Simviation Forums

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?

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..

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.

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

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.

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.

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.