Posts about: "Vmo" [Posts: 19 Page: 1 of 1]¶

Biggles78
You are right on the button. Under NORMAL circumstances, Concorde never flew supersonically in level flight. You would always follow the Vmo bug on the ASI during the supersonic climb. (The ASI pointer actually nudged into the bug; it was a beautiful design). Initially this would be at a constant Vc of 400 kts, the 400 KT segment then went off towards 530 KTS as you climbed. You then 'stuck' to 530 knots until a fraction over 50,000', when 530 KTS became Mach 2. You would then continue the climb at between Mach 2 and around Mach 2.02, depending on the temperature of the day. (the colder the temperature, the faster you tended to fly). There was an extremely complex AFCS mode for the supersonic climb, that I promise to cover in anaother post.
So yes, on the whole, TOC did equal TOD.
The 'subsonic climb' wasn't quite as you thought; you'd normally subsonic climb to FL280, staying there (at Mach 0.95) until the acceleration point. Mach 0.95 was 'subsonic cruise'. But you were on the right track.
Oh, and NOPE, they never boomed us either
Nick Thomas
Keeping the powerplants as separate as possible was a major design headache, but generally they were just that; there was a titanium centre wall between the two engines and a really substantial heatshield above the engine also, to protect the wing above. To give you an idea how all this worked in practice, in 1980 G-BOAF, flying at Mach 2 between JFK and LHR had a major failure of one of the engines, caused by a defective material ingot used in the forging of one of the 1st stage LP compressor blades; which was subsequently shed. (The analysis done by Rolls Royce ensured that no such incident ever happened again in the life of Concorde). The resulting mayhem terminated in a large amount of engine debris flying around, and a titanium fire burning in the engine bay also. The aircraft however decelerated and landed at Shannon safely. On inspection, although there was extensive damage found in the engine bay, the adjacent engine was completely unmarked, protected by the titanium centre wall, and more importantly, when the heat shield werer removed, the wing was found to be completely undamaged!
The only problem you ever had with the dual nacelle arrangement was if you had an engine surge above Mach 1.6 (These were relatively rare, but could happen with an engine or intake control system malfuntion). If one engine surged, the other would surge in sympathy, because of the shock system being expelled from one intake severely distorting the airflow into it's neighbour. These surges were loud, quite scary (to the crew that is, most passengers never noticed much), but in themselves did no damage at all. Delicate movement of the throttles (employed during the subsequent surge drill) would invariably restore peace and harmony again to all. (The intake on Concorde was self-starting, so no manual movement of the intake variable surfaces should be needed in this event). After this was over, normal flying was resumed again As I said before, these events were relatively rare, but when they did occur, they would be dealt with smartly and professionally; the engine and intake structure being undamaged. (Post surge inspetion checks were always carried out on the ground after an event, on both engine and intake, but nothing much was EVER found).
The reduction of fuel flow as you climbed was quite interesting. Although the throttles would be 'at the wall' (dry power remember), the electronic control system was constantly winding fuel off as a function of Static Air Temperature, as well as falling Total Pressure. The system was always 'tweaking' as you climbed, and you only used as much fuel as you really needed to stay at Mach 2. There were various ratings that would also be manually selected at various phases of flight; each rating change 'detuned' the engine slightly, so yes, you did not run the engine when flying fast at anywhere near the levels you did at lower speeds/altitudes. The engine final ratings were changed from 'Climb' to 'Cruise' manually at FL 500, just as you hit Mach 2).

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.

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

It was certificated - up to a point. Problematic? Maybe not, but it was a part of the flt envelope to be treated with respect.

Obviously there are no spoilers, and once you translate to 'vortex lift' (stalled in conventional terms) there is definitely no shortage of drag. (This happened at about 250kts at landing weight).

Supersonic - it was certainly no sailplane and an ability to increase drag wasn't required.

So - there is a bit of the flight envelope where you are subsonic, descending at about 350kts IAS, where you may need a bit of drag; e.g. to make the FL140 limit on the OCK 1A SID (as it then was) to LHR.

To facilitate this, engines 2 and 3 could be selected to reverse idle within certain strict limitations (most of which have now left my brain). The mechanism was to ask the SFE to arm the system on his panel and then to select reverse on the inboards. Where the system was slightly unreliable was that you were running the air-driven buckets with the engines at idle thrust - consequently they sometimes didn't make a full reverse selection, in which case you canx reverse on that engine and managed on one.

Clearly the big event would be if they didn't translate into fwd thrust, which is one of the reasons it wasn't done below 10 000'. I'm not aware of this happening.

