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LCDFlight
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Could an airliner save fuel by being unstable

Sun Jun 14, 2020 1:15 am

This topic is brought to you by MD-11 and Boeing 737 Max.

Starting with either a clean sheet or an existing airliner, is it possible to take some fairly obvious shortcuts on stability in order to get a better unit fuel efficiency? Is there any reluctance to use MCAS style remedies to cure an otherwise unstable aircraft? I have seen it claimed that major fuel savings can be had by doing that. Is it true for 787, A320 etc? Thanks
 
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Florianopolis
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Re: Could an airliner save fuel by being unstable

Sun Jun 14, 2020 1:32 am

Technically? Yes.
Legally? Maybe not. https://www.law.cornell.edu/cfr/text/14/25.672
 
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Starlionblue
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Re: Could an airliner save fuel by being unstable

Sun Jun 14, 2020 3:42 am

LCDFlight wrote:
This topic is brought to you by MD-11 and Boeing 737 Max.

Starting with either a clean sheet or an existing airliner, is it possible to take some fairly obvious shortcuts on stability in order to get a better unit fuel efficiency? Is there any reluctance to use MCAS style remedies to cure an otherwise unstable aircraft? I have seen it claimed that major fuel savings can be had by doing that. Is it true for 787, A320 etc? Thanks


Airliners have used relaxed stability to increase fuel efficiency for decades, by means of the "obvious shortcut" of moving the CG back.

The A330/A340 and A380 shift fuel back to the trim tank during the climb to move the CG back. This decreases stabiliser loading, and thus also decreases pitch stability, resulting in lower burn.

The A350 instead uses variable extension of the flaps in cruise to move the CL forward. The effect is the same.


Going unstable is technically possible, of course, but I think very far from what airframers, operators and regulators are currently comfortable with in a commercial operations.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
strfyr51
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Re: Could an airliner save fuel by being unstable

Sun Jun 14, 2020 5:28 am

LCDFlight wrote:
This topic is brought to you by MD-11 and Boeing 737 Max.

Starting with either a clean sheet or an existing airliner, is it possible to take some fairly obvious shortcuts on stability in order to get a better unit fuel efficiency? Is there any reluctance to use MCAS style remedies to cure an otherwise unstable aircraft? I have seen it claimed that major fuel savings can be had by doing that. Is it true for 787, A320 etc? Thanks

an unstable fighter/bomber is one thing as it might only kill a small number of people at a time, And? They volunteered. However? An Unstable Airliner? Those passengers Paid to fly and were it ever found out an airplane was Built Inherently unstable? And it Crashed? There wouldn't be enough money PRINTED to cover the lawsuits, Hell! .I'm not a Lawyer and I'd sue them! Or try and clean up as an "Expert witness" !! I've been on a few accident investigations. A lot of it is "speculation".
and I can speculate as well.
 
mxaxai
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Re: Could an airliner save fuel by being unstable

Sun Jun 14, 2020 12:54 pm

Technically, you could. Potential benefits could be smaller control surfaces, perhaps even completely without the horizontal or vertical stabilisers. At a minimum (if applied to existing designs) you could unload the horizontal stabiliser and reduce drag. The most benefit would probably be seen on a BWB, which is rather tricky to stabilise with conventional methods.

The primary benefit of unstable aircraft is improved maneuverability, especially in transsonic & supersonic flight, which is obviously not a priority for airliners.

However, all current civilian aircraft have to satisfy stability requirements in all conditions. AFAIK center of gravity (i. e. stability) has been an issue for some A320 operators so they had to block the last row of seats.
 
kalvado
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Re: Could an airliner save fuel by being unstable

Sun Jun 14, 2020 1:15 pm

Similar, but not exactly an instability:
As far as I understand, canards instead of tail mounted stabilizer could be a fuel saver at a cost of stall recovery and low speed issues.
Same as dynamic instability, found the way to military planes, but very little for civilian use.
 
hitower3
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Re: Could an airliner save fuel by being unstable

Sun Jun 14, 2020 4:48 pm

Interesting read!
This triggers one more question: how much fuel could possibly be saved by the means of giving up the inherent stability?

If we define "unstable" as "the horizontal stabilizer will not provide a continuous down force in normal flight" (I am not sure about this definition, please correct me if I am wrong!), there is probably little to gain here.
 
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Starlionblue
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Re: Could an airliner save fuel by being unstable

Sun Jun 14, 2020 11:48 pm

hitower3 wrote:
Interesting read!
This triggers one more question: how much fuel could possibly be saved by the means of giving up the inherent stability?

If we define "unstable" as "the horizontal stabilizer will not provide a continuous down force in normal flight" (I am not sure about this definition, please correct me if I am wrong!), there is probably little to gain here.


