Debate over Mcrit and Mach Trim

[mikepops]

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

[FE Hoppy]

Mcrit < Mno

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

[Mad (Flt) Scientist]

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.

[mustafagander]

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

[411A]

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.

[Jaguar Pilot]

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.

[BOAC]

mikepops - try 'search'?

[Old 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!

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

[gengis]

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

[FE Hoppy]

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

[mikepops]

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

[CaptainSandL]

Certainly can. Here (http://www.pprune.org/forums/showpost.php?p=3138523&postcount=9) are three of the best.

[blackmail]

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

[Mad (Flt) Scientist]

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

Regarding the listed books...

If you prefer, refer to this report (http://www.winggrid.ch/DO728JET%20type%20with%20WINGGRID.pdf), 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 (http://naca.central.cranfield.ac.uk/reports/arc/rm/3520.pdf) 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 ...

[john_tullamarine]

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

[Gullyone]

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.

[beamer]

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

[sikeano]

Quote from 411A

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

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

[Dick Whittingham]

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

[False Capture]

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

[747dieseldude]

A 777 was cruising along, everyone FDH, when suddenly an SLF calls the F/A and tells her the wing is bent.
The F/A looks out the window, and behold - the wing is bent.
She ring the flightdeck, a heavy F/O goes back to look, and yes, the wing is bent.
Now the Capt. goes back and has a look, and the wing is still bent.
Everything's normal, no vibrations, normal fuel consumption etc, but since they are close to home base and a long flight ahead of them, they decide to turn back and have that thing inspected.
During descent the wing goes back to normal and on the ground no evidence of it could be found.
They ring up Boeing, and they say:
"yeah, we are familiar with it, it's a shockwave which distorts the vision and makes the wing look bent".

Proving the point: Mcrit is below normal cruising speed for some airplanes.
In fact, any airplane who has a super-CRITICAL wing will have Mcrit below Mmo.
That's why they call'em "super-critical".

[Jaguar Pilot]

Sorry 747dieseldude.

"Supercritical" refers to the precise profiling required of the upper surface leading edge. The upper surface is relatively flat, delaying shockwave formation to a higher MFS. The loss in lift can be recovered by a pronounced reflex camber on the lower surface towards the trailing edge.

With a carefully profiled upper surface leading edge, expansion waves will reflect from the subsonic/supersonic boundary back into the shockwave,
thus reducing its' intensity and thus wave drag rise. Also the wing may now be made thicker, allowing for, amongst others, greater torsional strength and larger fuel tanks. Sweepback angle may now also be less,
reducing the undesirable slow speed charateristics of sweepback.

If you want a diagram PM me.

JP

[mikepops]

Some of the posts including their references have made interesting reading yet are inconclusive with respect to the original question:
"Mcrit < Mmo for subsonic jet transports?"
I have read and re-read Handling the Big Jets chapter concerned and it treats the subject very generically (i.e. not specifically from the point of view of subsonic aircraft).
With reference to the 2 reports:
1.The 728 target cruising Mach is 0.78, the wing Mcrit is 0.80. Also note that this number does not represent the aircraft Mcrit.
2.The Tornado si firstly a supersonic aircraft thus an inapropriate example, and secondly variable geometry implies a variable Mcrit.
Once again if anyone can shed some light on this, a logical argument based on aerodynamic principles, or a specific reference to a publication would be greatly appreciated. (Specific doesn't mean "read that book" it means a quote or a page, chapter, etc.).
I've started this thread in the hope of finding an answer from those in the know, and not to provide another platform for a slanging match by those that really need to get out more.
I am still hoping... :ugh:

[BOAC]

mike - I'm not sure whether you took my advice to 'search' but there are so many threads where this has been discussed it would take forever to cut and paste the links! I would, however, endorse a look at one thread, http://www.pprune.org/forums/showthread.php?t=131271&highlight=mcrit which goes some way to answer your initial query and in which our friend Old Smokey gives you an Mcrit (for the DC9). Unfortunately his helpful drag-rise graphs will not display now, but I'm sure we could encourage him to 'refresh' the link?:)

I think that describing Tornado as a 'supersonic aircraft' in the context of this discussion is misleading, unless of course its 'econ cruise' is supersonic, which I doubt. I know only of the Lightning where our 'econ cruise' was 0.9M, and was, I suspect, close to Mcrit if not slightly above. Cruising above Mach 1 is very 'fuel intensive', but there is often another supersonic 'window' for cruise where acceptable drag rise is traded against operational needs - M1.3 I seem to recall?

As you will see from the discussions on the various posts/threads on this topic, 'MCrit' is of little relevance in modern airliners and is more a concern for performance guys/girls in calculating econ cruise and in any failure cases of stab augmentation devices. It merely represents the point at which shockwave formation SOMEWHERE on the aircraft begins and there may well be subsequent effects on tail controls/pitch or roll trim and drag rise depending on where the shock/s form/s. Drag certainly increases above it on 'normal' aircraft, but modern aircraft design delays the critical drag rise (which is another definition).

[gimmesumviagra]

Buying into this Suicidal discussion:

Further to jaguar Pilot:
'Super Critical' was designed by Airbus and is noticeable on the A330/340 wing (maybe 777 - not sure). This permitted some airflow over the wing to be supersonic without resulting in an unacceptable rise in Drag.
This results in a higher MCrit, resulting in a higher Mmo/Vmo

In the older generation T-tail Jets (B727/DC9) there was a Mach Tuck effect. At high Mach Nos the nose tended to pitch down, and nose-up pitch trim was introduced to compensate.

Not so noticeable on new generations.

While flight testing our B767 SIM at high Mach No, I have observed:
1. Initially Auto-throttle response to compensate (as per systems);
2. A Nose-up pressure (maybe aerodynamic or perhaps some nose-up stab trim) [perhaps someone from Boeing can enlighten me]

[Gullyone]

surely mcrit and mmo are the same,tho in this case they are just ignoring mcrit.

[mikepops]

I have not seen the documentation but the Chief pilot of a UK operator that uses the Embraer 145 aircraft told me yesterday: Mmo = 0.76, Mcrit = 0.86