To be honest it was only really used when ATC threw an alt constraint at you during the descent, because in general if you just pitched down to 380kts (Vmo when subsonic at typical approach weights) you would get the height off comfortably.

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

Interesting & nostalgic thread. Nice to see this monumental aviation achievement still generates such passion...

In case it's of interest (and suitable health warning as the memory fades)...

The heat did evaporate water vapour in the airframe - reducing corrosion. I remember when the 5 BA aircraft were returned to service, after the post-accident mods, their weight and balance certificates were prepared and found to be out by (IIRC) more than a tonne. This represented water in the airframe present after a year on the ground, and was gone again after a couple hours of supercruise on return to service. Back to the weighbridge for new W&B Certificates....

Vortex lift caused buffet which felt very similar to a conventional wing's stall/low speed buffet. At landing weights (I hate the trend of using the term "mass": weight is a force, mass is not!) you felt the buffet start as you reduced speed (CAS: Vc) to about 250kts. It was handy as a reminder that you should select visor down / nose to five below 250kts (the recommendation was as you slowed through 270kts, but latterly we were in the habit of holding at 250kts nose/visor up - I think TCAS was quoted as a backup to the more limited visibility in that config). At takeoff weights, the buffet went at more like 270kts accelerating. So I'm pretty sure there was no vortex lift at AoA > 7 degrees (250kts at LW).

Recommended subsonic cruise at MTOW was F260 / M0.95 which was equal to Vmo of 400kts (CAS). It was best cruise because Vc=400kts was also min drag at MTOW. F280 meant a slightly more draggy speed of 384kts, but some preferred it because when cleared to climb & accelerate supersonic (the official expression was "go for it") it gave you a bit of slack against Vmo when eng put the reheats in. But we tended to ignore the overspeed warning anyway: it was supposed to go really really fast...

We never flew with visor down and nose up unless it was bust - that config was only used during pushback (except one captain who always thought it looked better visor up....). Visor down max Vc was 325kts/M0.8 so it would limit subsonic cruise, and besides it made a racket like that.

It was a beaut in x-winds - a total lack of yaw-roll couple meant you just straightened the 'plane up with rudder and carried on into the flare as normal. No roll to counteract, and the sideways "lift" created by the rudder deflection on the fin pretty much equalled the x-wind drift. Nice.

Wind limits were Crosswind 30kts (15kts contaminated or autoland), Headwind for autoland 25kts (or manual "reduced noise" approach: that's a technical way we used to reduce the noise footprint down to 800' by flying at 190kts then reducing to a target speed of Vref+7kts at that point). Tailwind 10kts. All these limits were, of course, subject to "on the day" performance limits calculated at the time. I seem to remember there was an over-arching limit of 6000' on r/w length, subject again to "on the day" performance limits. OK, I cheated on this paragraph and dug out FM Vol 2a.

There were loads of other limitations which were, by and large, more "esoteric" than a conventional airliner and which had to be learned for the conversion course. It really made the head hurt, and would have been impossible without a big loverrly picture of the beast on the wall chucking out yellow smoke and making noise. Even a static picture of her seemed to make noise...

No one who flew it could really believe their luck, but one thing for sure is "they don't build them like that any more"...

Ahhhhhhhhhhhhhh..........

They weren't used for take off because there was no relevant mode. The initial climb was to hold 250kts after takeoff until a predetermined time for noise abatement (a little over a minute ex-LHR, less ex-JFK) and then start to accelerate after the NA thrust reduction.

It was a balancing act and different for each departure - the RoC actually went up the faster you went so you were easing up, maximising the acceleration, while ensuring you (just) made the SID alt requirements.

So, apart from a few seconds holding 250kts, there really wasn't a mode that would work in pitch; you would have to take vert speed and be constantly asking the NHP to select different VS's and HDGs and they had quite enough to do already. No benefit, so don't use it.

Once you'd got to Vmo during the SID (it was 400kts at that point) you could use Max Climb mode (see earlier discussions) and that was generally when the FD was engaged.

It was permissable to engage the AP at an early stage, in which case HDG mode and Pitch Hold would be used, but it was more effort than hand-flying, less accurate and less fun so that was a rare event.

On approach, if an ILS was being flown, you are correct that the FD could be used, although for a typical approach it needed to be off at 300'. This was because wherever possible we flew a 'Reduced Noise Approach' (again, see earlier comments) which consisted of holding 190kts to 800' then reducing to final speed to be stable by 300'. AP/FD had to out by 300' in this case, owing to the very tight pitch control required for which it wasn't certificated (although it could carry out a very good coupled approach and landing if given a more stable approach).

