> That makes absolutely no difference at all.

That would be very surprising if it were true, given the very large effect density altitude has on airplane performance (engine and wing).

It's gonna make some difference, but air resistance goes with square of speed and the change in density due to temperature isn't that large.

It’s about -3% change in drag force per +10°C change in temp.

Which is like the same as 60 mph vs 59 mph.

In drag force, yes, but in this case, you are taking the 3% penalty and aren't getting the partially offsetting 1.7% increase in distance per unit time that 60 gives you over 59.

Note that it's 3% per 10°C delta so, for a 40°C swing (say 30°C/86°F summer to -10°C/14°F winter), that's a 12% difference in aero drag force.

Which is basically nothing.

Opening your window makes far more of a difference.

Over what sort of range of height and pressure?

It's just about observable with very light and very aerodynamic aircraft at ground level if you can get them cold enough.

It's well beyond "just about observable". Here's a sample landing distance performance chart: https://www.ascentgroundschool.com/~ascentgr/images/ct80802e...

At -10°C/14°F, with no other adjustments, in the hands of a test pilot, that aircraft will have a ground roll of around 1000' and clear a 50' obstacle and come to a stop in around 1500'.

At 30°C/86°F, with no other adjustments, in the hands of a test pilot, that aircraft will have a ground roll of around 1150' and clear a 50' obstacle and come to a stop in around 1750'.

Flying in and out of 2000'-2500' strips with trees, it's impossible to not notice the performance difference with temperature. Perhaps the people who can't notice those differences don't fly into anything other than very long runways.