On Monday, 22 April 2019 at 01:59:31 UTC, Tony wrote:
> On Sunday, 21 April 2019 at 19:52:58 UTC, Walter Bright wrote:
>
>> [snip]
>> He argues that airplanes are stable without augmentation. This isn't true for any jetliners, they have an active yaw damper:
>
> I don't know which part you are referring to as suggesting "are stable without augmentation" (a phrase not in the article

What Walter is referring to is the fact that planes in general are not machines that fly "in a straight line". They have a tendency to bank, roll, or yaw. These are fixed by using hardware, or as most modern designs do, in software. When the pilot yanks the controller, he dooesn't actually move any control surfaces, he just gives an input. This input is then translated into movement of control surface by software. If the pilot thinks that specific flight control surfaces need to be moved, he can do that too, but its not the default.

> , but I see him saying "The airframe, the hardware, should get it right the first time and not need a lot of added bells and whistles to fly predictably".

That is something that hasn't been true for a long time. The early planes were all unstable designs. Modern military jets are all "relaxed stability". The MD-11 was also similarly unstable. It had an "LSAS" to keep it stable. This design isn't something that's unheard of or bad. At least we can't say that based on publicly available info. The NTSB report will be necessary to say anything about this.

> I don't read that as planes should have "zero pilot augmentation". I think his point is you don't design an aircraft, and when you find it has a tendency to stall on takeoff more than a typical or historical aircraft, go ahead and produce it anyway. "Other than a higher than normal tendency to stall on takeoff..." is not what most people want to hear in a design review of a proposed production aircraft.

The point is that such designs really aren't as radical or unheard of as the article suggests. These things have been done before and will be done again. The MD-11 is an example of another commercial aircraft that did it. The MD-11 was also controversial in its decision. The real issue here is that the software (MCAS) which was supposed to fix the pitch up, was poorly designed.

On 4/23/2019 1:09 AM, Russel Winder wrote:
> The 737 MAX is, as far as I know, the first commercial airplane to be
> unstable.

They are all unstable in certain parts of the flight envelope. All swept wing airplanes are unstable.

https://en.wikipedia.org/wiki/Dutch_roll

All jetliners are augmented with a yaw damper.

There are other automatic systems in a jetliner to prevent the pilot from doing something stupid. For example, the faster the jet goes, the less authority the pilot has over the elevators. This is to prevent the pilot from literally ripping off the elevators. I'm not sure, but I think this device is controlled electronically.

The airplane will also become uncontrollable if you fly it too fast, like in a dive. Yes, this has happened, sometimes due to other factors, sometimes because the pilot wasn't paying attention.

All airplanes have a "flight envelope" that pilots had better stay inside of or they're gonna crash. The MCAS system was put in to prevent one bad part of the envelope from being accidentally entered. It is not near part of the normal flight envelope, likely why pilots had never encountered MCAS activation before.

On 4/23/2019 1:06 AM, Paolo Invernizzi wrote:
> Pressure over engineerings coming from management, I'm meaning (again, personal opinions).

I'm very curious what led to the software specification errors, but we don't have any facts, and there's been enough uninformed speculation.

We'll just have to wait for the NTSB report.

On 4/23/2019 2:42 AM, Uknown wrote:
> Modern military jets are all "relaxed stability".

The 1903 Wright Flyer was highly unstable. The Wrights didn't know anything about stability :-) The pitch instability is likely what made their first flight so short.

The WW1 Sopwith Camel was famously unstable, killing some high percentage of its pilots because of that (I forgot the percentage). But in the hands of a competent pilot, he could use that instability to enable high maneuverability in combat.

The P-51 liked to flip on its back if you applied power too fast.

The WW2 Me-262 had control problems with flying too fast, you could lose all pitch control.

The Korean War F-80 was also unstable, it would abruptly pitch up and the wings would come off if you flew it too fast. It had an engine powerful enough to do that in level flight, you just had to watch it.

