On their own nacelles and pylons produce a small destabilizing moment when mounted on the wing and a small stabilizing moment when mounted on the aft fuselage
Movement of CG
Longitudinal static stability depends strongly on the location of cg
denote the cg location when = 0 =
•
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or where the airplane becomes neutrally stable is called the stick-fixed neutral point Stick-fixed implies that elevator was held fixed during angle of attack disturbance
Influence of CG
Stick-Fixed Neutral Point
Setting equal to zero and solving for the center of gravity position yields:
Aerodynamic Center = − = − ℎ: = − Neutral point is in essence the aerodynamic center of the entire aircraft
For most aircraft designs, it is desirable to have a stick-fixed static margin between 5% and 15% of the mean chord
Static Margin
Low Static Margin gives less static stability but greater elevator authority, whereas a higher Static Margin results in greater static stability but reduces elevator authority. Too much Static Margin makes the aircraft nose-heavy, which may result in elevator stall at take-off and/or landing. Whereas a low Static Margin makes the aircraft tail-heavy and susceptible to stall at low speed, e. g. during the landing approach.
Static Margin for Canard Configuration
For a better longitudinal stability, the canard should have higher lift coefficient and stall at lower geometric Angle of Attack than the main wing.
CG Movement During flight the CG can move substantially: •
As CG moves forward the aircraft becomes more stable –
The forward limit to CG position is limited by the moment that the tail can produce
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This is a function of tail lift and the tail volume
While stability improves with forward CG movement –
Drag increases, this increase is known as Trim Drag
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Aircraft maneuverability can suffer, larger control movements are required, and response becomes sluggish
When CG moves backwards –
Aircraft eventually becomes unstable
–
Trim drag reduces
CG Limits •
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The absolute limit for forward CG position is determined by aircraft handling being too sluggish to control effectively The absolute limit for rear CG position is the onset of instability, and aircraft handling being too sensitive to control Aircraft Designers and Regulatory Authorities impose a more restricted CG range in practice
CG Limits
Example (Nelson 2.1)
=0.3
If the slope of the versus curve is -0.15 and the pitching moment at zero lift is equal to 0.08, determine the trim lift coefficient. If the center of gravity of the airplane is located at , determine the stick fixed neutral point.
Example (Nelson 2.2) For the data shown in Figure, determine the following: (a) The stick-fixed neutral point. (b) If we wish to fly the airplane at a velocity of 125 ft/sec at sea level, what would be the trim lift coefficient and what would be the elevator angle for trim?
Example (Nelson 2.3) Analyze the canard-wing combination shown in the Figure. The canard and wing are geometrically similar and are made from the same airfoil section.
= = 0.2 = 0.45 a) Develop an expression for the moment coefficient about the center of gravity. You may simplify the problem by neglecting the upwash (downwash) effects between the lifting surfaces and the drag contribution to the moment. Also assume small angle approximations. b) Find the neutral point for this airplane.
Solution Same airfoil section +
=
= = =
Neglecting drag and using small angle approximation
H.W. Assignment # 2 Solve problems 2.2 to 2.3 from Nelson’s book Submission date: 7 Apr. 2015 Submit at the start of class on due date (even if you plan to be absent). No credit afterwards. Do not copy any assignment.
AAE 1760 Lesson 02-5
Flight Dynamics Power Effects
Effect on Static Long. Stability •
Direct Effects: Caused by forces developed by propulsion units
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Indirect Effects: Caused by propeller slipstream passing over wing or tail surfaces
Direct Effects •
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Thrust Effect: Effect on stability from the thrust acting along the propeller axis. Normal Force Effect: Effect on stability from a force normal to the thrust line and in the plane of the propeller.
Conclusion: The direct power effects are destabilizing if the power plant is ahead and below the cg
Indirect Effects •
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Downwash Effect: Downwash caused by the jet/propeller makes the tail trim contribution to be less negative or less stable than the power-off situation Slipstream Effect: Increased speed of slipstream impacting tail increases the tail contribution to stability
