Originally Posted by downwind
Quick question up to what speed or mach number is a straight able to operate efficiently. (ie wings on a dash 8, saab 340, king air 200 etc...).
Your question is too vague, really.
Define "efficient" ....
The aircraft category you mention runs out of "puff" way before you run into Mach effects.
In that case a straight wing is the simplest and most efficient solution.
The only exception is when for some reason you want to get rid of the horizontal tail, with such extreme examples as delta-plane hang-gliders and microlights, the B2, the Me163, the Vulcan or the Rutan Vari-Eze. Swept-back flying wings, and tailless deltas can be made stable in pitch, unlike a straight wing.
At what point you run into "efficiency" problems with a straight wing depends mostly on the wing profile and the wing thickness.
Strictly off the top of my head, without looking up references, I'd say with a straight wing, a modern wing profile and about 10 to 12% thickness (your examples), you will find the trouble starts at about M0.65, where the airflow over the wing will go locally supersonic (Mcrit) causing increased drag.
Depends on the wing and the aircraft.... it would appear a Spitfire has been dived to Mach 0.92, although its critical Mach number was certainly less.
So when you run into "Mach" problems, you have two solutions.
Make your straight wing thinner. Oops. Less space for fuel, structurally heavier and more difficult to manufacture.
Or use a swept-back wing, which "looks like a thinner wing to the airflow", but isn't in reality. So you can still house the fuel.
Swept-back wing have less ideal flying qualities than straight wings, but we've learned to deal with that for a very long time.
So.... a Fokker F-27 has a straight wing. Anything else would have been a joke.
A Boeing 737 has a swept wing. Again, today, anything else would have been a joke.
Beyond Mach 1?
The F-104 and Concorde were two extremes. Yet the currently proposed SSBJs (supersonic business jets) are still looking at those extremes.
So the jury is still out.
CJ