Bjorn’s Corner: Fly by steel or electrical wire, Part 10

By Bjorn Fehrm

September 27, 2019, ©. Leeham News: In our series about classical flight controls (“fly by steel wire”) and Fly-By-Wire (FBW or “fly by electrical wire”) we started a discussion about the need for stability augmentation systems last week and how these are implemented.

We handled yaw augmentation and began the discussion on pitch augmentation. Now we dig deeper into the trickier form of pitch augmentation, the one needed because of regions of lower stability in pitch at higher Angles of Attack (AoA).

Figure 1. The pitch moment curve of a modern airliner. Source: Leeham Co.

Augmentation of the pitch moment curve

We covered the Mach tuck and Stick pusher pitch augmentation last week. Now we dig deeper into pitch augmentation for the flying envelope below the stall Angle of Attack (AoA), Figure 1.

Normal flying for an airliner keeps the angle of attack well below 10 degrees. Cruise is typically at three degrees AoA and normal turns in a climb, cruise and descent will keep the AoA below five degrees.

An airliner will fly at higher AoA just after takeoff and before landing. The landing configuration with full flaps and a gradually declining airspeed before touch down will cause the aircraft to fly with the highest AoA.

For such flying, the slats are extended. These increase the AoA region before stall upwards with typically five to eight degrees, Figure 2.

Figure 2. The Cl-AoA (also called Alfa) curves of an airliner with our without slats and flaps. Source: Leeham Co.

The angle of attack during approach is typically below 10 degrees with the short time transition to AoAs over just 10 degrees during landing flare or a go-around maneuver.

The pitch moment curve in Figure 1 is for a clean wing (no slat or flaps deployed) when flying at a high AoA like when turning in a circling situation while waiting for approach (trimmed flight is at nine degrees AoA in Figure 1). The influence from the new larger wing-mounted engines has made the aircraft less stable in the region before stall. When flying in take-off or landing configuration the form of the curve would be affected by the deployed slats and flaps. The important change will come from the slats. These make the leading edge of the wing less sensitive to disturbances of the airflow from objects ahead of the wing.

MCAS is not active on the 737 MAX when flaps are deployed. This is because when flaps are out the slats are out as well and these diminish the disturbance to the pitch moment curve from the larger and further forward-higher slung engine nacelles.

So for most aircraft, an augmentation of the pitch moment curve making the aircraft simple to fly for the pilots is restricted to clean wing flying. The difficulty to achieve such a straight, smooth curve varies significantly depending on what pitch flight control system we have.

Augmentation of FBW systems

For a feedback based FBW system, the pilot asks through the Yoke/Stick for a certain Pitch G at high speed or a certain Pitch Rate at a lower speed. The aircraft measures the aircraft’s actual G or Rate and automatically adjust its command to the elevator and pitch trim to reach the value the Pilot has commanded. A varying pitch moment curve is handled by the FBW systems feedback loop. The Pilot feels a consistent aircraft in pitch even in regions of AoA where the base aircraft has deficiencies.

For non-feedback (open loop) FBW system it’s a bit trickier but the aircraft designer still has good tools to straighten the curve. A good example is the E-Jet E1 system. A digital augmentation computer calculates and transmits augmentation signals which are merged with the Pilot’s command from the Yoke. The summed signal straightens any bumps in the pitch moment curve as computers can take input from several sensors and calculate a sophisticated augmentation signal.

Backup modes

For both FBW system types, degraded modes must present an acceptable aircraft to the Pilot. Now any bumps in the pitch moment curve will be felt by the Pilot as the feedback or injected augmentation is no longer present. This puts a limit to how much the pitch stability can be allowed to vary over the curve and how low the stability margin can be at any point.

Embraer’s new E2 system is the exception. It’s most degraded mode, Direct mode, still has pitch augmentation active. Therefore, it allows flying with reduced static pitch stability for the aircraft, which increases aircraft efficiency by reduced trim drag.

At first sight, this seems contradictory to what we have learned about backup modes for FBW. These revert to “fly by electrical wires” with no augmentation computers active. The signals go directly from the Yoke/Stick to the flight control surfaces.

This is an implementation decision for the systems which uses it. It’s not a law of nature. If the designers can implement a pitch FBW with augmentation which meets the availability criteria for all degradation scenarios there is no principle difference between it and systems which rely on one to one electrical transmission of the Pilots Yoke/Stick movements.

Mechanical flight control pitch augmentation

Classical mechanical systems make augmentation of pitch moment curves trickier still. This is the subject for our next Corner.

18 Comments on “Bjorn’s Corner: Fly by steel or electrical wire, Part 10

  1. Bjorn, I am very interested in the specifics of how an aircraft such as the A320 family is modified by FBW to ‘feel’ the same to a pilot. I understand that the A318 has a larger vertical stabiliser to compensate for the reduced moment arm.

    Are there any other aerodynamic changes across the family, or does the FBW system manipulate the primary flight controls to shape the moment curve so that the aircraft ‘feels’ the same to the pilots.

