Forming Imperative

"My" rules for forming Imperative, taken from here and there, are rather simple and up to now without an exception:
1) Stem in й (читай-, делай-): nothing is added -> читай, делай
2) The rest of cases
2.1) 1st. p. s. acute (писа-, пишý): и is added. пис(а)и -> пиши
and the previous vocal is suppresed
2.2) id NOT acute (ответи- отвéчу): ь is added. ответ(и)ь ->ответь
and the previous vocal is suppresed
unless accumulation of consonnts is present:
пoмни- пóмню where и is added. пoмн(и)и -> пóмни.

But I found now вылететь II, вылечу whose imperative should be according to what is previously said: вылеть, but I see it everywhere вылети. No mention to an irregularity is found nowhere. I suppose that simply it is like this and can be applied the Spanish proverb: "There are lentils, if you want, eat them; otherwise, leave them" ("Son lentejas, si quieres las comes y si no las dejas"). Is there any additional, and unknown by me, rule that makes this behaviour of вылететь normal or unless not unique? Thank you.

Same for any verb вы-prefixed : выйди, выговори, высмотри, вытерпи... In these cases you must form the imperative as you would without the prefix, i.e. you look at the stress in the original word. When the stress does not stand on the ending in the original word I think the use is floating. What would you say, Russian fellows? Вымучи или вымучь?

Thank you. I copy and paste your answer. Your answer fits perfectly with the case: летéть, лечý, лети.