Posts about: "Mmo" [Posts: 15 Pages: 1]

Re Mach 2 ....

In the earliest days of the project, Concord(e) was described as a Mach 2.2 airliner.

Once the RR58 alloy arrived, and the first thermal fatigue tests were underway, Mach 2.2 appeared as somewhat optimistic, and to assure an acceptable airframe life, the Mmo (maximum operating Mach number) to be certified was brought down to Mach 2.04.

Interesting question just asked by somebody on another forum....
Why Mach 2.04 ? Why not Mach 2.10, or Mach 1.96 ?
With thermal fatigue still being a field that was only starting to be explored, was that a fully technical choice.... or was there a commercial aspect ?

Mach 1.96 would again have meant a few more hours life for the airframes, and would not really have made a significant difference in the flight duration.
But think of the huge difference between "more than twice the speed of sound" and "not quite as fast as twice the speed of sound".....
Mach 1.96 would simply not have "sold"......

I have no answer to the question who finally decided on '2.04', and I don't think many of the people that wrote the "TSS spec" are still with us, so we'll probably never know.


And along the very same lines, another snippet.....

In 1985, during a major cabin upgrade, BA installed the "Marilake" displays, that showed Mach, altitude, groundspeed, etc. in place of the simple Mach-only displays that Air France kept until the end.
Nice display, complete with microprocessors.... you must have seen photos.

Of course everybody wanted their photo taken next to the display saying "Mach 2".
So these display were subtly programmed to read "Mach 2.00" as soon as the Mach number was above 1.98, and they stayed there....even if the aircraft went to Mach 2.03 or beyond.
A tiny bit of cheating... but commercially it made a lot of sense, of course.

Like the earlier BA cabin displays, the Air France displays only showed the Mach number, and they were little more than "rescaled" digital voltmeters that directly displayed the 0-12V Mach signal from the Air Data Computer. They tended to flicker a bit from 2.00 to 2.01 to 2.02 and back, but at least they didn't "cheat". And I still proudly have a photo of myself with a "Concorde grin", at Mach 2.03 !

Biggles78
This is the minimum Mach number that can be flown with the existing CG. (which would be around 59%). Just as the CG indicator (not shown in this photo) gave minimum and maximum CG for a given Mach number, the Machmeter gave a reciprical indication also). You can also see that as the aircraft is not flying at Vmo any more, being at Mach 2 cruise, that the VSI pointer is now away from the orange and black Vmo bug. At our 'not so coffin corner', now that the aircraft is at maximun alllowable altitude, Vmo would naturaly coincide with Mmo; the orange and black Mmo bug being shown at Mach 2.04. This really superb photo taken by Bellerophon gives a graphic illustration of what the panels looked like at Mach 2. Note that the with the TCAS VSI Concorde retained it's original linear VSI also. (Miust have beeen the only aircraft flying with FOUR VSIs. (The originals had to be retained due to the fact that the autopilot Vert' Speed Mode error was derived from the indicator itself. As far as TCAS goes, R/As werer inhibited above FL300 (on acceleration this would coincide with the aircraft becoming supersonic, and the mfrs would not countenance the aircraft doing extreme manoeuvrs as a result of TCAS RAs at supersonic speeds).
The tail wheel was lowered for all 'normal' gear cycles (not stby lowering of free-fall). It was designed to protect the bottom the nacelles in the case of over-rotation, but in practical terms the thing was a waste of space (and weight) and a simple tail skid (used on the prototypes) would have sufficed. Any time that the tail wheel contacted the ground, it would ALWAYS collapse, damage the tailcone structure and in fact aforded no protection whatsoever. Fortunately these events were EXTREMELY few and far between. The biggest problem with the tail wheel was a major design flaw: On gear retraction the assembly would retract in sequence with the nose and main gear, and as it entered the opening in the tailcone, it would release over-centre locks that were holding the spring-loaded doors open. The doors would then firmly spring shut behind the gear assembly and finish the job. UNFORTUNATELY this was a very poor design; if for any reason one of the two doors had not gone over-centre on the previous gear lowering, it would be struck by the retracting tail wheel gear and cause structural damage to the local skin area, that would have to have a repair done. Unfortunately these events were not quite so rare, and several measures were tried to reduce the chance of this happening. Although not a safety issue, it was an issue that was a total pain. (As a matter of interest, G-BOAC had this happen on one of it's first test flights out of Fairford in 1975).
Nick Thomas
As ChristiaanJ said, the last two BA aircraft WERE lighter than the others, and would be preferred aircraft for certain charters. But that is not to say that any aircraft could not happily do ANY sector. We fortunately had no distorted airframes in the British fleet, so this was never an issue. There was very little spread, regarding fuel consumption between different engines; one of the best parts about the Olympus 593 was that it hade very little performance deterioration with time, it was an amazing piece of kit.
Time on wing for the engines was a real variable. Each engine was built up of modules, each one of these had a seperate life. In the early days of operation, time on wing was quite poor, and MANY engines would be removed on an attrition basis. One of the early failure problem was the fuel vapourisers inside the combustion chamber were failing, taking bits of turbine with it!! A Rolls Royce modification that completely changed the design of the vapouriser not only solved the problem completely, but also increased the performance of the engine. As the engine matured in service time on wing greatly improved, and in service failures became a thing of the past. A 'trend analysis' was done after each protracted supersonic flight, where engine parameters were input into a propiatry RR computer program, that was able to detect step changes in the figures, and if this were the case, more boroscope inspections were carried out. The OLY time on wing was nothing compared to the big fan engines, but the conditions that it operated under bore no comparison. Not really sure about absolute figures on this one Nick, I'll ask one of my Rolls Royce friends and see if I can find a figure.

