The center is small curved wing want to turn in a straight line without affecting adjacent nodes.
Ok, I think I know what you are trying to do. With the "Anchor Point Tool" selected, hold down the "command" key (not sure for Windows) and click on the anchor point in question, then while still holding the "command" key, click on the hand you want to delete. That should accomplish what you need.
EDIT: Sorry the above was for Illustrator. For Affinity its somewhat similar, choose the “corner tool” and click on the anchor point, which should active the handles. Still having the “corner tool”, hold down the Command+Option keys and click on the handle you’d like to delete. That should do it.