Debate over Mcrit and Mach Trim
 

A few of us were debating whether on contemporary subsonic transports (A320, 737, etc) Mmo is above or bellow Mcrit.

It is my understanding that all subsonic transports are limited in speed by Mcrit, where the flow becomes sonic somewhere on the airframe and a shock-wave forms, thus in my understanding on theese aircraft Mmo is always smaller than Mcrit.

The point challenging this idea is that of the Mach Trim operation within the normal range of operating speeds of theese aircraft, to compensate for mack tuck which appears after the formation of the first shock wave, and therefore it must be that Mmo > Mcrit. Can those in the know shed some light on this subject?

I have re-read the Handling The Big Jets chapter on high speed, and the OAT POF manual, neither of which actually specifies whether or not Mmo > Mcrit for subsonic aircraft.

Does anyone have specific values of Mmo and Mcrit for a passenger jet?

Thank you in advance

Mcrit < Mno

The whole world doesn't end at Mcrit. In fact it's a rather trivial number.:rolleyes:

Indeed, Mcrit is WELL BELOW Mmo. As you noted, there are Mach effects happening at much lower speeds than Mmo - evidence of shockwave formation and movement.

How fast you want to fly is more often governed by how much fuel you are willing to burn to do it. :)

All this mach crit/Mmo nonsense is totally unnecessary, except for exams, and even there, it is debateable whether the exam originator knows the difference...:}

Gotta be an ARB (nee, UKCAA) exam question.
Or, maybe JAA.
In either case, doubt is cast on whether the question is meaningful.

MCRIT etc
 

MMO is indeed higher than MCRIT. It is not possible to find values of MCRIT for a particular aeroplane - only MMO is quoted as Indicated Mach Number.

Indeed, a supercritical wing may be cruised with a (weak) shockwave attached.

MCRIT will vary with weight. The heavier the aeroplane, the lower will be MCRIT.

A jet transport in the end is limited by its aerodynamic ceiling, which for most allows for an acceleration of 0.3G incremental. The values may be found by use of the Buffet Onset Boundary Chart.

mikepops - try 'search'?

I think that it's time that someone wrote a definitive story about the relationship between Mcrit and NORMAL jet transport operations, and made it a sticky!

Methinks that a few too many people have seen the 1950's "Sound Barrier" movies, and renamed them the "Mcrit barrier".

At normal cruising levels for civil jet aircraft -

(1) MMO is above Mcrit,
(2) Normal and Long Range Cruise Mach Numbers are above Mcrit,
(3) Maximum Range Cruise (the slowest that you'd want to fly en-route) is above Mcrit, AND

For high flying civil aircraft (like Learjets), Minimum Drag Speed (Holding speed) at high altitude is above Mcrit.

At lower than typical Flight Levels, of course, speed will be limited by VMO, unrelated to Mcrit.

Regards,

Old Smokey

Smokey:

<I think that it's time that someone wrote a definitive story about the relationship between Mcrit and NORMAL jet transport operations, and made it a sticky!>

Actually those stories HAVE been written - try "Mechanics of Flight" by AC Kermode, "Handling the Big Jets" by DP Davies or "Flying the Wing" by Jim Webb. Great write ups

I think the problem is the name. Critical:ooh:
I think it should be renamed Mtriv, then it would be banished into obscurity where it has belonged since the 1950s

Why? How?
 

Thank you for your reply. You've made a statement, can you support it with a logical argument or a specific reference to a publication?

Mike

Certainly can. Here are three of the best.

Mcrit
 

hello every one,
just some food for thought.
Mcrit = general subsonic Mach n° where a sonic point (M=1) develops over the wing, sonic point that, when accelerating any further, will become a full blown shockwave, resulting in a dramatic dragrise & possible mach tuck effect(requiring a mach trimmer).
Mcrit can be as low as 0.7
there is a formula : Mcrit + delta Mach = 1
for a straight wing Mcrit is quite low. so by sweeping the wing(>15°) we in fact lower the delta Mach in above formula & by doing so, increase Mcrit so as to keep the sum of the two = 1.
increasing Mcrit delays the onset of the shockwaves & the corresponding drag penalties.
shockwaves are a graveyard for (heat)energy & must be compensated by a thrust increase, resulting in a drag rise.
kind regards,
bm

Regarding the listed books...

If you prefer, refer to this report, specifically section 3.2, where the annotation to the figures indicates an Mcrit for the wing of 0.80. I'm pretty sure the 728 wasn't going to have an mmo below 0.80....

This report on high-subsonic drag contains, in figures 3 and 4 (page 16) an estimate of the theoretical Mcrit for Tornado - it's about 0.65! Again, I think that plane flies a tad faster than that ...

now, now, MFS ... do be gentle ...

I dont think the Tornado is a very good example in this discussion as when it is flying at its operational cruise I expect it will be flying above Mach 1 and the whole flow would be supersonic. M crit would be long gone and not an issue as it would just blast past it.

I think some of you need to get out a little more.............!!!!!!!!!!!!!!!!!

Quote from 411A

"Gotta be an ARB (nee, UKCAA) exam question.
Or, maybe JAA"

you are showing your age here Old Boy-ARB :p

But the Tornado refered to in the report has to be the 1942 version, the original brick outbuilding, and probably couldn't manage much more than M0.65anyway. Still, I agree the basic premise. Mmo will be higher than Mcrit.

Dick W

Mikepops,
Now that you've started a thread on PPRuNe this must surely be debating by the masses ie. mass-debating.:}