MEMORIES
Like so many in the Concorde family, I have millions, I'd like to share a couple here. I remember at Fairford in mid 1974, a CAA test pilot (I honestly forget the gentleman's name) was taking the British pre-production A/C 101 (G-AXDN) for a special test flight. The reason that this flight was so special was that for the first time, the CAA were going to do an acceptance flight trial of the brand new digital air intake system. This revolutionary system had been retro fitted to 101 barely a year earlier, and being a brand new (and totally unique, in electronics terms) system had been plagued with teething troubles. It was quite reasonable for any airworthiness authority to have serious misgivings about any system that was going to wave great big metal lumps around in front of the engine compressor face, and that if only a few degrees out from the commanded position out could cause the engine to 'backfire' etc.
So anyway, 101 took off and disappeared into the very blue sky and we waited, and waited, AND WAITED. (I'd only left the RAF and joined the project a few months previously, and did not want my new association with this amazing aircraft to end). I was biting my nails, drinking coffee, losing my hair... (without the help of M2V ). Anyway after about 2 1/2 hours the aircraft returned to Fairford, and everybody crowds around the crew for the debrief. A very stern faced CAA pilot looked at us all, broke into a grin and said "as far as I'm concerned gentlemen, you've got yourselves an airliner". At that point the room was a study of total happiness, blessed relief, and a need to go to the loo..... But from my point of view, I will remember those words forever.
101, which now resides at the Imperial War Museum Duxford was the fastest Concorde ever. She achieved Mach 2.23, which was an incredible irony, as Concorde can trace a large part of it's developement history back to the BAC 223, proposed SST.
As far as flying memories go, I just don't know where to start; My first ever Concorde flight was in November 1976, out of Fairford on a pre-delivery test flight on G-BOAD. (Now sadly bobbing up and down on the Hudson, next to the USS Intrepid). I was staggered how fast and high we flew (Mach 2.08, FL580). Most of my flying up to that date had been in C-130's in the RAF, at around 340 KTS and FL300; Concorde also being infinately quiter in flight than the good old Herc'. I remember a BA QA guy showing me how I could touch the skin of the aircraft at Mach 2 (You reached behind a door busstle flap, moved your hand through some insulation until you felt bare metal). OUCH!! it was hot, very hot.
But I think one of my most memorable flight memories was aboard G-BOAG, (now residing in the Boeing Museum of Flight in Seattle) returning from BKK, having stopped off to refuel in BAH. We were forced to fly subsonic over Saudi, and got caught in this amazing electrical storm, There was St Elmo's fire cracking and bubbling all over the visor panels, but just as incredible was the long blue electrical discharge coming off of the nose probe; it seemed to extend about 50' in front of the aircraft. The crime was, none of us on the F/D had a camera. Every time I bump into the captain on that day (are you reading this Ian?), we go back to remonissing about that incredible flight. Also, later on the same sector, after we had decelerated to subsonic cruise again, this time flying up the Adriatic, we had another fascinating sight: It was getting quite dark now, and here we were, travelling at Mach 0.95 at FL290, when above us was all this Mach 0.8 ish traffic at around FL330-350. All we could see were all these navigation and ant-coll' lights above us, seemingly travelling backwards. It was quite a sight. On the original BAH-BKK sector a week earlier, we flew through some of the coldest air I'd ever seen; The air was at ISA -25, and at Mach 2 our TAT was only about 85 deg's C. (You could feel the difference too; the cabin windows felt only warm-ish to the touch). The upside also of all this was that your fuel burn was much lower than usual. (The only downside of course is that your TAS is a little lower). Rolls Royce did some analysis on the flight, and were amazed at how well the propulsion systems coped with some of the temperature sheers that we encountered, sometimes 4 to 5 deg's/second. They said that the prototype AFCS had been defeated by rises of only 0.25 deg's/second ).
Not meaning to go off onto a (yet another) tangent; Negative temperature shears, very common at lower lattidudes, always plagued the development aircraft; you would suddenly accelerate, and in the case of a severe shear, would accelerate and accelerate!! (Your Mach number, quite naturaly, suddenly increased with the falling temperature of course, but because of the powerplant suddenly hitting an area of hyper-efficiencey, the A/C would physically accelerate rapidly, way beyond Mmo). Many modifications were tried to mitigate the effects of severe shears, in the end a clever change to the intake control unit software fixed it. (Thanks to this change the production series A/C would not be capable of level flight Mach numbers of any more than Mach 2.13, remembering that Mmo was set at 2.04).
There was one lovely story, involving the Shah of Iran, having one of MANY flights in a developmment aircraft. The aircraft encounterd quite a hefty series of temperature shears that plagued havoc with some Iranian F4's that were attempting to close on the Concorde, to act as an escort for the Shah. (or so the strory goes). I'm still trying to picture these F4's, on full afterburner trying to get close to a Concorde cruising away on dry power). It is said that the F4's were having such difficulties, due to their relatively crude powerplant, coping with the temperature changes, that the Concorde was ordered to slow down, 'so the escorting F4's could catch up'!! True or not, it is part of Concorde folklore.

