Posts about: "C of G" [Posts: 77 Page: 2 of 4]¶

Hi guys, here is a schedule showing CG against Mach number (It's very old just like the author here). I hope that it now completes our collection of flight envelope diagrams. ( Bellerophon, by the way, your diagram is precisely the one that I was scouring around for). Great explanations by everybody on the Mach/TAS/IAS etc issue, mostly all clear and concise ( a couple of minor goofs that were subsequently corrected, otherwise very good) .
If I were in the LEAST bit pedantic (and any here that know me would say that the b****d certainly IS pedantic), I would merely add that Concorde (like virtually all complex aircraft) relied on CALIBRATED airspeed (Vc) and not IAS, taking into acount plate and probe errors. Just as well I'm not pedantic .

Dude

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...

Concorde did actually have a four engine failure drill, which covered it's complete speed rsnge including Mach 2.0. There was one assumption made in this drill and that the engines would continue to windmill which would allow them to give you full hydraulic pressure

As you could imagine, If all 4 engines cut at Mach 2.0 the F/E would be quite busy and so the the non flying pilot would use his fuel transfer switch to start the fuel moving forward. This was a pretty basic selection where fuel would be pumped out of Tank 11 using all 4 pumps [2 electrical and 2 hydraulic driven] and into the very forward tank which was no 9.

As a rule of thumb transferring 1000kgs from tank 11 to tank 9 moved the Cof G forward by 1%. Now with all 4 pumps in tank 11 running the tansfer forward was so quick that the pilot had to keep switching the transfer off and then on to stop the Cof G moving forward too quickly. It was usually to everybody's relief when the F/E could find the time to take over the fuel transfer as he had the selections to allow him to be more selective as to where the fuel went and so slow the rate down
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This was quite a neat system, as the gear was retracted, a SHORTENING LOCK valve was signalled, allowing a relatively tiny jack to pull the entire shock absorber body into the body of the oleo progressively as the gear retracted. So the shock

Forther to M2dude's explanation Concorde's main landing gear consisted of 3 seperate metal castings . there was the normal two for the oleo and these two were fitted inside the outer casting, which was the one you could see.
As the gear retracted a mechanical linkage , which was driven by the gear's retraction movement, would lift the oleo assembly up into the outer casing, so shortening the length of the leg . If I remember the shortening jack was just to assist in breking the geometric lock of the linkage
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The other difference between AF and BA aircraft was the DC electrical system

AF had Nickel cadmium batteries with an automatic charging system

BA had the good old lead acid battery sysytem, well except for AG where the DC system was one of the systems they never changed when AG was incorporated into the BA fleet

The deceleration would be like very hard braking after landing, so - yes.

The drag incurred flying supersonic was once described to me as like flying through wood, not air. The only times I ever closed all 4 throttles at M2 was dealing with surges (see earlier posts on the subject). While not quite like flying into teak, the decel was very impressive - it more than once resulted in a member of cabin crew appearing in the flt deck in a semi-seated position, grimly trying to stop a fully loaded galley cart.......

As for four-engine flameouts - perish the thought. The checklists, like many, depended on flight phase;

Above M1.2 it was expected that windmilling would provide adequate eletric and hydraulic power so the c/list aimed to start a fuel txfr forward, use the spare hydraulic system to drive half the PFCUs, ensure a fuel supply to the engs and ensure cooling to equipment.

Below M1.2 the RAT would be deployed, it was less likely that the standard means of fuel txfr would work so valves were overridden and the hydraulic fuel pumps brought into use, and the Mach fell further the PFCUs were put on half-body use only, using the stby hydraulic system.

You weren't far from the ground, in time, at this stage so it was a good time to get an engine relit!

Given the Olympus' auto-relight capability a four engine loss was going to be caused by something fairly drastic.

Re the CG limits, here is the diagram from the 01 (preprod a/c) flying manual.



The production a/c diagram is slightly different, but it shows the same kind of "corridor".

Edit : here is the production a/c diagram.
Sorry for the distortion during the scan....



