What is the average wingspan of a aeroplane?

Answer 1

The wingspans of airplanes varies according to the amount of lifting area needed to sustain the weight of the fuselage, engine, fuel, cargo and passengers.  There is no "average".

Light sailplanes, for instance have very long wingspans. High performance aerobatic aircraft have short wings. A high aspect ratio defines an airplane design with long, narrow wings and a low aspect ratio defines an airplane design for short wings.  Airplanes with long wings have a low roll-rate when aileron pressure is applied.  Airplanes with short wings have a high roll rate and generally require greater speed.  Another component of the aspect ratio is the difference between wing chord and wing length.  The chord of the wing is the distance from the leading edge to the trailing edge.  This ratio is also called the planform.

If you were to design an airplane with the intent of carrying heavy cargo, you would probably want to start with a high aspect ratio wing design.