Dude

ChristiaanJ
aaah yes, Max Climb/Max Cruise modes. I'd not forgotten this my friend, I was going to say a few words about that in a future post, but maybe we can do that now. (And I'd love to hear more of your comments on this here too, ChristiaanJ). The intake and autopilot modifications were in a way complimentary it's true, but really dealt with separate problems, at least in my view:
The intake control unit software change (a change to the control law that limited engine N1 as a function of intake local Mach number, Mo, and inlet total temperature, T1) was able to put an absolute limit on aircraft achievable Mach number during Mmo overshoots, but it would not PREVENT Mmo overshoots occurring altogether, it was more of a safety brake. This particular overspeed problem manifested itself well before route proving, and in fact the intake system 'fix' resulted in the Thrust Auto Reduce System being deleted, electronic control boxes and all. The TAR system was fitted on all development aircraft equiped with the digital intake system, and it tried (in vain) to limit extreme Mach overshoots. The production aircraft retained the TAR wiring and locked out circuit breakers, as well as two vacant spaces on the electronic racks. The prime reason for all these efforts were that some of the rapid excessive Mach overshoots quite often drove the intake into surge; the modification to this N1 limiter control enabled engine mass flow to be controlled in such a way that these surges could be prevented during temperature shears. The aircraft Mach limit was an extremely useful fringe benefit.
The AFCS mode change from what was Max Op and Max Op Soft (always loved that name) to Max Climb/Max Cruise was at a stroke able to deal with the regular Mmo overspeeds that kept on occuring during, as you say, the route proving trials of 1975, when British aircraft G-BOAC and the French aircrfraft F-BTSD carried out pre entry into service evaluation flights, SD sadly was the aircraft that was tragically lost at Gonez in July 2000). The Max Climb/Max Cruise AFCS mode combo is a mode like no other that I've personally seen before or since anywhere, (it for instance resulted an elsewhere taboo; an autopilot and an autothrotte working together IN A SPEED MODE).
This problem encountered primarily at lower lattitudes, (for example, G-BOAC doing route proving flights out of Singapore), occurring initially as the aircraft reached Mach 2. It was termed 'the insurmountable problem', but the AFCS designers (such as ChristiaanJ) fortunately did not have 'insurmountable problems' in their vocabulary. The issue was that the aircraft would have been climbing rapidly at Vmo of 530 KTS, with throttles at the gate as usual, At exactly 50,189' we hit what was known as 'the corner point' in the flight envelope, where 530 KTS IAS equated to Mach 2 exactly. Max Op mode would then 'let go' of the Vmo segment, and try and control the aircraft to Mach 2. (As the aircraft climbed, Vmo itself would progreesively decrease in order to equate to Mmo, or 2.04 Mach). But in very cold conditions, the aircraft still 'wanting' to accelerate, and the simple Max Op/Max Op Soft modes just could not cope with gentle pitch changes alone. The problem became even bigger during the cruise/climb when severe temperature shears occured, and routinely regular Mmo exceedences occured. Something had to be done, and something WAS done and how; enter Max Climb/Max Cruise. It was really a classic piece of design, where the aircraft would do the initial supersonic climb in Max Climb mode. This mode itself was relatively simple, in that it was more or less a Vmo -Vc hold mode. That meant that the difference at selection between indicated airspeed, Vc and Vmo would be maintained, with a vernier datum adjust to this being available. In practice this mode was selected pretty much at Vmo, so datum adjusting was not always required. Now comes the clever part; the autothrottle, this would operate in standy mode at this point, just waiting there doing nothing, with the throttles at maximum as before. So the aircraft would now climb as Vmo increased to 530 KTS, and then following a now constant Vmo of 530 KTS until the magic 'corner point' (51, 189' remember). Now all hell would break loose; the mode would automatically change to Max Cruise, the autothrottle would also be automaically selected to Mach Hold mode (initially datumed here to Mach 2) and the throttles would retard, attempting to hold this Mach 2 datum, and the autopilot is commands a 'fly up' signal, over a 20 second lag period to 600'/minute. Now comes an even cleverer (?) part; the autothrottle Mach Hold datum is gradually increased over a 100 second period towards Mach 2.02, and so in stable conditions the throttles would now gradually increase again until they once more reach the maximum limit. At this point, the autothrottles now come out of Mach Hold mode and back into the waiting in the wings standby mode. The autopilot would now cancel it's 600' fly up, demand, returning to a datum of Mach 2. There was a little more complexity built in also, where the difference between the 'commanded' and actual vertical speeds offset the autoplilot Mach 2 datum. This would apply whether the autothrottle had cut in (+600'/min demand) or with the throttles back at maximum (0'/minute demand. A positive climb error tweaked the cruise Mach up slightly, a negative error (eg. in a turn) the converse was true. The effect of all of this complexity was that the aircraft itself could 'scan' until it settled at a point where the throttles could be at maximum, and the speed between Mach 2 and 2.02. On the North Atlantic, with warmer ISA temperatures, there was usually just the initial routine with the autothrottle as you hit the corner point. However at lower lattitudes (eg. LHR BGI) there could be a few initial autothrottle intercepts before things settled down. This whole incredible routine completely took care of the insurmountable problem, a problem that was shown not only to be insurmountable, but was put to bed forever, by people like ChristiaanJ.
I hope that my explanation here does not sound too much like gibberish.

