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Old 25th Jan 2012, 22:55   #1541 (permalink)
 
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Hello,

Because this is such an interesting plane, I have a few questions. Maybe someone can answer, would be interesting:

When a concord(e) is in supersonic cruise, is there one main compression/ expansion wave? If so, where is it located, on the longitudinal axis?

Also:

It is quite obvious that the plane doesn't have a horizontal tail. Now I know that a plane without a tail can be statically stable (I think), but is concorde dynamically stable?

If so, how is this achieved?
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Old 26th Jan 2012, 16:39   #1542 (permalink)
 
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Two shock waves - the main one located on the nose, then an expansion field over the wing and a final shock at the tail where the flow was recompressed. [That is ignoring all the intake shocks!] The two compression shocks are what gives rise to the characteristic Boom-boom on the ground.

The aircraft was dynamically stable (just) because it had natural pitch damping, but in practice to give good handling qualities some artificial damping was required via the autostabiliser.
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Old 27th Jan 2012, 07:14   #1543 (permalink)
 
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OK.. It is a FBW aircraft, right? Did the flight control software include this artificial damping?

I would not expect that it has a ,,direct law'' type FBW where a control deflection gives a control surface deflection, right?

Thanks CliveL
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Old 27th Jan 2012, 09:49   #1544 (permalink)
 
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One for Christiaan I think

Sure it had electrically signalled controls, but this is 1960s technology - it was (sshhh) an analogue system. No "control laws", just a direct relationship between stick and surface - the autostabiliser signals (also analogue) were just added to the stick commands.

The only bit of digital control on the airplane was in the hybrid intake control system.
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Old 27th Jan 2012, 23:59   #1545 (permalink)
 
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Quote:
Originally Posted by CliveL
One for Christiaan I think
Sorry all, this is now forty years back, literally....
So no, I don't have all the block diagrams and circuit diagrams in my head any more... I'll have to look through what I still have in the way of documentation.
i'll try to give some quick answers.

Da-20 monkey,
Yes, Concorde had "artifcial damping", or "autostabilsation" as we called it, on all three axes (pitch, roll and yaw), even if it could be flown without it.
On the prototypes there were three separate computers (one per axis).
On the preprod and production aircraft the A/S function was 'compressed' into a single unit (I still have one).

CliveL has given the basic answer.
Don't confuse the Concorde FBW (which we referred to as "electrical signalling") with the current "Airbus" digital FBW.

One, rather than in previous-generation aircraft, the pilot no longer pushed and pulled on cables and rods to move the control surfaces. Instead, when he moved the controls, those movements were translated into electrical signals that were sent to the electro-hydraulic control surface actuators (even if in the Concorde days there still was a mechanical back-up).

Two, the entire system was "analogue". A concept difficult to explain in these days, where nearly everything is digital.....
Very briefly, you can convert 'physical' data, like control positions, or altitude, or pitch or roll rate, or Mach number, into 'analogue' electrical signals. You can then perform all kinds of 'computations' on those signals, like filtering them, or add or subtract them, or even multiply them, using electronic circuits based on 'operational amplifiers'.

In digital systems you go one step further.... you convert all those data into digital values, and use a digital computer to perform all your calculations, in accordance with the 'system software', then convert all the results back into physical data, such as control surface commands.

In analogue systems there is no "software". The entire system is defined by 'control laws ' (not the same thing at all as in the Airbus FBW aircraft) that are fixed in terms of 'transfer functions' of the various control loops.
Those in turn are determined by the values of the components in the various electronic circuits (resistors and capacitors mostly). So in those golden days.... we didn't re-write and re-program software.... we changed resistor and capacitors, and re-wired logic circuits.

I admit, you almost have to have been there to understand it....

I'm not sure whether it's worth starting an entire new thread on 'analogue computing' (maybe there's something on wikipedia, I haven't looked)....

CJ
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Old 28th Jan 2012, 15:10   #1546 (permalink)
 
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There is Analog computer - Wikipedia, the free encyclopedia

I've done a few study exercises on a hybrid system; programming digital computers now. However when designing machines there still are tasks we delegate to analog electronics or mechanical non-linear transmissions.
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Old 28th Jan 2012, 16:48   #1547 (permalink)
 
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Quote:
Originally Posted by MathFox View Post
There is Analog computer - Wikipedia, the free encyclopedia

I've done a few study exercises on a hybrid system; programming digital computers now. However when designing machines there still are tasks we delegate to analog electronics or mechanical non-linear transmissions.
Thanks MathFox, for looking up the link to the Wiki article.
I just read it, and I wasn't impressed... it's more historical than a clear explanation of what analogue computing really is and does. I wouldn't advise it to somebody who's trying to get his head around the basic concept.

