 Twist that increases the local incidence from root to tip is less common and is called "wash-in".The X-29 had strong wash-in to compensate for the additional root-first stalling promoted by the forward sweep.Wing twist can also, rarely, refer to the deflection of the wing when it is made of insufficiently stiff materials; actuation of the flaps can, instead of deflecting air as intended, cause the wing itself to be deflected and is related to compressibility effects; this problem has mostly been eradicated however, with modern high-strength alloys and composites.