Posts by user "ChristiaanJ" [Posts: 266 Total up-votes: 0 Page: 9 of 14]¶

Maybe M2dude remembers the occasion?

First time that happened was on prototype 001 in the very early days, when an engine "spit out" the entire ramp (there's a photo in Trubshaw's book).
The ramps and actuators were 'beefed up' considerably afterwards... I didn't know an in-service aircraft had suffered a similar mishap.

Good question.... not being an "engine man" I've always been amazed how a nice steady Mach 2 flow, slowed down to Mach 0.5 at the engine inlet, is capable of totally choking off and even reversing itself in less than a second.... no wonder it's usually accompanied by a big bang!

CJ

PS I have no record of any of the British development aircraft ever having lost a ramp, notwithstanding the number of deliberate engine surges they went hrough. But then maybe I wasn't told....

I know exactly what you're saying, Nick...

And yes, Concorde evokes images like no other aircraft really does.

Funnily enough, to me the image you describe always evokes something totally different... although it does depend on the exact angle it's taken from.
Seen from exactly the right angle, she has this slight 'smile' on her face, saying "did it again, people... home soon ! ".


Otherwise, oh yes, I have two images.... both sad.

One is the air-to-air video of the Jubilee flypast with the Red Arrows (I don't have the link at hand), when she pulls up and away, saying goodbye.
For some reson, I have never been able to watch that one without a huge lump in my throat.

The other one is this one....



Last time ever.... and somehow one hoped an instant she would disappear from view, and return forever to the sky where she belonged.

And why should I now be furiously rubbing my eyes?
But maybe that answers your question, too....

Christian

Hi Nick,

A more cheerful image !



CJ

DavvaP , Nick , and the others,
If you haven't got it already, try and get a copy of Brian Calvert's "Flying Concorde", which has the story of those first landings and take-offs in New York in some considerable detail.

CJ

In case anybody is interested, I just started off a separate thread
Concorde Books .
Probably too late for Christmas, but maybe useful just the same.

CJ

Thanks, Bellerophon !

That just went into my 'archive'.

Maybe it should be printed out, plastified, and used as a briefing sheet for the Brooklands simulator, where the JFK 31L take-off is still one of the favourites !

CJ

You mean you actually were on one of the Concorde landings at Kai Tak?
You'll have everybody here green with envy....
Even if it was on a "blunty" it was still spectacular.
And I think Kai Tak is already in the Brooklands sim database.... so you can now come and fly it yourself....

I'll let Bellerophon answer.... but there once was a low-weight take-off from Cardiff straight out to sea, without any restrictions, that IIRC still holds the record for the shortest time to Mach 1 and Mach 2 from brake release....

No thanks needed... I hope you'll enjoy it as much as I did !

CJ

Welcome Clive !

See you again sometime after Xmas, when you've worked your way through the 45 pages this thread has already generated......

Clive,
Re the autotrim, tell us some more?
I wasn't directly involved with the control laws themselves, more with trying to assure those control laws were respected to well below 1%.

I thought it was the first civil aircraft, and that the Vulcan had already been there and done that...

I know what you're saying.....
Still, I think you'll agree that lessons were learnt, rather than totally ignored.
I would say Airbus can trace its history back to the lessons learned from Concorde.

CJ

Minor correction (typo) about quoting from a post. As, said, the address will end with "&noquote=1" (meaning 'noquote' is "on").
Either replace that last " 1 " with a " 0 " or just delete the "1", then press enter.
CJ

CliveL , check your PMs (private messages).
CJ

Both CliveL and EXWOK are of course right about the pitch-down into ground effect... I think I apologised for my bludner in a later post.

CJ

Here are the graphs that CliveL was referring to.

The Mach trim control law




The aircraft response to a double engine surge
Split into two halves (longitudinal and lateral response)






Note the almost immediate rudder response, long before the engine N2 rpm starts to wind down. I'll have something to say about that in a separate post....

CJ

A double engine failure, or even a double engine surge, could lead to a very nasty yaw, faster than the pilot, not necessarily instantly aware of exactly what was happening, could counter.
The designers were, right from the start, aware of this problem.

Hence, the prototypes were equipped with specific "contre automatique" (auto-rudder) computers, that would "kick in" a given rudder deflection as soon as they detected an engine failure (and twice as much in the case of a double failure).
Unfortunately... the manner of detecting an engine failure was based on pressure sensors in the engine, which proved to be notoriously unreliable.
Since the whole system was "fail-passive", in the case of a pressure sensor failure nothing happened, other than that I got the "suspect" computer dumped in my lap every time, since it was easier to swap a computer than test and swap pressure sensors....
In the end, it was always "no fault found", and the engineers had to go and test the sensors to find the failed one.

Already on the pre-production aircraft, this Rube Goldberg system was replaced by a single circuit board 'buried' in the autostab computer.
It used a lateral accelerometer to detect the abrupt yaw of a sudden engine failure or surge, and applied appropriate rudder. Look at the sudden rudder deflection 'peak' on the lateral response graph in the previous post.

Since there was no separate 'auto-rudder engage' control switch (the function was permanently active), and it was only mentioned very much in passing during training, some pilots were not even aware it existed.......

CJ

Allow me to nitpick a moment.....

