Hi, I've got a question and I thought this would be the place to ask it.

 

Heres the setup. I've been flying the Carenado 172N using Airhauler. Airhauler uses the reference airspeeds in the aircraft.cfg file to trip an overspeed warning. While flying the 172 at an indicated airspeed of 110 with a 25kt tailwind I get the overspeed warning even though I'm not putting the airframe under stress above 110kts. In this situation I can be showing 110 indicated and 135 or so ground speed and get the warning.

 

Heres the question. Is there anyway to list the reference airspeeds in the cfg file as indicated rather than true. It seems like the true is read off of ground speed which isn't correct at all. True airspeed takes indicated airspeed and adjusts it for tempature, it has nothing to do with ground speed.

 

If this question would be better placed somewhere else please bump it over there.

Thanks Mark

No, there isn't... and this:

True airspeed takes indicated airspeed and adjusts it for tempature, it has nothing to do with ground speed.

is only sorta correct... you can calculate true airspeed from indicated given temperature and a known calibration error table to determine first CAS and then true airspeed... but ground speed and true airspeed are the same, give or take a bit of wind. :wink:

Oh noes, I thought I got it (the speeds) and now i am confused.

For example: You fly at 15°C and 29.92Hg. Your IAS is 100kt and you have 50kt tailwind. So your TAS is 100kt, too (due to temp and pressure) and your groundspeed is 150 (due to 50 kt tailwind).

Is that correct?

 

Regards

Hirschi

Almost but not quite. IAS only approximates TAS because of instrument and position errors - it is the Indicated Air Speed. Ideally the two speeds would be the same at sea level but for those errors.

ah, ok. IAS is how he aircraft feels the speed depending on air temperature and pressure resulting in an certain air density, in't it? But for what do I need TAS? IAS tells me, how my airplane sees the speed (if i am correct with that) and for flightplanning I should use groundspeed because ranges between 2 points are measured at the ground. I know, this must sound stupid but Mark's question completely confused me.

If I understand Mark correctly and with what I thought to know, the overspeed alert is false because his IAS is well inside the limits and only IAS off limits should affect the airplane.

Another approach... 100kt headwind and IAS 100kt should result in 0 GS but i would fly. Regarding airhauler i would get a stall warning or a question from the dispatcher when I intend to deliver because he doesn't see me moving

 

Regards

Hirschi

And someone long ago told me that there are two different speeds used to determine if your aircraft is in danger. Indicated airspeed is the obvious one, which is used for aerodynamic overspeed (i.e. the air ripping something off). But true airspeed is the other one because if you hit some unexpected turbulence and your plane shifts suddenly, you are actually travelling at the true airspeed and that is what will provide the inertia to bend your airplane. So both values are important.

 

Hope this helps,

Thanks Tom and yes, it helps. I thought it was quite easy with the three speeds but it seems more complicated in detailed view. But every day without learning something new is a wasted day

 

Regards

Hirschi