In Aviation Week magazine, May 29, 1961
in an Aviation Week Pilot Report on the Convair 880-M* I found this quote:

"Power was left at maximum continuous, 97%rpm, and the aircraft allowed to accelerate to maximum normal operating speed, Vno of Mach .89. The level flight acceleration permitted a demonstration of the speed stability system installed on all 880 aircraft, which automatically trims the aircraft as speed changes, especially near the limiting Mach number. . . Although positive speed stability is required of all aircraft, its value in present day large jet aircraft with wide speed ranges is debatable among pilots. Speed stability was more important in the days of deep-muscle-sense flying than it is today when attitude flying is the order."*

Could someone explain this a bit to me? The author speaks of positive speed stability, negative speed stability and neutral speed stability. Since I am not a pilot or engineer, a kind of dumbed down explanation of this would be greatly appreciated.

_____________________
"Aviation Week Pilot Report: Takeoff Distance Cut on Convair 880-M" in Aviation Week Magazine, May 29, 1961. pp. 51, 53.

Quoting Boeing:

"Simply stated, speed stability is a measure of the control force required to hold the airplane at an airspeed other than the trimmed airspeed, with the throttles fixed at the trimmed thrust setting. Airplanes with positive static longitudinal stability require a pull force to maintain a speed below the trimmed speed, and a push force to maintain a speed above the trimmed speed. . "

Source with some good graphics: https://www.boeing.com/commercial/aerom ... 01txt.html



In practical terms, positive speed stability implies that when in trim and without a power change, the aircraft will tend to return to the same speed if there is a gust that changes the speed.

On the "front" of the drag curve, that is with a speed above minimum drag (max lift/drag ratio), airliners are speed stable. So if there is a gust and speed changes, the aircraft will return to the same speed.

On the other hand, on the "back" of the drag curve, that is with speed below minimum drag, the aircraft is speed unstable. If there is a gust and speed decreases, more thrust is required to return to the same speed. If thrust is not increased, speed will continue to decrease. Given the somewhat laggy thrust response of turbofans, you can end up in a situation where you need a LOT of thrust to get back to your target speed, and while thrust is coming up, speed continues to decay, requiring even more thrust. This is why airliners are typically not flown below min drag speed.

I'll have to chew on that. But thanks so much. Do modern jetliners all have positive speed stability? Are there differences in speed stability in different commercial aircraft such that one could graph them in relation to each other? Perhaps I am asking the wrong questions.

In the case of early jetliners, positive speed stability was important nearing limiting MN. As the Mach increases, the center of lift moves aft, which causes the nose to tend to pitch down. The desired state is, at a constant trim setting, increasing speed or MN, the nose pitches up trying to maintain the trimmed speed. So, if the MN increases, it naturally is stable to the trim speed. If it’s neutral, increasing MN, the pitch force doesn’t change. If the negative, increasing MN, pitches nose down and Mach continues to increase in a dive. Vice versa, at a constant trim, if speed or Mach decreases, the plane returns to trim speed.

I believe there were early cases where the MN increased in turbulence and the plane went into a dive, unrecoverable until reaching thicker air, reducing the Mach and full revovery.

See. Mach Tuck,

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

convair880mfan wrote:

I'll have to chew on that. But thanks so much. Do modern jetliners all have positive speed stability? Are there differences in speed stability in different commercial aircraft such that one could graph them in relation to each other? Perhaps I am asking the wrong questions.

Yes, all modern jetliners have positive speed stability. It is a regulatory requirement*. An airliner without positive speed stability is a bad thing, as evidenced by the 737MAX fiasco.


There are differences, which is fine as long as stick forces are not outside the regulatory limits. In FBW aircraft stability is enhanced in the control system.


* https://www.law.cornell.edu/cfr/text/14/25.173

Starlionblue wrote:

convair880mfan wrote:

I'll have to chew on that. But thanks so much. Do modern jetliners all have positive speed stability? Are there differences in speed stability in different commercial aircraft such that one could graph them in relation to each other? Perhaps I am asking the wrong questions.

Yes, all modern jetliners have positive speed stability. It is a regulatory requirement*. An airliner without positive speed stability is a bad thing, as evidenced by the 737MAX fiasco.


There are differences, which is fine as long as stick forces are not outside the regulatory limits. In FBW aircraft stability is enhanced in the control system.


* https://www.law.cornell.edu/cfr/text/14/25.173

I don't immediately see how the MAX issues were positive speed stability issue.

The big issue in positive speed stability in the last 20 years has been the slow retreat of aircraft with high sweep like the MD-11 and more aircraft with hard protections and/or stabilities in the Airbus sense (which interestingly, the MD-11 has a version.)

Flight engineer saved the aircraft. The investigation was a white wash. My good friend was the chief line check flight engineer (later) at PAA.

https://en.m.wikipedia.org/wiki/Pan_Am_Flight_115

FlapOperator wrote:

I don't immediately see how the MAX issues were positive speed stability issue.

