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# Concorde question

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# Concorde question

5th Sep 2010, 21:04

Join Date: Jun 2002
Location: UK
Posts: 116
I did experience reverse thrust (in flight) once and, as I recall, we needed to get down somewhat quicker than normal.

From my usual seat (25A) the airframe vibrated a good deal and you could feel that the descent was more rapid than usual.

However, after getting down we had to go twice (or maybe 3 times) round in the hold before lining up.

I always looked forward to holding as it was a little bit like being in a race car with the aircraft being powered round the loops all the time much like you would throw a go-kart round the track with full throttle and opposite lock. Normally I was out of luck as we usually went straight in.

6th Sep 2010, 09:17

Join Date: Jan 2008
Location: FL 600. West of Mongolia
Posts: 456
Coffin Corner

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

6th Sep 2010, 09:43

Join Date: Jan 2008
Location: London
Age: 46
Posts: 26
Thanks M2dude, I'm sure you and your ex-colleagues are enjoying contributing to the thread as much as the rest of us are enjoying reading it!

6th Sep 2010, 12:18

Join Date: Mar 2007
Location: Cardiff UK
Age: 64
Posts: 118
Thanks CJ and M2Dude for such complete answers. When I was typing "air pressure" I knew it was not quite the right term; so M2dude thanks for explaining that the correct term is "air density".
Would I be right in thinking that TAS is different from speed over the ground? I presume GS would be TAS plus or minus wind speed.
Regards
Nick

6th Sep 2010, 12:52

Join Date: Dec 2000
Location: UK
Posts: 255
M2dude

Is this what you're looking for?

Best Regards

Bellerophon

6th Sep 2010, 12:52

Join Date: Nov 2007
Location: Devon
Posts: 39
Although generally ignorant about aircraft, I have been absolutely rivetted by this thread, and can only sit in awe at the complexity and perfection that has been revealed here.
I've pretty much managed to keep up with the principals and technicalities involved, but there is one thing I don't understand. Could you explain to a complete novice the relationship between IAS, TAS ,GS and Mach no.?
Apologies if this is obvious to most here, but you can't get away with writing such a brilliant account of such a wonderful aircraft without attracting the odd ignorant byestander!

6th Sep 2010, 14:14

Join Date: Jun 2008
Location: Outer space
Age: 47
Posts: 3

I'm not a professional pilot, just a humble owner of a PPL with a very strong interest in aviation and a long time reader here (esp. in the Tech Log board). I've never posted here, because I prefer to read and learn from those who know it better, but this thread has finally managed to lure me out of lurking mode
If anyone has seen the video of AF landing at BZZ after the first post-grounding test flight, you may have noticed that you can hear the buckets translating to reverse even over the noise of the blustery wind and four Olympus 593's at idle.
Like in this video?

There is a strange high pitch sound that kicks in for about a second in the same moment the nose wheel makes contact with the ground and before the actual reverse thrust sound can be heard.

Thanks to all for sharing all this information about one of the most fascinating machines ever created by human mankind.

6th Sep 2010, 14:40

Join Date: Jan 2005
Location: France
Posts: 2,319
Hi all,

Re the flight envelope diagram, Bellerophon got ahead of me, and his scan is cleaner than mine!

I've got a second one, which is basically the same, but has the envelope for a CG of 55% and for a CG of 59% hatched in.

For anybody who wants the full scans to print them out in A4, use these links.

Flight envelope A4 format

light envelope w. CG limits A4 format

Apologies for the mediocre quality of the scans...

CJ

6th Sep 2010, 15:10

Join Date: Sep 2010
Location: Devon
Posts: 91
Nice to see another Devon person here. I agree this is a great thread. Here is a simple explanation of airspeed.

A stationary aircraft, just like anything else is subject to static air pressure, which varies from place to place, day to day and decreases with an increase in altitude and.or a rise in temperature.

Once that aircraft starts moving through the air, it also experiences dynamic pressure which is the force of the air particles it meets as it moves. Of course the static pressure remains too, so the aircraft is experiencing static + dynamic which is called pitot pressure or total pressure. The laws of physics say that total pressure remains constant.

Indicated Airspeed (IAS) is a measurement of dynamic pressure which is described as 1/2 rho (rho is air density) X V2 (V= velocity). This is very important when talking about principles of flight (thrust, drag, stall speed etc)

Air density is a function of pressure and temperature, so if density (rho) is reduced V2 which is True Airspeed (TAS) has to have increased at a given IAS. (ie the same number of particles hit the aircraft in a given time)

In a nil wind situation TAS would be the same as your speed over the ground (GS). Groundspeed is then calculated by adding or subtracting wind speed from TAS. eg TAS 150kts, tail wind 20 kts = GS 170kts.

Fairly simple at low speeds. At speeds of 300kts and above the compressibility of air becomes an issue and has to be allowed for - the air is compressed as it stops against the aircraft. So TAS also includes an adjustment to compressibility.

Mach no is a percentage value of the speed of sound ie 0.85 = 85% speed of sound. Unfortunately the speed of sound changes with pressure but at sea level is around 760 mph and decreases as pressure decreases.

