[Ref. Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 5-15]
Longitudinal Stability (Pitching)
Static longitudinal stability is dependent upon 3 factors:
increase of power may tend to make the nose rise – this can be offset by “high thrust line” – line of thrust passes above the CG;
In contrast, “low thrust line” would tend to add the nose-up effect.
Design for positive longitudinal stability – “Negative AOA on horizontal stabilizer” , “CG-CL-T”
- wing’s CL is to the rear of the CG – i.e CG-CL (Nose Heavy)
- By setting the horizontal stabilizer at a slight negative AOA – in order to let the downwash to balance the aircraft (conterbalancing the “heavy” nose) and prevent the nose continually pitching downward – i.e CG-CL-T
(Indeed, the downwash is air from the wings – This downwash strikes the top of the stabilizer and produces a downward pressure on tail)
Notes: Here, CL is also known as the center of pressure (CP)
HOW “Negative AOA on horizontal stabilizer” helps the aircraft back to its longitudinal stability?
Airspeed ∝ Downwash ⇒ Nose and Attitude Changed ⇒ Airspeed Changed ⇒ Downward force on the tail Changed
Airspeed ↑ ⇒ Downwash ↑ ⇒ Nose up, climbing attitude ⇒ Airspeed ↓ ⇒ Downward force on the tail ↓
Airspeed ↓ ⇒ Downwash ↓ ⇒ Nose down, pitch down attitude ⇒ Airspeed ↑ ⇒ Downward force on the tail ↑
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