Tactics and Vectors 98/99
A Comparison Of Cross-Country Soaring Performance Of The Grob 103 Glider And The Monarch Butterfly Motorglider
Comparison of Cross-Country Soaring Performance Of The Grob 103 Glider And The Monarch Butterfly 'Motorglider'
The Grob 103 ACRO is a 2 - seat, fiberglass, glider considered to have gentle enough
flying characteristics to be used for training purposes but is also certified for
aerobatics. It has a wingspan of 17.5 m and a Gross Weight (glider fully loaded with 2
pilots, parachutes, ballast, etc. ) of 580 kg. The normal speed range is 42 - 92
kts (48 - 106 mph) with a stall speed of 40.5 kts (46.5 mph) and a top speed is 135 kts
(155 mph). The best L/D (glide ratio) is 37:1 at an airspeed of 57 kts (65.5 mph),
compared to about 3.6:1 for a Monarch butterfly at an airspeed of about 5 kts (6 mph).
However, minimum sinking rates, which determine the speed at which both 'aircraft' will
ascend in thermals are 0.70 m/s for both the glider and the butterfly. In other words, if
the butterfly and the glider are gliding at the same altitude, say 300 m above the ground,
and both enter the same thermal at the same time and begin to circle in 3 m/s lift, the
two will soar upward together at the same rate (3 -0.7 = 2.3 m/s). As is often the case in
aviation, rates of climb in strong thermals are largely independent of aerodynamic design.
Lets assume that both leave the thermal at 1500 m above the ground. Since butterfly and
glider entered the thermal at the same altitude, 300 m above the ground, each will have to
circle for about 9 minutes to climb the additional 1200 m. At this point, the superior
aerodynamics of the the glider finally makes a difference. Due to its greater size, higher
wing loading, more efficient wing design (fewer constraints), and lower coefficient of
drag (fewer constraints again). While the glider can speed off at 57 kts and glide for 37
x 1.5 km = 55.5 km in search of a thermal before the pilot is forced to land, the
departing butterfly, flying at only 5 kts, can glide just 3.6 x 1.5 km = 5.4 km before
running out of altitude and either landing, or switching to muscle power and continuing on
by flapping its wings.