KNOTS-MACH conversion

[Guest Fabrizio Berretta]

At sea level conversion from knots to mach is 1 mach = 659 knots.But how this conversion change with air density (altitude) or air pressure?What codes are used in FS9 autopilot to convert speed from knots to mach?

[n4gix 4,947]

The "conversion" is internal and automatic...Simply use the correct FS variable... ;)

[Guest Fabrizio Berretta]

I know this, but i need the conversion codes because i'm working to a self-made XML autothrottle for my 737-300 panel.

[Guest _FSAviator_]

<The question is based on several misconceptions.Knots means nothing until qualified and all such comparisons must be made within the International Standard Atmosphere as defined by the Treaty of Chicago. The velocity of sound in air then depends only on temperature. The reference temperature in the ISA is 15 Celsius or about 288 Kelvin. Under those conditions Mach 1 is about 661 KTAS.The adiabatic lapse rate in the ISA is about 1.98 degrees C or K per Flight Level within the troposphere until reaching the tropopause. In the stratosphere above the tropopause temperature is defined as constant.Consequently the velocity of sound in air declines by about 2.44 KTAS per Flight Level in the troposphere of the ISA, but within the stratosphere of the ISA Mach 1 is constant at about 573 KTAS.The data published in any 'Boys Big Book of Wonder Planes' should have been corrected to ISA for purposes of normalised comparison. The algorithm used by the author's of such books to convert Mach 1 is akin to;KTAS = 661 - (FLIGHT LEVEL * 0.2438)IF KTAS < 573 THENKTAS = 573ENDIFYou are not producing a normalised performance comparison chart or some text for a book. As has already been explained you do not need to convert anything. You need to retrieve the values calculated inside FS9 because Microsoft know how to do it properly and allow for all the other things that lead to the KIAS/KEAS being false due to local transonic heating whilst nevertheless calculating the correct KTAS v Mach for non standard adiabatic lapse rates defined by the code the consumer imported into the weather engine however variable, realistic or crazy. The weather model within FS9 has to cope with all non standard conversions. Books do not. You do not. An autothrottle requires the user to declare a (speed) control mode before dialling the target that FS9 will seek. Nothing needs to be converted. You will need to create a code module for each target mode using the variable names defined by Microsoft so that the current value retrieved from FS9 can be compared to the target value held only in your gauge, like for like. Your gauge only holds the target value for comparison, it cannot calculate current Mach. That is Microsoft

[n4gix 4,947]

>I know this, but i need the conversion codes because i'm>working to a self-made XML autothrottle for my 737-300 panel.You still do not need to "calculate" IAS <=> M...As "FSAviator" - bless his heart, ask him the time and he'll build you a watch :) - wrote, you only need to retrieve the data from FS variables, and then code your "autothrottle" such that it will automatically switch from IAS-Hold trend monitoring to MACH-Hold trend monitoring at some pre-determined point, such as 0.68M...Armed with those trend calculations, then you can use that information to implement the type of mode you wish to model: FLC, VNAV, etc.

[mgh 89]

The equation for the speed of sound in the International Standard Atmosphere is SQRT(Gamma * R * T) where:Gamma is the ratio of specific heats = 1.4R is the gas constant = 287.1 J/Kg/KT is the air temperature in degrees K = degrees C + 273.15 The following link has a tabulation of the International Standard Atmosphere and gives the variation of the speed of sound with altitude.http://www.aeromech.usyd.edu.au/aero/atmos/atmtab.htmlDon't worry about the "bunch of lawyers" in Montreal. The US (NACA) adopted the ICAO standard atmosphere in 1952 and the only changes I'm aware of were its extention in height from 20 km to 32 km in 1964, and then to 80 km in 1993. There've been no changes since 1993.

[Guest Fabrizio Berretta]

Many many thanks, but unfortunately FS2004 don' t follow this equation.For example at 10.000 feets FS2004 gives mach 0.45 for 250 knotsThe ISA equation gives mach 0.39 for 250 knotsI try a lot but i cannot understant FS2004 equation !!! Bill Gates help me !!I need to convert knots to mach for my XML autothrottle.FS2004 converts knots to mach (A:AUTOPILOT SPEED VAR) and (A:AUTOPILOT MACH VAR) only if default autothrottle is active.So to have the conversion in a self made autothrottle i need to activate default autothrottle in "speed" mode, change autothrottle mode to "mach" and deactivate default autothrottle. Everybody can understand that is better if i find an equation to do the conversion in my gauge.

