Posts about: "Lift Drag Ratio" [Posts: 10 Page: 1 of 1]¶

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

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!

Landroger
SSBJ is Supersonic Business Jet Rog', there have been a few designs but the most famous (and had the most potential) was the Sukhoi-Gulfsteam S21. This aircraft would carry about a dozen passenges at Mach 2.2, with a range of 4,500 miles. Gulfstram pulled out of the partnership; there werer serious doubts about the viability of the Russian engine as well as serious aerodynamic issues too.
I would not personally utter 'Concorde and Tornado' in the same breath Rog; you need to carry this 6 tonnes over more than several HUNDRED miles. There is absolutely no comparison between the performance of Concorde and the Tornado I'm afraid, you'd need to base any military adaption on a far better design than that.
Although design of the powerplant for any future SST is pivotal to the whole design, you still need an aerodynamic model with a significantly higher lift/drag ratio than Concorde to make the project viable. And as good as the SR-71 was (I'm one of her biggest fans) she was still using afterburning/reheat at Mach 3 cruise.
galaxy flyer
Great to see you back here GF. I DO hope that you are wrong about mini-skirts
You are so right about the massive industrial collaboration required; it seems that there was so much more of a 'daring spirit' in the 1960's, makes you wonder where all the balls have gone today. (Oh I know, there are so much more deserving causes than aviation for us to spend BILLIONS of $'s and \xa3's on today).
Nick Thomas
No need to apologise for any thread drift Nick; this is such a diverse thread now; your points are perfectly valid here. And thanks for your kind comments again Nick; CJ the rest of the guys and myself are more than happy to bore the socks off of you and all the other posters and readers.
hoofie
So glad that you enjoyed your Concorde experience. The Jeddah flights were a fairly brief 'experiment',it would be great if one of my pilot/flight engineer friends here did a trip, we'll soon know. The double 'shove in the back' would indeed as you say have been the inboard/outboard reheat selection. Glad you are enjoying the thread, it is certainly bringing back memories for me about this seemingly eternal aereplane.
jodeliste
No problem Rod, I think most of us here agree about that one. A terrible waste and a giant leap BACKWARD in aviation.

Dude

quote:I don't know why this popped into my head but what was her glide ratio if all the engines stopped? Maybe because I remember from my early training being told the a B707 had a better glide ratio than the PA28-140 I was learning in. Now that was an eye opener at the time.unquote

A lot depends on how fast you were flying. You can get pretty good values of Lift/Drag ratio (that defines the potential glide slope with all engines operating, if that is not a contradiction in terms) from the Concorde B pages of the Concorde SST site. The actual glide slope with engines out would have been a lot worse than those numbers because of windmilling and (supersonically) intake spillage drag.

Nowhere near any subsonic values!

Clive

Further to skittles post - if anyone has links to HD footage of concorde at all, I'd love it see it. There are a couple of youtube videos:
YouTube - Concorde Final flight:HD Tranquility Base Razorfish ( Audio 320kb
YouTube - Saving Concorde HD

But it would be great to see nice HD footage of one of the ladies in flight.

One question for the people who designed & worked with her - was there any sort of thrust vectoring - not for any directional controls of course, but perhaps to give extra lift to save drag on the wing when at mach2? Or is that a completely daft idea? Perhaps I'm not explaining what I mean well - what I mean is was the direction of the exhaust gases completely 100% unchanged, or were they directed slightly downwards to generate some lift?

Clearly, I have no clue what I'm talking about :-)

No, I probably couldn't.

Let me ask it this way: Could a student in Aeronautical Engineering calculate the lift and drag for (non-vortex) Concorde using the same equations he would use to calculate lift and drag for say, a 777?

In other, other words... I understand that there's a very different phenomenon developing a chunk of lift at high AoA. But the wing still has a very unique shape and camber, so I wonder if-- when the AoA is *not* as high-- phenomena responsible for our 777 staying up fully apply to Concorde.

If I'm just missing the boat completely here, just give me the stern eye and ask me to reread some physics.

asc12


Well yes, there is no magic difference. You could calculate the lift and drag using the same methods as you would for any other delta winged aircraft, or indeed for aircraft with a 'classic' planform. You might have a bit of trouble handling the effects of camber, especially leading edge camber, if you were starting from a clean sheet of paper, but skin friction, form drag and lift curve slope (attached flow) are all calculable by standard methods. You might be pushed to get a decent estimate of lift dependent drag - I spent many hours in my youth looking for wind tunnel results on low aspect ratio delta wings to get some idea of what might be expected, but once you have tunnel tests on 'your' aircraft course there is no problem.


Again the answer is yes - in subsonic cruise (0.93M in this case) exactly the same sort of aerodynamics applies - different in detail, but the same in principle.


CliveL

For a mid-cruise weight of say 300,000 lb and a lift/drag ratio of 7.5 the thrust required would have been 40,000 lbf and the powerplant sfc was around 1 lb/hr/lbf, so 40,000 lb/hr is just about right.

CliveL

Out of interest with any successor to concorde, what lift/drag ratio is now technically possible, and likewise from more advanced powerplants that could be available now what lb/hr/lbf numbers could be achieved?

One other question if I may - how much of a compromise was concorde's wing with respect to the balance of supersonic vs sub-sonic efficiency? What I'm trying to ask is if the wing could be a variable geometry with no weight cost (impossible I know) how much more efficient could the supersonic wing have become - or was the compromise very much on the sub-sonic performance and not much to gain in terms of supersonic efficiency?

Once more - thanks for all the great answers to the really intriguing questions!! I'm fascinated by Concorde - and regret I never had the means to fly on her whilst she was still where she belongs!

I'll try!

The last time I had anything to do with it people were talking about L/Ds around 10.5 in cruise (up from 7.5).

There are technical issues why one cannot use high bypass engines for supersonic cruise, so the thermodynamic cycle would be much the same as the Olympus. That being so the only real gain would come from higher TETs today so the benefits would be limited - two or three percent sfc perhaps?

[Yes I know the USAF are flying supersonic cruise aircraft, but look at how much bypass their engines actually have and the supersonic cruise Mach Numbers]

Obviously the MOST IMPORTANT condition was supersonic cruise, so this dominated the compromise. OTOH, the reserve fuel was largely driven by subsonic performance, so one couldn't give too much away. It might surprise people, but the 0.93M specific range is much the same as the 2.0M value.

As for variable geometry wings (1970s style), the best I can offer is that Boeing started with a variable geometry design (with which they won the design competition), but as the design process progressed the amount of wing that varied got less and less until the Boeing aircraft looked very much like the Lockheed design that lost the original competition.

What do you think?

CiveL