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Originally Posted by Chris Hibner
Someone earlier mentioned that a drawback to the fan (and pitot tube) is that it only measures speed relative to the air and not the ground. This is NOT a drawback. An airplane flies relative to its surrounding air - NOT the ground. The airplane does not care whatsoever what is happening with the ground unless the plane is within ~1 wingspan above the ground (i.e. only takeoffs and landings).
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When I mentioned groundspeed vs. airspeed, that's exactly the situation I had in mind; operating near obstacles. Before he'd more fully explained the concept, I was envisioning something like the IARC, where small autonomous aircraft are operated near buildings, even (in the case of helicopters) inside of buildings. In that sort of situation, you have to know where you are in absolute terms to follow a clear path, or otherwise, know the position of all obstacles at all times, and know how to take evasive action when you do see one, while still heading for your destination.
If it's just going to fly around, airspeed is quite sufficient; it's just that you need to also know wind speed and direction at all points, in order to follow a ground-based course using velocity as a reference.