ILS,
About half of my collection is concentrated in US Airmail postage and covers, and the other half in regular US postage and covers with an aviation theme. The latter starts with Scott #'s 649-650, and the most recent is last year's Chief Anderson stamp (#4789). So this collection of covers is a nice fit in with the latter collection. I doubt that I would have ever tried to acquire all 20 of these at the usual price of $12-$13 each, but at this price I couldn't pass it up. Most of the newer items in my aviation topical collection consist of a stamp, a plate block, and a sheet, all mint, and a few covers (maybe half a dozen on average). For the Classic Aircraft sheet (Scott 3142), I've concentrated on covers with multiple stamps (Bevil has the 20 stamps on five covers, of which I have four, still looking for the fifth), or something novel, like this:

Collins covers are "okay," but sometimes overrated (and thus overpriced) in my opinion. But I'm still thrilled to have these!
Basil