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enter image description here

What’s the purpose of this design, and why only on one side?

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    $\begingroup$ You should add a reference to the original picture. I'm quite sure this is a testbed and its goal is to test the extra engine in flight (i.e. this engine is not here to provide thrust) $\endgroup$
    – Manu H
    Commented Mar 4, 2020 at 7:26
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    $\begingroup$ A good question would have been "why do they add a pylon instead of replacing one existing engine with the tested one?" $\endgroup$
    – Manu H
    Commented Mar 4, 2020 at 11:54
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    $\begingroup$ @ManuH True, but that question can only be asked once you know this 747 is a test bed. The additional engine might as well provide extra thrust or just get carried around, like Quantas did. $\endgroup$
    – PerlDuck
    Commented Mar 4, 2020 at 14:12

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It's an engine testbed aircraft. In this particular picture, the tested engine is probably one of the PW1X00G series (mainly used by Airbus A220, Airbus A320neo, Embraer E-Jets E2, Irkut MC-21, Mitsubishi SpaceJet) and the aircraft is one of two Boeing 747SP testbed aircraft owned by PW.

Engine testbed aircraft test the engines in-flight, with parameters and conditions that can not be tested in a static test on the ground (for example, low temperatures and non-static thrust).

Engine testbed aircraft often end up in unusual configurations. Other examples include (but not limited to):

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