Aerodynamically things start going pear shaped as an aircraft nears the speed of sound as the airflow over parts of the aircraft can go supersonic. Aircraft approaching these kind of speeds have to fly mach numbers. Airliners typically fly Mach 0.80-0.85.

6th Sep 2010, 15:25

Join Date: Mar 2007
Location: Cardiff UK
Age: 64
Posts: 118
Thanks Bellerophon and CJ for posting the flight envelope. With regard to the CofG of 59%, I notice that the upper part of the envelope abuts the MMO boundary.So is the Cof G of 59% the determining factor for the MMO or is it some other factor?
Regards
Nick

6th Sep 2010, 16:42

Join Date: Jan 2005
Location: France
Posts: 2,319
Originally Posted by Shanewhite
Could you explain to a complete novice the relationship between IAS, TAS ,GS and Mach no.?
I'll try....

TAS (true air speed) is simple, it's the true speed of the aircraft through the air.

GS (ground speed) is equally simple, it's the speed of the aircraft over the ground i.e., TAS plus the component of the wind along the flight path. If your TAS is 1300 mph and you have a 100 mph tailwind, your GS will be 1400 mph.

Mach no. is TAS divided by the local speed of sound.
The speed of sound in air depends almost exclusively on the temperature: in a 'standard' atmosphere it's 760 mph on the ground at +15°C, drops to 660 mph at 37000ft / -57°C and remains constant above that height.

It's IAS (indicated air speed) that's complicated....

Lift, drag, control forces, stability, etc. are all proportional to the 'dynamic pressure' that the aircraft experiences moving through the air.
This 'dynamic pressure' is proportional to density x TAS squared.

Now take an aircraft flying along horizontally at sea level, say at 200 mph.
Lift = weight, so the aircraft stays on a horizontal flight path.

Take this same plane, without changing anything else, to an altitude where the air density is half that at sea level.

Dynamic pressure is now half, so the lift is half as well, but the weight is still the same, so the aircraft can no longer fly horizontally.

So what do we do... we increase the TAS until the dynamic pressure is the same as it was at 200 mph at sea level.
Half the density, so (TAS squared) has to be double, so TAS has to be increased to 1.4 (sqrt of 2) x 200 mph = 280 mph.

This is somewhat confusing for the pilot.... He flies the same aircraft, same weight, same angle of attack, etc. but not the same TAS... he'd have to mentally juggle airspeed and density (altitude) the whole time to maintain horizontal flight at different levels.

It would be much easier if he had an indicator showing dynamic pressure... and maintain that constant for different altitudes.
This is where IAS comes in.

Stick your hand out of a car window. The force you feel is due to the dynamic pressure.
Stick a tube, closed at one end, into the airstream and measure the pressure with a basic pressure indicator, that's your dynamic pressure.

Now the 'clever' bit. Mark your indicator, not in bar or psi, but in mph, so that at sea level it will indicate the same speed as the TAS (200 mph in the example).

Now, same as above, go and fly at an altitude where the density is half, with a TAS of 280 mph. Your indicator will still show 200 mph, showing you that the dynamic pressure, hence the forces (lift, drag, etc.) are the same as those at sea level at 200 mph.

It's the IAS that tells you what happens to your aircraft in terms of the forces and aerodynamics, and that's why figures such as the Vne (never-exceed speed) or the stall speed are always in mph or knots IAS, not TAS.

As a matter of fact, a pilot is not very much interested in TAS as such, and most aircraft do not even have a TAS indicator.

It's not until you start approaching the speed of sound that TAS becomes important, and even then it's not TAS as such that's used but its relation to the speed of sound, i.e., the Mach number.

OK, Shanewhite, it's a long and complicated description, but maybe it helps?

CJ

Edit PS : I see mykul10 already had a go as well. So much the better, explanations from two different sides nearly always complement each other!

6th Sep 2010, 17:05

Join Date: Nov 2007
Location: Devon
Posts: 39
Thankyou guys for your lucid explanations. Things are now a great deal clearer, and I now understand how you can be doing 550 knots at Mach 2, which previously seemed impossible!

6th Sep 2010, 17:13

Join Date: Sep 2007
Location: UK
Age: 52
Posts: 124
Nick:

So is the Cof G of 59% the determining factor for the MMO or is it some other factor?
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.

6th Sep 2010, 17:26

Join Date: Jan 2005
Location: France
Posts: 2,319
Originally Posted by Nick Thomas
With regard to the CofG of 59%, I notice that the upper part of the envelope abuts the MMO boundary.So is the Cof G of 59% the determining factor for the MMO or is it some other factor?
Nick,
No... for the manual they've just hatched the limits for a couple of example CGs inside the existing overall limits.
It's the M=1.56 limit that's related to CG, the Mmo=2.04 boundary is purely certification-related.
I think the CG "corridor" is already posted, if not I'll do so.

It's more the other way around... the aft CG limit is 59% for anything above about Mach 1.5.