In summary - a very 'hands-on' aeroplane. And all the better for it

quote:Re the autotrim, tell us some more?unquote

It is a little complicated, but let me go back half a step.
Concorde was not certificated to FARs or BCAR (the French code was essentially a straight translation of FAR) but to completely new set of requirements known as TSS (Transport Supersonique Standards). The old UK ARB had initiated discussions about these even before cooperation negotiations had started. The result was that young, junior engineers got to debate the basics of airworthiness rules with older, experienced airworthiness specialists. In hindsight it was wonderful training!

But to get to the point, it was this thinking that allowed us to ignore some of the older rules which, although great for the aircraft flying at the time they were written, had little or no relevance to SSTs. We could interpret that as trying to find out what the pilots really wanted the aircraft to do and then to try and provide it.

In the particular case of trim/speed stability it was quite clear that what they wanted was an aircraft that could be flown with minimal trim changes and which once trimmed would not go wandering off all over the place. We also knew that in some cases the 'elevator angle per 'g' ' could get as low as one degree/g in some cases and that the pilot could not tell exactly where his hands were positioned to that precision, although he would always know if he was pushing or pulling. So we could abandon the old rules for stick movement and instead supply classic stick force stability for deviations from the trimmed state.

All this had to be matched to the varying aerodynamics through the transonic region (where everything varies rapidly) and the fuel transfer system characteristics. The resulting Mach trim laws were quite complex and were not, in fact just Mach Number sensitive. We also had two airspeed (Vcas) terms, one of which had a variable gain which was itself Mach dependent and kicked in above Vmo = 5kts and the other was a straight nose up elevator command as a function of Vcas. The Mach trim itself was highly nonlinear. The best way to illustrate this is probably a diagram but now I've run into another gap in my knowledge of this thread - how do I do that?
Anyway, the result was that the fuel transfer held the trim setting variation down to between 2 deg down to 1.5 deg up through the acceleration from 0.95M up to 0.5 deg down at Mach 2.0. Without fuel transfer the trim at Mach 2 would have been closer to 10 deg. The trim between say 0.95 and 1.2 varies in a nonlinear fashion and the Mach trim law shows roughly similar variations.

But the best measure of our success is the comments we are getting here from the guys who actually had to fly it.

Clive


[IMG]file:///C:/Users/Clive/AppData/Local/Temp/moz-screenshot.png[/IMG][IMG]file:///C:/Users/Clive/AppData/Local/Temp/moz-screenshot-1.png[/IMG]

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

I personally doubt very much if the Emergency Pilot would be the 'way in' for the sidestick input. EFC ROLL commands were inputed from the SFC computer to the Autostab computers as 'stab demands' and therefore drove the MIDDLE and OUTER elevons only for roll. To make matters worse, if your test flight was really 'exciting' and you found yourself at any time at Vmo + 20 KTS, roll control would be through the middle elevons ONLY. I'm with CliveL in that the most likely scenario would be for the demand would feed via a D/A converter somehow. (It would be great to find out though).
I would have thought that the whole venture was a proof of concept by SFENA for future implementation in the Airbus family. This excersise would have been both costly and highly complex at system level, any other reason would really have been quite daft.

Best Regards
Dude

gordonroxburgh
It matters not I'm afraid Gordon. I would not have thought that anyone would design ANY flying control system, experimental or otherwise, that does not have full potential authority in all axis. As we do not know what the PROPOSED flight regime was, on the part of SFENA and Aerospatiale,we also can not assume that any particular manoeuvr would not have been considered. (But as I said before, it would be great to find out the whole story).
The limited authority for roll autostabilisation (and hence Emergency Flight Control) was of course a very deliberate piece of design. (You could test the Emergency Pilot on the ground at ADC Test 2 (Which simulated several seperate overspeeds, including Vmo +20) and when you put in a roll demand (against some resistance), only the MIDDLE elevons deflected. It really looked wierd on the ICOVOL as well as outside the aircraft.
(To any chaps or chapesses who are not aware, above Vmo+20 KCAS, a system known as OUTER ELEVON NEUTRALISATION was invoked, where any input demand to the outer elevons was met by an automatic equal and OPPOSITE input, that of course completely neutralised the demand, giving a zero OUTER elevon deflection).

