Bjorn’s Corner: Pitch stability, Part 4

By Bjorn Fehrm

December 14, 2018, ©.Leeham News: Last week we introduced a horizontal stabilizer to make our DC-9 like aircraft stable in pitch. We got a pitch moment curve which was forcing the nose down of the aircraft if there was an increase in Angle of Attack (AoA) of the aircraft. Should the angle of attack decrease from a trimmed position, the aircraft would put the nose up to correct the disturbance. The aircraft is stable in pitch.

Now we take a closer look at how such a horizontal stabilizer is made and why.

Figure 1. The pitch moment coefficient curve of an earlyDC-9 candidate. Source: Stanford University.

The aircraft’s horizontal stabilizer

The first aircraft horizontal stabilizers had a fixed tailplane and a movable elevator at the back. This configuration is today kept for low-speed aircraft like general aviation or sports aircraft, Figure 2.

It’s a simple design, which gives enough pitch authority to handle takeoff rotation (a medium strong pitch up command), normal flight and the pitch up moment to counter the strong pitch down moment of full deployed wing flaps at landing and keeping the nose up during landing flare.

Figure 2. General aviation fixed horizontal stabilizer with movable elevator with trim tab. Source: Wikipedia

At the back of the elevator is a settable trim tab. The angle of this tab to the elevator is controlled by cables from a wheel in the cockpit. With this, the steady-state angle of the elevator can be set, to get the tail stabilizing force to trim the aircraft to zero pitch moment for a certain speed. The aircraft is in trim for this speed.

When jet aircraft was developed the speed range of the aircraft increased. Jet airliners started flying close to the speed of sound. A good example is the Boeing 747 which is cruising at Mach 0.85 from 1970.

To reach these speeds, the wings were swept, making the center of gravity changing as the wing fuel tanks were emptied. The long bodies of modern airliners also increased the possible range of load variations in pitch. The center of gravity of the aircraft varied over a wider range. A modern example is the acceptable Center of Gravity (CG) range for the Boeing 737 MAX 8 in figure 3.

Figure 3. 737 MAX 8 acceptable Center of Gravity range. Source: 737-8 Airplane Characteristics for Airport Planning (ACAP).

The range is measured as a per cent of the wings Mean Aerodynamic Chord, MAC. The MAX will have acceptable pitch stability margins when loaded at the aft limit of 35% MAC when taking off with 150,000lb or 68t. And it can still control the aircraft in pitch fast enough for takeoff when it’s loaded nose heavy at 12% of MAC for the same weight. At landing at a lower weight, the allowed forward CG is even wider. The aft limit is then a bit less. This is normal, there is less weight working on the pitch stability moment arm between center of lift and CG. The diagram is not unique to the MAX, other airliner diagrams are similar.

The wide range in speed and CG forces a better arrangement of the surfaces of the horizontal stabilizer. At high speed, the change of the aerodynamics on the wings moves the aerodynamic center backwards. At and beyond maximum operational Mach this movement can create a strong nose down moment. We need a strong stabilizer trim to counter this. And it cannot eat up the Pilot’s margin for manoeuvres which a trimmed elevator would do.

Jet aircraft, therefore, introduced a trimmable stabilator, Figure 4. With this arrangement, the pitch trim for the wide speed and CG range can be done with an all movable stabilator while leaving the Pilot the full elevator authority for pitch commands.

Figure 4. Trimmable stabilator, which frees the elevator for the pilot’s pitch manoeuvres. Source: D.P. Davies. Handling the Big Jets.

This strong pitch trim capability is also needed to counter the powerful slats and flaps such jet aircraft have. These produce a strong pitch down moment in landing configuration.

With such a strong trim function the safety precautions around the stabilizer increased. Trim systems faults like a stuck switch nose up or down introduced dangerous pitch moments. Double trim switches were introduced on the yokes. If either was going awry there would be no trim. Both had to function correctly and be moved in the same direction for trim. Trim cut-out switches were introduced and trim problems training entered the pilot’s education and recurrent simulator sessions.

In the next Corner, we look at the problems as we move out from the normal operating area of the pitch moment curve.

14 Comments on “Bjorn’s Corner: Pitch stability, Part 4

  1. Bjorn:

    The moving stabilizer was introduced on Jet fights first (F-86?)

    What was the need for those?

    • Hello TransWorld,

      Regarding: “The moving stabilizer was introduced on Jet fights first (F-86?)
      What was the need for those?”

      See below for Wikipedia’s answer.

