In straight and level flight, the lift has to remain constant.

Therefore from the lift formula:

L = 0.5 x rho x v x v x S x Cl

Now if we increase v by 41% the new speed is 1.41v

So now

L = 0.5 x rho x 1.41v x 1.41v x S x Cl
= 0.5 x rho x 2 x v x S x Cl

Therefore in order to get L back to its original value (0.5 x rho x v x v x S x Cl) we need to reduce Cl by a factor of 0.5