i guess it has alot to do
on what size plane, what type, and also maybe were u live
i have a 160 in my DP ULTIMATE 120 size spinning a 18x6 and it flys it pretty well, not break neck speed but more than this fragile airframe need's, i tried a 18x5 and it was ok but didnt have the grab (ummph) that the 6 did havnt tried the 18x8 i need to give that a whirl

ps... i only have about 3 gallons on the 160 so it's still breaking in