Posts about: "TAS (True Air Speed)" [Posts: 12 Pages: 1]

MEMORIES
Like so many in the Concorde family, I have millions, I'd like to share a couple here. I remember at Fairford in mid 1974, a CAA test pilot (I honestly forget the gentleman's name) was taking the British pre-production A/C 101 (G-AXDN) for a special test flight. The reason that this flight was so special was that for the first time, the CAA were going to do an acceptance flight trial of the brand new digital air intake system. This revolutionary system had been retro fitted to 101 barely a year earlier, and being a brand new (and totally unique, in electronics terms) system had been plagued with teething troubles. It was quite reasonable for any airworthiness authority to have serious misgivings about any system that was going to wave great big metal lumps around in front of the engine compressor face, and that if only a few degrees out from the commanded position out could cause the engine to 'backfire' etc.
So anyway, 101 took off and disappeared into the very blue sky and we waited, and waited, AND WAITED. (I'd only left the RAF and joined the project a few months previously, and did not want my new association with this amazing aircraft to end). I was biting my nails, drinking coffee, losing my hair... (without the help of M2V ). Anyway after about 2 1/2 hours the aircraft returned to Fairford, and everybody crowds around the crew for the debrief. A very stern faced CAA pilot looked at us all, broke into a grin and said "as far as I'm concerned gentlemen, you've got yourselves an airliner". At that point the room was a study of total happiness, blessed relief, and a need to go to the loo..... But from my point of view, I will remember those words forever.
101, which now resides at the Imperial War Museum Duxford was the fastest Concorde ever. She achieved Mach 2.23, which was an incredible irony, as Concorde can trace a large part of it's developement history back to the BAC 223, proposed SST.
As far as flying memories go, I just don't know where to start; My first ever Concorde flight was in November 1976, out of Fairford on a pre-delivery test flight on G-BOAD. (Now sadly bobbing up and down on the Hudson, next to the USS Intrepid). I was staggered how fast and high we flew (Mach 2.08, FL580). Most of my flying up to that date had been in C-130's in the RAF, at around 340 KTS and FL300; Concorde also being infinately quiter in flight than the good old Herc'. I remember a BA QA guy showing me how I could touch the skin of the aircraft at Mach 2 (You reached behind a door busstle flap, moved your hand through some insulation until you felt bare metal). OUCH!! it was hot, very hot.
But I think one of my most memorable flight memories was aboard G-BOAG, (now residing in the Boeing Museum of Flight in Seattle) returning from BKK, having stopped off to refuel in BAH. We were forced to fly subsonic over Saudi, and got caught in this amazing electrical storm, There was St Elmo's fire cracking and bubbling all over the visor panels, but just as incredible was the long blue electrical discharge coming off of the nose probe; it seemed to extend about 50' in front of the aircraft. The crime was, none of us on the F/D had a camera. Every time I bump into the captain on that day (are you reading this Ian?), we go back to remonissing about that incredible flight. Also, later on the same sector, after we had decelerated to subsonic cruise again, this time flying up the Adriatic, we had another fascinating sight: It was getting quite dark now, and here we were, travelling at Mach 0.95 at FL290, when above us was all this Mach 0.8 ish traffic at around FL330-350. All we could see were all these navigation and ant-coll' lights above us, seemingly travelling backwards. It was quite a sight. On the original BAH-BKK sector a week earlier, we flew through some of the coldest air I'd ever seen; The air was at ISA -25, and at Mach 2 our TAT was only about 85 deg's C. (You could feel the difference too; the cabin windows felt only warm-ish to the touch). The upside also of all this was that your fuel burn was much lower than usual. (The only downside of course is that your TAS is a little lower). Rolls Royce did some analysis on the flight, and were amazed at how well the propulsion systems coped with some of the temperature sheers that we encountered, sometimes 4 to 5 deg's/second. They said that the prototype AFCS had been defeated by rises of only 0.25 deg's/second ).
Not meaning to go off onto a (yet another) tangent; Negative temperature shears, very common at lower lattidudes, always plagued the development aircraft; you would suddenly accelerate, and in the case of a severe shear, would accelerate and accelerate!! (Your Mach number, quite naturaly, suddenly increased with the falling temperature of course, but because of the powerplant suddenly hitting an area of hyper-efficiencey, the A/C would physically accelerate rapidly, way beyond Mmo). Many modifications were tried to mitigate the effects of severe shears, in the end a clever change to the intake control unit software fixed it. (Thanks to this change the production series A/C would not be capable of level flight Mach numbers of any more than Mach 2.13, remembering that Mmo was set at 2.04).
There was one lovely story, involving the Shah of Iran, having one of MANY flights in a developmment aircraft. The aircraft encounterd quite a hefty series of temperature shears that plagued havoc with some Iranian F4's that were attempting to close on the Concorde, to act as an escort for the Shah. (or so the strory goes). I'm still trying to picture these F4's, on full afterburner trying to get close to a Concorde cruising away on dry power). It is said that the F4's were having such difficulties, due to their relatively crude powerplant, coping with the temperature changes, that the Concorde was ordered to slow down, 'so the escorting F4's could catch up'!! True or not, it is part of Concorde folklore.

