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Biggles,
The Braniff crews [ I think it was 5 sets of crew] were trained for Concorde with some of crews trained in France whilst the others were trained in the UK. Flying training was done using an Air France Concorde F-BVFA with flying being at Shannon initially but when they ran out of fuel it was moved to Montpellier. As their operation was to be subsonic they were only trained to operate the aircraft subsonically, but they were given a supersonic trans Atlantic trip as an observer. ChristiaanJ If I remember correctly ground effect tended to force the aircraft nose down, so requiring the pilots to pull back on the stick as if they were flaring ,but in fact what they were doing was as you say maintaining the pitch attitude constant. I have to say that in the early days the landing could be a bit of a hit or miss affair with some being perfect and some less so. The crews were originally taught to pull the power off in one stroke at about 15ft, but later they used to bleed it off and in my opinion this improved the landings greatly. The problem with landing Concorde was when it got into ground effect if you let the nose drop you lost a lot of lift and arrived somewhat heavily. However if you pulled too hard you could raise the nose too much and suffer a big loss of speed causing a subsequent un-attractive landing, and you could also touch the tail wheel. This touch would be noticed by the ground engineer after landing as a scuff mark on the tail gear tyres. Therefore your friendly F/E on his external check prior to departure would always check the tail wheel tyres for scuff marks and if there were any you could inform the engineers at the other end of the trip that they were there prior to you taking the aircraft, and they would have to go and find another crew to blame At touch down the pilots eye height was similar to that of a 747 pilot at touch down. Below 800ft when the aircraft had slowed down to landing speed the pitch attitude was such that the F/E could not see the runway ahead:eek: |
ChristiaanJ
During landing, Concorde isn't flared at all, it is flown onto the ground at a constant pitch attitude. The autoland on Concorde was both extremely accurate and reliable, and an awful lot of guys said they hated using it 'because it can land the aircraft :mad: better than I can'; their words NOT mine. (Personally I never bought that one, the guys were just modest as far as I was concerned:ok:). This in my opinion is an absolute testament to the AFCS designers; ChristiaanJ and his colleagues at SFENA and GEC Marconi. To give the complete final approach story; as the aircraft tracked the glideslope in LAND mode, the autopilot G/S deviation, like most aircraft, was geared as an inverse function of radio altitude, and at 75' radio this deviation was flushed down the loo altogether, leaving the A/P to hold radio rate for just a few feet. At 50' the flare was instigated, and at around 35' DECRAB was commanded, where the yaw channel would use a rudder input alone to 'kick off drift' and align the aircraft with the runway centreline. (Concorde did not employ a fwd slip manoeuvre in crosswind conditions, being a slender delta). The 'final' command was at 15' radio, when the autothrottle smartly retarded the throttles. (The Pitch Computer flare law of course continuing to control decent rate all the way down). On touchdown the autopilot would be manually disengaged and the nose gently (usually :p) lowered to the ground. (Concorde was only designed and certified as a CAT 3A system, so there was no automatic rollout guidance. However there was a runway guidance symbol on the ADI, which used a combination of Localiser deviation and lateral acceleration, to give you runway rollout track). Now the flare law was tested every autoland, at G/S capture, and failure of this test resulted in the loss of LAND 3 status on the landing display panel. The most common defect of all with the Concorde autoland was in fact failure of the flare test, when at G/S capture, the previously illuminated LAND 3 indication would drop all of it's own to LAND 2. A simple changeover of autopilot paddle switches would nail the offending Pitch Computer, which would then be replaced before the next trip. Dude :O |
Bigggles78
Brit312 has certainly answered your query about the Braniff crew issue; I remember being told that one of their captains, a Texan who allegedly wore big cowboy boots while flying, had an ambition at '3,2,1, NOW..' to KICK the throttles open with the sole of his right boot. Never did find out if this ever happened. :ooh: The Braniff operation does seem a little crazy now, I must admit. They were supposed to have had long term ideas of serving the Pacific Rim with Concorde, it's a pity that we never got the chance to find out if that could have worked. Dude :O |