      “Stabilators were developed to achieve adequate pitch control in supersonic flight, and are almost universal on modern military combat aircraft. All non-delta-winged supersonic aircraft use stabilators because with conventional control surfaces, shock waves can form past the elevator hinge, causing severe mach tuck.

      The British wartime Miles M.52 supersonic project was designed with stabilators. Though the design only flew as a scale rocket, its all-flying tail was tested on the Miles “Gillette” Falcon. The contemporary American supersonic project, the Bell X-1, adapted its variable incidence tailplane into an all-moving tailplane (based on the Miles M.52 project data) and was operated successfully in 1947. The North American F-86 Sabre, the first U.S. Air Force aircraft which could go supersonic (although in a shallow dive) was introduced with a conventional horizontal stabilizer with elevators, which was eventually replaced with a stabilator.”

      Also note that F86 in particular, and supersonic aircraft in general, have a stabilator, which is not the same as a moveable stabilizer. A stabilator has no separate elevators, a moveable stabilizer does.

      “Most modern airliners adjust the tailplane angle of incidence to trim during flight as fuel is burned and the center of gravity moves. These adjustments are handled by adjustable incidence horizontal stabilizers. However, such adjustable stabilizers are not the same as stabilators; a stabilator is controlled by the pilot’s control yoke (or stick), whereas an adjustable stabilizer is controlled by the trim system.”

      The above quotes are from the following link.

      • Q: What does a 150 or 180 horsepower Piper Cherokee single piston engine light plane, with a top speed of 140 to 150 mph, have in common with F86’s and many supersonic military fighters?

        A: They have in common use of a stabilator instead of a stabilizer, and not much else. I believe that Piper Comanches and Cessna 177’s also use staibilators; however, most light planes do use the elevator and non moveable stabilizer combination that Bjorn described as being typical of light planes.

        Another Piper pitch control idiosyncrasy, is the use in older models of roof mounted trim cranks instead of a control panel or console trim wheel. See demonstration below.

        Oh my god, they are flying without an autopilot or electric trim motor, and the control panel is full of round gauges instead of auto flight computer screens!

        • Bf109 had its horizontal surfaces supported by a strut, so it wasnt all moving, just a trailing elevator. Messerschmidt’s earlier M20 airliner and M23 sports plane had a single piece
          horizontal stabiliser with trailing elevator

          • ME 109 early had a strut.

            The 109-f ones did not but still had a stabilizer not a staibilitator.

  2. Phew, a whole new world. I keep learning new things.

    Still did not answer why they put it on a light aircraft, more like because they could and it was cool?

    So you staiblitor on a Piper and then add a surface anyway to make it feel-able. V Tail Bonanza anyone?

    Maybe just because you can does not make it a good idea? Certainly a more costly one with no return.

    • Why was it used ?
      “The piper selection of the stabilator instead of the conventional stabilizer/elevator configuration was done for several reasons. The stabilator gives a wider range of pitch control over all flight speeds. The stabilator is lighter with lower drag. The use of the anti-servo trim design causes the tab to move with the stabilator but the combination requires more pilot input with any increase in speed or deflection. . The stabilator utilizes an “antiservo” tab that deflects upward on the trailing edge of the stabilator as the controls come back. This antiservo tab generates the necessary control feel and feedback to the pilot to maintain the necessary “stick force per G” to keep a hamfisted pilot from easily breaking the airplane with excessive control movement. This is a safety device improves longitudinal stability while at the same time limiting the pilot ability to cause structural damage. …

      Lighter and lower drag would be a good reason for a light plane.

      • Yea I got all that.

        The point was it was not an aircraft that justified whiz bang fancy. Forrest for the trees and all that.

        Beech Bonnaza finally gave up the V tail. Not worth the complexity and cost.

        HAm fisted has nothing to do with it, it added an element that required another elements to counter.

        Like taking an tranquilizer to calm down and an upper to stay awake.

  3. I assume the more the larger percentage of weight on the main gears, (aft) increases pitch moment coefficient across all AOA.

    In Fig. 4, “weight aft of CG”. Is that allowed in flight, to shift fuel and shift the weight aft and use the horizontal stabilizer for upward lift?

    • Hi ted,

      it’s not weight aft “of” CG, it’s weight aft CG = a CG at the rear of the allowed range.

      An aft CG reduces the static stability, it lowers the angle of the pitch moment curve (less moment per degree of AoA change).

  4. side topic (sort of)

    Flew home from Miami on a 737-8, first flight for me on a MAX….

    it is just a slightly quieter 737 with that new car smell and a hint of eau de aviation geek panic as I count to 10 repeatedly after every pitch change…..

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