What were allophone rules for [r] in Old English and Middle English?

Question

I gather that [r] (trill) was realized as [ɹ] in different dialects of Old English and Middle English, but when [r] was used, was it an allophone?

In other words, did [r] vary predictably with [ɹ] (or maybe the flap [ɾ]) in specific phonetic contexts, e.g.,

word initial? between vowels? when long /rr/? in consonant clusters? word final?

Recommendations on articles/books with more info are also welcome.

Answer 1

There's no certain answer to this question, as the various realizations of /r/ in OE/ME (and in early Germanic languages generally) are far from straightforwardly clear. A good recent article by Piotr Gąsiorowski, arguing that /r/ was already highly variable in OE, can be found here.