# 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.

##### The aircraft’s horizontal stabilizer and how it trims the aircraft

A trimmable horizontal stabilator like in Figure 2 will angle the stabilator so the pitch moment is trimmed to zero at different Angles of Attack (AoA).

Figure 2. Trimmable stabilator, which frees the elevator for the pilot’s pitch manoeuvres. Source: D.P. Davies. Handling the Big Jets.

At cruise, the needed AoA on the wing to lift the weight of the aircraft is around 2°-3°. The pitch moment curve for an aircraft trimmed for cruise is shown in Figure 3.

Figure 3. Pitch moment around the Center of Gravity for cruise. Source: Leeham Co.

The stabilator is trimmed until the forces on the aircraft in pitch are at equilibrium at this AoA. If the aircraft is disturbed to a higher AoA there is a pitch down moment generated which bring the aircraft back to trimmed cruise.

In Figure 4 we have an aircraft which has lower pitch stability. The curve is still trimmed at 2°-3° but the pitch down moment from an AoA increase is lower.

Figure 4. A less stable cruise pitch moment curve. Source: Leeham Co.

This can happen at high Flight Levels where the air is thinner and the stabilizing force from the Stabilizer is weaker. Such an aircraft can require a pitch stability augmentation system for cruise at high Flight Levels. We will discuss augmentation systems more in following Corners.

Finally, in Figure 5 we have a pitch moment curve which is trimmed at 9° AoA for low-speed flight during the landing. The stabilator has now changed position to trim the aircraft in pitch at low speed with landing flaps (which create a strong pitch down moment).

Figure 4. Pitch moment around the Center of Gravity for landing. Source: Leeham Co.

As the flight speed is lower the forces generated by the stabilator and elevator are lower and the pitch moment curve has a lower angle. The nose up and down moments from changes in AoA are weaker.

In the next Corner, we go into the more difficult part of the curve in Figure 1. A real-world pitch moment curve for an airliner can have several areas of lower pitch stability dependent on speed, altitude, Angle of Attack and configuration.