Leeham News and Analysis
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Leeham News and Analysis
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- Better transparency needed on Boeing’s 1Q earnings call April 22, 2024
Bjorn’s Corner: Pitch stability, Part 10. Wrap up.
Feb. 15, 2019, ©. Leeham News: We now summarize the problems around an airliner’s pitch stability and why a good pitch stability is so important before we go to the next subject, yaw stability.
The pitch stability and how it works in different parts of the flight envelope is the trickiest of the aircraft’s stability problems around its three axes; Pitch, Yaw and Roll. Here is why.
Figure 1. A thought pitch moment curve for Boeing’s 737 MAX. Source: Leeham Co.
Bjorn’s Corner: Pitch stability, Part 9
Feb. 08, 2019, ©. Leeham News: In our run through of the pitch stability problems of an airliner we covered high and low-speed problems in the last Corners and before it deep stall.
Now we go back to the region just before and during a stall, to look at more measures to help the pilot besides stall warning and stick pushers.
Figure 1. A thought pitch moment curve for Boeing’s 737 MAX. Source: Leeham Co.
Bjorn’s Corner: Pitch stability, Part 8
By Bjorn Fehrm
Feb. 01, 2019, ©. Leeham News: In last week’s Corner, we covered the high-speed pitch stability problem most airliners have, Mach tuck.
Now, we continue with low-speed problems which made Boeing introduce Speed trim on the 737.
Figure 1. The block diagram for Boeing’s 737 Speed Trim system. Source: Boeing.
Bjorn’s Corner: Pitch stability, Part 7
By Bjorn Fehrm
Jan. 25, 2019, ©. Leeham News: We have covered the pitch stability for an airliner including some well know problems around deep stall.
We now continue with other frequent problems as the aircraft covers a large envelope in speed and altitude.
Figure 1. The pitch moment coefficient curve of an early DC-9 candidate. Read more
Bjorn’s Corner: Pitch stability, Part 6
Jan. 18, 2019, ©. Leeham News: We have now covered the basics of pitch stability for an airliner and how a stable or unstable pitch moment curve looks. Now we look at different trouble areas.
Straight and stable pitch moment curves are difficult to achieve at all flight situations. We will discuss some well-known problems, how these were detected and what the solutions were.
Figure 1. The pitch moment coefficient curve of an early DC-9 candidate. Source: Stanford University.
Bjorn’s Corner: Pitch stability, Part 5
By Bjorn Fehrm
Jan. 11, 2019, ©. Leeham News: The week before Christmas we discussed the pitch stability of an airliner. We covered how a horizontal stabilizer made the aircraft stable in pitch, and why transonic airliners used a trimmable horizontal stabilizer rather than trimming with the elevator.
Now we look at some different flight situations with different trim needs before we move into the more troublesome parts of a pitch moment curve.
Figure 1. The pitch moment coefficient curve of an early DC-9 candidate. Source: Stanford University.
Bjorn’s Corner: Pitch stability, Part 4
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.
Read more
Bjorn’s Corner: Pitch stability, Part 3
By Bjorn Fehrm
December 7, 2018, ©. Leeham News: Last week we looked at the pitch stability of an aircraft wing with fuselage. We could see the combination was unstable. Now we add a rear wing called a horizontal stabilizer to get the whole aircraft stable in pitch.
We use the DC9 as our example of a pitch stable airliner (Figure 1) as it has some interesting pitch stability problems outside the normal flying envelope. This we will discuss in coming Corners.
Figure 1. The DC-9, the airliner we use to study pitch stability. Source: Wikipedia.
Bjorn’s Corner: Pitch stability, Part 2
By Bjorn Fehrm
November 30, 2018, ©. Leeham News: Last week we started a series on pitch stability of aircraft. It has actuality as the reason Boeing introduced the now well-known MCAS (Maneuver Characteristic Augmentation System) was to improve the pitch stability of the 737 MAX.
We discussed the pitch stability of the basic wing last week. This week we add the fuselage and see what happens.
Figure 1. The pitch stability of two different wings, Violet (Classic profile) and Red (Supercritical profile). Source: Leeham Co.
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.