Question about airliners

Discussion in 'Aviation' started by Thorlifter, May 9, 2014.

  1. Thorlifter

    Thorlifter Well-Known Member

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    Can your typical airliner (757, A300, etc) break the sound barrier? If so, why don't they fly faster? Not fuel efficient?
     
  2. pbehn

    pbehn Well-Known Member

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    No way, the dreamliner has a higher cruise speed than most airliners but still subsonic.
     
  3. eWildcat

    eWildcat Member

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    Drag increases dramatically when speed gets too close to speed sound. So it can be done (Concord did it for many years), but it's definitely not fuel-efficient.
     
  4. pbehn

    pbehn Well-Known Member

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    I think I read on here that Concord was quite efficient when travelling supersonic, it was the fuel used during take off and climb that was the killer.
     
  5. m37b1

    m37b1 Member

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    #5 m37b1, May 9, 2014
    Last edited: May 9, 2014
    I doubt they could. Transonic drag would be too great, given that the wing and fuselage are not optimised for supersonic flight. A good study in this is the development of the F-102. Also, a generic google of "The Area Rule" will explain the limiting factors.
     
  6. tyrodtom

    tyrodtom Well-Known Member

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    Most airliners are rated by passenger miles per gallon. When a Concorde was at capacity (they weren't always full), it was rated at 15.8 pmpg. A 707 was 33.3, a 747 was 46.4, a DC-10 was 53.6.
     
  7. BiffF15

    BiffF15 Well-Known Member

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    Thorlifter,

    Planes are like cars, each are designed for different things. F1 racecar, BMW 3 series and Jeep can all drive on the road, but each has it's forte. Your sedan doesn't do well where 4x4 trucks excell since it wasn't designed for it. F-15, Cessna 152, and a B757 all fly, but each has it's limitations. An airliner might be able to go supersonic, but it won't do it more than once and it probably won't be pretty where it "lands".

    Cheers,
    Biff
     
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  8. cherry blossom

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    Discussions of the efficiency of Concorde are confusing. Some people, such as Stanley Hooker, will tell you that Concorde's engines were much more efficient than those of a 747 and of course they were because the high speed supercharged them so they were giving almost twice the thrust per unit of fuel burnt. However, Concorde was flying twice as fast as a 747, so the drag had gone up by a factor of four. Thus Concorde needed to burn fuel at twice the rate of a 747. However, the flight would only last half the time.

    What hurt Concorde was that they had to land at almost the same speed as a subsonic airliner. Thus they needed wings with about the same wing loading for take off and landing. If they were a cruise missile, they would have suffered much less drag. They could avoid some of the drag by flying higher but then the engines became less efficient as there is less air up there and it is hotter (the lowest temperature is roughly where the 747s fly).

    The figures for passenger miles per gallon are distorted because Concorde was all first class.
     
  9. tyrodtom

    tyrodtom Well-Known Member

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    That 15.8 Pmpg was figured with 128 passengers, a configuration it rarely flew at. I noticed you left out the fact that this Concorde would be delivering 1/2 to 1/3 rd the passengers on that flight in half the time.

    With a 8'7" cabin width there was never any possibility of anything greater than 4 abreast seating. A approximate 18 in width seat is uncommonly narrow seat for first class.
     
  10. pbehn

    pbehn Well-Known Member

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    That was my understanding, Concorde used more fuel per mile but was travelling so much faster. Are those figures an average over all usage or the best at cruising speed/altitude?

    The wings on an A380 are almost as thick as a Concorde fuselage, its freakin enormous.
     
  11. GrauGeist

    GrauGeist Well-Known Member

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    Perhaps you meant it is colder?

    The higher in altitude, the lower the temperature :)
     
  12. cherry blossom

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  13. GrauGeist

    GrauGeist Well-Known Member

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    Ok, you got me...yes the temps increase to about 32F/0C at 55km/180,000ft. at the stratopause.

    However, even the SR-71 doesn't stray high enough to experience that "heat", as it's maximum altitude recorded was 25,929m/85,069ft. and that altitude is well within the negative temps.

