Expert: Marc Ebelini - 11/10/2008

Question
The early jet airliners tended to have clean wings, both on the top and underside except for the occasional fence and vortex generators ie 707, 727, 737 DC 8, DC 9.   However, by the 747, flap track fairings were added and are now standard additions to wings.  What changed from the early designs that allowed for these fairings with regards to drag and efficiency.  thanks very much.  Kevin

Answer
While I don't have any detailed information to answer your question because I never worked for Boeing or Douglas I do know this. When jet aircraft came into being they needed to add flap area to reduce landing speeds. Fowler type flaps where the answer. Each company kept adding to the flap extension that if you recall appeared to have the wings coming apart. As these extending arms became longer and turbulence  increased in those open areas. Any turbulence results in vibration that that cause metal fatigue. The amount of benefit the fairings have with respect to turbulence I don't know. Because fairings are also used to protest certain areas from flying debris on take off and landing.
Sorry I cannot supply aerodynamic details for your question. This data would have to come from the aircraft manufacturers engineering department. The ratio of added lift Vs drag would be in their data. Adding lift at the expense of drag is complex because it requires at what speed. What may be efficient at one speed may not be at another because of the square law involved.