I think I understand! An ND filter may still be handy though. Sometimes in really bright conditions (not that they happen too much in the UK) I find the only way I can get shutter speeds slow enough to get prop blur is by stopping down below f/16, by which point diffraction is starting to kick in so the lens is actually getting softer. An ND filter will help in these circumstances by allowing the aperture to be a little wider.

Just a thought anyway, now I’ll go check your Duxford post out! 🙂

Paul