For those dealing with the CAS vs IAS issue

[DHenriques_]

The following link I have used many times when dealing with this issue. I hope it proves useful here.
Dudley Henriques


https://www.faasafety.gov/files/gslac/l ... randed.pdf

[Oliver Branaschky]

Thanks, Dudley, this really is most helpful.


Viele Grüße/Best regards
Oliver

Oliver Branaschky

[Tug002]

That was a great and very informing read. Thanks Dudley

Keep smiling
Tug

[taildraggin68]

Need to sticky this Dudley...good read

[Nick - A2A]

Yes, thank you for the link Dudley. That was indeed a useful little document.

I must admit, the whole CAS vs IAS discussion has been a bit of a revelation for me. Coming at this without any experience of flying for real, it's very easy to ignore the fact that there's likely to be a difference between the speed your ASI shows and your actual airspeed as you slow down in the landing config. This is because in FSX/P3D there isn't! Well not until the Accu-Sim ASI error modelling was introduced anyway...

What really struck me with the C172R was that as we approach our touchdown speed during the landing flare—hopefully somewhere close to that VS0 of 33 KIAS—the aircraft would still actually be travelling at around 47 knots. That's a big old difference!

I'm guessing that before this code was introduced, it was necessary to introduce a sort of 'fudge factor' with the flight model to try and get the indicated airspeeds showing something like correctly with respect to the coloured arcs and stalling speeds. This update seem to really free up the potential for a FDE which allows the aircraft to travel at the correct speed (so that appropriate landing distances and the 'sensation of speed' are preserved) and with an ASI displaying the readings it would in reality too.

I was also interested to note from Dudley's link that the MPH ASI in a 1960s Cherokee that many here will be familiar with is actually marked in CAS. Therefore I'm guessing that the readings we see on it will tend to agree more closely with that FSX ctrl+z readout of "ASI". Not too sure about the pitot head in the PA-28, but maybe it's less prone to AoA error than the pitot tube on the C172 as well? In my experience with the Cherokee in FSX, it's very forgiving in terms of published stall speeds anyway, and I'd guess this is a result of the benign handling qualities of that constant-chord wing?

Anyway, apologies for rambling on about this so much. I realise that CAS and static port position error generally doesn't make too much difference at cruising speeds, but it's the whole low speed, high AoA thing which I find so interesting!

Cheers,
Nick

P.S. I agree Taildraggin'. Maybe Dudley would consider moving this topic to the Flight Academy forum where it won't tend to disappear too quickly.