Vne and TAS versus IAS

From Matronics

According to Ken Krueger of Van's Aircraft, the speed at which an airframe will experience flutter depends on TAS, not IAS. If you equip your RV with an turbonormalized engine, or a more powerful engine, allowing higher IAS at high altitudes, you may exceed Vne (based on TAS) and therefore risk encountering flutter.

Ken's article is available here.