Simple newb question here. Do I need a prop spinner to go with my prop adapter, or are they just cosmetic add ons?

You dont have to, but they do add some aerodynamic help. And cosmetic too.
And it also helps the plane plant itself deeper when it goes in, if it has one.

robertdownes25::All your better lawn-darts should have um!!( Makes the tail stick-up so you can find it easyer) your bub, scoooper

Also not a bad move from a safety perpsective, particularly if you are flying indoors or somewhere where there may be some people around. A spinner will stop the prop shaft from penetrating the skin.

Thanks people. However, I am flying a modded F27B, so unless I end up going backwards I think I can live without one.
Thanks for the advice!