EXWOK
I think you've guessed right as far as my identity goes; it's great that it's not just Concorde pilots I can bore the socks off now
PS. I bet the ex-SEOs LOVED your comments

Dude

Nick Thomas
Just like Christiaanj I'm trying to dig up an accurate flight envelope diagram. (A lot of my Concorde 'technical library' is out on long term loan), but I would suggest that anywhere within Concorde's published flight envelope you never hit any equivilant to Coffin Corner, a la' U2. The whole issue is really one of air DENSITY, rather that pressure, where as you climb at a given Mach Number, your Indicated airspeed (IAS) falls away with altitude. (Velocity of sound being primarily tied to static air temperature). Now if you are climbing in the stratosphere, where temperature is more or less constant up to around 65,000', you can say that your TRUE Airspeed (TAS) is also constant with climb at a given Mach number. But lift and drag are functions of IAS (the equivalent airspeed that the aircraft would 'feel' at sea level) and not TAS. Because the U2 had a very low Maximum allowable Mach number (Mmo) as IAS fell away with altitude, it would get to the point where it's lowest permitted airspeed (we called this VLA) got to within a few knots of Mmo and severe aerodynamic buffering. i.e. you were screwed with nowhere to go but down .
In the case of Concorde, Mach 2 at FL500 was 530KTS, falling to 430KTS at FL600. Although we have less lift due to 100KTS lower IAS, the aircraft is now much lighter (this is the whole principal of cruise/climb) which keeps the universe in balance, but drag is now significantly lower too, getting us better MPG .
On the ASI, the only limitation displayed was Vmo; however the Machmeter did display fwd and aft CG limits at a given Mach number. The ONLY time that Concorde would experience relatively low speeds at altitude was at Top of Descent. I'm a little fuzzy here how it all worked exactly (it's an age thing you know), I'm sure one of the pilots can correct me, but I seem to remember that the autothrottle was disconnected, ALTITUDE HOLD was selected on the AFCS, and the throttles slowly retarded. (If you pulled back too far you'd often get a gentle 'pop surge' from the engines, and you had also to be wary of equipment cooling airflow too). The aircraft was then allowed to gently decelerate, still at TOD altitude, until Mach 1.6, when power was tweaked to give 350KTS IAS and IAS HOLD was selected. The aircraft was now free to carry out her loooong descent to 'normal' altitudes. VLA on Concorde was not directly displayed as you never flew anywhere near it, and also every pilot knew his VLA . (Stray into this and you'd get a 'stick' shaker warning.
I hope this blurb helps Nick

Dude

Nick:

The top of the boundary is FL600, largely an artificial number - the airframe is good for rather higher than this, but I believe air supply and ramp scheduling could become an issue not so far above this level.

Mmo - ditto. As others have said, Mmo was originally going to be higher (M2.2) but was reduced to extend fatigue life as the aircraft design 'grew'.

The significance of the shaded 59% portion of the graph is that it shows the envelope at that CG - in this case the relevant line is the bottom of the shaded area - M1.56. This is the MINIMUM mach number that can be flown with the CG at 59% (normal for supersonic cruise). You will see it represented on the Machmeter (a few pages back) as the "AFT" bug. i.e. you can't fly slower than this without moving the CG forward.

So it can be seen that the decel must be done in concert with CG transfer - and as (mostly) always the designers had made it as straightforward as possible. Transferring forward from Tank 11 using the two electric pumps the rate of txfr pretty well matched the standard decel profile, leaving the FE to make the occasional tweak to keep the flight envelope in concert with the CG envelope through the decel/descent.