CJ
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Old 28th Jan 2012, 19:20   #1548 (permalink)
 
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A collection of links to videos, articles and much more on the rise and fall of analogue computing should prove interesting.

For those cutting their teeth in the computing field in the last twenty years, the analogue concept was already well and truly buried. As we all know, the Concorde project epitomized analogue computing which peaked in the decade between 1960 and 1970. One can only but wonder how much weight could have been saved and performance improved if current FBW digital computing was available at the time. That the aircraft type soldiered on well past its "use by date" is a credit to the designers and engineers who implemented the "state of the art" systems of the time.
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Old 28th Jan 2012, 19:59   #1549 (permalink)
 
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Analogue computers are still very active on the Airbus A300-B4 100/200 srs.
A lot of the Concorde technology was transferred to the Mark 1 Airbus, and it still works very well. We are Cat IIIa certified and I have never had a failure when it was needed.
Yes we get a lot of drop out of things, but usually they work after a reset.
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Old 28th Jan 2012, 23:18   #1550 (permalink)
 
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Thanks Cristiaan,

From the way you explain this autostabilisation it looks similar to the way a yaw damper works. With a rate gyro, ac current that is phase advanced, filtered and then amplified, but on all 3 axes?

Not being a real bright light on engineering I can see that such a system works to give ,,apparent'' dynamic stability..

Another question perhaps: Does concorde have real trim tabs? Or is it just an artificial feel unit that ,,changes the neutral point in the stick?

I've seen the (awesome, by the way) ITVV documentary but don't remember this being mentioned...


mm43 That's a nice link
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Old 28th Jan 2012, 23:46   #1551 (permalink)
 
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Da-20 monkey


Quote:
... Does concorde have real trim tabs?...
No, there were no trim tabs on Concorde.


Quote:
...is it just an artificial feel unit that changes the neutral point in the stick?...
Yes, just so.

Concorde had conventional trim controls (electric trim in pitch, manual in roll and yaw), but operating a trim control merely changed the artificial feel datum position and thus the neutral position of the flying controls.

The electric pitch trim not only operated automatically whenever an autopilot was engaged, but could and would also operate automatically in manual flight, independently of any pilot input, to provide pitch stability corrections in various situations.

Best Regards

Bellerophon
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Old 30th Jan 2012, 22:51   #1552 (permalink)
 
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Thanks all
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Old 31st Jan 2012, 11:44   #1553 (permalink)
 
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Perhaps more an operational than a technical question, but it's nice to see one of the sky-gods back on this thread!

I confess to occasionally firing up Microsoft Flight Simulator and SSTSIM, and am particularly impressed by the Barbados route - especially as half-way between the Azores and Barbados, you are literally a thousand miles from anywhere with limited diversion options and marginal fuel in case of engine failure and subsonic cruise. While I'm sure you had all the angles covered, was it really a nail-biting moment and what sort of contingency plans were in place?
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Old 31st Jan 2012, 13:33   #1554 (permalink)
 
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fizz57

Quote:
... Barbados route...half-way between the Azores and Barbados...a thousand miles from anywhere with limited diversion options and marginal fuel in case of engine failure and subsonic cruise...what sort of contingency plans were in place?...

As well as carrying sufficient fuel to arrive at BGI with standard fuel reserves remaining, there was also a requirement that sufficient fuel be carried to ensure that, following an engine shut-down at any stage in the flight, Concorde could divert, on three engines, to a suitable diversion airfield, and still arrive there with standard fuel reserves remaining.

It was this requirement - the three-engined diversion requirement - that often required more fuel to be loaded - above the basic LHR-BGI flight plan fuel figure - often bringing the total fuel required up to or over the full tanks figure and so became the limiting factor on this route.

Perhaps the main difference between Concorde and most subsonic aircraft, following an engine shutdown in cruise, was that Concorde would suffer a much greater loss in range. From four-engined supersonic flight to optimum three-engined subsonic cruise the loss in range would have been in the order of 30-35%.

This was mainly because Concorde, following an engine shut-down in cruise, would have to decelerate and descend, and thus leave a very efficient flight regime, at M2.0 and 55,000-60,000ft, with relatively low drag, low winds and very cold outside air temperatures, for a much less efficient regime, at M0.95, at around 30,000ft, in a higher drag subsonic cruise with warmer outside air temperatures and much stronger, probably adverse, winds.

The forecast weather at the principal en-route diversion airfields of Santa Maria, Lajes, Bermuda and Antigua, along with the calculated wind components at subsonic cruise levels to these airfields, were all taken into account at the flight planning stage, with the forecast subsonic cruise wind component to Antigua generally being the most critical factor.