For the full A4 page, use this link :

Prod CG enevlope A4 format

CJ

Christiaan,


Since you were discussing the scenario of a nosewheel not lowering and that the CG was over the main wheels may I suggest a rather (amusing at least)
possibility ?!!



With a nose gear jammed up but all other gear lowered normally could the Flight Engineer pump fuel rearward adjusting the CG aft sufficiently to allow the Concorde to settle back on her 'Tailwheel'



I realise there would be some damage, especially in light of what has been said about the occasional tailwheel contacts but I imagine it would be less than lowering the unprotected forward fuselage onto the runway.





Of course some pax might have to move to the back of the cabin too !



If the CG was adjusted this far aft would there be controllability issues ?




There could finally be a use for the 'full down position of the visor' landing in this attitude !

Why not?
Let's admit that being faced with that nasty situation in reality would not have been amusing....
But kicking the idea around a moment, why not? It's what I did seeing the question at first.

Obviously depends a bit on the fuel remaining, but yes, I think one could have move the CG sufficiently rearward.

LOL, don't you think they'd all would have moved as far back as possible anyway?

Possibly, but not enough that an experienced pilot couldn't handle it, I would think.

I don't quite follow you there...

One thing that promptly occurred to me for this 'no nose wheel' scenario is braking, since both engine reverse and main gear wheel braking act below the CG, so they'd both act to pull the nose down.
At some point, stick fully back, hence elevons fully up, will no longer be enough to fully counteract that, so you'll have to cancel reverse and braking, and probably commit to an overrun.
Your scenario of doing it as a 'three-pointer' on the tail might wel be the better one !


And while we're at it, what about ditching a Concorde?
It's hinted at in the Safety Cards.
It's been tried lots of times, with models in water tanks.
It wasn't really feasible.......

CJ

Hi again Stilton. We really need one of the flying folk to answer this one fully, I am not sure what drill there was for this scenario, but I'm sure there was one. The Concorde flying manual had a drill for everything, from a four engine flame out at Mach 2 to a blocked toilet (ok, maybe not the loo thing ), and one of my winged friends EXWOK, Bellerophon, SEO Brit312 would remember one.
As far as your point about moving the CG further aft; you never had oodles of fuel to play with , and I'm sure that the guys will mention about handling the aircraft on approach with the CG too far aft. (After landing four tonnes of fuel were transferred from Tanks 5 & 7 into the empty fwd Trim Tank 9, 'to aid ground stability'. ie, help stop the aircraft from trying to sit on it's rear end as the passengers got off).
As far as your visor query goes, well the visor is either up, or retracted into the nose. The nose itself (which I suspect is what you are really referring to) would already be at the fully down 12 1/2 degree setting for landing anyway.
Oh, and back to the ground stability issue, was Concorde ever sat on it's tail by accident? Oh yes, just once to my knowledge. In May 1977, aircraft G-BOAA was returned to Filton for some modifications that were required, and part of these 'mods' was some improvements to the main trim-transfer pipes connecting the three trim transfer tanks 9, 10 & 11, as well as the trim tanks 5 & 7. Now the flow into tank 11 (the rear tank) had to be checked, but there was insufficient fuel at the front of the aircraft for stability. This shortcoming was passed on to the BAe manager in charge of everything, who stuffily refused to listen, and INSISTED that these transfer checks were carried out, 'do as I tell you, I am the manager here'. The man's sole concession to any sort of common sense was to allow a BAe employee to sit on the flight deck 'and watch the CG indicator', what the point of this was, well your guess is as good as mine. The name of the guy sitting on the flight deck was... John Thomas. (Hilarious I know, but true). So in goes the fuel, and in a very short period of time, John Thomas notices that the roof of the Filton assembly hangar seems to be slowly getting closer, and closer, and BANG!! The aircraft nose is high into the roof section of the hangar, but fortunately because the hangar is so huge, the nose did not hit anything, it was just stuck up there, complete with a very worried/terrified John Thomas who is sitting terrified in the captain's seat, staring at the hangar roof. The rear of the aircraft however was not so lucky. The right hand inner elevon came down on top of a hydraulic rig, damaging the elevon badly, as well as FLATTENING the rig. The opened #3 engine bay door came down on some large access steps, tearing the corner of the door. (not much left of the steps either). The rear fuselage, in the area of the hydraulic tanks, was holed quite badly by some access staging, entire spectacle coming to a very 'grinding' halt.
So now we have this Concorde G-BOAA, due to be returned to BA the following day, sat down on top of a lot of equipment, it's nose high in the air with a terrified John Thomas requiring a change of underwear. (The brilliant manager of course was nowhere to be seen). The aircraft was eventually returned to it's rightful attitude by someone WITH some sense instructing Mr Thomas on how to slowly, a little at a time, pump the fuel from Tank 11 forward into Tank 9.
And was OAA returned to BA the following day? errr no. The best skin repair man that BAe had to offer was sent from Weybridge to sort out the holes in the rear fuselage (he did an amazing job) and the crunched bits of aeroplane were repaired or replaced. OAA flew back to Heathrow four short (??) days later.

