A flanger is a form of modulated delay in which two audio signals are combined. The second signal is slightly delayed which causes the combined signal produces a “swirling“ effect. In electronic flanging, the amount of delay is modulated by a low-frequency oscillator, which causes random shifts and sweeps in the frequency spectrum of the original signal, thus causing the "flanging" effect.
Because the copies of audio are identical, interference in the signal will occur in the form of harmonically-related notches (notch filters). Due to its shape, this is sometimes called a comb filter. 
The name “flanging” came about in the 60s. The effect was derived by having a song recorded on two tape machines while slowing one of the machines down by placing your finger on the tape flange. The effect was used extensively by artist such as The Beatles and Jimi Hendrix. 
Comparison of common modulated effects
(this section is copied from another article)
The similarities and differences between the common modulated effects (flanger, chorus, phaser) are summarized below:
- Flanger has the strongest and most noticeable effect. Chorus is more subtle. Phasers are subtler still.
- Flanger and chorus effects create delayed copies of the original signal and mix them back. Phasers use a series of all-pass filters to create phase shifts at various frequencies.
- Flanger uses shorter delay times (<7ms) than chorus (<20ms).
- Phasing can be used in a similar way to chorus, but, whereas chorus creates the impression of two slightly detuned instruments playing the same part, phasing sounds more like a single sound source being filtered. 
- Chorus (typically) is not fed back, whereas flangers often are.
- Phasing and flanging both produce comb filters. However, in flangers, the individual notches are linearly spaced. In phasers, they are uneven. 
- Flangers sound more pronounced and natural than phasers (like the jet plane "whoosh" effect), whereas phasers tend to sound more subtle and "otherworldly". 
- Wet/Dry - the balance between the processed and dry signals
- Delay Time - the delay time of the parallel signal
- LFO Modulation Amount - the amount by which the delay time should be modulated according to the LFO
- LFO Rate - the rate at which the LFO modulates the delay time parameter
- Feedback - controls the depth of the 'comb filtering' produced when the delayed signal is added back to itself
- Flanger generally relies on an equal balance of dry and wet signal to achieve the strongest effect, so it is easier when working with plug-ins to adjust the wet/dry balance using the plug-in controls, rather than adding the wet only signal via a send/return loop.. 
- Most of the time, low modulation depths tend to work well for faster LFO speeds (often also referred to as the rate), while deeper modulation works better at slower modulation rates. 
- A flanger can be used to make a mono track stereo by introducing a Haas effect. This technique is useful for 'faking' mono instruments to sound stereo and is widely used in the mixing stages. 
- Subtle flanging can give life to some instruments. For example, flanging a bass can add weight or depth to it. Flanging strings can make them sound thicker without destroying the intonation and tuning of the string ensemble. 
- “Barber Pole” or "Infinite” flanging is a sonic illusion similar to the Shepard tone effect. It is equivalent to an auditory “barber pole” in that the sweep of the flanged sound seems to move in only one direction (“up” or “down”) infinitely, instead of sweeping back-and-forth. 
- https://splice.com/blog/effects-101-phasers-chorus-flangers/ Cite error: Invalid
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