The "mildly relaxed stability in cruise" on the A330 gives maybe 1% fuel savings. I guess if you went completely unstable you might gain a little more. But I don't think we're talking any radical gains.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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jetmech
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Re: Could an airliner save fuel by being unstable

Mon Jun 15, 2020 4:42 am

kalvado wrote:
Similar, but not exactly an instability:
As far as I understand, canards instead of tail mounted stabilizer could be a fuel saver

I think this is an interesting idea. The wings of commercial types need to be bigger than that required to lift the mass of the aircraft due to the negative lift produced by the tailplane. This is especially acute at rotation; the moment the aircraft needs as much lift as possible is the exact same moment when we get a huge negative lift from the tailplane!

kalvado wrote:
at a cost of stall recovery and low speed issues.

Interesting. What are the stall and low speed characteristics of canards?

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
kalvado
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Re: Could an airliner save fuel by being unstable

Mon Jun 15, 2020 5:11 am

jetmech wrote:
kalvado wrote:
Similar, but not exactly an instability:
As far as I understand, canards instead of tail mounted stabilizer could be a fuel saver

I think this is an interesting idea. The wings of commercial types need to be bigger than that required to lift the mass of the aircraft due to the negative lift produced by the tailplane. This is especially acute at rotation; the moment the aircraft needs as much lift as possible is the exact same moment when we get a huge negative lift from the tailplane!

kalvado wrote:
at a cost of stall recovery and low speed issues.

Interesting. What are the stall and low speed characteristics of canards?

Regards, JetMech

Disclaimer: my background is in physics, but not aero. I may look at things differently and misunderstand some

With that, the question is what happens at critical AoA and what stalls first. For swept wing, wing root has to stall first. That creates nose down momentum and helps plane to avoid full stall - if that plane is not 737max. That also allows pushing flight envelope to maximal AoA, hence lower speed
For canards, center of lift of main wing goes further back, so canard has to stall first, and AoA envelope is narrowed, requiring bigger wing or higher speed.
If wing stalls first, nose is pushed up by canards and things go really wrong.

Very remote analogy would be shadowing of T tail by the wing. While effect is pretty non obvious and not affecting most of operations, it still ends up a significant factor affecting entire design paradigm.
 
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jetmech
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Re: Could an airliner save fuel by being unstable

Mon Jun 15, 2020 6:02 am

kalvado wrote:
With that, the question is what happens at critical AoA and what stalls first. For swept wing, wing root has to stall first. That creates nose down momentum and helps plane to avoid full stall - if that plane is not 737max. That also allows pushing flight envelope to maximal AoA, hence lower speed
For canards, center of lift of main wing goes further back, so canard has to stall first, and AoA envelope is narrowed, requiring bigger wing or higher speed.
If wing stalls first, nose is pushed up by canards and things go really wrong.

Thanks! Yes, I just had a read of the Wikipedia article which mentions this phenomena amongst others. It's interesting that many things may behave counter to first appearances. No doubt we would have seen a canard airliner by now if their were realisable benefits to the configuration.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
kalvado
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Re: Could an airliner save fuel by being unstable

Mon Jun 15, 2020 2:46 pm

jetmech wrote:
No doubt we would have seen a canard airliner by now if their were realisable benefits to the configuration.

Regards, JetMech

Maybe not. it is about stall entry, which is tightly regulatred, so to be certifiable such plane must have MCAS-like logic running on a very reliable platform. Probably more reliable than FBW, as at least Airbus has a fallback mode with all electronics off and plane still reasonably flyable.
As always - in 50 years we'll have a better answer...
 
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Starlionblue
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Re: Could an airliner save fuel by being unstable

Mon Jun 15, 2020 3:03 pm

kalvado wrote:
jetmech wrote:
No doubt we would have seen a canard airliner by now if their were realisable benefits to the configuration.

Regards, JetMech

Maybe not. it is about stall entry, which is tightly regulatred, so to be certifiable such plane must have MCAS-like logic running on a very reliable platform. Probably more reliable than FBW, as at least Airbus has a fallback mode with all electronics off and plane still reasonably flyable.
As always - in 50 years we'll have a better answer...


There is no "no electronics" fallback in newer Airbus models.

In the older Airbuses, e.g. A330, rudder and trim are mechanically connected to the cockpit, so you could use the pedals and pitch trim for control in "Mechanical Backup"

In the newer ones, e.g. A350, all flight control is electronic. Backup Control is run by the Backup Control Module. So if you really lose all electronics, you're toast.