Dude

First of all, THANKS to all you from Concorde family for this fantastic topic. Started reading last night and almost slept in front of computer trying to read everything!

As a Concorde fan for 10 years (since I bought FS2000), and passionate developer of SSTSIM Concorde and FSLabs ConcordeX (flight dynamics, weight and balance), it's simply awesome to have you guys and gal here sharing your memories.

Regarding the CG corridor, here's a fantastic graphic from online Concordepedia, aka ConcordeSST.com, Technical/Fuel System section:



Interestingly, it doesn't show a warning for CG>59.1% above M1.6, opposite to what M2Dude said earlier on the topic.

I got curious on the Max Climb/Cruise and ALT ACQ not being primed. How the levelling at FL600 was done? Manually?

Regarding the fuel tanks, specially tanks 6 and 8: did these tanks' lateral center of gravity change with quantity? Due to their completely assymetrical shape, I'd expect some change in it.

Operationial question: did BA use the 380kts descent profile? Have heard that only AFR used it, but Haynes' book says that BA started using it too.

There are many doubts regarding procedures as manuals and informations available on the internet are mostly from BA 1976 entry-into-service era. But i understand many things changed along the years, as I can see on a Aug 2000 manual I've got, with percentages showing differences from the 76 era, or even completely new tables.

Well, that's it, hope to be able to contribute on the topic, but mainly learn from you that flew the real thing.

BlueConcorde ,
Look at my post #260, second diagram, taken directly from the BA Flying Manual.
There is a "first stage warning" (Mach/CG lights and gong) above Mach 1.6 and below Mach 0.45 for the aft CG limit.
It's only in the "corridor" that there is a "second stage warning" (flashing Mach/CG lights and stick shaker).

CJ

Blue Concorde

The logic of the C of G limit warnigs were

1st stage warning then the F/E rectifys it by moving the fuel

2nd stage warning was considered a more dangerous exceedence and would be remedied by the F/E moving fuel and the pilot slowing or speeding up the aircraft depending on which limit had been triggered

The aft limit second stage warning was a flashing light and a stick shaker to which the natural response from the pilot is to speed up Now you can see from your diagram that above M1.6 increasing will not improve the siuation if the aft boundry has been infringed. Therefore to prevent an auto response from the pilot to a stick shaker [ push the nose down and speed up] the 2nd stage aft warning was not available after M1.6


Tanks 5 and 7 were operated as a pair as were tanks 6 and 8
Because of the odd shapes of the tanks when you were transferring from the set 5 and 7 the F/E had to pump fuel across the ship to keep lateral trim.
Once they were empty and 6 and 8 were being used then again due to their shape the F/E had to transfer fuel across the aircraft to keep lateral trim ,but this time in the opposite direction . well it stopped him getting bored

Thanks on the CG info!!

Right, do this everytime I fly on the sim!

Yes, so my not-so-trivial questions, aimed more for F/E and Ground Engineers are:
1) with the same quantity on tanks 6 and 8, for example, 10 tons, there would be a roll tendency? I suspect yes, but not sure.
2) Using valves 6/7 and 5/8 would make lateral unbalance gone or they just leveled the fuel height on each pair of tanks? (Assuming that all these 4 tanks had the same height, what sounds logical to me)
3) Is there any table with these tanks quantities to reach lateral balance or the F/E did fine tune just by making elevons level?

