How can best glide speed be lower than best rate of climb speed? I was looking up V-speeds for C172. I don't have easy access to an official copy of the POH, but online I found:


$V_y$, best rate of climb speed (at sea level): 76 KIAS (p22)
$V_{glide}$, maximum glide speed: 65 kts (p15).


But I always thought that $V_y$ is the speed for highest L/D, which should give both highest rate of climb and lowest rate of descent, and that longest glide is achieved by flying slightly faster than that, because drag grows with square of speed, and therefore it initially grows slower than speed above the minimum drag point.

So why is the $V_{glide}$ quoted lower than $V_y$?
The best glide speed is the speed where tangent line from the origin (zero horizontal and vertical speed) touches power curve (for unpowered airplane).

Best rate of the climb is the highest point of the power curve.

When we start with unpowered airplane, the direct equivalent of best rate of climb is lowest rate of sink. The speed for lowest sink will be clearly lower than the best glide speed.

If we would assume that aircraft engine is a magical device, which simply adds some fixed amount of mechanical energy to the airplane regardless of what, then the "powered" power curve would be simply the "unpowered" one shifted up in the graph. Under such assumptions the speed for best climb would be the same as the speed for minimum sink and in both cases lower than best glide speed.



Nevertheless mentioned Cessna converts fuel to mechanical energy through not-so-magical fixed pitch propeller which pushes some amount of air around. Such propeller would be typically optimized for highest efficiency at the cruise speed, so with decreasing speed the amount of available mechanical energy dereases. Which results in changed shape of power curve and maximum point moves towards higher speeds compared to the "unpowered" power curve.



This shift can be high enough that Vy ends up higher than Vg.

The Vy is always higher than the speed for the best climb angle, because these two speeds are found on the same (max power) power curve. For the same reason best glide speed will be always higher than speed for minimum sink. But speeds for best rate of climb and for the least sink does not need to be the same because of effects of real propeller.

For related images and more information, have a look at great online text See How It Flies.