Either way, the A350 is probably more redundant and reliable than the A330 in this regard.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
kalvado
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Re: Could an airliner save fuel by being unstable

Mon Jun 15, 2020 4:05 pm

Starlionblue wrote:
kalvado wrote:
jetmech wrote:
No doubt we would have seen a canard airliner by now if their were realisable benefits to the configuration.

Regards, JetMech

Maybe not. it is about stall entry, which is tightly regulatred, so to be certifiable such plane must have MCAS-like logic running on a very reliable platform. Probably more reliable than FBW, as at least Airbus has a fallback mode with all electronics off and plane still reasonably flyable.
As always - in 50 years we'll have a better answer...


There is no "no electronics" fallback in newer Airbus models.

In the older Airbuses, e.g. A330, rudder and trim are mechanically connected to the cockpit, so you could use the pedals and pitch trim for control in "Mechanical Backup"

In the newer ones, e.g. A350, all flight control is electronic. Backup Control is run by the Backup Control Module. So if you really lose all electronics, you're toast.

Either way, the A350 is probably more redundant and reliable than the A330 in this regard.

Thanks for correction. I actually heard about A320/330; but it never crossed my mind that 350 would be different. What about 380, while we're at this?
 
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Starlionblue
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Re: Could an airliner save fuel by being unstable

Tue Jun 16, 2020 1:22 am

kalvado wrote:
Starlionblue wrote:
kalvado wrote:
Maybe not. it is about stall entry, which is tightly regulatred, so to be certifiable such plane must have MCAS-like logic running on a very reliable platform. Probably more reliable than FBW, as at least Airbus has a fallback mode with all electronics off and plane still reasonably flyable.
As always - in 50 years we'll have a better answer...


There is no "no electronics" fallback in newer Airbus models.

In the older Airbuses, e.g. A330, rudder and trim are mechanically connected to the cockpit, so you could use the pedals and pitch trim for control in "Mechanical Backup"

In the newer ones, e.g. A350, all flight control is electronic. Backup Control is run by the Backup Control Module. So if you really lose all electronics, you're toast.

Either way, the A350 is probably more redundant and reliable than the A330 in this regard.

Thanks for correction. I actually heard about A320/330; but it never crossed my mind that 350 would be different. What about 380, while we're at this?


Not really sure.

The A380, rather logically, seems to have systems that are halfway between the A330 and the A350. For example, it has a trim tank like the A330 for CG control, and it has EBHAs and EHAs (electrohydraulic actuators) for backup control like the A350.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
hitower3
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Re: Could an airliner save fuel by being unstable

Wed Jun 17, 2020 2:02 pm

Starlionblue wrote:

(A380 backup flight control)
Not really sure.

The A380, rather logically, seems to have systems that are halfway between the A330 and the A350. For example, it has a trim tank like the A330 for CG control, and it has EBHAs and EHAs (electrohydraulic actuators) for backup control like the A350.


Dear Starlionblue,

There has been a constant evolution in back-up flight controls in Airbus FBW aircraft:
- A320: Mechanical pitch trim and rudder
- A330/340: As 320, but with an additional backup yaw damper unit (BYDU) to control dutch roll
- A340-500/600: As A330/340, but the rudder has now electric control with a back-up power supply (BPS) and control module (BCM).
- A380: Full electrical control (BPS & BCM) on all 3 axes, trim possible by switches instead of wheel.
- A350: Like A380.

Best regards,
Hendric
 
Tango-Bravo
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Re: Could an airliner save fuel by being unstable

Thu Jun 18, 2020 2:02 am

In watching flight sim video reenactments of actual airliner crashes based findings of investigations by NTSB, FAA, and other agencies, based on CVRs, FDRs and other evidence, it seems there is a razor thin line between correctable instability and instability to the point of no return where a crash becomes inevitable, often with 100% fatalities. Seems to me that 'unstable' and 'airliner' do not mix well...ever.

Todd
BJI
 
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Starlionblue
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Re: Could an airliner save fuel by being unstable

Thu Jun 18, 2020 4:24 am

Tango-Bravo wrote:
In watching flight sim video reenactments of actual airliner crashes based findings of investigations by NTSB, FAA, and other agencies, based on CVRs, FDRs and other evidence, it seems there is a razor thin line between correctable instability and instability to the point of no return where a crash becomes inevitable, often with 100% fatalities. Seems to me that 'unstable' and 'airliner' do not mix well...ever.

Todd
BJI


Any examples of said crashes? There are no unstable airliners so there isn't, by definition, a razor-thin margin in this respect.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
LCDFlight
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Re: Could an airliner save fuel by being unstable

Thu Jun 18, 2020 8:18 pm

Tango-Bravo wrote:
In watching flight sim video reenactments of actual airliner crashes based findings of investigations by NTSB, FAA, and other agencies, based on CVRs, FDRs and other evidence, it seems there is a razor thin line between correctable instability and instability to the point of no return where a crash becomes inevitable, often with 100% fatalities. Seems to me that 'unstable' and 'airliner' do not mix well...ever.

