Select records by time in interval between 12:00:00 and 18:00:00 on every day

I've tried to select all records in a table with the timestamp in the dateformat 2011-08-01-

12:00:00

Using the following code:

SELECT f.`fly_reg`, RIGHT(f.`start_tid`,8) AS st, f.`start_hight`
FROM vbsk_dk_02.fab_master_flyvedata f 
Where st between 12:00:00 AND 18:00:00

But can't get it to work