In the case of abnormal procedures depriving one of electrical power then some other way had to be found to enable a descent (which required a decel) and that is why there are also two hydraulically driven fuel transfer pumps in tank 11.

It's a bit confusing at first, but there are two overlapping flight envelopes - the speeds/alts drawn on the basic envelope and those determined by the CG postion at the time.

In practice - one had a takeoff CG, a landing CG, a subsonic crz CG, a supersonic cruise CG and the only area one had to keep a close eye on was the transition between the last two. There were several visual and aural warnings to back up the CG and Machmeter bugs.

I have to admit that some of the subsonic fuel burn figures for Concorde were truly eye watering, and without massive engine and airframe modifications there was precious little in service that could be done to improve things. Paradoxically improvements to the supersonic efficiency of the powerplant were easier to implement, and several modifications were implemented, tried or proposed to improve fuel burn:
Way back in the late 1970's we did a major modification to the intakes that increased capture area by 2.5% and gave us typically a 1.6% improvement in trans-Atlantic fuel burn, and although this was our biggest performance improvement modification, there were more:
The famous elevon and rudder trailing edge extension modifications (that due to poor design, produced in later life the water ingress induced honeycomb failures) together with the re-profiled fin leading edge modification, I never saw the performance gains quantified (anyone have any ideas?).
Can anyone here remember the riblet trial? In the mid 1990's Airbus supplied 'stick on' plastic riblets, applied to various areas on the under-side of the wing on G-BOAG. These riblets had very fine undulations moulded into the surface; the idea being that as the air flowed through and around the riblet patches, boundary layer turbulence, and hence induced drag would be reduced. Now, the performance gains (if any) were never quantified, mainly because the riblet patches either peeled off or the surface deteriorated with the continuous thermal cycle. (I was over in JFK when the aircraft first arrived after having the riblets fitted, and as the crew were trying to proudly show me these amazing aerodynamic devices, they were sadly embarassed, as several had dissapeared in the course of a single flight).
There was one modification, proposed by Rolls Royce in the late 1990's that did have quite a lot of potential; this was to increase the engine N1 by around 1.5%. This would have had the effect of increasing engine mass flow and therefore reducing the drag inducing spill of supersonic air over the lower lip of the intake. Depending on the temperature, the performance gains were in the order of a 1.5% improvement in fuel burn at ISA Plus upper atmosphere temperatures ('normal' LHR-JFK) to none at all at significant ISA Minus temperatures (LHR -BGI). The modifacation had been trialed on G-BBDG before her retirement in the early eighties, and was proven in terms of performance enhancement and engine stability. In order to keep TET at the pre-modification level, there was a small increase in N2 commanded also. (The higher N1 required an increase in primary nozzle area, reducing TET). The main reason for the modification not being implemented was one of cost; The Ultra Electronics Engine Control Units were analog units, and the modification was a simple replacement of two resistors per unit. However because ultimate mass flow limitation was also controll by the digital AICU (built by British Aerospace Guided Weapons Division) the cost of getting a software update for this exremely 'mature' unit was found to be prohibitive.
A certain 'brainy' SEO and myself were working on a modification to improve fuel burn on ISA minus sectors. The idea was to force the autopilot, in Max Cruise at low temperatures only , to fly the aircraft close to Mmo, rather than at Max Cruise speed of Mach 2 - 2.02; this would have given us gains of up to 1%, depending on the temperature. The basic electronics involved for the modification were relatively straightforward, but it was never pursued due to the complexity of dealing with temperature shears and the cost of certification.

