Thanks John, Because I fly almost entirely off grass or dirt I used to use those air filters on anything with an open (non cowled) engine bay. As pointed out, they didn't fit in most cowls so it was trainers and non scale sport planes that had the air filters. I still have several planes in the hanger with the filters on and two unused new filters still in the package. Not much need for them these days as I am using much larger engines and planes with cowls now. Thought maybe my LHS dropped them cause they now sell only electric planes! Yes, no glow or gas powered planes just electric, mostly RTF foamies. Time marches on but I don't, I still fly glow power and even build a kit or two now and then.