Nick Thomas
... I think am right to assume there were no spoilers... Correct. ...so on landing did the act of bring the nose down spoil the lift... Yes, as with most conventional aircraft, reducing the aircraft pitch attitude (once the main wheels were on the runway) would reduce the angle-of-attack and therefore reduce the amount of lift being generated by the wing. Modern aircraft wings are very efficient and will still be generating a considerable amount of lift during the landing roll, even as the aircraft slows down. Put simply, spoilers and/or lift dump systems are required to destroy this lift, in order to get as much of the aircraft weight as possible on the main landing gear, which, in turn, allows greater pressure to be applied to the wheel brakes before the wheels start to lock-up and the anti-skid units activate to release the applied brake pressure. Concorde’s wing however developed very little lift at zero pitch attitude, so, once you had landed the nose wheel, there was no need for spoilers. ...is that the reason why the non flying pilot pushed the yolk forward once she was down?... No. The reason was that using reverse thrust on the ground on Concorde caused a nose-up pitch tendency, strong enough to lift the nose. The procedure was the handling pilot would call Stick Forward as soon as she had landed the nose wheel and the NHP would apply forward pressure on the control column to make sure the nose didn’t rise. If the handling pilot applied reverse thrust before the nose wheel was on the ground, things could get very awkward very quickly. Firstly, the nose would probably rise, quite possibly beyond the power of the control column to lower it. Secondly, the wing would still be generating (some) lift and so only reduced wheel braking would be available before the anti-skids kicked in, and the amount of runway left would be diminishing faster than normal. The solution was to reduce to Reverse Idle power until the nose wheel was back on the runway, however, in the heat of the moment it was very easy to go through Reverse Idle and on into Forward Idle. Not only would this again hinder the deceleration of the aircraft, but it would also run the risk of scraping the reverser buckets on the runway (as the buckets moved from the reverse thrust position to the forward thrust position) so tight were the clearances between the buckets and the runway on landing. Best Regards Bellerophon |
M2dude,
The Braniff crews were great characters and yes many did wear cowboy boats, but the story I like is the one that goes as follows After hours of briefing prior to going on the simulator [for the first time] the Braniff crew got on the sim and went through all their checks, started the engines and taxied out to the end of the runway for their first Concorde sim take-off. Everybody was strapped in with seats in the correct position and all checks complete. The Captain called out "3,2,1, now" and all the throttles were moved sharply to full power and away we went with the visual showing the runway passing by at an ever increasing rate. Now the F/E had a couple of calls to make prior to V1 relating to how good the engines were performing the most important being at 100 kts, however before we got that far the Braniff F/E stood up in his harness and let out the cry " Gee Whiz look at the son of a bitch go". Needless to say that take off was stopped and we went back to start again at the end of the runway |
Awesome thread guys...special thanks to M2dude etal
Hey folks, I too have a wonderful love of the beautiful lady "Concorde"
I jsut fly the little ole 73 for Southwest. A former Braniff Captain, working as an instructor now(age 60 hit him); named Jerry white...has told me many fascinating stories of flying Concorde.:ok: I could sit and listen to him all day long. :p Not too long ago, fslabs released a version of Concorde for Microsoft flight sim FSX. Don't know if any of you "Concorde" folks as well as others have seen it. If not, take a look at Flight Sim Labs, Ltd. From what I understand, this is as close to flying a real Concorde as you can get. Marvelous bit of programming. I know a lot of the systems, flight model etc..were verified by former Concorde crews for accuracy. Check it out when you have the time. :ok: Now back to reading.... Best, David |
db737,
I remember Jerry White, who did his Concorde training in Toulouse. He was a great bloke and and a good pilot. If you see him again send him by best wishes from the Brit F/E in Toulouse |
Originally Posted by Brit312
If I remember correctly ground effect tended to force the aircraft nose down, so requiring the pilots to pull back on the stick as if they were flaring ,but in fact what they were doing was as you say maintaining the pitch attitude constant.
