Expert: Mark Janus - 8/30/2008

Question
QUESTION: Vmo is defined as the max indicated airspeed /mach no which can be intentionally flown and at which normal strength and normal level of handling qualities are provided , and a book also says speed above Mmo or Vmo also alarms the overspeed warning ,,,, can u pls tell me what is the difference between Vmo and Mcrit , Which comes first ?

Thanks
Praveen , India

ANSWER: Mcrit is the Mach number at which supersonic flow first appears on a body....  Mcrit occurs before Vmo....  Aircraft can safely operate with small regions of supersonic flow (and do routinely), so the max operating Mach number is higher than the critical Mach number....

mj

---------- FOLLOW-UP ----------

QUESTION: Thanks Mark ,
Mark as per ur explanation Mcrit come first and Mmo follows , but how is it possible for an a/c to get to Mmo when at Mcrit shock wave forms and this would lead to compresibility effect leading to Mcdr and and sever buffeting , how can a transonic a/c cross Mrit and get to Mmo ..
Handling Big jets ( by DP davies ) also says that there is Mdf and VDF which comes after Mmo and Vmo .... can a transonic aircraft really reach this regime ( without shock wave drag troubling the a/c ) ???

Praveen

Answer
Mcrit is the speed when sonic flow first appears shock waves (of significance) only form after you've exceeded this speed by a bit... also, the Mcrit is attitude dependent so different angles of attack will yield different Mcrit...
On commercial airliners you can actually see (if the conditions are right and the sun in the right place) the shock waves on the surface of the wing....they are weak and hence do not produce significant losses

Certainly in a dive the speed can increase beyond the Mmo...  in WWII I believe the P38 aircraft was nicknamed the "widow-maker" because in a dive the control authority could be lost due to shock-induced separation (and that was a prop plane)....