Effects of Controls – Design and Stability

Types of stability:

• Positive Stability: The Aircraft wants to return to a stable state
• Neutral Stability: The aircraft will remain in its disturbed state
• Negative Stability: The Aircraft  will continue away from its stable state

• Tailplane has a –ve Angle of Attack, creating a downforce on the Tailplane
• Wing creates downwash onto the Tailplane
• Tailplane creates a Downward Force

When the Nose is Disturbed Up:  

•   Airspeed decreases
•   Lift reduces
•   Downwash on Tailplane reduces
•   Downward force of Tailplane reduces
•    Nose returns down to stable position

When the Nose is Disturbed Down:

•   Airspeed increases
•   Lift increases
•   Downwash on Tailplane increases
•   Downward force of Tailplane increases
•   Nose returns up to stable position

Stability in Small Aircraft

• Aircraft in normal flight suffers a pitch down disturbance
• Increased angle of attack on tail plane compared to main plane:
  • Causes greater % increase in lift on tail plane compared to main plane
  • Creates downwards force restoring moment about the C of G
• “Positive dynamic stability” means the aircraft will attempt to “level out” by itself
• Loading with CoG beyond aft limits could make vehicle uncontrollable in pitch

• Aircraft with dihedral in normal flight suffers roll disturbance:
  • Causing sideslip toward downward wing
    Dihedral causes an angle of attack on the downward wing to generate lift
  • Creating restoring roll moment
• Positive dynamic stability at small angles of bank
• Negative dynamic stability at high angles of bank (spiral tendency)

Dynamic Inherent Stability

• High wing aircraft in normal flight suffers roll disturbance:
  • Causing sideslip toward downward wing
  • Which creates a small sideways drag force
• The resulting aerodynamic force is displaced sideways i.e. no longer in direct alignment with C of G:
  • Which creates a restoring roll moment

• Aircraft experiences direction disturbance
• Resulting air flow acting between tail and C of G creates restoring yaw moment
• Positive dynamic stability

Washout is a twist in a wing that causes the wingtip to meet the airflow at a lower angle than the root in normal upright flight. It is usually applied as a design characteristic built into the wing, where the angle of attack is reduced span-wise from root to tip, typically 1° to 2°.

Washout creates a situation where the root of the wing stalls before the tip, softening the stall and allowing the ailerons to be functional deep in the stall.

Washout can be added after construction by slightly raising both ailerons. This is recommended for the maiden flights of a new model.

Reduce Angle of Attack to un-stall the aircraft!

Using Aileron to pick up a wing drop will increase the Angle of Attack on the dropping wing and stall it further creating a further wing drop.

Remember the aerofoil has stalled.

Therefore use rudder to keep straight!

How to un-stall

Simultaneously:

• Lower the nose
• Full Power
• Rudder to stop further wing drop (if needed)
  • Do Not use Aileron (more drag, loss of airspeed)!
• When recovered
  • Level the wings with Aileron
  • Regain normal flight

Power Off Stall Recovery

Power On Stall Recovery