M2dude, Reading your description of the autoland, you must be quoting from documentation, no? I've still got the AFM, but i don't think it's as detailed as that? And for me it's now too long ago to remember it in full detail, without any documentation to cross-check. This in my opinion is an absolute testament to the AFCS designers; ChristiaanJ and his colleagues at SFENA and GEC Marconi. BTW, it was still Elliott at the time.... both SFENA and Elliott have gone through a few permutations since. CJ |
Hi Brit 312
I've got training at the end of Sept....I'll be sure and tell Jerry you said hello.:ok:
Best, David |
ChristiaanJ
Yes, as Brit312 has suggested, the pitch change as Concorde entered ground effect on landing was nose down. In simple terms (i.e. as it was explained to me ;) ) due to the nose up approach attitude, (10˝° PA) the wider trailing edge of the wing entered ground effect before the much narrower leading edge of the wing, meaning more lift was now being generated at the rear of the wing than before, aft of the CG, resulting in a nose down pitch change. As you say, she landed at (or very nearly at) her approach attitude, with ground effect responsible for killing most of the rate of descent, but, during the last fifty feet or so, in order to maintain that steady pitch attitude against the increasing nose down pitch change, a definite, progressive, rearward movement of the control column was required, a movement that felt remarkably like a “flare” to the pilot. The available pitch attitude range on landing was very tight. Depending on the approach speed selected, touchdown attitude typically would be 11° - 11˝°, with a warning call of “Attitude” from the NHP at 12˝° PA. On Concorde, effectively you only got one attempt at making a good landing, after that, with little room to manoeuvre (literally), you had to settle for a safe one, by making sure the wings were level, the rate of descent reasonable, and the pitch attitude within limits and just accepting whatever sort of touchdown she gave you! The speed, pitch and tone of the F/E's voice, as he made the radio height calls, were as reliable an indication as any as to what sort of arrival was imminent! Best Regards Bellerophon |
Bellerophon,
Many thanks for filling one of the many current gaps in my memory! I only "flew" her once, and that was on the original Filton development simulator. Took her up to Mach 2.11, well into the "cricket" zone... never forgot... (I think that was some development issue we were trying to settle at the time). I've been involved in a minor way with the restoration of both the Filton/Brooklands sim and the CDG/Toulouse sim, and with the SSTSIM and FlightSim Lab Concorde simulator programs, but never yet "flown" any of them. Maybe it's time I should ! CJ |
BRIT312
Now the F/E had a couple of calls to make prior to V1 relating to how good the engines were performing the most important being at 100 kts, however before we got that far the Braniff F/E stood up in his harness and let out the cry " Gee Whiz look at the son of a bitch go". It's generally known that the BA aircraft were temporarily re-registered to facilitate Braniff's operation out of IAD to DFW; G-BOAA, B, D & E were re-registered from G-BOAA and so on, to G-N94AA etc. Being an older registration, G-BOAC was re-registered as G-N81AC. At IAD, the 'G' part of the registration was covered over, leaving a now perfect 'American' tail number. Only five aircraft were involved in the operation (at the time BA operated just six aircraft, G-BOAF was still at the manufacturers at Filton, and G-BFKW (later to become G-BOAG) was on loan from British Aerospace. In order for the necessary FAA certification, required for operation by a US airline, a modification package were required by the FAA. Some of these modifications seemed a little 'picky' and irrelevant at the time (they still do). However some modifications were certainly not in this category, and quite honestly should have been 'picked up' by the CAA & DGAC during original certification of the aircraft. As an example, if the flying controls had been operating on GREEN or BLUE hydraulics only (due to an indicated spool valve jam) and that particular hydraulic system was subsequently lost, there was originally no automatic switching to select the standby YELLOW system into the flying controls; the controls would have been completely unpowered until a manual selection was made by the pilot. . One of the 'FAA Mods' was to facilitate just that, so if this (extremely unlikely I grant you) scenario had occurred, then YELLOW would automatically been selected into the controls, and at no time would the controls have been in an unpowered state. The Braniff operation ended in May 1980, due to heavy losses on the subsonic only route, and it's a rather sad irony that aircraft G-BOAF had been modified and reregistered at Filton, from it's original registration of G-BFKX to G-N94AF. Unfortunately the aircraft was delivered to BA in June 1980, one month too late to participate, and prior to delivery it's registration was converted to it's 'normal' British registration; all other aircraft also reverted to original registrations also. ChristiaanJ Reading your description of the autoland, you must be quoting from documentation, no? Bellerophon A brilliant description of the mechanics of final approach. It's so easy for us mere mortals to forget just what an involved and skilled process it was, to fly, and in particular land our totally amazing aircraft. Dude :O |
British Airways/Air France
Hi guys, just another SLF enjoying the thread...
