Like the more popular and longer-lasting 'monster' line of cereals from General Mills (Count Chockula, Frankenberry, et al), the two biplane-piloting characters shilled for cereals that tasted like a certain type of fruit. Like many popular cereals of the time, they also featured similarly-tasting marshamllows in the mix, which would turn the milk a satisfying purple or red.
Virtually nothing is remembered about these two cereals except that (a) they tasted pretty good, and (b) they smelled strongly. I recall this vividly - adults were repulsed by the strong odor of a bowlful of Sir Grapefellow , while I myself found it none too pleasing (although I held my nose long enough to eat the cereal). This is probably one of the reasons the product was so short-lived.
All that remains today are a handful of boxes that turn up on eBay now and again, as well as a small assortment of themed merchandise consisting mainly of iron-on patches, toy planes, and four mail-away posters to color.
Here's hoping General Mills one day sees the light and returns Sir Grapefellow and his dastardly Hun antagonist to the airways of a hero-starved America. Because if there's one thing that children love, it's fruit-flavored World War I air aces.