If the weather conditions at and en-route to these diversion airfields were favourable, flight planning was straightforward. If the weather conditions were unfavourable, flight planning got more difficult, but the necessary fuel was always carried, passenger numbers limited or a re-fuelling stop planned.


Quote:
...While I'm sure you had all the angles covered, was it really a nail-biting moment...
No, not really.

LHR-BGI was certainly the most demanding route on Concorde, and required careful planning, good tactical awareness and diligent in-flight monitoring, however the flight planning procedures and tactical decision making processes were standard and would have been very familiar to any ETOPS rated pilot.

With one exception.

Concorde would still have got you to a diversion airfield following a second engine failure!

Best Regards

Bellerophon
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Old 1st Feb 2012, 07:07   #1555 (permalink)
 
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Thanks for your insightful reply Bellerophon. Business as usual then, at Mach 2 1000 miles from anywhere and only an hour to destination. If it wasn't nail-biting, it must at least have brought a smile to your face.
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Old 1st Feb 2012, 20:43   #1556 (permalink)
 
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Quote:
Two shock waves - the main one located on the nose, then an expansion field over the wing and a final shock at the tail where the flow was recompressed. [That is ignoring all the intake shocks!] The two compression shocks are what gives rise to the characteristic Boom-boom on the ground.
Thanks for this, CliveL. Could you please explain 'expansion field over the wing '?
Thanks.

Also, how was the flow re-compressed at the tail? What is the aerodynamic explanation of this?

As the intakes produced shocks, what about other protruberances such a aerials and drain masts etc? Did these also produce (small) shocks?
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Old 3rd Feb 2012, 13:32   #1557 (permalink)
 
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Down here in Devon, the only contact we had with Concorde was the spine-tingling "whomp-bomp" every evening as flights out of CDG lit up down the channel. Replaying that sound in my memory, I would say that the time between the two thumps was between 1/3 and 1/2 sec, and yet a bit of maths tells me that at 1300 MPH, Concorde travelled her own length in less than 1/10 sec. This always puzzled me at the time, and I still can't explain it. Any ideas, anyone?
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Old 3rd Feb 2012, 18:10   #1558 (permalink)
 
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Although the nose and tail shocks are 204 feet apart at the point they leave the aeroplane, I'd guess that they diverge away from each other with distance, so the further away you are from Concorde when they reach your ears, the longer the gap between the shocks.
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Old 3rd Feb 2012, 18:31   #1559 (permalink)
 
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Concorde shock waves

SSD: [quote]Thanks for this, CliveL. Could you please explain 'expansion field over the wing '?
Thanks.

Also, how was the flow re-compressed at the tail? What is the aerodynamic explanation of this?

As the intakes produced shocks, what about other protruberances such a aerials and drain masts etc? Did these also produce (small) shocks?[unquote]

I was generalising a bit and referring to the shocks as heard on the ground which take the form of a classic 'N' wave. This is a sudden rise in static pressure followed by a drop in static pressure to below atmospheric and then another sharp pressure rise. In supersonic flow any increase in static pressure is associated usually described as some sort of compression and conversely a drop in static pressure as an expansion. Since this happens over the region where the wing sits I described it as an expansion field over the wing.

The N wave is OK for the 'far field' shock characteristics, but as you hint, the flow near the aircraft is more complex than that - all the separate shocks gradually. merge into the bow and tail shocks. In the near field all the aerials, drain masts etc have their own little shock waves of course.

I have never seen a completely satisfactory explanation for the drop in static pressure, but if you will accept a much simplified description .....

I think there are two parts to the explanation.

The pressure over the upper wing surface will be below atmospheric static in the usual sense when lift is being generated - this will depend on the amount of lift (and hence weight)

Besides that there is a term related to the volume of the aircraft. One graphic description I have seen is that flying along at 2.0M the aircraft 'tears a hole' in the atmosphere that the surrounding air, being limited to 1.0M cannot fill. Consequently there is a drop in static pressure around the airframe. When the aircraft is past the 'void' is filled up and the resulting rapid increase in static pressure goes with the rearmost shock. This tail shock is sometimes described as the rarefaction shock.

This may not be scientifically accurate, but it satisfies me

CliveL
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Old 3rd Feb 2012, 18:40   #1560 (permalink)
 
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Time between 'thumps'

I think the explanation is that the sound you heard was (and could only be) travelling through the air to reach your ear at the speed of sound. The distance between the shocks would be 204 ft, but the speed would have been 1100 ft/sec - i.e.about 0,2 seconds between them
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