I have to agree with Deck Knight on Ghost being an excellent secondary typing.
With flying as the main type, at least two of rock, electric, and ice will be super effective unless we go with ground or steel. ground already has two different momentum gaining pokemon in the metagame with Gliscor and Landorus, and steel lends itself very heavily to being defensive, which is already exemplified in Skarmory.
With this in mind, I also think that it would be more beneficial to try and capitalize on additional immunities a second type could bring to the table, while hopefully adding at least decent coverage, as well as non-damaging ways (trick, sub+pain split/encore, trick room, will-o-wisp, hypnosis, etc.) to gain momentum, which ghost does very well.
Basically, it seems better to try and enhanced what potential for offensive/indirect means of momentum flying gives, by coupling it with ghost, than to fret over being SR weak.