How much cooperation was there between the two airlines in terms of training etc? Did any BA crews fly Air France aircraft for any reason for example? Were cockpit proceedures pretty standardised across the two airlines? Cheers! |
Great thread which I am thoroughly enjoying.
I seem to recall that Concorde was certified for the use of reverse thrust in the air. I also recall that it was "problematic". Would any of the contributors like to expand? :ok: |
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. |
Question for engineering types:
I remember being told in my conversion course that the motors driving the secondary nozzles (buckets) were the fastest rotating devices on the aircraft. Is it true? Have you got a number for it? Was it really more than the gyro in the stby horizon? 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. |
In-Flight Reverse (A case of Bucket and See)
Capt Chambo
Concorde was, as EXWOK says, could use reverse in flight, on the inboard engines only, and only as far as reverse idle, the mechanism of which was quite complex and did on occasion not do work as advertised. Bear in mind here that the Rolls Royce Olympus 593 was a pure turbojet with no bypass, and so a hot stream reverser only had to be used; the reverser buckets acting directly on the efflux as it did any reverser in the 'old' days. Also the same buckets that were used for reverse were also progressively opened up between Mach 0.55 and wide open at Mach 1.1, this giving a vital control enhancement to the divergencing efflux. The overall effect of this was to give a true overall convergent/divergent nozzle assembly, the ideal for any supersonic aircraft. As far as inflight reverse goes, the amount of HP compressor delivery air (P3) required to actuate the bucket airmotor in flight at an idle thrust settings, was quite minimal to say the least, and some help was definitely needed here. The moment that inflight reverse was selected (on the inboard engines only remember) the OUTBOARD engines would have their idle N2 automatically increased, and some of THEIR P3 air supply was also automatically ported over (via an isolation valve) to the inboard buckets. This whole process was required in order to give a little added muscle to the bucket airmotors, and give the system a fighting chance. Even this however was still not quite enough, the inboard travelling buckets required minimal air loading on their surface, and so the primary nozzles for the affected engines (the primary nozzle lived just aft of the LP turbine, aft of the reheat assembly) was automatically signalled wide open in order to assist matters here, by reducing gas velocity. One the buckets had reached full reverse the primary nozzle was then signalled full close (this applied for normal ground reverse also) and the automatic increased idle on the outboard engines was cancelled. To enable the described process to occur, provided all four engines were at idle, a solenoid latched button on the F/E's station could be selected. This signalled a circuit that enabled the selection of idle reverse on the inboard engines only, the opening of the P3 isolation valve, the raising of the outboard engine's idle and maximum primary nozzle angle for the outboards as soon as reverse was then selected.. The whole system was just a little fragile here; failure of either the extra air supply, or the raised idle on the 'other' engine was usually enough to stop the process working correctly. EXWOK While flying 'up front' I only ever experienced the use of inflight reverse once. (The captain was a bit of an Animal, if you flying guys see what I mean ;)). I would not say that it felt as if we'd hit a brick wall, as I'd expected the sensation to feel, more like we were flying into the dumped contents of a very large manure truck:O . The whole operation was so slick, we'd dumped the required amount of IAS more or less within a second or two, and normal thrust was immediately selected. As so often happened with you guys, you made it look too easy. As far as the speed of the airmotor goes, I seem to remember that it was something in the order of 80,000 RPM at max chat; as you say faster (around twice as fast) as the standby horizon motor. The basic core airmotor (not the whole assembly) was the same Garrett unit used on the P&W JT9 as well as the RB-211. Dude :O |
telster
