trees with straight unbent trunks make the best lumber.
Examples of unbent
unbent
For once the Northerners have unbent and are not afraid to show their emotion.
From the telegraph.co.uk
Outward from the axis of the tube, the first surface is the unbent portion of the tube.
From the en.wikipedia.org
The straighter the opening which has to be filled, the more the panel needs to be unbent.
From the newscientist.com
The brim of his baseball cap is unbent, spread wide atop his head.
From the newsday.com
This is when the leg is being unbent and is decelerating, just before the foot touches the ground.
From the newscientist.com
To do a Back layout, you must arch during the flip in order for the unbent legs to come around.
From the en.wikipedia.org
We could see only two other fisherman on the lake, both as still as the night heron, their rods unbent by fish.
From the sfgate.com
Mr Netanyahu would be able to point to it as evidence of his unbent neck, in the hope of allaying nationalist ire that could erode his rightist-religious coalition.
From the economist.com
But, instead of making each flexible panel straight and then forcing it into a curve, the Boeing panels are made already curved, and are unbent by force for installation.
From the newscientist.com
More examples
Straighten: straighten up or out; make straight
Unfasten, as a sail, from a spar or a stay
Free from flexure; "unbend a bow"
Relax: make less taut; "relax the tension on the rope"
Relax: become less tense, rest, or take one's ease; "He relaxed in the hot tub"; "Let's all relax after a hard day's work"
Release from mental strain, tension, or formality; "unbend the mind from absorbing too much information"
(unbending) inflexible: incapable of adapting or changing to meet circumstances; "a rigid disciplinarian"; "an inflexible law"; "an unbending will to dominate"
(unbent) not bent; "looking for an unbent nail"; "trees with straight unbent trunks make the best lumber"
B: to take the sails off from their yards and stays. To cast loose the anchor from the cable. To untie two ropes.