Think of air speed as simply a performance statistic of the craft. The powered parachute flies through the air at 26 mph, regardless of whether the throttle is at full setting or cruise. The powered parachute does not increase in air speed with an increase in power because as power is increased the angle of attack is also increased, which produces more lift and drag coinciding with the increase in thrust. Therefore, the powered parachute climbs but does not fly at a greater air speed. The air speed must be used in conjunction with the wind speed in order to compute the ground speed.
Ground speed is the speed which the craft is actually moving ouer the ground.
It varies with the wind speed and the direction the powered parachute is facing in respect to the wind. Going into a 10 mph head wind produces a ground speed of 26 mph minus 10 mph or 16 mph ground speed. A 10 mph tail wind produces ground speed of 26 mph plus 10 mph or 36 mph ground speed. This must be considered when flying cross country. For instance, you cannot fly to a given point with a tail wind using half your gas supply and expect to get back on the other half facing into a head wind.