[Roger Mazengarb 0]

G'day,>For example at 10.000 feets FS2004 gives mach 0.45 for 250>knotsWhere are you sourcing the 250 kts from. You MUST be using TAS (true airspeed) and not IAS (indicated airspeed)At 10,000 ft 250Kts IAS is approximately 300 Kts TAS which would account for the higher mach number of 0.45Are you using 250 kts TAS??>The ISA equation gives mach 0.39 for 250 knotsThis equation is using TASRoger

[Roger Mazengarb 0]

Checked up on sim using Hawker XP 800@ 10,000 ft ISA250 KIAS = 293 KTAS = M 0.46211 KIAS = 250 KTAS = M 0.39Make sure you use KTAS in your conversion and all should be well.

[n4gix 4,947]

>I need to convert knots to mach for my XML autothrottle.>FS2004 converts knots to mach (A:AUTOPILOT SPEED VAR) and>(A:AUTOPILOT MACH VAR) only if default autothrottle is>active.>So to have the conversion in a self made autothrottle i need>to activate default autothrottle in "speed" mode, change>autothrottle mode to "mach" and deactivate default>autothrottle. >Everybody can understand that is better if i find an equation>to do the conversion in my gauge.For Pete's sake! Use the correct variable and you won't have these issues!A:AIRSPEED MACH,machA:AIRSPEED INDICATED,knotsA:AIRSPEED TRUE,knotsThese variables will return your speed whether the autopilot is ON, OFF or not even installed... ;)If you don't have the Panels & Gauges SDK, you really, really need to get it... :)

[WarpD 824]

I have to toss in some disagreement with the posts saying 'use the correct variable'.First, the approach to switch to Mach when it's at .68... can actually cause a significant increase in airspeed at a lower altitude. As example, in FS, .78M ~= 270kts IAS at around FL360. So, the 'catch' to the autothrottle process is to KNOW when your KIAS is equal to a desired Mach value. Since the KIAS 'target value' can be variable (set by pilot), there's no tried and true 'static' approach that will work.FS does not calculate this switch-over point for you, they keep this data to themselves.However, it is possible to calculate the correct point of switchover. The only thing you have to do is convert KIAS to KTAS and then KTAS to Mach. When the desired KIAS->KTAS->Mach is equal to the actual airspeed in Mach... it's time for the autothrottle to monitor the speed via Mach and not KIAS.KTAS = KIAS+((0.15*KIAS)*alt/1000))KIAS is the desired IAS, as set by the pilot. Once you have KTAS, it's rather simple to convert it to the correct Mach value based on altitude.Here are three "C" routines used to convert KTAS to Mach and Mach to KTAS:FLOAT64 speed_of_sound(FLOAT64 tgt_altitude){ return sqrt(518.7f - (3.57f * (tgt_altitude/1000.0f))) * 29.06f;}FLOAT64 CalcMach(FLOAT64 tgt_tas, FLOAT64 tgt_altitude){ return tgt_tas/speed_of_sound(tgt_altitude);}FLOAT64 CalcTAS(FLOAT64 tgt_mach, FLOAT64 tgt_altitude){ return tgt_mach*speed_of_sound(tgt_altitude);}I use this process myself and it's fairly reliable within FS.

[taguilo 33]

""KTAS = KIAS+((0.15*KIAS)*alt/1000))""I understand the rule of thumb for IAS to TAS conversion isTAS = IAS plus 2% of IAS per 1000 ft Using your formula TAS = IAS +((0.02*KIAS)*alt/1000))Tom

[WarpD 824]

>""KTAS = KIAS+((0.15*KIAS)*alt/1000))"">>I understand the rule of thumb for IAS to TAS conversion is>TAS = IAS plus 2% of IAS per 1000 ft >>Using your formula >TAS = IAS +((0.02*KIAS)*alt/1000))>>Tom>>>>>>>>>That may be, however I coded it to match FS... not the rule of thumb. ;)