Is it necessary to glue this piece of wood? What is its function? Thanks

I would say yes, it should be glued on. It looks to be a bit of a fin (not sure how much effect it would have) but mostly seems to protect the end of the rudder. It would be a shame to hit a bump on landing and have the rudder catch on a clump of grass and tear off...

Happy Thanksgiving

Does the plane use a tail wheel or a tricycle configuration? If it is a trike, this could be a skid that protects the rudder on those beautiful landings where you roll out on the mains and hold the nose wheel off the ground until the last second.. Regardless, I'd glue it on. It just don't think it will look "finished" without it.

Oh, right. Duh. It's a trike. Definitely to protect the rudder. Thanks! Happy T-day.

some full scale use a stabilizer as the one in your picture..to prevent turbulent air from causing drag as the rudder is turned...I knew of some King Airs that have a turret back on the fuselage and were getting some yaw action when the turret was turned...the answer was to remove the one stabilazing fin and add two an equal amount off the center line

Hey yetti if you got the sonic low wing i advise to not use the riser block on the ailerion servo, i did and i had the remount my throttle servo so it didn't hit it. The low wing flies quite nicely, i've only had it up 3 times before the snow came this year. And to answer the question, yes glue it on, your rudder will thank you, it looks tricky to glue on but i used a couple of small pcs of small linkage rod to help, i drilled in the pc and inserted the rods, then lined it up on the fuse to mark the holes, then drilled in the fuse and then epoxied it in place.

Thanks everyone. Micro, I didn't use the blocks. Thankfully I read the reviews and discovered this same advice.