Pilot demands in manual flight produced electrical signals corresponding to the control position, which were sent to the 'servo control amplifiers' (eight in all, one per control surface) which in turn commanded the PFCUs (power flying control units) that hydraulically moved the control surfaces.

Autopilot demands directly moved the pilot's controls (stick and rudder) via hydraulic cylinders (the 'relay jacks') so that the same signals as in manual flight then went to the servo control amplifiers.

The purpose of the autostab was to provide proper dynamic stability over the full flight envelope. The aircraft could be flown without autostab, but over some of the speed range it was only marginally stable.
The electrical signals from the autostab computing were fed directly into the servo control amplifiers, so there was no feedback to the pilot's controls, unlike the autopilot demands.

There was occasional confusion about exactly what did what and how and where.... because the servo control amplifiers - although a function independent of the autostab as such - were housed... in the autostab computers.

To complete the tale, this is what those servo control amplifiers look like.
The one of the left is from prototype 002, the one on the right from a production aircraft. To give them scale, the one on the right is about the size of a box of large kitchen/fireplace matches.



CJ

Re TURB mode
That reminds me.....

We spent a lot of time, and did a lot of flights, to get the VOR (capture and hold) autopilot mode to work satisfactorily.

It was not until years later, and on another type, that a pilot told me "VOR mode? We never use it. We use TRK or HDG mode, and monitor on the HSI that we're on the VOR radial."

IIRC, in service it was the same on Concorde, but maybe EXWOK or one of the other pilots here can confirm it?

CJ

"Superstab" ?

CJ

Before adding my own little bit to Clive's earlier reply about the autotrim, I will try to explain, for those not fully familiar with the subtleties of automatic flight control, the difference between "closed loop" and "open loop".

Closed loop

As an example, let's look (very simplified) at how the autopilot maintains a selected altitude.



On the one hand we have the desired altitude as selected on the autopilot controller (here 40,000 ft).
On the other hand we have the true altitude , as measured by the altimeter (let's say 39,000 ft).
We subtract the two to obtain the altitude error (in this case 39,000-40,000=-1,000 ft).
We 'multiply' the altitude error by a factor, the gain (for the discussion, let's assume this gain is 1 degree elevon per 1000 ft altitude error), and send the resulting elevon position command to the elevon.

So, the elevon moves 1\xb0 nose-up, the aircraft starts to climb, the altitude increases and the altitude error decreases until it becomes zero, by which time the elevon position has also returned to zero.

What we have now is a "closed loop" : any deviation from the selected altitude results in an elevon command in the opposite direction, until the deviation is again reduced to zero.
Another commonly used term is "feedback" : any error is fed back in the opposite sense until it's reduced to zero.

The significant figure here is the 'gain'.
If the gain is too small, the autopilot response to a disturbance (say turbulence) will be sluggish ; the aircraft takes too long to return to the desired altitude.
If the gain is too high, a small disturbance will cause the aircraft to start climbing too rapidly, and to overshoot the desired altitude, then descend to correct the new error, etc.
In other terms, the control loop is no longer stable, but starts to oscillate.

Both theory and practice show that the exact value of the gain is not all that critical, a few percent more or less do not markedly change the response of the loop.

Note: a "closed control loop" as described above can be implemented in just about any way you like.
It can be done purely mechanically, with a few clever clockwork mechanisms 'computing' the altitude error and controlling the elevator pneumatically or hydraulically. It's how the earliest autopilots worked.
After that came electromechanical systems, analogue computers and then digital computers... but the principle has remained unchanged.


Open loop

As already described in earlier posts, the situation with the automatic trim is the opposite.



We now need to compute a neutral elevon position from several data, such as Mach number or airspeed, but without any feedback as to whether our computations are correct.

We're now working in "open loop".
To complicate matters... that neutral elevon position is not a simple linear function of Mach and airspeed, but far more complex (see the earlier posted graphs).
And because of the large response of the aircraft to small changes in trim, in particular in the transonic regions, those computations have to be far more accurate : a one degree error is simply not acceptable.


In the end.....

The AICS (air intake control system) also uses several "open loop" functions.
The early development aircraft still had an analogue system, which proved all too soon to be inadequate, so, at a very late stage, it was replaced by a digital system (one of the rare digital systems on Concorde).

The "open loop" functions of the autotrim system initially had the typical "a few" percent" accuracy of the other flight control systems, which, for the autotrim, also proved inadequate.
We managed to "save the furniture" (as they say in French) by using 0.1% components in all the critical computing paths, so the autotrim computers remained analogue until the end.

But, a slide rule is not accurate to 0.1%... So that's when I had to buy my very first pocket calculator.
\xa342 at 1972 prices... just as well the firm paid.

CJ

There was, and maybe still is, a video....
Shortly before the French end-of-service, a small group of French enthusiasts set out in a boat (fishing boat I think) to waypoint 'TESGO' in the Channel, to hear and capture the supersonic bang one more time (TESGO is already beyond the supersonic accel point).
They brought back a video which has become a classic, with both Concorde and a contrail overhead and a recording of the supersonic bang.

A quick search on the net for "TESGO" or "Operation TESGO" didn't produce anything, but I will make some enquiries.

CJ

Thanks Biggles78 and SilverCircle , that's it !

Those are the essential 30 seconds from a longer video that tells some more about the preparation, etc.

CJ