A periphrastic construction with εἰμί and an aorist participle characterises the state of affairs as a condition.
The poets use ἔσομαι + aor. part. as a periphrasis for the future perfect.
This periphrasis is very common in Koine, too, usually with an imperfect: ἦν + aor. part. This construction is not necessarily a continuation of the classical periphrasis (Aerts 1965: 76).
This is the least frequent periphrasis with εἰμί.
There is some debate about the force of εἰμί + aor. par. Most researchers equate this periphrasis with the perfect, but Willem J. Aerts considers this improbable due to its aspect. Thus he does not define the state of affairs as a condition in this case, but as 'a factual meaning that establishes action'.