Debate over Mcrit and Mach Trim
Joined: Mar 2006
Posts: 113
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From: TLV
Mcrit Story...
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".
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".
Joined: Feb 2007
Posts: 129
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From: Bournemouth
Supercritical Wing
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
"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
Thread Starter
Joined: Feb 2004
Posts: 61
Likes: 0
From: UK
OK
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...
"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...
Per Ardua ad Astraeus
Joined: Mar 2000
Posts: 18,575
Likes: 4
From: UK
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/showthr...ighlight=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).
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).
Joined: Mar 2007
Posts: 8
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From: on the beach
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]
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]
