 aspect air-helium simul hyperson approxim aspect air-helium simul hyperson approxim illustr differ expect hyperson wind tunnel emploi air emploi helium test medium imperfect-ga effect consid compil present simpl express present demonstr possibl simul air helium test transform helium data equival air data nonvisc viscou simul consid case method gener form express simul deriv applic ideal gase ratio specif heat 