I was recently helping my son in law get his flat ready to sell, and I
noticed that the Critall metal windows have unusual hinges, like in the
photo. These are similar to parliament hinges.

https://www.deltaglazing.co.uk/wp-co...ow-repairs.jpg

The effect is that, with the window open there's a gap at the hinge side
that's wide enough to get your arm through for cleaning the outside of
the window.

That's such a simple idea, and so easy to implement that I'm surprised
it's not more commonplace.