    I presume that when AB were designing the A320 family they knew they would do an A319, A320, and A321 so that aerodynamically the wing, stabiliser, rudder etc. would be designed at the outset to work across the family.

    Would they need to make any major aerodynamic changes if they went ahead with a stretched A321 as a possible A322 lower end NMA challenger ?

    • The shortest member in the family decides the size of the vertical and horizontal tail. More than the pitch moment stability we discuss here it’s typically rotation at MTOW, full flaps and most forward CG which set’s the size for the horizontal tail and worst instance of one engine out for the vertical tail. Once the maximum force needed can be attained the FBW doses what’s needed for a smooth response at all times for all family members.

      A new longer member has longer tail arm as you say, thus it doesn’t need any aerodynamic changes. A 322 with a new wing can force an updated tail as a new wing can have a stronger basic nose-down moment at the above cases and can require a larger horizontal tail. Stronger engines can also require a larger vertical tail to handle the stronger engine out moment.

  2. Great explanation! I flew the P-3 Orion and we had a mechanical “aerodynamic bandaid” for the pitch curve…a spring loaded force link tab on the elevator to improve pitch control feel and prevent stick force decreases at higher airspeeds.

  3. Thank you Bjorn as always for an excellent article. This is a fundamental question, so apologies if you have covered it elsewhere. If I’m reading Figure 1 correctly, then there is zero moment on the pitch axis at approximately 9 degrees AoA. You reference that a typical cruise AoA is 3 degrees. I’m assuming therefore that the tail is trimmed to provide an offsetting moment? Is (one of?) the primary reason for having cruise AoA well below Cm,cg of zero to provide sufficient margin before the stall characteristics?

    • Hi Tom,
      Figure 1 is from a previous series on pitch stability, it shows a case where the aircraft is trimmed for nine degrees AoA, a typical low-speed case like a circling with no slats or flaps. The cruise case would have the curve pass zero Cm at three degrees, so your question is motivated. The diagram does not show a cruise case, rather the turn case in a circling situation. I changed the text to make this clear, thanks.

  4. I see the red line and the green line, and I wonder what line in between is required for certification. Obviously the MAX fell somewhere above this line to require MCAS to be certified. Both lines have a downward slope, indicating increased righting force as angle of attack increases. But apparently, the red line the slope is not enough. So what slope is required by the certifying authorities?

    • The certification text in FAR 25 is written like legal text, thus hard to digest. This is an extract from an FAA test pilot instruction on how to test compliance with the certification rules, it’s easier to understand re. what’s required:

      “At all points within the buffet onset boundary (read normal flight envelope, my comment) … the stick force should increase progressively with increasing load factor. Any reduction in stick force gradient with change of load factor should not be so large or abrupt as to impair significantly the ability of the pilot to maintain control over the load factor and pitch attitude of the airplane.”

      It’s, in the end, a judgment call from the test pilots but these have rules and experience for what’s acceptable. They must deem the least capable pilot can handle the aircraft without problem for it to pass certification for this item.

      • I also read that the critique from NTSB was the single discrete failure events being tested fro not the multiple cascade ones as was seen with Lion and Ethiopian.

        And have they updated standards to deal with women who while fully capable pilots are likely to be at the bottom of the upper body strength of even a smaller male pilot.

        • Yes, body strength is one parameter. In the case of the reduction of pitch force for gaining Gs after 12 degrees in Figure 1, it’s more speed of reaction to reduce Yoke force before the aircraft swings into stall AoA or pulls too many Gs. I have flown this kind of aircraft (red curve, it was 1950s design fighters) and it’s all about feel for what the aircraft is doing and quickness in reaction. It’s tricky stuff especially if you have other distractions.

      • They do mention the speed and altitude issue though not focused.

        I think if they keep counter trimming they would stay even or ahead, but you had to be persistent and it may have seemed like no gain.

        No question a god awful stupid implementation of what should have been simple.

        With the Manual Trim issue brought to light as well as digging down and finding out what the link is between the AOA and speed/altitude busts, lives may be saved in the future.

        A heavy price has been paid, sadly that is how we make programs all too often.

        You hope its the ones no one foresees that occur, hard reality but to have one you can clearly see implications do two planes in, that is hard to accept.

        I would put the 737 Rudder issue into the no one saw it. That does not mean when it was question if it could happen, Boeing should have taken a new look at it trying to find whole not vindicate it.

    • My early guess was that postprocessing indroduced errors via some software bug or other. one of the dumber ones. write into some other routines memory, lack of initialization. something really worthy of an intern let loose.
      ( and MCAS as it was implemented made full sense in an environment of synthetic AoA value generation ( i.e. checked and augmented with other infos: always correct ).
      The late increase in MCAS “reach” indicates that the effect was marginally understood and deemed more dangerous than
      has been allowed as truth by Boeing.

  5. I think that device is called an anti servo tab or probably in this case an anti servo spring tab. Spring tabs were very effective I’ve read in reducing forces on large aircraft (or in this case increasing them)

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