Delta wee, as HebaruSan wrote, build the ramp as part of the plane. For Example connect a decoupler to the plane, right click the decoupler and set ejection force to zero. Then press 2 (Offset tool) click the decoupler, hold shift key and move the decoupler where you want to build the ramp. It might be difficult to get enough distance between the plane and the ramp in the SPH, but I think there are mods to allow you to build outside (EDIT: such a mod is mentioned here: https://forum.kerbalspaceprogram.com/index.php?/topic/182945-vab-size/). Using a ramp to launch a plane has never occured to me, but it sounds exquisitly kerbal. Thank you for the idea.