An invention allows viewing of strange new worlds. One of the engineers alters a frequency on the equipment and suddenly they can pass through and back.
They discover other civilizations and trade with some of them for technology that doesn't exist in their "universe/reality/world" including flying technology.
They set up a company trading the "off world" tech.
Any help would be great and thanks in advance for any help.
I've read this series a couple of times.
https://storiesonline.net/universe/831/d-hopping
I don't think it matches the original description very much.
vaguely remember something with a similar premise. is the flight tech used on a retired russian submarine and used to go into space/low earth orbit? that's the only memory i have of a portal type story with flight.
-think there was a scene in same story where inventor is using a telescope at night to see through portal because adjustment had something to do with a crystal/diamond. and an alien species traded for knowledge of paint and/or makeup knowledge to allow him to marry. but this might be a second story creeping into the memory of flying russian sub.-
no clue on name of story either way but maybe the info can trigger a memory for someone else.
I really think that Dinsdale nailed it. The TransDimensional Portals series, by Porlock fits your criteria.