ORIGINAL: Gray Beard
It's called an incidence meter.
He needs his wifes help or he will have to grow another arm. You have to wet down the wood then hold and twist it while there is a heat sorce like a covering heat gun when trying to remove a warp. An extra set of hands really comes in handy. If it isn't too bad you can also remove a warp with the covering, sort of the same thing as the wet and heat but you twist one side of the wood and shrink the sag in the covering to straighten out the warp. Brian already mentioned that. It's something I have done a lot when I assemble ARFs for people, works pretty well too.
Brian, if you hold the wing at both ends and flex it, can you get the warp out of it with the flex?
As we know, wetting and drying is a standard method to remove or induce warp in wood, Gene above alludes to the fact it can be done with a interconnected/tied structure such as a wing, so you might as well give it a shot - but obviously only if you can easily remove the warp by flexing it while it's dry.
If you can get the warp out by flexing it, can you flex it a little more in an opposite warp? You'd probably want to bend it past the point of the warp, then as it dries occasionally loosen the flex to see how it's doing because you wouldn't want to induce opposite warp of what you already have.
I'd use a spray mist bottle to wet it. If you do try it, I'd be very careful about the amount of pressure when you flex it wet. I don't think soft wet balsa is something you'd want to really torque on. And if the warp is induced by the spar make sure that spar is totally soaked and hit the spar with the heat first and foremost. If you attached a vice grip to each opposite ends of the spar, does twisting the spar take out the warp? If so, maybe the spar is what you want to concentrate on.
Like I wrote before, this is a total bummer. I've never had a warp building (or maybe I have and didn't notice). I have my fingers crossed!