Dude

Hi DavvaP, and welcome. As far as ice on the wing goes, I'm sure as any of my pilot friends here will agree that she was treated just like a subsonic in that regard; any ice or snow build up on the surfaces of the wings would not be tolerated and would have to be removed before flight. (She may have had a revolutionary wing design, but still this was a wing nonetheless ). She would also require pre-flight chemical anti-icing/de-icing treatment from a ground truck just like the rest, in shall we say, 'less than tropical conditions'. (Winters in Prestwick during crew base training... such fond memories ). As far as active ice protection on the wings, there was a highly sophisticated Lucas electrical 'spraymat' system fitted, but only the wetted areas of the wing, forward of the engines were 'covered'. Two digitall cyclic timers (CTPUs) would automatically regulate cyclic switching on and off of 115 VAC for various load areas of the wing at a time at pilot pre-selectable intervals (2, 4 or 8 seconds). Also as part of this system, there was continuous de-icing for certain other load areas too, so you had a mix of cyclic and continuous de-icing in operation. The whole idea here was to prevent chunks of ice entering and damaging the engines, the only other areas of this electrical de-icing system were the intake lips and side-walls and also the D Box area above the auxilliary inlet vane, built into the spill door. (This would only operate if the auxilliary inlet door itself was open). The whole shooting match would automatically switch itself off, for obvious reasons, above a TAT of 15\xb0 C. (ie. the vast majority of the flight). The only other de-icing system (apart from the galley drain masts) was on the engine inlet guide vanes, but this was purely pneumatic and again would swith itself off above 15\xb0 C.
I think you will find that precious little of Concorde is now not generally available in the public domain, some control software and laws are still I would expect covered by some sort of patent. (That is why when I publiished here the engine 'E Schedule' graphs I deliberately deleted the equations for the various running lines.
Your efficiency question was a valid one; as IAS and Mach number increase the aerodynamic drag (in all it's forms) will generally increase, but the efficiency OF A WELL DESIGNED powerplant wil also increase, and Concorde was definately no exception here. The real beauty of Concorde was just HOW MUCH the powerplant efficiency increased with increasing speed and more than totally eclipsed the aerodynamic drag rise with this increasing speed. At supersonic speeds, the closer you could fly to Vmo/Mmo the lower the fuel burn was. (Especiall true at Mach 2, although the autopilot would hold you Mach 2 (ish) in Max Cruise mode, flying closer to Mmo, Mach 2.04, would save fuel, assuming the static air temoerature was low enough to sustain this). This fact (along with about a million others) produced what we all like to call 'The Magic of Concorde'

Best Regards
Dude

quote:Rolls Royce did some analysis on the flight, and were amazed at how well the propulsion systems coped with some of the temperature sheers that we encountered, sometimes 4 to 5 deg's/second. They said that the prototype AFCS had been defeated by rises of only 0.25 deg's/second ).unquote

Just for the record, the intake control system was designed to cope with a temperature shear of 21 deg C in one mile (about 3 seconds)

quote:Not meaning to go off onto a (yet another) tangent; Negative temperature shears, very common at lower lattidudes, always plagued the development aircraft; you would suddenly accelerate, and in the case of a severe shear, would accelerate and accelerate!! (Your Mach number, quite naturaly, suddenly increased with the falling temperature of course, but because of the powerplant suddenly hitting an area of hyper-efficiencey, the A/C would physically accelerate rapidly, way beyond Mmo). Many modifications were tried to mitigate the effects of severe shears, in the end a clever change to the intake control unit software fixed it. (Thanks to this change the production series A/C would not be capable of level flight Mach numbers of any more than Mach 2.13, remembering that Mmo was set at 2.04).unquote

Not temperature shears, and not AICU modifications (which I see has been discussed in a later posting). But back to the 'shears':

Most of Concorde's flight testing was, naturally, done out of Toulouse and Fairford, i.e. into moderate latitude atmospheres where the tropopause is normally around 36,000 ft so that the supersonic flight testing was done in atmosphers where the temperature doesn't vary with altitude. The autopilot working in Mach hold would see an increase in Mach and apply up elevator to reduce IAS and recover the macg setting. But at the lower latitudes around the equator the atmosphere is different in its large scale characteristics. In particular the tropopause is much, much higher and can get as high as 55,000 ft. Nobody had been up there to see what it was like! Now when the A/P applied up elevator to reduce IAS it went into a region of colder air. But the speed of sound is proportional to air temperature, so as the aircraft ascended the IAS dropped alright but since the ballistic (true) velocity of the aircraft takes a while to change and since the speed of sound had dropped the Mach number was increased, so the A/P seeing this applied more up elevator and the aircraft went up and the speed of sound dropped and ........

Like solving crossword clues, the answer is obvious once you have spent some time finding it!

This phenomenon rather than temperature shears (encountered mainly over the tops of Cb clouds) was the reason for the autopilot modifications which included that clever use of autothrottle (I can use that adjective since it was my French colleagues that devised it)

And before anyone asks; yes, the same problem would relate to subsonic aircraft operating in Mach hold driven by the elevators and flying below the tropopause, but:
a) Subsonic aircraft are old ladies by comparison with Concorde in that they fly at only half the speed. At Concorde velocities even modest changes in pitch attitude can generate some pretty impressive rates of climb or dive!
b) Subsonic aircraft are normally constrained by ATC to fly at fixed flight levels - the use of elevator to control Mach number is not really an option - you have to use an autothrottle.

There was that other problem, also described in later postings, where the aircraft regularly 'rang the bell' when passing through the Vmo/Mmo corner in the lower latitudes, but this was simply due to the additional performance one got in these ISA minus conditions in comparison to the temperatures encountered around the same corner in higher temperatures.

Anyway, the flight test campaign got me my first sight of sunrise over the Arabian desert and my first trip to Asia, so it goes into my Concorde memory bank.

Have a look here :
Flight envelope
Those are the official certified limits.
I doubt somehow anybody went beyond those "for the press".....
But during certification, yes, each of those limits was exceeded, slowly and carefully, to be able to draw those official operational limits on the document.
For instance, the certified Mmo is 2.04. During certification, G-BBDG (202) went to Mach 2.21....

