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Guest Tom Goodrick

Airspeed Confusion in FS

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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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I've always found the max realism settings in FS defaulted to KIAS.The reference speeds in the reference documentation are in KIAS.The reference speeds in the aircraft.cfg file are KTAS for AI aircraft, but not for user flown aircraft. They are only for aircraft flown at empty weight with no fuel or pax/cargo. And they are only reference targets, not hard limits because the other aircraft flight dynamics control the actual stall behavior of AI aircraft.The one, actually two, numbers in the reference speeds section which apply to user flown aircraft are the max indicated and the max mach speeds - because those trigger the overspeed threshold.The speeds in the cessna172sp_ref.htm are the ones for the user aircraft - and again they are only approximations and only valid at standard altitude and temperature and pressure. Aircraft weight, altitude and pressure will cause them to vary slightly.

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My main point was that, with that [airspeed_indicators] line in the aircraft.cfg file for the Skylane 182S, the indicated airspeeds are off by 30%. Hence stall speed references are screwed up. I had to take out that line and found that stall was then occuriing at 64 KIAS. Then i reset the lift curve so that stall now occurs at 54 KIAS as it should. The use of the incorrect calibration function gives a 30% error in the KIAS. I am only concerned about user-flown aircraft.

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Tom,I know this is an old post... and yes... I think some people say it is bad "internet etiquette" to reply to a 4 year old post and dredge it back up to the top of the board. However, I was so pleased to see you define the exact same problem I was having... that I just had to reply. Don't know if you come to this site anymore... but if you do... I want to say thank you for posting this.I am not a real pilot and don't understand mathematical curves and stall speeds the way you probably do... so the 30% error thing you mentioned was a little over my head. But I too did notice that the overall speed was off on the C182. How I noticed it was when I throttled up from a dead-standstill for takeoff, the airspeed indicator hung on 0 knots while I'm going down the runway at what had to be at least 25 or 30 kts before the air speed indicator started counting up. I thought it very strange myself, and noticed that no other plane in the sim did this. Digging through the aircraft.cfg file (and comparing head-to-head with the .cfg file for the similar C172), I too noted the rather odd "airspeed indicator" command line in the .cfg that seems rather oddly misplaced in there. Simply put, if no other plane needs a special line to define numerical offsets in the ASI, then why did MS think that this one C-182 needed it?So I too removed the line, and the kts now start counting up from 0 right as soon as the wheels start creeping forward down the runway-- AS THEY SHOULD! I'm so glad that the fix was this simple. And that simply copying and pasting the command line into a Google Search brought me to your post 4 years after the fact. The internet is an amazing thing... and you helped me confirm that I took the right action by removing the airspeed line in the .cfg altogether.Thank you again.-- John

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