Practice test
Unit 3 · Aerodynamics · Lesson 4 of 7

Stability and center of gravity

8 min readLesson 13 of 593D lab

By the end of this lesson you'll be able to:
  • Explain static and dynamic stability
  • Describe how the CG location affects stability and control
  • Compare forward and aft CG effects

Static and dynamic stability

Static stability is the airplane's initial tendency after it's disturbed. Positive static stability means it tends to return to where it was; neutral means it stays in the new position; negative means it keeps going away. Dynamic stability is what happens over time: whether the oscillations get smaller (positive), stay the same (neutral) or get bigger (negative). Training airplanes are designed with positive static and dynamic stability, so they settle down on their own.

Longitudinal stability (pitch)

In most airplanes, the CG is ahead of the center of lift, so the airplane naturally wants to pitch nose-down. The horizontal tail produces a small downward force to balance it. If the nose pitches down and airspeed increases, the tail's down force increases and pulls the nose back up. That's what makes the airplane stable in pitch.

Lateral and directional stability

Lateral stability (roll) comes mostly from dihedral, the upward angle of the wings. Directional stability (yaw) comes mostly from the vertical fin, which acts like the feathers on an arrow.

Where the CG is matters

Forward CGAft CG
StabilityMore stableLess stable
Stall speedHigherLower
Cruise speedSlower (more tail down force)Faster
Main riskHard to raise the nose for landingHard or impossible stall/spin recovery

Outside the CG limits, the airplane may not be controllable. An aft CG beyond the limit is especially dangerous, because the elevator may not be able to lower the nose to recover from a stall. That's why weight and balance (Unit 11) isn't optional.

Try it in 3D: Weight & balance in 3DLoad people, bags and fuel and watch the CG move on the envelope.

Key takeaways

  • Static = initial tendency; dynamic = what happens over time.
  • The CG is usually ahead of the center of lift; the tail pushes down to balance.
  • Forward CG: more stable, higher stall speed. Aft CG: less stable, harder stall recovery.

What the examiner may ask

Practice answering out loud before you open each answer.

Why is an aft CG dangerous?
It reduces longitudinal stability and can make stall and spin recovery difficult or impossible because the elevator may not be able to lower the nose.
How does a forward CG affect performance?
Higher stall speed, slower cruise (more tail-down force), and it can be hard to raise the nose for landing.
What gives an airplane directional stability?
Mainly the vertical fin.

Common mistakes

  • Loading heavy baggage aft without computing the CG.
  • Assuming that if the weight is under max gross, the CG must be fine.
  • Mixing up static and dynamic stability.

Read it in the FAA sources

PHAK · Chapter 5Aerodynamics of FlightOpens the PHAK at this chapter ↗PHAK · Chapter 10Weight and BalanceOpens the PHAK at this chapter ↗

Lesson check

Get every question right to complete the lesson. Take your time; wrong answers explain why.

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