Recommended Posts

When I am in cruise in the Flight 1 Citation Mustang up at FL320 I'm doing .585 Mach but on little navmap it says .572 Mach which does not match what the G1000 is showing

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Hmm. Just a wild &@($* guess, but...

 

Could it be that both readings are correct depending on what they are referencing? i.e. owing to the curve of the Earth, you are covering a larger radius at 32,000 feet than you would be if at ground level? A bit like having to go faster on the outside lane of a running track to keep up with someone running on the inside lane.

Share this post


Link to post
Share on other sites

Mach number depends essentially upon TAS and temperature. There are other factors but I don't think the curvature of the Earth is one of them :wink:

 

In a flight simulator, the temperature comes from the weather model.  Commonly, temperatures are calculated from the altitude using the International Standard Atmosphere (ISA).  A correction for local weather conditions - the "ISA Deviation" - should then be applied.

 

So depending on the weather model, the weather sources used, and indeed whether the ISA Deviation is applied at all, it is quite likely that two different programs will give slightly different results.

 

As a rough calculation, the figures you gave differ by about 2% in mach and hence correspond to a 4% difference in absolute temperature, or some 8 degrees C at FL320.

Share this post


Link to post
Share on other sites

Hi All,

I just checked my code and ran a few tests with the default Learjet and could not find any differences. I just use what SimConnect passes over to my code and do not apply any calculations there.

 

Maybe the Mustang is doing its own mach calculation overcome simulator limitations?

 

I found a lot of especially analogue instruments inaccurate. Sometimes even the tooltips do not match what the instrument shows. I have a Learjet here where the mach number sometimes differs by 0.05 which seems to be small but is a big difference if you want to cruise at e.g. 0.75 and VMO is 0.81. In that case I'd rather trust my own value than the needles in the model. :smile:

 

Alex

Edited by albar965

Share this post


Link to post
Share on other sites

Using this:

 

TAS        Alt        Delta-ISA           IAS         Mach

334         32000       0                  202         0.571

334         32000      -8                  206         0.582

 

seems to indicate that the Citation Mustang IAS and Mach use an ISA correction, while the mach value from FSX (and used by LittleNavMap) does not.  Would be interesting to know what the FSX IAS was!

Share this post


Link to post
Share on other sites

Very interesting.

 

I could calculate and display the corrected mach number too.

 

Alex

Share this post


Link to post
Share on other sites

After consulting the curve I used when I was a flight engineer, if you read 206 KIAS at FL320, you should get M.58! What kind of temperature indication do you have in the Citation, SAT, TAT?


With a supposed -41°C at FL320 and  M.58, I read 204 KIAS at this FL for a TAS of 320 KT on my Jeppesen computer. Don't forget FSX is just a game and you can't get the precision of real life.

Share this post


Link to post
Share on other sites

Hi Folks,

 

One thing I've wondered about - since we seem to have people in the know here - I've seen the Mach specification for various biz jets displayed as Mi - which I'm assuming means "Mach Indicated" - - - I thought Mach was already corrected for pressure and temp - - - so is there a different Mach also associated with an aircraft that differs from what's indicated - such as we do when using knots ???

 

Thanks...

 

Regards,

Scott

Share this post


Link to post
Share on other sites

Don't forget FSX is just a game and you can't get the precision of real life.

 

I would say the exact opposite is true. Because FSX is a game, calculations should 100% precise and repeatable.

Share this post


Link to post
Share on other sites

Hi Folks,

 

One thing I've wondered about - since we seem to have people in the know here - I've seen the Mach specification for various biz jets displayed as Mi - which I'm assuming means "Mach Indicated" - - - I thought Mach was already corrected for pressure and temp - - - so is there a different Mach also associated with an aircraft that differs from what's indicated - such as we do when using knots ???

 

Thanks...

 

Regards,

Scott

In real life, we only have one mach number, which is the mach number you read on your machmeter(or whatever indicator). The mach number is TAS/Speed of sound, which speed of sound depend only of air temperature.

 

I would say the exact opposite is true. Because FSX is a game, calculations should 100% precise and repeatable.

 

I flew jets like DC8 and DC10, it was part of my FE job to check performances, and with the indications I get from the plane instruments I always confirmed the perf given by the AFM. I tried the same with the some planes in FSX, Carenado amongst them, and I always found discrepancies. Anyway at this level, I mean for a game, it was not so bad :wink:

 

Cheers

 

Pat

  • Upvote 1

Share this post


Link to post
Share on other sites

@Scott:

    Indicated Mach is indeed corrected in that it relates to TAS rather than IAS, but is still subject to instrument and measurement errors.  In particular, the placement of the pitot/static probes with relation to the airflow over the fuselage and the formation of shockwaves can have a significant effect on  pressure measurements.  Indeed, the same is true for IAS and TAS, which are both "indicated" values in the sense that "they are what the indicator says they are".  The "true" values are called "calibrated" values, and are used in the design phase and carefully measured during flight test, but are not used in normal operations.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this