What you need to do is reference the current value.
So always - set 'sprite.angle' to 'sprite.angle' +20 * timedelta
I don't know how to use this actually within one event, because it is not the same object, it needs to change its angle actually rotate based on other object.
angle set angle lerp(.angle, newangle, 1-0.5^timedelta)
Course you'll get the barrel roll when you go from 360 to 0....
And all i get is barrel roll all the time