Longitudinal Stability (Pitching)

[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:

1. Location of the wing with respect to the CG
2. Location of the horizontal tail surfaces with respect to the CG
3. Area or size of the tail surfaces

 

Notes:

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

So…
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 ↑

 

Read More –

# How CG and CP position affect nose pitching tendency?

CP, AOA , AC, CG – WHAT is…. and HOW is the relationship between

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