    The Concord/SST flew at about 16,764m/55,000ft. which puts it just above the tropopause boundary. The temps in this region are around -60C/-76F.
     
  14. pbehn

    pbehn Well-Known Member

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    No wonder pilots wear sunglasses they must have been close to roasting up there!

    In school I was taught that temperature was an average of molecular collisions and their velocity. On the edge of space it is a moot point what is "temperature" there arnt many molecules to collide with.
     
  15. nuuumannn

    nuuumannn Well-Known Member

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    Temperature decrease is around two degrees C every thousand feet. That's what we are taught when studying this sort of thing. You couldn't fly a 757 or any modern airliner at supersonic speed at all; it's not just airframe limitations, but engine limitations also; the big fans are not designed for those types of specifics. At supersonic speeds the air does funny things and it completely changes in nature, subsonic laws between temp, pressure and velocity, which govern gas turbine operation go out the window. The Concorde's engines were optimised for operation at supersonic speeds with variable intake doors to limit and slow down the flow of air to the engines. I can go into the specifics of it, but I don't have my text books handy.

    Concorde also was fitted with afterburners, which it used on take off - that's where the fuel burn goes, where the air is the densest, and loads are at their heaviest. Concorde's ogival wing design was also a compromise between high subsonic and supersonic regimes; it was a masterful piece of designing, but is inefficient at subsonic speeds, resulting in a very high landing speeds etc. I remember Braniff leased a Concorde for trans continental flights over the USA, but it cost the company millions because its fuel burn was so high; they lost gazillions on it because it wasn't designed for lower alt operations at subsonic speeds.

    Obviously, the longer into a flight, an aircraft gets lighter because of fuel burn and this contributes to engine efficiency also. On take off, a B747 burns 20% of its total fuel load, just to get off the ground. I'd imagine Concorde was similar. At height the big fans - and Concorde's turbojets were designed for maximum efficiency. With a Trent for example, or any big fan engine, the N1 compressor is doing all the work; some 90 percent of thrust is being produced by that fan alone - considering the latest variants of the Trent are generating some 100,000 lbs thrust, that's still 10,000 lbs by the hot stuff coming out the back, but that makes for a very low fuel burn at height. Modern high bypass gas turbines are the most efficient types of engine produced in terms of the amount of power generated compared to the amount of fuel used to power them.
     
  16. BiffF15

    BiffF15 Well-Known Member

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    Nuuumannn when I went through school on the Eagle they taught that supersonic airflow could not hit the face of any current jet engines (hence the variable intake ramps on the Eagle, Phantom, Super Hornets, and "Chines" on the SR-71). The fixed ramp jets have curved intakes to take care of this I believe (F16 / F22 / F35).
     
  17. tyrodtom

    tyrodtom Well-Known Member

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    Could some of the airliners break the sound barrier in a shallow dive and stay in one piece ?

    I've seen some of the models of Boeing 720's top speed listed as 625-640 mph, that's no very far off from the speed of sound at 35,000 feet.
     
  18. nuuumannn

    nuuumannn Well-Known Member

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    Yeah, Biff, the big fan at the front wouldn't like the speed of the air at all. EPR would be through the roof. Supersonic aircraft need convergent/divergent ducts as intakes.

    A DC-8 did, Tom, in 1961.
     
  19. Koopernic

    Koopernic Active Member

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    I'm not quite sure that is entirely correct, the F-16 deliberately avoided a variable geometry intake on cost and weight grounds, I imagine otherwise much higher speeds would otherwise be possible. The Chines are not part of the SR-71 intake system AFAIKT but part of the wing.
     
  20. BiffF15

    BiffF15 Well-Known Member

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    Koopernic,
    I should have typed "spike", instead of chines. I am speaking of the pointed cone at the front of each engine. As for the F16 I can only repeat what I've been told, but have heard it from different sources in the USAF (no supersonic airflow to the engine face). As for reasons, I could understand it costing less (and having fewer moving parts probably more reliable).
    Cheers,
    Biff
     
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