Dude

Nick Thomas
Just like Christiaanj I'm trying to dig up an accurate flight envelope diagram. (A lot of my Concorde 'technical library' is out on long term loan), but I would suggest that anywhere within Concorde's published flight envelope you never hit any equivilant to Coffin Corner, a la' U2. The whole issue is really one of air DENSITY, rather that pressure, where as you climb at a given Mach Number, your Indicated airspeed (IAS) falls away with altitude. (Velocity of sound being primarily tied to static air temperature). Now if you are climbing in the stratosphere, where temperature is more or less constant up to around 65,000', you can say that your TRUE Airspeed (TAS) is also constant with climb at a given Mach number. But lift and drag are functions of IAS (the equivalent airspeed that the aircraft would 'feel' at sea level) and not TAS. Because the U2 had a very low Maximum allowable Mach number (Mmo) as IAS fell away with altitude, it would get to the point where it's lowest permitted airspeed (we called this VLA) got to within a few knots of Mmo and severe aerodynamic buffering. i.e. you were screwed with nowhere to go but down .
In the case of Concorde, Mach 2 at FL500 was 530KTS, falling to 430KTS at FL600. Although we have less lift due to 100KTS lower IAS, the aircraft is now much lighter (this is the whole principal of cruise/climb) which keeps the universe in balance, but drag is now significantly lower too, getting us better MPG .
On the ASI, the only limitation displayed was Vmo; however the Machmeter did display fwd and aft CG limits at a given Mach number. The ONLY time that Concorde would experience relatively low speeds at altitude was at Top of Descent. I'm a little fuzzy here how it all worked exactly (it's an age thing you know), I'm sure one of the pilots can correct me, but I seem to remember that the autothrottle was disconnected, ALTITUDE HOLD was selected on the AFCS, and the throttles slowly retarded. (If you pulled back too far you'd often get a gentle 'pop surge' from the engines, and you had also to be wary of equipment cooling airflow too). The aircraft was then allowed to gently decelerate, still at TOD altitude, until Mach 1.6, when power was tweaked to give 350KTS IAS and IAS HOLD was selected. The aircraft was now free to carry out her loooong descent to 'normal' altitudes. VLA on Concorde was not directly displayed as you never flew anywhere near it, and also every pilot knew his VLA . (Stray into this and you'd get a 'stick' shaker warning.
I hope this blurb helps Nick

Dude

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

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!

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.

I'll try....

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

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

Mach no. is TAS divided by the local speed of sound.
The speed of sound in air depends almost exclusively on the temperature: in a ' standard ' atmosphere it's 760 mph on the ground at +15\xb0C, drops to 660 mph at 37000ft / -57\xb0C 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!

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\x92t know what the wind was, nor what the TAS or OAT were, but we can easily deduce that:

• If the OAT was standard at FL600, at -56.5\xb0C, then, as at that temperature M2.00 equates to 1,147 kts TAS, in order to have a GS of 1,139 kts, she must have been flying into an 8 kt headwind.

• If the wind at that altitude was calm, then her GS of 1,139 kts must have been the same as her TAS. For M2.00 to be 1,139 kts TAS, then the temperature at FL600 must have been 3\xb0C colder than standard, at -59.5\xb0C.

• If , as was typical on a LHR-BGI sector, the OAT at FL600 was 10\xb0C colder than standard, at -66.5\xb0C, then M2.00 would equate to 1,120 kts TAS, so to have a GS of 1,139 kts, she must have been flying in a 19 kt tailwind.