How much cooperation was there between the two airlines in terms of training etc? Did any BA crews fly Air France aircraft for any reason for example? Were cockpit proceedures pretty standardised across the two airlines? On the technical side of things there were meetings between the two airlines, both together and jointly with the airframe and engine manufacturers, but on a day to day basis there was precious little exchange of information, and you'd have thought that we (BA) were the only operator of Concorde, as I'm sure the AF guys felt the same also. In all of my 30+ years on Concorde, I personally went to CDG only once for an exchange of technical views and to help them out with an air intake defect. I'm so glad that you are enjoying this post, it's great to have you here telster. (It's certainly forcing me to look deep into the dark corners my poor old grey matter). Dude :O |
Hi
I have yet another question! Last year I watched a programme where James May went up in a U2. He explained that at FL700 the plane was flying in "coffin corner" and that the difference in IAS between the stall and the max speed was only 10 knots. I understand that it's due to the very low air pressure at such heights. As Concorde could fly up to FL600 I wondered what this safe airspeed window was during the cruise/climb phase of flight and if this window was framed by the air pressure and/or the CofG position? Once again thanks to everyone for such great answers and also for the background information. Regards Nick |
Nick,
Again I'll leave the full details to the real experts.... I saw that program with James May in the U2 too, so I know what you mean. The "coffin corner" is the point for a subsonic aircraft where stall speed and limiting Mach number "meet" ; about 70000 ft for a U2, and considerably lower for an airliner. Either you stall, or you run into Mach buffet with control problems. I'll try and find you a proper picture of the Concorde flight envelope (I have a few but one isn't scannable, the other is too ancient - preprod, yet another is 'buried' on a CD). But nearly all of the edges and corners in that flight envelope "window" are a matter of certification. You're not supposed to exceed M=2.04, Tt=127°C, IAS= 530kts, simply because of the "wear and tear" on the aircraft. You're not supposed to go above 60000ft because your passengers might no longer survive a window blow-out. You're not supposed to go below 300kts above 41000ft, don't know if that's a minimum control speed or linked to the engines... Yet Concorde has been flown to Mach=2.23 and 68000ft without ill efect.... So the basic limitations are not linked to the classic "coffin corner" at all. CJ |
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. |
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 :p. (Stray into this and you'd get a 'stick' shaker warning. I hope this blurb helps Nick Dude :O |
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! :O
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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 |
M2dude
Is this what you're looking for? http://i303.photobucket.com/albums/n...s/scan0005.jpg Best Regards Bellerophon |
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! |
This thread deserves an award...
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. YouTube - Concorde late 32 landing at Leeds/Bradford Airport 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. |
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. http://img.photobucket.com/albums/v3...envelope2w.gif 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 |
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. |
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 |
Originally Posted by Shanewhite
(Post 5917085)
Could you explain to a complete novice the relationship between IAS, TAS ,GS and Mach no.?
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. So the 200 mph is your IAS, your "indicated air speed". 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! |
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!
Now, more about that beautiful machine, please... |
Nick:
So is the Cof G of 59% the determining factor for the MMO or is it some other factor? 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. |
Originally Posted by Nick Thomas
(Post 5917441)
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?
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 |
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. |
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. Doesn't change any of your other comments, though. CJ |
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. |
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:
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 |
Supersonic - it was certainly no sailplane and an ability to increase drag wasn't required. |
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.
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 |
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