Why don't jets fly higher?
Guest
Posts: n/a
Probabley because a B747 operater at M.86 doesnt want to see only 208 knots on his clock at FL500, and a B737 driver doesnt want to see only 178 knots at M.75 at the same level either.
I must admit its rediculous all these new types of long-haul aircraft are still dribbling around at subsonic speeds. But you can blame the politicians, greenies, and traveling public for that.
I must admit its rediculous all these new types of long-haul aircraft are still dribbling around at subsonic speeds. But you can blame the politicians, greenies, and traveling public for that.
Guest
Posts: n/a
I belive the problem is what is known as "Coffin´s coner" (sorry for the spelling)
With increase in alt the stallspeed increase, and the highspeed (Mcrit) decrease. At a point they will meet so you either fly to fast or to slow. I think the first Learjets had a problem with that, being able to fly very high. (I´m no Lear driver)
So one would have to fly supersonic, wich again means new engines, new intake design, new wings, new......
Another problem would be antennas. I belive to have heard that the Concorde is the only aircraft (comercial airliner) that does not carry TCAS, because the antennas would melt at high speed, and I would to expensive to develop new antennas for 8?? aircraft.
With increase in alt the stallspeed increase, and the highspeed (Mcrit) decrease. At a point they will meet so you either fly to fast or to slow. I think the first Learjets had a problem with that, being able to fly very high. (I´m no Lear driver)
So one would have to fly supersonic, wich again means new engines, new intake design, new wings, new......
Another problem would be antennas. I belive to have heard that the Concorde is the only aircraft (comercial airliner) that does not carry TCAS, because the antennas would melt at high speed, and I would to expensive to develop new antennas for 8?? aircraft.
Guest
Posts: n/a
I guessing part of the reason is the pressurization of the aircraft. At FL350 I get a cabin altitude of around 8000 ft. The press controller is nearly maxed out at 7.76 psid and the emergency pressure relief valves will open at 8.06 psid, so I guess I couldn't go much higher without sucking an O2 hose. I assume designing an airplane that can handle the pressure without popping like a balloon presents it's own challenges and the fix may not be cost effective.
Guest
Posts: n/a
The weight and dimensions of the aircraft are also factors. The amount of thrust and lift required to allow something like to A3XX to cruise higher would be enormous. There is also the increase in drag created by such large aircraft that is not as much of a problem a higher speeds with something as sleek and slender as concorde. The local mach numbers would increase and cause buffeting. Fuel consumption would be massive and increase the operating costs dramatically. The list goes on......
Guest
Posts: n/a
I took a 737 test ride to check some of these ideas. OK it was MS Flight Simulator but it's probably a satisfactory model of the effects.
I could get it to 43000 feet with 100 fpm still possible. But it was very difficult. N1 was higher than perhaps is operationally realistic and EGT was over the red line a number of times.
At this height (in ISA presumably) we were in 'Coffin Corner' with the Barber's Pole at something like 225 kts - not much margin there for turbulence etc.
More significant was the lack of power above about 35000 feet.
I still can't quite get my head around why power falls away. The plane itself just goes faster as the air gets thinner until it thinks it's the same as at low level. Why does the engine run out of puff in the same circumstances?
I could get it to 43000 feet with 100 fpm still possible. But it was very difficult. N1 was higher than perhaps is operationally realistic and EGT was over the red line a number of times.
At this height (in ISA presumably) we were in 'Coffin Corner' with the Barber's Pole at something like 225 kts - not much margin there for turbulence etc.
More significant was the lack of power above about 35000 feet.
I still can't quite get my head around why power falls away. The plane itself just goes faster as the air gets thinner until it thinks it's the same as at low level. Why does the engine run out of puff in the same circumstances?
Guest
Posts: n/a
TES, the engine runs out of puff for the same reason humans do at altitude. An engine designed to process fuel/air efficiently at 35000 will be unlikely to have the compressor/turbine aerodynamic properties, nor the fuel flow to operate in the more rarified 50000 ft alt. You can imagine the inefficiency of the Olympus on the Concorde if it were cruising at 39000 ft (the norm for most modern airliners today). It would use fuel at an alarming rate compared to a CF6/4060/211 I should imagine.
However, as alluded by 9freighter, the real reason is the pressurization loads required to keep the cabin at 8000ft would need a vastly heavy structure to cope. This is the very reason the fuselage dia on the Concorde is about the same as an F27. If you increase the internal volume of the pressure vessel at the same pressure, you have the X psi's acting on more square inches of area, requiring a much stronger structure/thicker skin to withstand the load.
For the sake of simplicity here, I am ignoring the supersonic wave form effects and surface heat a large physical size fuselage would need to have catered for it.
------------------
When all else fails, read the manual!
However, as alluded by 9freighter, the real reason is the pressurization loads required to keep the cabin at 8000ft would need a vastly heavy structure to cope. This is the very reason the fuselage dia on the Concorde is about the same as an F27. If you increase the internal volume of the pressure vessel at the same pressure, you have the X psi's acting on more square inches of area, requiring a much stronger structure/thicker skin to withstand the load.
For the sake of simplicity here, I am ignoring the supersonic wave form effects and surface heat a large physical size fuselage would need to have catered for it.
------------------
When all else fails, read the manual!
Guest
Posts: n/a
Big aeroplane supersonic = big sonic boom = overwater (or desert) flights only.
