Difference in pronuncation: ape vs chapel

Trying to find pronunciation rules for English orthography is an exercise in futility. Questions about this on English Language & Usage (ELU) usually get closed. See e.g. Are there rules of pronunciation for words in English?.

The tag wiki for pronunciation-vs-spelling on ELU) contains the poem "The Chaos" (1922) by the Dutch scholar Gerard Nolst Trenité that illustrates how hopeless this is. There are several readings of this poem in YouTube, e.g. this shortened version spoken in British English and this longer version (over 11 minutes!) read by Brumplum.

However, there is an ELU question specifically about the grapheme a.

Note also the following pronunciations:

• chapel /ˈtʃæpəl/ does not rhyme with lapel /ləˈpɛl/, and neither of these words rhyme with label /ˈleɪbəl/. (So I see no reason to assume that the 'l' at the end of the word determines the pronunciation of the 'a');
• but chapeless /tʃeɪpləs/ rhymes with shapeless /ʃeɪpləs/.

How did we (i.e. students of English linguistics) deal with "The Chaos" at university? Well, we were made to transcribe literally hundreds of words into International Phonetic Alphabet (IPA). You'll need a good dictionary with IPA transcriptions, e.g. the Oxford Advanced Learner's Dictionary, or a pronunciation dictionary such as the Longman Pronunciation Dictionary. (Note: these references focus primarily on British English, but American pronunciation is also mentioned.)

There is a "rule" I learned in second or third grade, though often pooh-poohed of late by "authorities":

A vowel in an "open" syllable (one that ends with the vowel) is "long", while a vowel in a "closed" syllable (one that ends with a consonant) is "short". However there's a special case for a word that ends in a consonant followed by a silent "e" (as in "ape") -- in that case the vowel (the "a") is "long". (This rule works pretty well for "a" sounds and "i" sounds, and not so well for "o" sounds.)

And, for those that have forgotten what was common knowledge 50 years ago, a long "a" sounds like the one in "late" or "gate" or "ape", while a short "a" sounds like the one in "cat" or "hat" or "chap".

And you know that the word "chapel" is (probably) not pronounced "chape-l" because English spelling is at least fairly regular about having syllables that contain at least one vowel in the standard spelling.

Even people who have never explicitly been taught these rules tend to learn them by osmosis over the decades, and so most experienced English speakers can usually approach a "normal" but unfamiliar word and do a pretty good job of pronouncing it, without having to consult a dictionary.

But, of course, in English there are no rules.