Watch the Flanger online guitar lesson by Jeff McErlain from Guitar Effects Survival Guide

The flanger is one of the more distinct effects out there, known for its jet-like sweeping sounds, it can also be very subtle as David Gilmour and Andy Summers have shown. It is similar to a chorus pedal in that it is a modulation time based effect. The flanger delays a copy of the original signal and mixes it in with the dry signal. The displacement of the time causes the swooshing effect. This can be done in multiple stages to produce a more dramatic flanged effect. “Originally flanging was done with tape machines" as explained here in a quote from Wikipedia. "The name "flanging" comes from the original method of creation.

Originally, a signal would be recorded to two tape machines simultaneously. The playback-head output from these two recorders was then mixed together onto a third recorder. In this form, minute differences in the motor speeds of each machine would result in a phasing effect when the signals were combined. The "flange" effect originated when an engineer would literally put a finger on the flange, or rim of one of the tape reels so that the machine was slowed down, slipping out of sync by tiny degrees. A listener would hear a "drainpipe" sweeping effect as shifting sum-and-difference harmonics were created. When the operator removed his finger the tape sped up again, making the effect sweep back in the other direction.” Famous tunes using flange effects are “Unchained” by Van Halen, “Spirit of Radio” by Rush and “Bold as Love” by Jimi Hendrix. The flange on “Bold as Love” is credited as being the first recorded use of the effect in stereo.