9 ms·

>if you do as the pilots want and achieve very high aerodynamic stability through the air-frame instead of stability control systems (fly by wire essentially) i

>if you do as the pilots want and achieve very high aerodynamic stability through the air-frame instead of stability control systems (fly by wire essentially) it reduces the aerodynamic efficiency of the airliner It's crazy to me that that would be an acceptable compromise.

AWildC182 8y ago
How so? When I say stability control, we're not generally talking about unstable or "relaxed stability" airframes where the system failing would cause a pitching moment to accelerate rather than converge and the aircraft would tumble and disintegrate. From an efficiency and handling standpoint, this would be ideal, but it's only used in tactical military aircraft where the crew can bail if there's a problem. In most cases, we're talking about preventing stall in a swept wing aircraft. Swept wings are necessary to cruise efficiently beyond ~300mph at high altitude so they have to stay obviously, however they have VERY poor stall characteristics. As such, we have to do some things to prevent the aircraft from stalling such as playing with trim, using a stick pusher, etc.
- mehrdadn 8y ago
  > Swept wings are necessary to cruise efficiently beyond ~300mph at high altitude Layman here, but 737, 747, etc. don't have swept wings, right? So they all cruise inefficiently... but are in fact stable... which is the opposite of what you wrote earlier? Sorry, I'm just really confused.
  - dragonwriter 8y ago
    > Layman here, but 737, 747, etc. don't have swept wings, right? They do, in fact, have swept wings.
  - stickfigure 8y ago
    You're picturing a fighter jet or something with severely swept wings. Do an image search - 737s, 747s, et al do indeed have gently swept back wings. They don't stick straight out at 90 degrees like aircraft from the piston era.
    - mehrdadn 8y ago
      Ohh I see, okay thanks.
      - tpowell 8y ago
        I like the LAX screensaver on Apple TVs—you can pick out the more organic/bird-like curve of the newer generation carbon fiber 787 planes from the old ones. I’ve never thought about it, but they do look less stable.
        AWildC182 8y ago
        From a stability perspective there's likely no difference insofar as the stall response will be bad regardless. You can have complex wing geometry that stalls gracefully, for instance carbon general aviation aircraft have similar traits but require much cleaner stall response for certification generally. BTW since it's been tossed around a lot, good stall response is when the whole wing stalls at the same time and both wings tend to stall together, therefore you get a clean lurch downward in a straight predictable line. Bad stall response is one part of one wing stalling before the rest such that the wing drops and the plane has to be fought to avoid a spin or if extreme enough, a tail slide or extreme side slip.
    - banku_brougham 8y ago
      Totally off topic but I see some of those WWII era piston craft flying and its quite a nostalgic experience.
anovikov 8y ago
Obviously stability has to be achieved through fly by wire tech. Doing it through natural aerodynamic stability is a waste of resources of insane proportions as such airframes induce more drag and burn more fuel. If this sounds too scary for people in a forum of software developers it only puts a shame on our profession, from ourselves.
- joe_the_user 8y ago
  No one knows better than software engineers how difficult it is to make inherently reliable software and how much complexity can add to the difficulty of making reliable software. That said, the cost of not using latest fuel efficient airplane would indeed be huge and the actual reliability of modern aircraft is very high and has been increasing over the years in which fuel efficiency also increased. Sometimes, human can hit on a formula that produces objects that satisfy all the given parameters more fully rather than compromising on any of the requirement. But it's quite plausible that these formulas cannot be milked forever - thus the "Max" may be the point where tradeoffs stop working.
  - cco 8y ago
    Could you clarify your assertion that it would be massively more expensive? A quick look at the numbers suggests that a 737 MAX 8 is about 10% more efficient on fuel burn compared to a 737 300. That is not "massive" in my book and I'm more than happy to pay a little bit more per ticket if it means a higher safety margin. Did you mean something older and less efficient than a 737 300?
- edoloughlin 8y ago
  such airframes induce more drag and burn more fuel As a curious bystander, I assumed using fly by wire tech to achieve stability would involve using control surfaces, which increase drag by their nature. How would an airframe that's naturally stable and doesn't require control inputs burn more fuel?
  - AWildC182 8y ago
namirez 8y ago
Fly-by-wire means the control surface actuators are connected to cockpit controls electronically rather than with hydraulic lines or metal cables; it's not necessarily related to stability and control augmentation system. With regard to the other point, I don't think anyone is advocating for very high aerodynamic stability. That would be a B52 carrying nuclear weapons. It was designed to be extremely stable and forgiving. That being said, you don't want to rely solely on stability augmentation for trimming an airliner. It reminds me of Air Canada flight 143, the pilots lost both engines and power. yet they were able to land the plane safely on an abandoned airport. I'm not sure if that would be possible with a 737 Max.
- sandworm101 8y ago
  Yes it would. There are backup systems to keep things working even with both engines dry (ram air turbine etc). In fact, if you really want to stretch our a glide, all those automated systems are probably a good things. They will keep the aircraft more perfectly trimmed for a glide than the pilots ever could by hand. https://en.wikipedia.org/wiki/Ram_air_turbine https://en.wikipedia.org/wiki/Ram_air_turbine
  - cududa 8y ago
    Yeah as long as the sensors actually work
  - StreamBright 8y ago
    And in case of the electrical system being down how do you control the flight when you have fly by wire controls?
    - zrobotics 8y ago
      If the electrical failure is severe enough to lose control authority, it seems just as likely that a hydraulic system would have failed. These planes are just too large to operate the controls mechanically, so in practice there is just as much to fail in a hydraulic system as electrical control, since both require power.

