To calculate the peak speed you need to know the launch height.
You are converting kinetic (movement) energy to potential (how high) energy. Ignoring air resistance and the drag you create when you rotate the DLG...
Mass x Acceleration due to Gravity x Height = Mass x 1/2 x (Velocity Squared ). Solving for Velocity for a given height....
V= Square root of ( 2 x 9.8 x H)
50m = 31.3 m/s or 113 km/h
75m = 38.3 m/s or 138 km/h
Someone check my maths..... My physics O level was a while ago