On Tuesday, 23 April 2019 at 11:05:46 UTC, Walter Bright wrote:
> On 4/23/2019 2:42 AM, Uknown wrote:
>> Modern military jets are all "relaxed stability".
>
> The 1903 Wright Flyer was highly unstable. The Wrights didn't know anything about stability :-) The pitch instability is likely what made their first flight so short.
>
> The WW1 Sopwith Camel was famously unstable, killing some high percentage of its pilots because of that (I forgot the percentage). But in the hands of a competent pilot, he could use that instability to enable high maneuverability in combat.
>
> The P-51 liked to flip on its back if you applied power too fast.
>
> The WW2 Me-262 had control problems with flying too fast, you could lose all pitch control.
>
> The Korean War F-80 was also unstable, it would abruptly pitch up and the wings would come off if you flew it too fast. It had an engine powerful enough to do that in level flight, you just had to watch it.

Indeed. i'm agreeing with you here. The F-16 was supposedly controversial for being both relatively small and more manoeuvrable (used "relaxed stability"). Infact wiki states that its the first to use it to achieve better manoeuvrability. Pretty much all fighters since have used this. The Su-27/MiG29 families have both used this along with TVC for some fantastic post stall manoeuvrers at aero-shows.
The Eurofighter Typhoon used it to achieve better manoeuvrability at most dog fight relevant speeds, and is rumoured to be among the best dog fighters because of its agility.

The F-22 similarly is relaxed stability. The F-117 is probably the most obnoxious designed fighters to have flown, and is similarly unstable. The B-2 is also unstable. The LCA Tejas is supposedly the most longitudinally unstable.

In short I'm having a hard time finding exceptions to military fighters that don't have an unstable design, especially those designed after the F-16 came out.

On Tuesday, 23 April 2019 at 08:54:30 UTC, Uknown wrote:
> On Tuesday, 23 April 2019 at 08:09:58 UTC, Russel Winder wrote:
>> On Mon, 2019-04-22 at 13:35 -0700, Walter Bright via Digitalmars-d wrote: […]
>>> [snip]
>> Surely, Boeing need to remove the MCAS system by reverting the 737 design to be stable rather than unstable.
>>
>> The 737 MAX is, as far as I know, the first commercial airplane to be unstable.
>
> The McDonal Douglass MD-11 was also unstable and was used for freight transport.
>
> Wiki link : https://en.wikipedia.org/wiki/McDonnell_Douglas_MD-11
>
> It was designed to have a smaller stabiliser, to reduce drag and thus improve fuel efficiency.
>
>> All military fighter airplanes are unstable but that is a military fighter. To date, again as far as I know, the strategy had been that all commercial aircraft were stable. Boeing broke ranks on this strategy with the 737MAX simply because they were trying to put an engine that was too big for the space available and so moved the engine forward and upward creating an aerodynamically unstable configuration.
>
> Not all are, the ones that are unstable are generally designed in that way so that they are more manoeuvrable. They use use digital fly by wire to make sure that the plane is seemingly stable. This decision by Boeing would have been fine if they had designed their software properly.


You need a stable design for a passenger plane. It happens every now
and then that the sensor fails and the plane needs to be able
to fly without them. The Dutch roll effects should go away if you just
fly straight, without active dampening, otherwise it is a bad design.

Airbus planes have standard 3 angle of attack sensors all connected
to the flight computer and you don't have to purchase any additional
safety measures like Boeing is doing with their angle of attack
sensors. It is one complete package. So with a single angle of attack
sensor failing an airbus plane keeps flying.

In case of an emergency you can switch to other "flight control modes".

Lufthansa A321
FL310 Two of the angle of attack sensors frozen in the same position. Nose dive.
FL270 Plane recovery.

You need time to recover.

In the Dreamliner Boeing places its angle of attack sensor nicely close
to the door so when a jet bridge is attached then they have to be
extremely careful. Just weird design decisions Boeing.

On Tuesday, 23 April 2019 at 12:28:21 UTC, sclytrack wrote:
> In the Dreamliner Boeing places its angle of attack sensor nicely close
> to the door so when a jet bridge is attached then they have to be
> extremely careful. Just weird design decisions Boeing.


https://www.cbsnews.com/news/boeing-787-dreamliner-prompted-nearly-a-dozen-complaints-from-whistleblowers-to-faa/