AWildC182 8y ago

How so? When I say stability control, we're not generally talking about unstable or "relaxed stability" airframes where the system failing would cause a pitching moment to accelerate rather than converge and the aircraft would tumble and disintegrate. From an efficiency and handling standpoint, this would be ideal, but it's only used in tactical military aircraft where the crew can bail if there's a problem. In most cases, we're talking about preventing stall in a swept wing aircraft. Swept wings are necessary to cruise efficiently beyond ~300mph at high altitude so they have to stay obviously, however they have VERY poor stall characteristics. As such, we have to do some things to prevent the aircraft from stalling such as playing with trim, using a stick pusher, etc.

mehrdadn 8y ago
> Swept wings are necessary to cruise efficiently beyond ~300mph at high altitude Layman here, but 737, 747, etc. don't have swept wings, right? So they all cruise inefficiently... but are in fact stable... which is the opposite of what you wrote earlier? Sorry, I'm just really confused.
- dragonwriter 8y ago
  > Layman here, but 737, 747, etc. don't have swept wings, right? They do, in fact, have swept wings.
- stickfigure 8y ago
  You're picturing a fighter jet or something with severely swept wings. Do an image search - 737s, 747s, et al do indeed have gently swept back wings. They don't stick straight out at 90 degrees like aircraft from the piston era.
  - mehrdadn 8y ago
    Ohh I see, okay thanks.
    - tpowell 8y ago
      I like the LAX screensaver on Apple TVs—you can pick out the more organic/bird-like curve of the newer generation carbon fiber 787 planes from the old ones. I’ve never thought about it, but they do look less stable.
      - AWildC182 8y ago
        From a stability perspective there's likely no difference insofar as the stall response will be bad regardless. You can have complex wing geometry that stalls gracefully, for instance carbon general aviation aircraft have similar traits but require much cleaner stall response for certification generally. BTW since it's been tossed around a lot, good stall response is when the whole wing stalls at the same time and both wings tend to stall together, therefore you get a clean lurch downward in a straight predictable line. Bad stall response is one part of one wing stalling before the rest such that the wing drops and the plane has to be fought to avoid a spin or if extreme enough, a tail slide or extreme side slip.
  - banku_brougham 8y ago
    Totally off topic but I see some of those WWII era piston craft flying and its quite a nostalgic experience.

anovikov 8y ago

Obviously stability has to be achieved through fly by wire tech. Doing it through natural aerodynamic stability is a waste of resources of insane proportions as such airframes induce more drag and burn more fuel. If this sounds too scary for people in a forum of software developers it only puts a shame on our profession, from ourselves.

joe_the_user 8y ago
No one knows better than software engineers how difficult it is to make inherently reliable software and how much complexity can add to the difficulty of making reliable software. That said, the cost of not using latest fuel efficient airplane would indeed be huge and the actual reliability of modern aircraft is very high and has been increasing over the years in which fuel efficiency also increased. Sometimes, human can hit on a formula that produces objects that satisfy all the given parameters more fully rather than compromising on any of the requirement. But it's quite plausible that these formulas cannot be milked forever - thus the "Max" may be the point where tradeoffs stop working.
- cco 8y ago
  Could you clarify your assertion that it would be massively more expensive? A quick look at the numbers suggests that a 737 MAX 8 is about 10% more efficient on fuel burn compared to a 737 300. That is not "massive" in my book and I'm more than happy to pay a little bit more per ticket if it means a higher safety margin. Did you mean something older and less efficient than a 737 300?
edoloughlin 8y ago
such airframes induce more drag and burn more fuel As a curious bystander, I assumed using fly by wire tech to achieve stability would involve using control surfaces, which increase drag by their nature. How would an airframe that's naturally stable and doesn't require control inputs burn more fuel?
- AWildC182 8y ago

namirez 8y ago

Fly-by-wire means the control surface actuators are connected to cockpit controls electronically rather than with hydraulic lines or metal cables; it's not necessarily related to stability and control augmentation system. With regard to the other point, I don't think anyone is advocating for very high aerodynamic stability. That would be a B52 carrying nuclear weapons. It was designed to be extremely stable and forgiving. That being said, you don't want to rely solely on stability augmentation for trimming an airliner. It reminds me of Air Canada flight 143, the pilots lost both engines and power. yet they were able to land the plane safely on an abandoned airport. I'm not sure if that would be possible with a 737 Max.

sandworm101 8y ago
Yes it would. There are backup systems to keep things working even with both engines dry (ram air turbine etc). In fact, if you really want to stretch our a glide, all those automated systems are probably a good things. They will keep the aircraft more perfectly trimmed for a glide than the pilots ever could by hand. https://en.wikipedia.org/wiki/Ram_air_turbine https://en.wikipedia.org/wiki/Ram_air_turbine
- cududa 8y ago
  Yeah as long as the sensors actually work
- StreamBright 8y ago
  And in case of the electrical system being down how do you control the flight when you have fly by wire controls?
  - zrobotics 8y ago
    If the electrical failure is severe enough to lose control authority, it seems just as likely that a hydraulic system would have failed. These planes are just too large to operate the controls mechanically, so in practice there is just as much to fail in a hydraulic system as electrical control, since both require power.