Currently QCAD doesn`t have an option in the circle tool set to draw a circle (C) tangent to two other circles (snapshot A), the following tip may help as a workaround if you need to do this.
1.Draw two given circles A and B of radii 50 and 40.
2.Set up a new layer - circle construction.
* Note - third circle ' C ' to be drawn is of radius 30.
3.On layer ' circle construction'
First construction circle with a radius of 80 (combined radii of circles A - 50 + C - 30).
Second construction circle with a radius of 70 (combined radii of circles B - 40 + C - 30).
4.Draw circle C (radius 30) using the intersection of the construction circles as the center!