The Role and Distribution of -ly Adverbs in Middle English.
The traditional view of the sounding of final -e is essentially correct both for poems in foot-counted verse forms and those written in the alliterative long line. Final -es had been lost after unstressed syllables well before the middle years of the fourteenth century. Chaucer and Gower retained a number of such -es endings on verbal and nominal forms, but never sounded them on -ly adverbs. The evidence strongly suggests that WL too had no use for -e on -ly adverbs.
Houwen and MacDonald, Loyal Letters. 131-54.
Duggan, Hoyt N.