I have these doubts for a looooooooong time, as I never found the lateral arm of the tanks, just the longitudinal (in % MAC that is equal to root chord in Concorde). So I assumed in FSLabs ConcordeX that if all these 4 tanks were FULL and symmetrical as a group, there would be no imbalance, that means: different quantities and different arms gives the same momentum.

Due to this, I always keep 300~500 kgs more on the 7 and 8 (right) tanks than on 5 and 6, but I'm really not sure if it's a realistic value.

The fuel system was just FANTASTIC... and making it work engineer-less under any abnormal condition would be something VERY difficult, in my humble opinion.

Nice week for everyone!!

I was Googling to see if I could find some info on how the CG indicator on Concorde functioned and found the following two links. The first one is an AAIB report from 2003 regarding a minor fire aboard G-BOAC whilst in transatlantic cruise which resulted in misreadings and failure flags on fuel guages. The second one is from the PPrune archives also dated 2003.

The last two comments at the bottom of that old PPrune thread are interesting since as evidenced by this ongoing thread (as well as the others running in the tech forum), seven years later, many many people are still fascinated by this gorgeous aircraft.

http://www.aaib.gov.uk/cms_resources...pdf_029047.pdf

http://www.pprune.org/archive/index.php/t-109948.html

Kangerlussuaq? Wow, that should have been awesome. Ronivaniemi flights were supersonic? I built and flew a EGLL-EFRO flightplan on the flight simulator once, supersonic along the Norwegian coast, descending and entering continent to reach Ronivaniemi from Northwest. According to my calcs, the difference to a subsonic-only trip using normal airways would be small, so that got me wondering how you actually did that.

Nice! So, by what I already read on this topic, you only worked with Concorde, right? But do you know if Concorde had the same issues as other airliners, or even for you guys "you're not (working) in an airplane, you're in Concorde" quote hold true?

For BA001 and BA003, 2 Concordes were prepared for the same flight, right? Did ever happened some situation that required a ready-for-takeoff Concorde be brought back? How long a cargo and passengers transfer would take? The backup Concorde was fueled?

Please, Landlady, that would an even bigger pleasure! Very interesting to read about Concorde from another point-of-view, much more human than Tons, Kgs, CG %, Celsius degrees, etc.

Hi again
I think the management of fuel on Concorde is fascinating.One of the reasons that this thread has been so interesting to me has been the explanations on how the fuel not only provided a potential energy source but was also used as a cooling medium and especially it's use in moving the CofG.
Am not sure but I think that some baggage was stored aft of the cabin. If that was the case on landing was fuel pumped forward to balance this out? If that was so when deciding on the amount of fuel needed plus diversion fuel etc was there a minimum amount of fuel that had to still be in the tanks on landing?
Regards
Nick

That's why those two small planks on Concorde work so well.

Quite a common trick of the trade eg with fighters eg the leading edge extension (LEX - the narrow aspect delta at the front of the wing) on the F18 and others. There are plenty of pix around with the vortex made visual due humidity and it can be seen to be tight, curly and designed nicely to interact with the fins - which, for the F18 has caused much in the way of fatigue related grey hairs in the boffin fraternity.

and especially it's used in moving the CofG.

again, a common observation eg 747-400 tail tanks .. just a matter of how much the movement is required to be.

Cron -

One of the downsides to flying a pointy aeroplane is that the front is somewhat narrow, as you have identified. It wasn't too bad getting in and out, but it was easier for the pilots to get in before the FE was in situ.

Once in, it was fine. The roof and especially the side window was much closer than one finds in other types, but there was adequate space. It helped to be less than 6' tall (I'm not.....)

As for the engineer's space being 'spacious', well that's relative. It was a bigger space than the pilots'; however a few of the FEs were quite .....spacious .......themselves so they had the same problems as us.

Nick Thomas -

There were, as you say, baggage holds under the cabin at the front, and one aft of the cabin (the bigger one).

