Adjectives that end in -able and -ible like, permissible and allowable are nominals. Words that end in -ed like, permitted and allowed are verbals. Verbals are semantically tied to a subject (e.g., a doer), whereas nominals are not. So, when you use -ed adjectives, a subject is implied, whereas when you use -ible or -able adjectives the subject isn't important or necessary. That's probably why -ible and -able adjectives often occur in expletive-It sentences. The act is more important than the person doing the act:
Ex: Is it permissible to copy my friend's work?
=> Is copying a friend's work permissible? <subject>
Ex: Is it allowable to ask students to participate in experiments?
=> Is asking students to participate in experiments allowable? <subject>
The difference between allow and permit is tolerance and authorization, respectively.
Something is permitted (by someone is implied).
Something is permissible.
Something is allowed (by someone is implied).
Something is allowable.
Here are a few dictionary entries on the difference between effective and effectual:
effective means 'having a (desired) result or effect,' effectual is 'succeeding in producing a desired result or effect,'
If something is effective it has an noticeable effect:
Ex: The actor made a most effective entrance.
If it is effectual it produces a particular effect, usually the one intended:
Ex: We took effectual steps to redress the situation.
Effectual overlaps somewhat with effective, especially in the sense “producing the desired effect,” but it also means “valid, legal,” as in The law offers no effectual remedy in this case.
All the best.