Even while staying within the limits, she could could exceed Mach 1 just below 30,000 ft.

Also, during the acceptance flights after major overhauls, most of those envelope limits would be exceeded by a given margin, to prove the aircraft still met all the certification requirements. Whether any of that was done 'closer to the deck', I wouldn't know.

The 'issues' would vary. Performance would be one issue... Delta Golf (and 01, who went to Mach 2.23) basically "ran out of steam" at that speed.
Another issue would be that 'going beyond the borders' shortened the fatigue life of the airframe, and it was difficult to assess exactly by how much. It was one of the reasons why Delta Golf and Sierra Bravo (the certification aircraft) in the end never went into service.

CJ

For convenience, I repeat Bellerophon's diagram of the flight envelope here.



Mike's earlier question had me scratching my head too, hence my question.
What are the fundamental reasons for each of the limitations, and what were the consequences of going outside them?

Going clockwise from the left, we have :

VLA (lowest admissible speed)
One would expect a curve for constant alpha max against IAS and altitude, not the staircase in the diagram.
Was this for simplicity of use of the diagram?

Max altitude (60,000ft)
This is the 'simplest' one: it was the highest 'safe' altitude from which an emergency descent could be made, in the case of a window blowing out, without having the blood of the pax boil....
Test flights (without pax, and with the crew pressure-breathing oxygen) did go as high as 69,000ft.

Mmo (max operating Mach number)
Mach 2.04 is usually quoted as having been chosen to assure an adequate life of the airframe.
But what effect does a higher Mach number as such have?
Or are Mmo and Tmo (127\xb0C) directly related?

Vmo (max operating speed) = 530kts until 43,000ft
I suppose this is related to structural limits (qmax)?

Vmo reducing to 380/400kts at about 33,000ft
What is the limiting factor here (other than qmax)?

Vmo constant at 380/400kts down to 5,000ft
What is the limiting factor here? The answer will no doubt also explain why this is slightly weight-dependent.

Vmo reducing to 300kts between 5,000ft and 0 ft
Why the sudden change below 5,000ft?

CJ

[quote=ChristiaanJ] VLA (lowest admissible speed)
One would expect a curve for constant alpha max against IAS and altitude, not the staircase in the diagram.
Was this for simplicity of use of the diagram?[quote]

I don't have a complete explanation for all the regions - it was a long time ago and I'll need to dig, but:

Below 16000 ft Vla obviously needs to go as low as Vref to cover landing at elevated airfield altitudes. At present I don't have a satisfactory explanation for 250 kts between 16000 ft and about 45000 ft (250kts/Mach 1.0) A constant value in IAS is what you would get for a constant CLmax (the alphamax is not really the driver). Vla should give a margin above stall, and a quick sum suggests that 250 kts would be consistent with a 1.3Vs condition and a CLmax of about 0.8 up to Mach 1.0, which is not unreasonable, but I am not saying that is the correct interpretation.

From 45000 ft to 60,000 ft I think Vla may be set by manoeuvre requirements. Certainly the forward CG envelope boundary between 1.0M and 1.5M discussed in earlier posts is very close to the requirement to be able to pull 1.2g with half hinge moment available at Vla and heavy weights. Again not certain, but best guess at the moment.

Yes

[quote ]Mmo (max operating Mach number)
Mach 2.04 is usually quoted as having been chosen to assure an adequate life of the airframe.
But what effect does a higher Mach number as such have?
Or are Mmo and Tmo (127\xb0C) directly related?[quote]

I have always been puzzled by this statement as one does not normally associate Mach Number with a life limit. Going through my collection of lectures I found another, more plausible explanation:

quote" The scheduled cruise mach Number was 2.0. associated with a structural total temperature of 400 degK. Above ISA +5 Mc was cut back to maintain 400 degK.
To cope with variations of flight mach Number about Mc associated with often rapid and significant changes in wind and temperature which occur particularly in the vicinity of the tropopause (which can of course be as high as 60,000 ft in the tropics) a maximum operating Mach Number (Mmo) of 2.04 is selected" unquote [Leynaert, Collard and Brown, AGARD Flight Mechanics Symposium October 1983]

This is much more in line with my memory on this subject.

Yes in principle, but it is a bit chicken and egg, since 530 kts also represents a very good choice for best performance, and I am sure that the stucture would have been built to cope if a higher speed was needed for performance reasons. To the best of my knowledge there is no structural design case that would be critical in this flight regime (other than flutter of course)

Same as earlier - transonic manoeuvre requirements with failed hydraulics, matched to aircraft weight and CG envelope possibilities.

Same again.

I'm not entirely sure, but:
a) there is absolutely no advantage is having a high Vmo at low altitudes as it could not be exploited even if one wanted to because of ATC limitations to 250 kts below 10,000 ft (in the USA at least)
b) there are a lot of things that get rapidly worse if you encounter them at high speed and which are anyway more likely at low altitude - hail, birds etc.
So why store up trouble for yourself!

