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A Comparison Of Cross-Country Soaring Performance Of The Grob 103 Glider And The Monarch Butterfly Motorglider

Glider and me
David Gibo and the Grob 103 glider after a two hour flight
Note fair-weather cumulus clouds in background marking tops of thermals

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.