I am a great believer in answering the questions posed.
Originally Posted by p tango
Can anybody here explain why wings with high aspect ratio have reduced rate of roll as compared to with low aspect ratio.
This question is probably too generic to admit of much of a contentful answer. The straight answer is: because they are designed that way.
But why are they designed that way? Structural reasons. Building something that is long and thin and light versus building something that is short and light.
Say you had a 27m-long half-wing (that is, span of 54m+fuselage-width).** That is not a lot compared with some aircraft flying. And say you wanted to achieve the 760°/second roll rate mentioned by bearfoil. Let's just take 720°/s, because that is twice around full circle. Let's rotate the airplane about the longitudinal fuselage axis so the tips are describing a circle, radius 7.5m + 1/2 fuselage width. The tips would be going through two circumference lengths, about 353m, in 1 second. The speed of sound at 15°C is about 340 m/s. So you would be wanting to accelerate your wing tips from 0 to over the speed of sound and back to zero again in 1 second.
Guess what? People don't try to do that. They want to roll fast, they generally stick to stubby.
Originally Posted by p tango
What are the limiting factors for increasing the aspect ratio of an aerofoil?
If you are talking just about designing aircraft with no context, then mostly structural. But if you are talking transport aircraft, they have to fit into gates at airports, and that limits the wing span, which again limits the achievable aspect ratio.
** Edit Note: I originally had a (much!) lower figure here, which bearfoil points out (below) is a mistake.
PBL