Bjorn’s Corner: Pitch stability

By Bjorn Fehrm

November 23, 2018, ©. Leeham News: In the spring I ran a series of Corners which dealt with aircraft stability on a basic level (April 13 to June 8). It covered the aircraft’s basic stability modes in normal flight and described the basic helper systems one finds on aircraft, such as yaw dampers and autopilots. But we did not go deeper into aircraft stability problems and more advanced helper systems.

Given recent events, it can be interesting to dive a bit deeper into the pitch stability of an aircraft and common helper systems.

Figure 1. The Boeing MAX 8 which introduced the MCAS system to help with pitch stability. Source: Boeing.

Pitch stability fundamentals

An aircraft’s airfoil is characterized by the air’s pressure distribution around the airfoil when the air flows past it.

Figure 2 shows two different types of airfoils, one conventional and one supercritical airfoil. The conventional is an often used historical airfoil and the latter the type of airfoils used in airliners flying today. The supercritical one has a different shape and therefore a different pressure distribution.

By curving less abruptly in the first part of the airfoil, the under pressure compared to ambient air (observe the Y-axis has the lower pressure at the top) is kept at a lower value than for the conventional airfoil and therefore it speeds up the air less passing over the airfoil. This gives it better high Mach characteristics.

Figure 2. A conventional and supercritical airfoil and their pressure distributions. Source: Embraer.

We can also see the two airfoils have very different lift distributions over the chord of the airfoils (chord= the length axis of the airfoil). If we sum the pressures on the top and bottom sides (the top and bottom curves for both airfoils) we get different moments on the airfoils should we pivot them at half chord length. The conventional airfoil would like to pitch up and the supercritical pitch down.

Airfoils are characterized by their lift characteristics but also how their pitch moments vary when flown with a different angle of attack against the airstream. Figure 3 shows a typical lift force (here characterized by the lift coefficient Cl curve) versus angle of attack curve for an airfoil or aircraft wing.

Figure 3. Lift coefficient versus angle of attack curve. Source: Wikipedia.

At high angles of attack, caused by low speed or a high aircraft load factor, the wing flies closer to stall = maximum lift. Before reaching stall most wings start to shake from a partially separated flow. This is called buffeting.

When an airfoil or wing is characterized by CFD tools (Computer Fluid Dynamics programs) and later tested in wind tunnels, the lift curve but also the pitch moment curve is measured. Figure 4 shows a generalized pitch moment curve for two different airfoils or straight wings using such airfoils (the pitch moment is shown by the pitch moment coefficient Cm).

Figure 4. Generalized pitch moment curve versus Alpha for airfoils/wings. Source: Wikipedia and Leeham Co.

A flying wing will move in pitch around its Center of Gravity. The red wing is called pitch stable as any increase of Alpha from for example a gust would increase the lift (Figure 3) and at the same time increase the pitch down moment of the wing, Figure 4. This returns the wing to a lower Alpha. The wing is correcting the gust disturbance by itself.

The blue wing, on the contrary, will pitch up when hit by a gust which increases Alpha . This increases the Alpha further, which increases the pitch up moment, which….. This wing is unstable, it would ultimately flip over backwards.

Next Corner

We have now covered the basics around lift and pitch stability for a wing. In the next Corner, we start building an aircraft and look at its pitch stability.

8 Comments on “Bjorn’s Corner: Pitch stability

  1. The VC10 was the first airliner to have a specific design super critical wing foil shape- which caused them some problems considering the state of the art of computer processing and wind tunnels at the time. I imagine the 737 had a wing out of the catalogue of profiles when first flown, that would have all changed with the complete new wing for the NG models from the early 90s, so it was possible to have the stability ‘built in’. That all changed again with the Max version as we have recently seen, as reprofiling the major part of the wing would be a highly expensive operation.

    • Kind of ??????

      Nothing changed hugely. All we know is it changed.

      We still need to wait to find out the % affect.

      As its listed as a pitch up issue, I can see the edges of the issue, if you are stalling, you really don’t want to go into more or full stall.

      But we also do not test Commercial aircraft flying upside down (or barrel rolls) because they are not supposed to go there though both have happened.

      On the other hand, how bad the issue is let alone the alerts you have you are getting there?

      It breaks down to

      1. Was it so bad that it really needed to be seriously countered?

      2. Was it in between?

      3. Was it low level and more a tweak than a serious issue (Boeing take seems to be in that line and while I don’t agree with their not emphasis it, it may explain the non challance of it as a no issue, pilots react, it never kicks it, it keeps the FAA happy – unless its a fake one and why no one went there is ??????????????)

      Did the longer 10 drive it?

      Its going to be some time before the whole reports are in, we will get the details of Lions Crash shortly, the tech end is going to be some time coming.

      Also where in the approach to or stall does the pitch up occur?

      And said pitch up should also have been in the book and taught as that is new.

      Also a huge marker that pitch up did not occur, AH is fine, then kill the trim.

      This has a lot of angles to it and a long time to run. We won’t have a full definite for months, maybe longer.

      • An intrinsically unstable airframe is IMU not certifiable.
        ( with or without a layered on stability program like MCAS )
        The MAX now seems to conform to that description.

        Boeing has a knack for cloaking necessary fix-ups in “this is a Boeing Uber Feature, hooray” PR.

        Boeing not proceeding on that path here seems to indicate that they know about the problematic character of this FAA pushover thing.

        NG supercritical wing or not.
        All I could ever find is that NASA tags the NG wing as “elements of a supercritical wing”. That sound more like some hybrid design than “fully supercritical”.

  2. I don’t know if you can over emphasize the issue with what went on with Lion Air.

    They had something occur that to their training should not.

    Having a stall alert (both a Voice e and stick shaker your first reaction is to confirm you are not (total confusion as nothign would have done that but that is what the aircraft is telling you)

    The shift is to deal with that, its your current crisis.

    Sneaking in along with that is a trim dumping the nose down that is masked by it.

    The remedy is the same, but the sequence of it happening and what you are focused on is not as its never been combined.

    The other aspect gets into why it was not published as both MCAS was put in and the characteristic of the aircraft at stall changed.

    Both have to be incorporated into the manuals, its a huge change even if the issue itself is fairly low % impact.

    And why is not a pilot action enough? That argues its so powerful that its deemed needed, but then back to, its a huger tech detail as well as actions and function and how could you not incorporate it into the manuals and training?

  3. My memory of observing testing of a 747 at stick shaker is that at the first increment of flap extension natural buffet will be felt before the stick shaker, whose actuation is based on a factor relative to stall. (Buffeting was not strong but the flight crew knew what it was.)
    At other flap settings natural buffet won’t occur before stick shaker.

    (Testing was for wind shear recovery.
    747SP may be different as its TE flaps are simpler than the full size 747.)

Leave a Reply

Your email address will not be published. Required fields are marked *