I'm a paragliding pilot. At school we learn that the trim speed of a paraglider is about 38km/h airspeed, the accelerated airspeed is about 45km/h and the minimum airspeed is 22km/h - being inside the weight range of the wing.
If I fly with a headwind of 25km/h, my groundspeed should be: 38-25=13km/h.
Well, from my flying experience (1200 flights in total) with that kind of headwind I never went forward +13km/h groundspeed but much less, near the minimum relative speed (22km/h).
Why there's a difference between the calculated groundspeed 13km/h and the actual groundspeed?