Todd
BJI


I guess I am having a hard time telling the difference between "basic stability" and electronic-augmented stability, especially when the control path is electronic, and no mechanical connection exists anyway. Isn't it just another mission critical electronic item? It's not the only one.
 
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Starlionblue
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Re: Could an airliner save fuel by being unstable

Thu Jun 18, 2020 11:39 pm

LCDFlight wrote:
Tango-Bravo wrote:
In watching flight sim video reenactments of actual airliner crashes based findings of investigations by NTSB, FAA, and other agencies, based on CVRs, FDRs and other evidence, it seems there is a razor thin line between correctable instability and instability to the point of no return where a crash becomes inevitable, often with 100% fatalities. Seems to me that 'unstable' and 'airliner' do not mix well...ever.

Todd
BJI


I guess I am having a hard time telling the difference between "basic stability" and electronic-augmented stability, especially when the control path is electronic, and no mechanical connection exists anyway. Isn't it just another mission critical electronic item? It's not the only one.


Speaking for the A330, you can have one out of three primary computers inop at dispatch. In case all primaries fail in flight you still have control as long as at least one computer (primary or secondary) is operative.

This isn't directly related to basic aerodynamic stability, however. The aircraft is aerodynamically stable regardless. As in, if you replaced the FBW with mechanical control (rods and cables) and no flight control computers the aircraft would be flyable. You don't need the computers to make it basically stable.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Tango-Bravo
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Re: Could an airliner save fuel by being unstable

Fri Jun 19, 2020 1:38 am

Starlionblue wrote:

Any examples of said crashes? There are no unstable airliners so there isn't, by definition, a razor-thin margin in this respect.


Examples of crashes caused by instability leading to loss of control that come to mind were, in every case I can recall, due to pilot error, load imbalance, icing, mechanical failure (sometimes causing collateral damage) to name those that come to mind. The original post seems to implicitly ask whether an inherently unstable airliner could save fuel. To which I would again reply, with further clarification, inherently 'unstable' and 'airliner' do not mix well.

Todd
BJI
 
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Starlionblue
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Re: Could an airliner save fuel by being unstable

Fri Jun 19, 2020 4:24 am

Tango-Bravo wrote:
Starlionblue wrote:

Any examples of said crashes? There are no unstable airliners so there isn't, by definition, a razor-thin margin in this respect.


Examples of crashes caused by instability leading to loss of control that come to mind were, in every case I can recall, due to pilot error, load imbalance, icing, mechanical failure (sometimes causing collateral damage) to name those that come to mind. The original post seems to implicitly ask whether an inherently unstable airliner could save fuel. To which I would again reply, with further clarification, inherently 'unstable' and 'airliner' do not mix well.

Todd
BJI


:checkmark: :checkmark: :checkmark:
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
mxaxai
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Re: Could an airliner save fuel by being unstable

Fri Jun 19, 2020 8:50 pm

LCDFlight wrote:
I guess I am having a hard time telling the difference between "basic stability" and electronic-augmented stability, especially when the control path is electronic, and no mechanical connection exists anyway. Isn't it just another mission critical electronic item? It's not the only one.

Stability is separate from electronic, or electronically augmented control. For stability, consider what happens after being disturbed by a gust, assuming fixed control surfaces:

A stable aircraft will return to steady flight.
A dynamic unstable aircraft will oscillate around the steady state with increasing amplitude.
A static unstable aircraft will amplify the disturbance and veer off course immediately.

Electronic controls can react to small disturbances fast enough to counteract such destabilising disturbances and keep the aircraft on course. Examples would be fighter jets like the Eurofighter, that require electronic controls.

However, in their most basic form electronic controls can simply replace a control cable, e. g. analogue FBW on the Concorde or Direct Mode / electric backup on Airbus models. For that to work, the aircraft needs to be inherently stable without electronic help.

Normal Mode on FBW airliners introduces additional augmentation to improve handling even though the aircraft is already stable. For example, swept wing aircraft have a tendency to enter dutch roll; electronic yaw dampers counter this oscillation. The airliner is still stable and the dutch roll would eventually stop on its own but electronic augmentation accelerates the process. FBW can also make the airliner behave similar in varying conditions. So a certain stick deflection always gives you the same roll rate regardless of altitude, speed or aircraft weight. This can increase comfort or reduce pilot workload but you don't need it to fly the plane safely.

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