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

Best Regards

Bellerophon

Quick link to Bellerophon 's post #66 and photo to save you having to 'leaf' back...
G-BOAE at Mach 2

Much as I look at that picture, I can't see the groundspeed.....

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

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

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

CJ

ChristiaanJ
It was one of the strange little differences between the BA and Air France aircraft that the French had a small digital TAS indicator (on the lower F/O's instrument panel) and BA had none.
As you rightly say, as an indicator TAS is not that much use to you, BUT TAS is vital for calculating wind speed/direction within an INS/IRS system, hence that is why any air data computer gives a TAS output to the INS or IRS.
Nothing dumb about the question (I wonder if you are even capable of such a thing ChristiaanJ ). Yes, the distance window on the HSI related to the next INS waypoint.

Dude

A memory which really stuck re. hand flying supersonic was how solid it felt - and how the extreme TAS (about 1200kts) vividly demonstrated the relationship between TAS / angle of bank and turn rate: you rolled on 15 degrees of bank and it seemed like the HSI heading reference had stuck - it just didn't turn. From memory turn radius with 30 degrees AOB at M2 was about 50nms...

Re the autostabs: it felt like the puppet's strings were cut with them off, but switching to Mechanical Signalling as well made it truly horrible. I only ever flew the sim in this config, I understood they used to do it at base but stopped it because they feared lives would be lost. A pal once memorably described it as like trying to fly a dustbin lid around. And so it was! Plug it all back in (Electrical Signalling and Autostabs) and it all snapped back into shape beautifully. So ahead of its time...

There were no doubt pitch trim changes transonic due to the mach trim system, but a more obvious effect in the 0.99-1.3 range was due to shockwaves forming and fading assymetrically causing minor oscillations in all axes: if you watched out front while hand flying the accel or decel you could make out the nose descibing what felt like figures of eight as the trim changed in pitch and yaw. Roll too, and gentle pressure was required on the cc to avoid overcontrolling and PIO - a bigger problem with the Conc than some other types...

Memory lane - this is fun!

Are there any concorde simulators that are still working and retain their certification?

Regarding LHR JFK routes.

What was the avarage fuel load and how close to full tanks was it.

At FL500-600 what sort of wind was usually encountered. So high above the tropopause I would think very little.

Flying magazine from the US did a spread on the concorde many years ago. Theye stated that the wind component was such a little percentage of TAS that the block times rarely differed by more then 10 mins. True or false.

They also said that the type rating course was so hard that only the top performers (pilots) were selected for the training and even then there was a 50% washout rate. True or false.

Does anyone still have a complete set of ground school notes?

stilton

Actually not technically correct mate. The generation(s) of aircraft after Concorde does not use INS as such at all. They either use an Inertial REFERENCE System (IRS) or an Air Data and Inertial Reference System (ADIRS). In both cases inertial data, such as attitude, present position, heading (both true and SYNTHESISED magnetic) acceleration data etc. are output to various user systems. (eg. FMS, EFIS, Autopilot etc.). Wind data, being a function of True Air Speed (TAS) and Ground Speed (G/S)requires in the IRS case TAS data to be input into the IRS from an Air Data Compter, whereas in the ADIRS case we can have muliple ADCs/IRUs effectively crammed into one box, and so wind is kept 'in house'. In either case the autopilot steering signals (LNAV/VNAV) come from the FMS and NOT the IRS. (If you like you could say that an INS knows where it is and where it wants to go, where an IRS just knows where it is and hasn't a CLUEwhere it's going to. In all cases GPS data is fed into the FMS itself, as 'just another input'.
Although Concorde HAD no GPS, (The most difficult part was always finding a part of the upper fuselage where chunks could be cut out for locating antennae) it was coming! EGPWS was being mandated, which required a simplex GPS antenna mount, and GPS updating for the INS was being seriously looked at. In the first case, the EGPWS requires accurate present position to check agaings it's terrain database for known obstacles and the latter case was because the Concorde INS navigational accuracy fell outside of future (now actually) long range navigational accuracy requirements.
Sorry for such a long winded blurb, but I've been away for a while and am gradually looking back over our wonderful thread to see if there is anything I can contribute to/prattle on about.