FAA Flight Instructor Airplane (FIA)Aerodynamics and Principles of FlightMedium
A CFI explains to a client that moving the center of gravity aft, toward the rear limit, will have which effect on the airplane's aerodynamic characteristics?
- ADecreased longitudinal stability and a lower stall speed
- BIncreased longitudinal stability and a lower stall speed
- CIncreased longitudinal stability and a higher stall speed
- DNo change in longitudinal stability, but increased stall speed
Show answer & explanationAnswer & explanation
Correct answer: A. Decreased longitudinal stability and a lower stall speed
As CG moves aft, the moment arm of the tail's stabilizing force is shortened, reducing longitudinal stability and making the airplane more prone to divergence in pitch. An aft CG also reduces the download normally required from the tail, allowing the wing to fly at a slightly lower angle of attack for a given speed, which slightly lowers stall speed but at the cost of reduced stability and spin recovery difficulty.
Why the other options are wrong
- B. Incorrect direction for stability effect.
- C. Reverses both effects; aft CG decreases stability and lowers stall speed.
- D. Incorrectly states no stability change and wrong stall speed direction.
CG Effect on Stability and Stall Speed
An aft CG decreases longitudinal stability (shorter tail moment arm) and slightly lowers stall speed, but increases spin/stall recovery difficulty.
- Aft CG = shorter tail arm = less stability
- Aft CG = less tail download needed = lower stall AOA/speed
- Aft CG increases risk of unrecoverable spins
Memory trick: Aft CG = 'tail loses its grip' on stability