Ground effect is all about reduced downwash and (lift) induced drag:
Prandtls lifting line theory: CDi (induced drag) reduces to ~ 50% at 10% of the wingspan gnd clearance.
Weiselberger’s theory (graph) for a 747-400 (213’ wingspan), increased by 12’ for the winglets:
https://qf32.files.wordpress.com/2014/01/induceddrag_vs_alt.jpg
CDi is down to 50% in the flare! So an aircraft flared in ground effect will have significantly reduced total drag so will cruise to the end and beyond the end of the runway
The reduction in CDi explains why:
- Do not allow the airplane to float, fly the airplane onto the runway.
- Do not attempt to extend the flare by increasing pitch attitude in an attempt to achieve a perfectly smooth touchdown.
- Do not attempt to hold the nose wheel off the runway.