One would try to distribute the load to minimise any pre-take off fuel txfr, and especially to minimise any burn reqd (we're getting into a new subject here....).

Having done this one knew the empty CG and then managed the fuel accordingly. Min reserve fuel was 6500kgs and that was more than enough to manage landing and taxying CG. After landing a chunk of fuel was pumped forward for taxying purposes and there would always be ample for this unless one was seriously low on fuel (low enough for a 'Mayday' to be mandatory rather than just a bit tight).

EXWOK
This is the Mach Trimming that was incorporated into the Electric Trom Computers for certification purposes, that I mentioned in a post here about a million years ago (same thread though). With the CG being controlled correctly you'd always notice it of course when you were hand flying, and would get a sore thumb winding the trim down to fight off the nose up Mach trim demand.
And you a steely eyed supersonic jet pilot too

Dude

Hi everyone
Earlier in this thread there was an interesting discussion on emergency depressurisation. During the rapid descent I would guess that the FE would be very busy find out "what was what" etc.

So I have been wondering if there were any special procedures for managing the CofG in a rapid descent especially as there could also be many other factors needing the crews attention?

BTW it only seems like yesterday when I was sat in front of my parents TV watching Concorde take off for the first time from Filton and in fact it's now nearly 42 years ago. I like most people watched the event in black and white which just goes to prove how far ahead of her time she was.
Regards
Nick

CRON
I can only speak here from the Concorde ground engineering school that I attended over a total of 13 weeks at Filton in 1980 and 1981; the pilot/flight engineer questions there were I'm sure FAR nastier (and also more operationally specific); we did get to share simulator time though, which was really useful. Like the aircrews, we stayed up in a hotel in Bristol during the week. (I personally had only left BAC, as it was then, for BA at Heathrow in late July 1977, so I was returning to familiar pastures). The exam format would be several dozen multi-choice questions per week/phase; a typical question would go something like:

The Inner Elevon Light, plus 'PFC' red Master Warning is triggered by:
a) The Green Flying ControlComparator
b) The Blue Flying Control Comparator
c) Either Comparator
The correct answer is (b).

Another flying controls question I can remember is:

Outer Elevon Neutralisation is triggered at:
a)Vmo + 10 KTS
b)Vmo + 15 KTS
c)Vmo + 25 KTS
The correct answer here is (c).

The pass mark in these exams was 75%, with penalty marking applied for any wrong answers. I always found the worst part was the fact that the exams were on a Friday afternoon after lunch

Nick Thomas
Hi again Nick, one really for the likes of BRIT312, EXWOK etc, but there was, as was mentioned before, an emergency forward transfer switch in the roof panel above the pilots (F/O's side if I remember correctly). When placed to the emergency poition two electric and two hydraulic fuel pumps for the rear trim tank #11 would start up automatically, as well as the forward tank inlet valves being opened also.
From what you said about the 'lady' being ahead of her time, I would certainly agree with you here; in my view she was generations ahead of everything else.

nomorecatering
The BA simulator that resided at Filton has been re-located to Brooklands Museum, and has been re-activated, but without motion and I'm not sure about full visuals either. I've not seen it myself yet, but I'm told that things have progressed really well with the operation. Obviously it is no longer certified as an active simulator; I'm not sure about the situation in France, perhaps my friend ChristiaanJ can answer that one.
I seem to remember typical loads for LHR-JFK being around 93-96 tonnes, depending on the passenger load and en-route conditions. Full tanks, depending on the SG was around 96 Tonnes. High fuel temperatures in the summer were a major pain; restricting maximum onload due to the low SG.

As far as ground school notes, mine are all out on long term loan (MUST get them back). The ground school are totally priceless and I am sure that there are many complete sets lying around in atticks/bedrooms/garages/loos etc.

Dude

I have been thinking (always dangerous!) about CofG movement. I understand the concept that the CofG must be positioned for the particular phase of flight. What I have been wondering is on shorter charter flights was there a mininium ammount of fuel that had to be loaded just to always have enough fuel for CofG movement ie was it possible to be in a position where trip fuel, fuel to an alternate etc was less than the fuel required for CofG movement after take off?
Regards
Nick