The document is really clear. I suggest that you re-read Background section. Pages 2 and 3 are essential pieces and the plots on pages 5 & 6 are the result of that math.
I can do some math here if math helps you at all.
L = rho/2*V^2*Cl*S
where L is lift, rho is density of the fluid, V is velocity of the fluid, Cl is coefficient of lift, and S is foil plan form area. We can solve for Cl required at any given airspeed and Lift:
Cl = 2*L/(rho*V^2*S)
If L and S and rho stay the same, but we change V, the Cl changes. Double the airspeed and the Cl needed drops to a quarter of what it was...
Cdi = 0.318 * Cl^2/AR
Cdi is induced drag coefficient, and AR is Aspect Ratio. 0.318 has come out of the analysis of wing lifting theory, and, as nearly as we can tell from a lot of wind tunnel work, it fits really well.
Hmm, let's stick the earlier form of Cl in the Cdi equation:
Cdi = 0.202*(L/(rho*S))^2*(1/V^4)
so, all other stuff being the same, Cdi goes with 1/V^4. Yep, Induced drag coefficient goes down rapidly with speed.
Now to get Induced Drag from all this
Di = rho/2*V^2*Cdi*S
Di = rho/2*V^2*S*0.202*(L/(rho*S))^2*(1/V^4)
Simplifying terms
Di = 0.101*L^2/(rho*S*V^2)
So, if lift and rho and S remain constant, then induced drag goes with the inverse of V^2, just like it shows on the plots on pages 5 & 6.
Low AR wings thus pay little drag penalty at speeds substantially above the min total drag point. The ultimate on this is an airplane designed for speed between two points at low altitudes. Little time in climb, lots of airspeed, who needs aspect ratio. Some folks will say "oh, IF1 airplanes", but I say they spend a lot of time in a steeply banked turn on the course, so lift required is more like 2 times airplane weight, so, induced drag is bigger for them then you might think. On the other hand airplanes that climb at airspeeds near their min total drag point can benefit substantially if they climb to high altitudes or otherwise spend large amounts of their flight time at low indicated speeds. Think airliners and sailplanes here... Yes many airliners are big fractions of Mach, but air density (rho) is much lower at their preferred altitudes, and thus their q (rho/2*V^2) and indicated airspeed that goes with it are much lower than you might think. So, airliners and sailplanes do spend much of their time in places where induced drag matters a bunch.
Billski