CJ

6th Sep 2010, 17:34

Join Date: Sep 2007
Location: UK
Age: 52
Posts: 124
For the sharp-eyed who may have actually gone back to look at Bellerophon's picture, you may note that the AFT bug is lower than M1.56, contrary to the flt envelope above. Billy ruffian will know for sure, but here's my surmise:

FL600 level flt means he was going to BGI. The length of this sector was, in crude terms, about 200NM more than the quoted max range of the aircraft, so the range envelope was being pushed a little.

Because there was no land you could stay supersonic all the way, so at the end of the cruise you would be supersonic, but with relatively little fuel in the tanks, and most of it in Tank 11 (at the back) to keep the CG aft. Even with a tweak to tanks 1&4 to run them at 50% level, eventually the CG would come forward as you burnt fuel out of tank 11. That's what is probably happening in Bellerophon's photo, hence the 'AFT' Mach bug being at a lower Mach. If the FCPI ('ICOVOL') was in the frame I wager you would see the elevons a somewhat above the optimum 1/2degree down position

The bugger was this; if you were a little tight on fuel, just when you wanted to maximise the time spent supersonic you'd have to start an early decel because there just wasn't enough fuel left to maintain the CG far enough aft to sustain M2.

All part of the fun, and why every sector was interesting and rewarding.

6th Sep 2010, 18:14

Join Date: Jan 2005
Location: France
Posts: 2,319
EXWOK,
On Bellerophon's picture, it looks as if the FWD bug on the Mach meter is still at about M=2.2.... and just the barber pole at about M=2.04 as you would expect.
At that exact point in the picture I think he just hit his head against the ceiling... (FL600) but could still stay there a bit more.

CJ

6th Sep 2010, 20:49

Join Date: Sep 2007
Location: UK
Age: 52
Posts: 124
Oooops - thanks for pointing out my AOT logic there. Note to self: don't post after returning from a night flight!

The other reason for the disparity of bugs on the Machmeter vs the flt envelope is whether they relate to the first or second M/CG warning. I can't remember and don't have the manuals to hand. I do recall that it was more accurate/practical to monitor the speed-driven limits on the CG indicator rather than the CG-driven limits on the machmeter.

6th Sep 2010, 22:18

Join Date: Dec 2000
Location: UK
Posts: 255
shanewhite

Following the excellent explanation given by ChristiaanJ about the relationships between OAT, Mach number, TAS and IAS (which I have now copied and shall shamelessly pass off as my own work in future ) if you wish to see how these relationships work in practice you might look back at the photo posted at reply #66.

You will see that at FL600 the aircraft had a GS of 1,139 kts whilst flying at M2.00 and an IAS of 429 kts.

We don’t know what the wind was, nor what the TAS or OAT were, but we can easily deduce that:
• If the OAT was standard at FL600, at -56.5°C, then, as at that temperature M2.00 equates to 1,147 kts TAS, in order to have a GS of 1,139 kts, she must have been flying into an 8 kt headwind.
• If the wind at that altitude was calm, then her GS of 1,139 kts must have been the same as her TAS. For M2.00 to be 1,139 kts TAS, then the temperature at FL600 must have been 3°C colder than standard, at -59.5°C.
• If, as was typical on a LHR-BGI sector, the OAT at FL600 was 10°C colder than standard, at -66.5°C, then M2.00 would equate to 1,120 kts TAS, so to have a GS of 1,139 kts, she must have been flying in a 19 kt tailwind.

For obtaining Mach/TAS/Temp values quickly and easily, as well as other useful information on the atmosphere, this Standard Atmosphere Calculator website is extremely useful.

Best Regards

Bellerophon

6th Sep 2010, 22:22

Join Date: Jun 2002
Location: Geneva, Switzerland
Age: 49
Posts: 1,138
Supersonic - it was certainly no sailplane and an ability to increase drag wasn't required.
Makes me wonder... In the event of a complete loss of thrust at Mach 2 (say fuel contamination) would the deceleration be significant ? If so I guess the fuel redistribution / pumping to maintain acceptable CG would become interesting...

6th Sep 2010, 23:08

Join Date: Jan 2005
Location: France
Posts: 2,319
Quick link to Bellerophon's post #66 and photo to save you having to 'leaf' back...
G-BOAE at Mach 2

Originally Posted by Bellerophon
You will see that at FL600 the aircraft had a GS of 1,139 kts whilst flying at M2.00 and an IAS of 429 kts.
Much as I look at that picture, I can't see the groundspeed.....

Ah, oh, ooooops!!!! Of course it's there, in the little window on the top right of the HSI (Horizontal Situation Indicator, the lower one of the two big central instruments).

Shanewhite, in a way, that illustrates that for flying the aircraft things like TAS and GS are not really that important... that's why there are no big instruments indicating TAS or GS, but only IAS and Mach, with only a little digital window for GS, which IS important for navigation (largely handled by the inertial navigation system, which is the system where the GS display comes from), but not for the minute-to-minute handling of the aircraft.

Bellerophon, dumb question from a techie... the 373 miles is presumably just the distance to the next INS waypoint?

CJ