Aerobatics --- How to conduct constant speed tricks?

Hi there!
I am now trying the ArduPlane’s new feature of Aerobatics.
My first schedule flight trial was a bit success but it was not beautiful because the speed change was so big, say plus/minus 10m/sec around the specified 20m/sec by TRIM_ARSPD_CM during tricks.

Could anyone help me how to conduct constant speed tricks?