Big aeroplane which cruises at >M1.5 not efficient <M1.0
Same aeroplane = megabucks.
Market too small to make business case.
(If you made transpacific the market it may work, except that you've now emptied the premium cabins of the transpacific a/c which is where the airlines make the money)
To date, any engine which works at high speed WILL be noisy at low speed (departure/arrival). Chapter 3 marginal, no hope of meeting regs in force at time of entry into service.
If there is ever a Conc replacement it will almost certainly be a biz-jet. 'til that time they're likely to remain king of the castle.
Big aeroplane which cruises at >M1.5 not efficient <M1.0
Same aeroplane = megabucks.
Market too small to make business case.
(If you made transpacific the market it may work, except that you've now emptied the premium cabins of the transpacific a/c which is where the airlines make the money)
To date, any engine which works at high speed WILL be noisy at low speed (departure/arrival). Chapter 3 marginal, no hope of meeting regs in force at time of entry into service.
If there is ever a Conc replacement it will almost certainly be a biz-jet. 'til that time they're likely to remain king of the castle.
twistedenginestarter,
Up to the tropopause the temperature of the air decreases, offsetting some of the loss of the lower pressures. Above the tropopause (about 36,000') the temperature stops dcreasing (and in fact begins to increase in the mesosphere above ~65,000') so the specific fuel consumption of the engine begins to increase above the troposphere, and thrust decreases.
Concorde is designed to operate at supersonic speeds. The engines need to have subsonic air at the compressor inlet, and this requires a different intake to that on fan engines fitted to subsonic airliners. The engines are not as efficient, but the speed increase is worth the loss.
Up to the tropopause the temperature of the air decreases, offsetting some of the loss of the lower pressures. Above the tropopause (about 36,000') the temperature stops dcreasing (and in fact begins to increase in the mesosphere above ~65,000') so the specific fuel consumption of the engine begins to increase above the troposphere, and thrust decreases.
Concorde is designed to operate at supersonic speeds. The engines need to have subsonic air at the compressor inlet, and this requires a different intake to that on fan engines fitted to subsonic airliners. The engines are not as efficient, but the speed increase is worth the loss.
Guest
Posts: n/a
Just a little addition to checkboard's response - all the stuff about Conc engines true except they are at their MOST efficient at M2.0. There are huge benefits in pressure recovery attained by decelerating the flow from M2.0 to M0.4 (in 10 feet!). Like a supercharger upstream of the LP compressor.
MPG at M2.0 compares very favourably with any other heavy jet in CRZ.
MPG at M2.0 compares very favourably with any other heavy jet in CRZ.
Guest
Posts: n/a
Just looking at the logistics of supersonic flight makes one marvel at the fact that Concorde was ever built at all, using the technology of 30-odd years ago, when we were driving Ford Anglias and were still watching black and white telly!
All these years on, it still works perfectly, looks divine and has a faultless safety record.
Cost a few quid, though...
All these years on, it still works perfectly, looks divine and has a faultless safety record.
Cost a few quid, though...
Guest
Posts: n/a
Concorde was indeed a technological success. However at $4000US or whatever a ticket costs, it's a plaything for the rich business man/woman, rock/movie stars, etc. Economically it's a flop which is one reason the American's put a bullet in their SST program. If Concorde could make money, there'd be alot more of them flying around the world. As it is, it's only a drag on BA an AF's other transatlantic business. More than a fair chance that the A3XX will be the Concorde of the future - great airplane that can't recoup it's development costs. Sure am glad I'm not subsidizing it. Maybe the Brits and French think the prestigue points are worth it.
Guest
Posts: n/a
Wondering hit the nail on the head.
Higher alt = thinner air, but
Higher alt = colder air too.
As altitude increases and air density decreases, thrust reduces.
As altitude increases, temperature reduces, offsetting some of the effect this density decrease has on thrust produced.
Rate of climb is dependant on excess thrust available (that over and above the amount required for straight and level flight). Since ~FL370 is theoretically where the tropopause is, jets find that lavel their theoretical optimum, since climing higher results in thinner air with no offsetting temp decrease. As a result, excess thrust diminishes above the tropopause. Unless the aircraft has buckets of excess thrust, (ie concorde)and is more efficient AND still aerodynamically stable in the thinner air above the tropopause where aerodynamic damping is also reduced, the amount of excess grunt required to get higher, and the amount of stability augmentation required to keep you upright negate the cost benefits of reduced drag and fuel flows.
That's what I reckon.
Higher alt = thinner air, but
Higher alt = colder air too.
As altitude increases and air density decreases, thrust reduces.
As altitude increases, temperature reduces, offsetting some of the effect this density decrease has on thrust produced.
Rate of climb is dependant on excess thrust available (that over and above the amount required for straight and level flight). Since ~FL370 is theoretically where the tropopause is, jets find that lavel their theoretical optimum, since climing higher results in thinner air with no offsetting temp decrease. As a result, excess thrust diminishes above the tropopause. Unless the aircraft has buckets of excess thrust, (ie concorde)and is more efficient AND still aerodynamically stable in the thinner air above the tropopause where aerodynamic damping is also reduced, the amount of excess grunt required to get higher, and the amount of stability augmentation required to keep you upright negate the cost benefits of reduced drag and fuel flows.
That's what I reckon.