speed discrepancy

[RAFAIR100]

Currently, I'm flying at 190 mph TAS (160 mph IAS) at 12,000 feet in zero wind conditions.     According to my rapid conversion, I ought to be doing 165 K.      Yet my GPS is reading 143 K.     Are my mathematics out or is there really a discrepancy?

[Brett_Henderson]

Your math is correct..  ( 1:1.15)  (kts:mph), so I'd guess that your airpseed indicator is off a bit.


OR (less likely)...

I always fly real-weather, so I'm not sure... but I do remember having some wind aloft, even if I set user-defined weather to no wind.  Hit ctrl+Z when your at 12,000 and make sure there's no wind..

[DaveSims]

Altitude also has an affect on airspeed versus groundspeed.  The air gets thinner as you get higher, which means your airspeed will be lower than your actual groundspeed.

[beaky]

It's more likely your TAS calculation is incorrect... where did you derive that TAS?

[RAFAIR100]

Thanks for the replies.       Brett, at least, confirms my mph/knots conversion.     I have used 76 mph = 66 knots for the past 50 years and it seems to have worked out alright.        I take your point Davysims but I believe that you are thinking of Indicated Air Speed.        In nil wind conditions, at any altitude, True Air Speed= Ground Speed.     In terms of the IAS/TAS calculation, I didn't calculate because I didn't have a calculator handy..   I read off the airspeed gauge values with the realism settings at both TAS and IAS.        Still, the values don't look unreasonable            Thanks again.

[Brett_Henderson]

What plane and gauge is it ?

The gauge get's it airspeed information accurately from the sim (obviously) and if it's a "dial" gauge, it's a couple of bitmaps (background.. ie.. the scale arcs and numbers  &  needle).. An XML code decides where the needle will be displayed on the arc (NOT calculated to point at a specific number ON the arc)....   SO.. if the background isn't exactly perfect (most aren't),, you cannot rely on that gauge for perfectly accurate readings.

I've had to re-do the background bitmap on more than one gauge... Airspeed, manifild pressure.. fuel-flow.. you name it.

[RAFAIR100]

Thanks Brett.    I think I'll  give up all thoughts of flight simulator accuracy and just fly the thing for fun.      Anyway, my 400+ K  Anson with two supercharged 12 cylinder in-line engines driving four bladed propellors is a lot more lively than the real thing ever was.