Interesting. I get in the case of the Spit MKI DH 2 Pitch and Spit MKIA CSP a 20MPH IAS diff at FTH. With the DH 2 Pitch being 20MPH IAS faster with Coarse pitch +6lbs. Compared with real life this is a little bit out as they should be a lot closer than this. Converting to TAS results in my test to a 16MPH TAS differrential
TAS in figures below.
