No, you can’t cheat the physics: If the flute was “in-tune” to begin with (a subject in and of itself), with the slide extended at a certain distance for the person who tuned it, then the scale will be “out-of-tune” for the same player as they move the slide. However, IF we have a decent ear, then we compensate and blow/lip differently enough to get the scale back in tune, whether we realize it or not. Alternately, some people simply don’t have a good enough ear to notice the out of tune scale, at least not until it becomes egregious. More likely for many people, there is a combination of both going on - we don’t hear the scale differences when they are a little off, because we don’t have “perfect pitch” or we simply aren’t LISTENING closely, but we unconsciously lip/blow the flute back into tune as we are playing once we have moved the slide enough that our ear can hear things getting (rather a lot) out of tune.
Truth is, many ITM flute players have a lousy ear for tuning, as noted by the number who play sharp in the second register and don’t adjust to have a really good cross fingered cnat on keyless flutes, for example. So these players will often say a flute is in tune (when it isn’t), but really can’t hear the problem, and hence can’t correct it. The opposite will be the players who say a flute is out of tune, and it may, or may not be, but they lack the technique to bring it into tune, even though they can hear it is out of tune. But then their lack of ability to bring the flute into tune signifies in itself that they likely lack the proper technique to play a perfectly good flute in tune to begin with…
The good player however hears and compensates, automatically playing the flute into tune as necessary, and it is more or less unconscious, at least with a flute they have become used to. The great player will hear AND be very conscious of exactly what is going on with the scale when they pick up a flute, particularly if it is different from what they normally play.
But we also have to ask what is “in-tune” - not just what pitch, but what tuning was intended - Equal, Just, etc. Also, what venting (on keyed flutes) was intended in order to achieve an in-tune scale? Etc., etc., different subject though.
But yes, those old flutes are out of tune at A=440 if they were tuned for a different pitch. So, either your ear isn’t good enough to hear that the scale is out of tune, or it is being blown/lipped back into tune by the player, whether that’s you or someone else.
But then that’s the nature of the flute isn’t it?
Loren