Firstly, it has
EVERYTHING to do with the prop tips!
This is such a common one that it should almost become a Sticky!
Every aircraft has two basic Speed limits, Vmo and Mmo.
(1) Vmo is a structural limit, defined as an Equivalent Airspeed (EAS) which is a direct measurement of the dynamic pressure of the air stream. Aircraft do not have EAS indicators (they should have), and, instead, we refer to CAS indication, which aircraft do have. For a given EAS, CAS increases with increasing Pressure Height, thus,
Willy Miller, from the information you've given us, the EAS limit for your aircraft is 284 KEAS.
(2) Mmo is an aerodynamic limit. It is a Mach Number beyond which control problems, and severe loss of thrust from propellers occurs. It has
NOTHING to do with the aerodynamic load on your aircraft, but may contribute to undesirable propeller vibration and flexing. High speed propeller aircraft usually do not have Mach Meters, they should have, and the Mmo limit is usually reflected as a decreasing Vmo with increasing Pressure Height.
Mmo is determined at a Mach Number somewhat beyond Mcrit (
NOT at Mcrit), where the control problems and/or loss of propeller thrust due to Mcrit exceedance is acceptable. Thus, it is related to Mcrit.
Every component of the aircraft has it's own Mcrit, wings, propellers etc., and Mcrit is usually found at the fastest moving part of the aircraft. For the propeller aircraft, the fastest moving part is the propeller tips, their speed being the VECTOR SUM of their own speed due to rotation and the aircraft forward speed (TAS).
From the information you've given -
Originally Posted by Willy Miller
red line FL180 284 IAS = 360 TAS
red line FL250 245 IAS = 340 TAS
Prop tips travelling at 350 kts ish at 850RPM)
We can establish several numbers for the aircraft (for ISA) -
284 CAS at 18000 Ft P.Ht. : EAS = 278.1 : TAS = 368.4 : Mach No. = 0.595
245 CAS at 25000 Ft P.Ht. : EAS = 238.6 : TAS = 356.5 : Mach No. = 0.592
Of interest is that whilst all of the other data varies, Mach Number is almost the same!
If we now calculate the Vector Sum of the Prop tip speed and the TAS, we can establish several numbers for the propeller, accepting your prop tip speed of 350 Kt at 850 R.P.M. (for ISA) -
At 18000 Ft P.Ht. : TAS = 508.1 : Mach No. = 0.821
At 25000 Ft P.Ht. : TAS = 499.6 : Mach No. = 0.830
Mach Number again is very similar.
For both examples the Mach Numbers are very similar. Don't forget that in aircraft certification, the highest temperature approved in the Environmental Envelope (Typically, ISA + 35°C) is used to find the TAS, and the highest permissible propeller speed is considered (Maximum R.P.M. is probably above the 850 R.P.M. that you quoted) to find the Vector Sum speed of the propeller tips. M0.82 to M0.83 does sound about right for a thin propeller blade, as compared to a much thicker wing. (If the certification criteria had been used in lieu of the example worked at ISA, I'm sure that the closely matching Mach Numbers would become equal).
Hudson Bay, you seem to have resurrected the old "aircraft don't fly above Mcrit" bogie again -"
your aircraft would exceed crit mach and I'm sure you have seen the TV footage of aircraft that exceeded crit mach!!!". As recently as yesterday I exceeded Mcrit during Climb, Cruise, and Descent in a normal sub-sonic airliner. Tomorrow, along with Tens of thousands of other pilots around the world, I intend to do the same again.It's normal operations.
Regards,
Old Smokey