Reread, he never said MAX was not positive speed stability. He said it showed how much not having such stability could be bad thing. And sure, MAX is positive speed, but issues with human-machine interface made its pitch-down maneuver effect quite similar.

Cheers,
Adam

gloom wrote:

FlapOperator wrote:

I don't immediately see how the MAX issues were positive speed stability issue.

Reread, he never said MAX was not positive speed stability. He said it showed how much not having such stability could be bad thing. And sure, MAX is positive speed, but issues with human-machine interface made its pitch-down maneuver effect quite similar.

Cheers,
Adam

Seriously, I'm not trying to be "that guy" and its been awhile since I read "Handling the Big Jets" (which for the original poster, who is interested in transport category certification history is a MUST READ) but yeah, the MCAS was pitch/handling augmentation like Landing Stability and Augmentation System (LSAS) on the MD-11 or the subtle Airbus "stabilities" (which can be confusing because we are using the same term, "stability" in related but subtly different but obviously related aerodynamic and handling/certification.)

Positive speed stability is ultimately a handling issue that says "at known power/pitch settings, there will be limited and self-dampening airspeed excursions." This is especially important due to the drawbacks of wing sweep at very low speeds or high altitude/speed, or in other words, at extremes of performance.

The Convair 990 was certified long before 14 CFR Part 25 (FAR Part 25 for some people) existed so any references to it are not what the aircraft was certified to. It was certified under CAM 4b SR-422B "Airplane Airworthiness; Transport Categories" as far as I can determine. For details search for "Civil Aviation manual (CAM) 4b Airplane Airworthiness; Transport Categories" on google.
The regulation as far as I can determine that was in affect at the time is:

Stability
[4b.150 General. The airplane shall be longitudinally, directionally, and laterally stable
in accordance with sections 4b.151 through 4b.158. Suitable stability shall be
required in other conditions normally encountered in ser­vice if flight tests show such
stability to be necessary for safe operation.]
[(Amendment 4b-12. published Z1 F. R. 2986, Mar. 30, 1962, effective May 3,
1962.)]
[4b.151 Static longitudinal stability. In the conditions outlined in §§ 4b.152 through
4b.155, the characteristics of the elevator control forces including friction and· the
elevator control surface displacement shall comply with paragraphs (a) through (c) of this
section.
[(a) A pull shall be required to obtain and maintain speeds below the specified trim speed,
and a push shall be required to obtain and maintain speeds above the specified trim
speed, except that if the elevator control forces are not dependent upon the hinge moments of
the elevator control surface it shall also be shown that upward displacement of the
elevator trailing edge is required to obtain and maintain speeds below the specified trim speed
and a downward displacement of the elevator trailing edge is required to obtain and
maintain speeds above the specified trim speed. These criteria shall apply to any speed which can be obtained,
except that such speeds need not be greater than the landing gear or the wing flap
operating limit speed or VFC/MFC, whichever is appropriate at minimum
speed in steady unstalled flight.]

(b) The airspeed shall return to within 10 percent of the original trim speed when
the control force is slowly released from any speed within the limits defined in paragraph
(a) of this section.
[ (c) The stable slope of the stick force versus speed curve shall not be less
than 0.5 pounds per 3 knots nor shall it exceed a value beyond which control of the
airplane is difficult.]
[(Amendment 4b--12, published 27 F.R. 2986,
Mar. 300 1962, effective May 3, 1962.)]

Longitudinal Stability and Speed stability are not exactly the same thing so the standards that have to be met are for longitudinal stability and speed stability is related to but not necessarily the same. It sounds to me like you must have positive longitudinal stability by regulation but only need neutral or positive speed stability as the speed before the test condition can be slightly different from the airspeed after the Test Condition (i.e. it does not have to be the same after as long as it is within 10 per cent).
As far as using "Handling the Big Jets" goes. It is an excellent book but keep in mind that the author of that book was a British Civil Aviation Authority Test Pilot and the standards for Stability and Control for British Certification were for many years tougher than the FAA standards. Some aircraft that had been certified by the FAA were unable to be certified in Britain due to their higher standards. I am not aware of the Convair 990 ever being certified in Britain, but I might be wrong about that.

Substitute 880 for 990 in my comments. If you need to see the certification standards that the Convair 880/990 was certified to check the FAA Type Certificate Data Sheet 4A27 now held by AeroXin Industries, Inc.and the Aircraft Model is called 22 (Convair 880) and 22M (Convair 990). For more details go to:
https://na01.safelinks.protection.outlo ... reserved=0

Another correction:
The type certificate 4A27 is for the Convair 880 only. The model 22 has General Electric Turbojet CJ805-3 or CJ805-3A Engines with 11200 lbs. rated thrust. The model 22M has General Electric Turbojet CJ805-3B engines at a higher rated thrust of 11650 lbs. There are additional changes in the Autoflight, Air Conditioning and electrical systems.
The Convair 990 is on a separate type certificate 4A30 and is referred to as the model 30 and is now held also by AeroXin Industries, Inc...