CliveL

As usual Dude you beat me to it! I really must give up having another life

As Dude says, the 'cruise' condition was set by the aircraft specification for transatlantic range on an 85% (ISA +5) day and the chosen mach Number was 2.0 (of which more anon). This gives a Total Temperature of 400.1 deg K. [Dude, I know your pipe-smoking thermodynamicist and he was having you on - he is quite capable of memorising the square/square root of 407.6 or whatever!]
To give margins for sudden changes in ambient temperature (we had to cater for a 21 deg change in one mile) the Mmo was set at 2.04 which matches 400 degK at ISA +1. In theory then we could have flown faster than our chose Mmo at anything colder than this, but there are two limits:

1) The object is not to fly as fast as you can but to fly with minimum miles/gallon. If you have a nice cold day and enough thrust to go either faster or higher which do you choose? For best specific range you go higher every time.
2) The thing that everyone forgets is that civil aircraft have to have margins around their authorised envelope. In Concorde's case these were set principally by the intake limits and engine surge.

Dude also says quite correctly that 101 flew to 2.23M but the production aircraft was limited to 2.13M. Now you may not believe this, but 101 could fly faster than the production aircraft because she (101) leaked like a sieve!.

I doubt I will get away with that without some explanation

Once you get past a certain Mach Number the airflow into the intake is fixed. The performance (intake pressure recovery and engine face flow distortion) then depends on how this air is shared between the engine and the throat 'bleed'. This bleed was ducted over the engine as cooling air and then exhausted (in principle) throught the annulus formed between the expanding primary jet and the fixed walls of the con-di nozzle. But if you took, or tried to take, more bleed air the intake pressure recovery went up and the primary jet pipe pressure went up with it. This meant that the primary jet expanded more and squeezed the available annulus area which restricted the amount of bleed air one could take.

Obviously if there are alternative exit paths between intake and final nozzle then you can take more bleed air off and the engine face flow distortions will benefit along with the surge margin. 101 was fairly 'leaky' in this respect, particularly around the thrust reverser buckets on the original nozzle design. This meant that 101's intake distortions were lower than the production aircraft so she could fly faster without surge - at least with the first attempt at intake control 'laws'. We managed to tweak most of the margin back eventually. Engine bay leaks were good for surge margin but VERY bad news for m.p.g.!

Here are a couple of diagrams to show what I mean. the first shows the surge lines for the various aircraft variants and also the N1 limiter Dude was talking about. NB: the X-axis is LOCAL Mach Number not freestream. The difference comes from the compression of the underwing flow by the bit of the wing ahead of the intake. Mmo + 0.2 is shown

">The next shows the surge free boundaries in sideslip and normal acceleration. You can see the zero 'g' capability Dude was enthusing over. ">

As for 'high speed stall', I don't think we ever contemplated trying it! Our requirements in 'g' capability were defined and that was it. Besides, the aircraft would fly like the proverbial stone-built outbuilding at those sorts of conditions so I don't think one would have been able to get anywhere near a stall in the conventional sense. Stall as commonly defined for subsonics (deterrent buffet) might have been another matter, but I don't remember anything.

Cheers

At low altitude think 455 ktCAS.
Tmo was a long exposure structural limit
Mmo was an intake limit
Vmo was a structural (flutter) limit

Bit of a hypothetical question requiring a judgemental response!
My short answer would be not much more than the certified limits - at least not without significant modifications.
FL680 was achieved at the end of a zoom climb, so the Mach No was a lot less than 2.0
M2.23 was in a shallow dive. The object was to demonstrate sufficient margin to avoid surge following the worst temperature transient specified in the TSS regulations. To that end both the intake laws and engine operating lines were tweaked as functions of Mach No to minimise intake flow distortions and maximise surge margin. The result was a long way from the performance optimum one would need for steady cruise.
The power plant was being pushed to its limits at this Mach No.
(As an aside, the subsonic rules make no mention of temperature transients as a cause of Mach exceedences. Some recent incidents suggest this could usefully be reviewed)
The altitude limit could perhaps be more readily expamded. The aircraft normally flew a cruise climb bcause at Concorde cruising altitudes there was no ATC conflict. The altitude was very sensitive to ambient temperature and aircraft weight. FL600 would be associated with end of cruise on a coolish day.
To usefully increase cruise altitude would require more engine thrust, but this could only be obtained by increasing engine TET which would screw the engine fatigue life.
Increasing Mmo from 2.04 would need an increase in Tmo (400 deg K) at any temperature above (from memory) ISA. This in turn would affect the airframe fatigue life unless the structural material were changed. Even then, there were a lot of nonmetallic bits (seals etc) that would also have needed replacement.
Sorry if this is a gloomy assessment, but that is the way I see it!