Definition
A number, always less than 1.0, that describes how closely a wing's lift distribution matches the ideal elliptical pattern. It appears in the induced drag equation as a correction factor: a wing with a span efficiency factor of 1.0 would produce the theoretical minimum induced drag for its span and lift, while real wings produce more drag because their span efficiency factor is lower.
Plain English
A score between 0 and 1 that tells you how well a wing spreads lift across its length. The closer to 1, the less drag the wing makes while producing lift.
Context Anchor
Seen in aerodynamics, aircraft performance, and wing-design discussions, especially when explaining induced drag.
Derivation
From 'span' (the distance from wingtip to wingtip) and 'efficiency' (how much useful output you get for a given input). The factor measures how efficiently a given wingspan is being used to produce lift with minimum drag.
Why Pilots Care
Directly affects fuel burn, climb rate, and range by influencing how much extra drag is created while producing lift.
Grounding Statement
When a wing makes lift, air tends to curl around the wingtips; a better span efficiency factor means the wing loses less energy to that tip-related airflow.
Intuition Check
Span efficiency factor is not the same thing as wingspan. Wingspan is a physical length; span efficiency factor is a calculation number that shows how efficiently that span is being used.
Example Sentence 1
Adding winglets raised the aircraft's span efficiency factor, cutting induced drag during climb.
Example Sentence 2
Winglets can raise the span efficiency factor and improve cruise performance on the same airframe.