So perusing AAF documents on ww2aircraft performance.com I cannot find any mention of correcting for compress-ability in any of the documents for the AAF on the P-51. I am curious if anyone has any P-51D testing that indicates compressability was corrected, or if anyone knows whether or not it was considered standard practice to correct for compression. I have a hard time imagining all the official documents of the AAF dont correct for this, as they would be essentially useless if they didnt. Secondly: Regarding 51 tests the critical altitude changes. 25,000-443 26,000 442 24500 440 24500 437 So im assuming that crit altitude is affected by weather or something, but the top speed doesnt seem to change much, in fact the curves seem to be proportional regardless of the FTH change. Is this because whatever temperature or humidity difference affected the results also resulted in proportional conditions at the different FTH? IE: thinner air at 26k resulted in less ram so FTH moved to 25k, but air at 25k was also thinner due to weather and resulted in higher speed at altitude?