# Interesting challenge...

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Hi,I'm looking for a formula that converts mach to ias within FS. I know it's possible since the FS autothrottle converts when you switch between IAS and Mach hold.None of the formulas I've tried thus far offer correct results. Basically I'm looking to convert back and forth between IAS and Mach.

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Well, there are two Mach's.I'm guessing you are refering to the actual mach number through the atomphere at the current PA and temperature, vs. just simply the indicated airspeed in Mach vs. knots. Right?Why not get the true airspeed from a MODULE_VAR, and then convert that to the mach using straight math?

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I think you're missing the point of my post... I'll try to explain better.On a PFD there's an indicator for the FMS hold airspeed. At a specific altitude the FMS automatically switches between IAS and Mach hold. It needs to set the indicator based on the value. However, the airspeed tape is always in IAS. So, to get the correct position for the indicator on the tape, I have to convert mach to IAS.FS provides the mach value, I don't have to calculate it. I need to convert it to it's IAS equivalent.

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Hi,Maybe this "rule of thumb"?MACH = IAS / 60phjvh

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>FS provides the mach value, I don't have to calculate it. I>need to convert it to it's IAS equivalent.Why not just continue to set the tape position based on the IAS MODULE_VAR AIRSPEED? Just because you are displaying the mach number does not stop you from using a different var for positioning needles and such.Sorry if I'm not helping any. =)

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Hi Ed,You will find all the necessary calculations in the outstanding WM Olson's Aircraft Performance Flight testing paper (chapter 4)Here is a direct linkhttp://perso.orange.fr/hsors/docs/WMOlson_...on_Handbook.pdfIf you are interested in the way the changeover FL can be calculated (interestingly, it doesn't depend on outside air temperature), please email me so as I can send you the code I used in my Adacalc calculatorhttp://perso.orange.fr/hsors/adacalc.htmlHerv

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Ed,These two macros work good for me:(L:AP Mach Speed, mach) (A:Ambient Temperature, celsius) 273.15 + sqrt 38.967854 * * (A:Airspeed True, knots) (A:Airspeed Indicated, knots) - - (L:AP IAS Speed, knots) (A:Airspeed True, knots) (A:Airspeed Indicated, knots) - + (A:Ambient Temperature,celsius) 273.15 + sqrt 38.967854 * / It shouldn't be difficult to translate them to C++ Tom

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>>FS provides the mach value, I don't have to calculate it. >I>>need to convert it to it's IAS equivalent.>>Why not just continue to set the tape position based on the>IAS MODULE_VAR AIRSPEED? Just because you are displaying the>mach number does not stop you from using a different var for>positioning needles and such.>>Sorry if I'm not helping any. =)>It's not the tape position that's at issue. It's the speed bug that's displayed alongside the tape. That speed bug represents what speed the FMS wants to hold. Once the aircraft reaches the FL crossover point, KIAS is no longer used to maintain a velocity. At that point Mach is used to maintain a specific velocity. Accurately indicating that position is the issue. Thus the need to convert any given mach value to the correct KIAS.

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Another way is ratio/proportion. Your ratio of indicated speed to indicated mach at any given alt/WX condition, will give you the conversion factor to find either mach/IAS bug position.(A:Airspeed indicated, knot) (A:Autopilot airspeed hold var,knots)----------------------------- = --------------------------------------(A:Airspeed mach, machs) (A:Autopilot mach hold var, mach) Regards,Roman

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>Hi Ed,>>You will find all the necessary calculations in the>outstanding WM Olson's Aircraft Performance Flight testing>paper (chapter 4)>Here is a direct link>http://perso.orange.fr/hsors/docs/WMOlson_...on_Handbook.pdf>>If you are interested in the way the changeover FL can be>calculated (interestingly, it doesn't depend on outside air>temperature), please email me so as I can send you the code I>used in my Adacalc calculator>http://perso.orange.fr/hsors/adacalc.html>>Herv

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Hello!For me the key stuff is to get the speed of sound var. The Mach number is equal to Airspeed / Speed of Sound. I use IAS for my HUD

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