Leeham News and Analysis
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Bjorn’s Corner: Yaw stability, Part 2
By Bjorn Fehrm
March 01, 2019, ©. Leeham News: We now continue our discussion of the yaw stability of an airliner.
Last week we defined the basic conditions of yaw stability. The aerodynamic side force from an angled airflow stemming from an aircraft yaw angle or sideslip must be higher behind the center of gravity than ahead of the center of gravity.
This is why aircraft have a large vertical wing at the rear of the aircraft called the vertical tailplane. Figure 1.
Figure 1. The surfaces affecting yaw stability on an aircraft. Source: Leeham Co.
Bjorn’s Corner: Airliner yaw stability
By Bjorn Fehrm
Feb. 22, 2019, ©. Leeham News: After discussing an airliner’s pitch stability we now turn to the yaw stability. A stable aircraft in yaw means we don’t want the aircraft to wag its tail sideways while flying.
The airliner shall fly straight ahead during take-off, climb, cruise, descent and landing, even when we have problems with an engine and must throttle it back or shut it down on one side.
Figure 1. The surfaces affecting yaw stability on an aircraft. Source: Leeham Co.
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.
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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.