keeps you safe.)

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