A Comparison Of Cross-Country Soaring Performance Of The Grob 103 Glider And The Monarch Butterfly Motorglider
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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.
The Grob 103 has been designed for soaring cross-country. It climbs quickly in thermals
and can fly a good distance at a relatively high speeds before the pilot has to pause and
circle in another thermal to regain lost altitude. The high airspeed of the Grob
allows a pilot to make progress under a wide array of crosswind conditions and even to to
make reasonable progress up wind. These traits greatly expand the tactics available to
glider pilots. For example, glider pilots planning an out and return flight will usually
try to select a route that takes them up wind during the middle of the day, when thermals
are likely to be both abundant and strong, and to make the return leg a downwind dash
during the late afternoon when thermals are fading. As thermals begin to fade and
soaring conditions deteriorate, the pilot can make much better progress gliding in a tail
wind. Since thermals always drift downwind, even the time spent circling to gain, or even
to just hold on and maintain, altitude still results in steady progress towards the
airfield. In contrast, the monarch butterfly is a 'floater', optimised for keeping aloft
by simply staying within thermals and drifting downwind. The low airspeed of the monarch
butterflies limit their ability to compensate for crosswinds and almost eliminates
the possibility of making progress by soaring cross-country against a head wind. In short,
monarch butterflies are not well designed to travel continental distances to a defined
location by soaring cross-country. The fact that they manage to overcome their limitations
as they make their way to the overwintering sites in Mexico each year, accomplishing a
large part of the trip by soaring cross-country, makes their achievement all the more
remarkable.