Lets look to the I153

It looks like it was made in order to make space for pilot to see its front just like here

But What about in the aerodynamics study ?

Are Gulled wing design very important in order to make wings farther away from each other ?
Also what would happen if it was straight just like this ?


from the wiki

The I-153 was a development of the I-15, of which 671 were built. However:

The upper wing of the I-15 was unpopular with some pilots, as it was felt to restrict visibility, so Polikarpov's design bureau produced a revised version, again powered by the M-25, with a longer span un-gulled upper wing.2 This version, the I-15bis, commenced production in 1937,[7] a total of 2,408 I-15bis' being delivered by the time production finished in 1940.2

So the straight upper wing version was built to improve visibility, and many more units were produced than of the I15.

Structurally and aerodynamically, straight or gulled in this way makes little difference. Structurally, any bend is a weak spot, but the upper wing of biplanes usually had pylons in that position anyway to fix the upper wing to the fuselage. Aerodynamically: the bit missing in between the gull pylons reduces drag a tiny bit - the cross wires have phenominal drag though, and that fat little body also drags a lot of air with it.


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