With airplanes, it’s almost always about weight. Building the plane right and putting in the air is simple enough (given proper engineering and piloting). *Keeping* it there as efficiently as possible is the bugaboo. Ultimately, it’s the cost of fuel (which you need more of for more weight) — not to mention the added effort for the pilot keeping a behemoth (weight-wise, in this case) in the sky.
Latest Answers