Originally Posted by jxk
If best glide speed means that the glide range is 5 miles from 2500 ft then what is impact of increasing/decreasing this speed by say 10 knots???
The answer to your question depends entirely on a) the aerodynamic properties of the glider, and; b) the speed range over which the 10kt difference is applied.
The rate of descent of a glider does not increase uniformly with speed (Because drag increases exponentially). At very slow airspeeds (Just above the stall) the rate of descent is quite high. As airspeed increases, the rate of descent reduces to a minimum point known as the 'minimum sink speed'. After that, as airspeed increases further, the rate of descent increases (almost)exponentially.
So, your rate of descent at 5kts below the minimum sink speed is probably about the same as it is at 5kts above the minimum sink speed. In other words, increasing speed by 10kts has no effect whatsoever on your range. At higher speeds, the 10kt difference will be much greater, especially in older gliders whose performance at extremities is poor. You might expect your rate of descent to increase by 1m/s when going between 80 and 90kts for example.
The easy answer to your question is that in a typical glider at a typical cruising speed, an increase of 10 kts will result in a small reduction in range - perhaps between 5 to 10%.
Incidentally, 5 miles from 2500ft is pretty poor. The average glider will do 40:1 so that's about 20 miles from 2500ft. Unless you factor in circuit and landing (Assuming you won't glide straight in to your intended landing site) in which case 10 miles is probably reasonable. In still air of course.