Stability – Straight and Level flying

One characteristic of an aircraft is that it is stable about the lateral axis (runs wingtip to wingtip). This is known as “pitch stability” or “longitudinal stability”.

If the nose drops in flight due to turbulence of bumping the controls, the aircraft will be in a “nose-low” attitude (nose goes down). Because the nose is going down, the wings will have a lesser angle of attack (pointing down) which will increase airspeed.

As airspeed increases, more airflows over the horizontal stabilizer in which air pushes the tail downward. This downward force on the tail pushes the nose up resulting in the aircraft returning to climb.

Since the aircraft is climbing, the airspeed decreases and thus, less air is flowing over the horizontal stabilizer and the pattern is repeated until the aircraft returns to its original airspeed and altitude.

The first time the nose comes up in this pattern is called “Positive Static Stability

The repeated nose up over time during this pattern is called “Positive Dynamic Stability

In order to have dynamic stability, the aircraft must first have positive static stability in which the aircraft initially pitches up in the series of the pattern.

There are three different terms for “stability”

Positive Static Stability

Aircraft returns to its original position.

Neutral Static Stability

Aircraft stays where is is displaced to meaning that the aircraft stays & holds in the disturbed position.

Negative Static Stability

The aircraft continues in the direction that it is being displaced and continues in the direction of the disturbance.

The Cessna 172 Skyhawk is designed to be inherently stable in that the aircraft will return to its original position if disturbed. This is said to have positive and dynamic stability.

Longitudinal (Pitch) Stability

The Longitudinal or Pitch Stability is determined by both the center of gravity as well as the center of lift. In the Cessna 172 Skyhawk, the center of gravity is located in front of the center of lift which makes it stable. This helps keep the nose of the aircraft from pitching down and there is a slight downward force on the tail which helps to keep the airplane stable regardless of pitch and speed.

When the aircraft is disturbed in flight and begins to “dive”, it will pick up airspeed which increases the downward force on the tail. This downward force causes the tail to go down and the nose up, ultimately balancing out and returning the aircraft to its original speed.

The similar effect is true for the opposite. When the aircraft encounters a disturbance that causes the aircraft nose to pitch up, the force on the tail is decreased and the tail will go up, causing the nose to go down. This pattern is repeated over several oscillations until the aircraft returns to its original speed.

Testing the Level Flying Attitude

In order to test the airplane’s ability to return to a level flight, first make sure the aircraft is trimmed for level flight. Once trimmed, briefly pull back on the control wheel which will cause the nose to go up and begin a series of dampening oscillations. Ultimately the aircraft will return to level flight.

Airflow

Airflow over the horizontal stabilizer and elevator creates an opposite lift and acts to push the tail downward in order to balance the lift produced by the wings. The more speed and airflow, the greater the downforce on tail so pushing the control stick forward at higher speeds will counteract this downforce and bring the nose down. Tail down =nose up.

Center of Gravity

The further forward the center of gravity is in comparison to the center of lift, the more stable the aircraft is

The further back (aft) the center of gravity is, the less stable the aircraft is.

If the aircraft is loaded so that the center of gravity is too far back (rearward) the aircraft will become negatively stable and when disturbed, the aircraft will not return to its original condition. Additionally, stall could become difficult or impossible. It is important to load according to the center of gravity.

The aerodynamic downward force on the tail means the nose pitches upon when the power is increased and pitches down when the poser is decreased.

The horizontal tail surface could be thought of as an upside down wing in that it creates a downward lift (tail downforce).

Center of Gravity Limits

The center of gravity (CG) location determines the general stability of the aircraft and certain limitations are set by the manufacturer. These are the forward limit and the rear (aft) limit for the center of gravity.

The center of gravity (CG) limits can be found in the POH (Pilot’s Operating Handbook) Weight and Balance (Section 6)

The center of gravity changes when any weight distribution within the aircraft changes including fuel weights, passenger weights and baggage/cargo loading.

Forward Center of Gravity

If more weight is in the front of the aircraft the center of gravity will more more forward an more tail downforce will be required to balance the aircraft and with more tail downforce, the aircraft will become more stable.

The further forward the Center of Gravity, the more the aircraft becomes “nose heavy” which requires more back pressure on the control wheel to raise the nose. If the Center of Gravity is ahead of the forward limit set be the manufacturer, the aircraft may become difficult to maneuver and have a hard time taking off, if not impossible.

Rear (aft) Center of Gravity

If the center of gravity is moved to the rear but loading more weight in the back, the less tail downforce is required to balance the aircraft which could cause the aircraft to become less stable.

The further back the rear CG is, the more the airplane becomes “tail heavy” which required more forward control to lower the nose to maintain level flight.

If the center of gravity exceed the aft limits, the plan may become stable and lowering the nose could cause it to continue lowering, while raising the nose could cause the nose to continue raising. Additionally, stall recovery could become difficult and there may be an inability to push the nose forward to recover from a stall.

Airspeed and Downward Tail Force

If flying at straight and level and only power is added, the aircraft nose will pitch up because the prop pushes more air back over the surface of the tail which creates a greater tail downforce.

The opposite is true when it comes to decreasing the power. Less air movement over the tail results in the nose pitching down. This effect is less noticeable in t-tail aircraft with the horizontal stabilizer at the top of the tail since the air flow from the prop wash is below the tail surface.

Controlling the Tail Downforce

The tail downforce can be controlled using the elevator (by pulling or pushing the control wheel)  in which you match the amount for force that is required for speed, power setting and the flap position. Additionally, the elevator trim can be set to achieve the appropriate tail downforce in which it is not necessary to hold the control wheel in position.

Trim settings may need to be adjusted and re-trimmed whenever there is a change to speed, power or flap position.