Best regards
Dude

<< I'm guessing you mean rate of climb rather than IAS? >>
<<No, I meant the airspeed you'd be flying at while climbing (post takeoff)>>

OK, then the answer to your Q's:
Also what was the typical climb speed
- At lift-off? About 200kts
- Once 240 kts is achieved? 240kts
- At minimum maneuvering speed at typical takeoff weight? Vla after takeoff was V2 until 15,000'. I.E. about 220kts
- At MTOGW? V2 didn't vary much by weight

Out of JFK we flew at Vmo once further than 12nms from the coast. Vmo=400kts IAS at low level.

Out of LHR overland the IAS restriction was 300kts until past the speed limit point early in the SID - much less draggy than 250kts and hence better climb rates. But you'd quickly be released to get to 400kts (barder's pole) where it was designed to be flown.

<<Why higher speed? That have to do with shockwaves and the resulting pressure distribution differences?>>
The flight envelope was bigger and more complex than subsonic types: it was developed in flight test and probably had many considerations involved. I think someone posted it earlier in this thread in graphical form (from the flight manual) if you want to see it. In practice, you had to be aware of three basic parameters - IAS, Mach and CG position (the CG "corridor"). Once understood, it wasn't that difficult to keep up with it...and the IAS and Machmeters had barber's poles handily programmed to show the limiting values (including, cleverly, max temp on the nose Tmo=127 degrees celcius).

Regarding climb rates - best ROC was at 400kts (MTOW) or 380kts (MLW). As speed reduced below that, drag increased and ROC reduced. At MTOW and 400kts you'd get about 4000fpm max dry power. At 250kts it was all noise and very few feet per minute - after noise abate procedures you had to lower the nose, just barely climb, and get IAS up toward min drag as soon as possible. With an engine failed go for 300kts minimum - Vmo as soon as you can.

<<shockwaves and the resulting pressure distribution differences>>
You had to avoid the "transonic" region due to these effects: maximum subsonic cruise was 0.95M due to the auto-stabilised flying controls become over-active as shockwaves started to "dance" around the airframe (usually asymmetrically). This calmed down by about 1.3M in the acceleration (when the intake ramps started to do their thing). To accelerate to 2.0M you needed reheat until 1.7M so you didn't hang around between 0.95M and 1.7M. FL260 was best for subsonic cruise because at that level 400kts IAS = 0.95M...

Last time I logged in someone was asking why the Vmo/Mmo was the way it was, but it seems to have disappeared along with Bellerophon's suggestion that someone else might be able to throw some light on it. This might help.

SORRY - senior moment - this should have been posted on another thread!


To be honest I can't remember exactly why 530 kts was chosen for the supersonic Vmo, but it was probably the best climb speed.
Mmo/Tmo was limited by a combination of intake and structural temperature.
The 'cut-off' in the sloping/530 kts boundary was, if I remember correctly, to avoid a minor aeroelastic problem at the Vd/Md condition one arrived at from that corner.
The variation of Vmo with weight was a device which, when associated with the CG corridor, allowed the aircraft to meet the manoeuvre requirements when flying on half hydraulics.
400 kts CAS gave 0.93M at around 28000 ft if I recall correctly, which was just below drag rise and gave optimum subsonic cruise performance

I'm betting low altitude limits for Concorde are less about hull temps and possibly more about Vmo (airframe) limitations. (...and surely not thrust!)

Meanwhile, I await the truth as you do Octane, from our resident experts, to whom I offer warm thanks and have the greatest admiration!

I've no idea about testing, but in line operations the Vmo peaked at 530kts, having started off rather lower depending on alt and mass. (Normally 380-400). I'm pretty certain there's at least one flt envelope earlier in this thread.

VAPILOT is spot on regarding the reasons for Vmo, of course.

Doesn't answer your 'during flight testing' question, I concede. Sorry!

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

Sorry, you will have to excuse my confusing statement as I am writing from. hospital bed and not at my sharpest!

To be clear, the original question related to maximum speed which I took to be Vd - 455 kts from about FL 60 up to about FL 360
This was the flutter clearance and was usually acheived in a dive. Vmo was thenwhat you got by backing off to give the statutory margins. Not strictly a flutter limit though limited by flutter! Vmo of course could be flown dry

It's actually a window, with a red warning annunciator/flag behind it, that flags a problem with the functioning of the Vmo needle (yellow and black "speed-limit" barber pole.)

See section 4 on this web page: Heritage Concorde

On the extreme right - microscopic explanatory type (sorry!)

Since one of its triggering events is a loss of electrical power, the red flag will be showing unless the aircraft electrical system is powered up (my guess). So it shows red in most cockpit photos.